NUREG/CR-7228

Open Secondary Testing of 

Window-Type Current 

Transformers 

Office of Nuclear Regulatory Research 

NUREG/CR-7228 

Open Secondary Testing of 

Window-Type Current 

Transformers

Office of Nuclear Regulatory Research 

Manuscript Completed: July 2016 

Date Published: May 2017 

Prepared by: 

M. Subudhi, P. Joshi1, S. Dimaiuta1 and W. Gunther2

Department of Nuclear Science and Technology 

Brookhaven National Laboratory 

Upton, New York 11973 

1 Superconducting Magnet Division 

2 Retired BNL Staff 

G. Taylor, NRC Project Manager 

iii 

ABSTRACT 

Two AMRAN Current Transformers (CTs) were tested to characterize the propensity of an opencircuited CT secondary to cause a fire in a location remote to the open circuit. Also evaluated was 

the magnitude of the secondary winding peak (crest) voltage1 when the CT’s secondary circuit 

transitioned to an open circuit condition, and its dependencies on the primary voltage, primary 

current and the CT’s turn ratio2. Parameters identified as having an effect on the CT’s open 

secondary crest voltage include: 

 Material and construction of the CT’s magnetic core, 

 CT’s primary voltage, 

 CT’s primary current, and, 

 CT’s turn ratio. 

The primary objective of this testing, however, was to better understand the following scenario: 

Will fire-induced open circuit in the secondary circuit of a wound-type CT, which is operating 

within its rated continuous primary current limits, result in an excessively high voltage in the 

secondary circuit sufficient to start a fire in the form of explosion or arcing in the circuit’s 

insulation at the location of the CT itself or at some other location in the secondary circuit? 

For the configurations tested, the results indicated that an open circuited CT was incapable of 

starting a secondary fire based on the criteria established in the test plan. This observation applies 

to the CT, CT taps, cabling, and burden devices. Additionally, when an arc was deliberately 

initiated in the secondary circuit, it was observed that the crest voltage diminished significantly. 

The test results provide supplemental information to the JACQUE-FIRE Volume 3 working group 

for developing technical recommendations to possibly extend the limit on the 1200:5 CT turns 

ratio limit as stated in the NUREG/CR-7150, Volume 1 

 

1 Throughout this report the open secondary “peak voltage” is identified as open secondary “crest voltage” consistent 

with the IEEE Standard C57.13-2008. 

2 Turn ratio is defined as the ratio of the number of secondary coil turns to primary coil turns around the magnetic core. 

v 

TABLE OF CONTENTS 

ABSTRACT ......................................................................................................................................... iii 

LIST OF FIGURES ............................................................................................................................... vii 

LIST OF TABLES ................................................................................................................................ ix 

EXECUTIVE SUMMARY ..................................................................................................................... xi 

ACKNOWLEDGMENTS ..................................................................................................................... xiii 

ACRONYMS ....................................................................................................................................... xv 

1 INTRODUCTION .............................................................................................................................. 1 

2 CURRENT TRANSFORMER CHARACTERISTICS UNDER OPEN SECONDARY 

CONDITION ...................................................................................................................................... 3 

3 TESTING APPROACH .................................................................................................................... 7 

3.1 Test Objectives ..................................................................................................................... 7 

3.2 Test Configuration and Setup .............................................................................................. 9 

3.2.1 Testing Facility ....................................................................................................... 9 

3.2.2 CT Test Circuit Configuration .............................................................................. 10 

3.2.3 Periodic Monitoring of the CT .............................................................................. 13 

3.2.4 Periodic Monitoring of the Secondary Cable ...................................................... 15 

3.2.5 Test Equipment Functionality Check .................................................................. 16 

3.3 Assumptions and Limitations ............................................................................................. 17 

3.4 Baseline CT Tests (No Secondary Open Circuit) .............................................................. 17 

3.5 Open Secondary Tests ...................................................................................................... 18 

4 TEST RESULTS ............................................................................................................................. 23 

4.1 Baseline Test Results ......................................................................................................... 23 

4.2 Open Secondary Tests ...................................................................................................... 27 

5 EVALUATION OF TEST RESULTS ............................................................................................. 35 

5.1 Effect of Primary Voltage/Current ...................................................................................... 35 

5.2 Effect of Primary Voltage and Turn Ratio .......................................................................... 36 

5.3 Effect of Primary Voltage/Current and Turn Ratio ............................................................. 36 

5.4 Impact of Arcing and Intermittent Opening ........................................................................ 37 

5.5 Other Effects ....................................................................................................................... 39 

5.6 Evaluation of Data to Test Plan Criteria ............................................................................. 40 

6 SUMMARY OF TEST RESULTS .................................................................................................. 41 

7 REFERENCES ............................................................................................................................... 43 

APPENDIX A OPEN SECONDARY TESTING OF 

WINDOW-TYPE CURRENT TRANSFORMERS – TEST PLAN ...................... A-1 

APPENDIX B DATA FROM ALL TESTS PERFORMED ......................................................... B-1 

vii 

LIST OF FIGURES 

Figure 2-1: Application of Window-Type CTs in Nuclear Power Plants ......................................... 3

Figure 2-2: Excitation Curve for a Typical Current Transformer .................................................... 4

Figure 3-1: Voltage versus Air Flash Gap ...................................................................................... 8

Figure 3-2: Test Power Supply Configuration .............................................................................. 10

Figure 3-3: Photograph of 2000:5 CT (Left), and 4000:5 CT (Right) ........................................... 10

Figure 3-4: Secondary Circuit Layout of the CT Testing .............................................................. 12

Figure 3-5: CT Testing Configuration and Data Logger ............................................................... 13

Figure 3-6: DC Resistance Testing of CT’s Secondary Winding ................................................. 14

Figure 3-7: ECG-Kokusai Model DWX-05 Impulse Tester .......................................................... 15

Figure 3-8: Example of Turn-To-Turn Short Indicating Compromised Insulation 

in the Winding ............................................................................................................. 15

Figure 3-9: HiPot Testing of Cable Insulation ............................................................................... 16

Figure 3-10: Typical Test Duration at 10,000 Steps/Sec ............................................................... 22

Figure 4-1: AM4CT - Secondary Voltage Under Normal CT Operation ...................................... 23

Figure 4-2: Inductance of Secondary Coil .................................................................................... 25

Figure 4-3: CT 2000:5 (X1-X2) Impulse Test Signature .............................................................. 25

Figure 4-4: CT 4000:5 (X1-X5) Impulse Test Signature .............................................................. 26

Figure 4-5: AM4CT01 Test – Primary Voltage at 500V (Typical) ................................................ 28

Figure 4-6: AM4CT01 Test – Primary Current Approximately at 400A RMS (Typical) ............... 28

Figure 4-7: AM4CT01 Test – RMS Primary Current Approx. at 400A (Typical) .......................... 29

Figure 4-8: AM4CT01 Test – Typical Secondary Current Harmonics for an 

Approximate Primary Current of 400A (or 4000AT) .................................................. 30

Figure 4-9: AM2CT01 Test – Open Secondary Crest Voltage (Typical) ..................................... 30

Figure 4-10: AM4CT01 Test – Open Secondary Crest Voltage (Typical) ..................................... 31

Figure 4-11: AM4CT01 Test – CT Temperature Rise Over 10 Minutes ........................................ 34

Figure 5-1: AM2CT - Open Secondary Crest Voltage Versus Primary 

Voltage/Current with Turn Ratio Fixed at 2000:5 ...................................................... 35

Figure 5-2: AM4CT - Open Secondary Crest Voltage with Varying Primary 

Voltage and Rated Turn Ratio .................................................................................... 36

Figure 5-3: AM4CT - Open Secondary Crest Voltage with Varying Primary 

Current at 500V .......................................................................................................... 37

Figure 5-4: Secondary Crest Voltage Characteristics Under Various Conditions ....................... 38

Figure 5-5: AM2CT03: Secondary Crest Voltage Under Intermittent Arcing ............................... 39

Figure 5-6: Secondary Crest Voltage in Tests AM2CT01 and AM4CT05 ................................... 40

ix 

LIST OF TABLES 

Table 3-1: Specification for Current Transformers Used in Testing ........................................... 11

Table 3-2: Maximum Current Achievable in the CT’s Primary Circuit ........................................ 18

Table 3-3: Test Matrix for AMRAN 2000:5 CT Tests .................................................................. 18

Table 3-4: Test Matrix for AMRAN 4000:5 CT Tests .................................................................. 19

Table 4-1: Sample Data Sheet for Open Secondary Tests ........................................................ 24

Table 4-2: Baseline HiPot Test Results of Instrument Cables .................................................... 27

Table 4-3: Test Results for AM2CT Test Series Given in Table 3-3 .......................................... 31

Table 4-4: Test Results for AM4CT Test Series Given in Table 3-4 .......................................... 32 

xi 

EXECUTIVE SUMMARY 

In October 2012, the Phenomena Identification and Ranking Table (PIRT) panel of the Joint 

Assessment of Cable Damage and Quantification of Effects from Fire (JACQUE-FIRE) program 

published NUREG/CR-7150, Volume 1 documenting an exercise conducted to better understand 

the state of knowledge related to fire-induced electrical circuit failures. As part of the work, the 

panel of technical experts reached a consensus on several long-standing fire protection circuit 

analysis issues. These recommendations were based on experts’ knowledge, experience, and 

insights from the PIRT and technical information (data) presented to the panel. One of those 

topics was the need to consider a fire-induced open-circuit on the secondary circuit of a CT that 

could result in a second fire at a different location than the initial fire that caused the damage. The 

PIRT panel recommended in Volume 1 of NUREG/CR-7150 that the secondary fires of those CTs 

with a turn ratio of 1200:5 or lower in power distribution systems are not a credible risk when the 

secondary circuit is opened. The PIRT panel also felt that the risk to fire safety is low for CT’s with 

higher turn ratios. The data to support this position, however, was not available. To resolve this 

concern, the PIRT panel recommended that additional testing be performed. 

Under the guidance of the JACQUE-FIRE Volume 3 working group, Brookhaven National 

Laboratory (BNL) performed testing on two AMRAN CTs, one rated at 2000:5 fixed-ratio and the 

other rated at 4000:5 multi-ratios, to characterize the propensity of an open-circuited CT 

secondary to cause a fire at a location remote to the open circuit. It was expected that the test 

results would provide a sound technical justification for treatment of CTs with higher turn ratios 

(>1200:5) in post-fire safe shutdown analysis. 

To address the above concern, BNL evaluated the test results associated with the magnitude of 

the secondary winding crest voltage when the CT’s secondary circuit transitioned to an open 

circuit condition, and its dependencies on the primary voltage, primary current and the CT’s turn 

ratio. Parameters that affect the open secondary crest voltage include the following: 

 The material and construction of the CT’s magnetic core It varies between 

manufacturers and even between different models from the same manufacturer. 

 Primary voltage source Crest voltage increases with increasing primary voltage 

under open secondary circuited conditions. This is contrary to CTs under normal 

operating condition, where primary voltage has little effect on secondary 

voltage. 

 Primary current level Crest voltage increases with increasing primary current 

under open circuited secondary conditions. 

 CT’s turn ratio Crest voltage increases with increasing CT’s turn ratio. 

The primary objective of this testing, however, was to better understand the following scenario: 

Will fire-induced open circuiting of the secondary circuit of a wound-type CT, which is operating 

within its rated continuous primary current limits, result in an excessively high voltage in the 

secondary circuit sufficient to start a fire in the form of explosion or arcing in the circuit’s 

insulation at the location of the CT itself or at some other location in the secondary circuit? 

Engineering principles and testing confirm that as long as current is flowing in the primary circuit, 

an open-circuit in CT’s secondary can cause high crest voltage on the secondary circuit as the CT 
xii 

attempts to maintain the current relationship dictated by the transformer’s winding turn ratio. This 

condition, in addition to shock hazard to personnel, can result in CT damage, potentially can 

generate voltages that may exceed the dielectric strength of the CT’s insulating materials, and 

may cause arcing to connected or nearby components. 

Additionally, high voltage induced on the secondary winding of a CT as a result of open-circuiting 

the CT’s secondary circuit due to a fire has the potential to cause the CT and/or the connected 

components to fail in a manner that could potentially start a secondary fire. A secondary fire, as 

used in this report, refers to a fire at a location remote from the original fire that is responsible for 

the initial open-circuit in the CT’s secondary circuit. 

From the CT’s physical location in the plant to the main control room instrument indications, the 

secondary circuit may consist of long (e.g., hundreds of feet) instrument wires whose insulation is 

susceptible to both initial and secondary fires. The resulting high voltage condition in the 

secondary from an open-circuited CT introduces a potential concern for fire protection strategies 

in nuclear power plants. Since the post-fire safe shutdown analysis is based on postulating a fire 

in one fire area at a time, the possibility of a second fire in a separate fire area can impact the final 

outcome of the fire protection strategies. 

For the configurations tested, the results indicated that an open circuited CT was incapable of 

starting a secondary fire based on the criteria established in the test plan. This observation applies 

to the CT, CT taps, cabling, and burden devices. Additionally, when an arc was deliberately 

initiated in the secondary circuit, it was observed that the crest voltage diminished significantly. 

This report describes the testing performed by BNL and documents the results. The data and 

video are available in electronic format in the CD-ROM provided with this publication. The 

information contained in this report is expected to support a technical recommendation by the 

working group for treatment of higher turn ratio CTs in post-fire safe shutdown analysis. 
xiii 

ACKNOWLEDGMENTS 

This test program was completed under the technical guidance of the Joint Assessment of Cable 

Damage and Quantification of Effects from Fire (JACQUE-FIRE) Volume 3 working group, which 

included staff from Brookhaven National Laboratory (BNL), the U.S. Nuclear Regulatory 

Commission (NRC), and the Electric Power Research Institute (EPRI). This project was 

sponsored by the NRC’s Office of Nuclear Regulatory Research (NRC-RES) with support from 

EPRI under a collaborative research agreement termed in the NRC-RES/EPRI Memorandum of 

Understanding (MOU). 

The authors thank the members of the working group, Harold Barrett, Robert Daley, Daniel Funk, 

Thomas Gorman, Shannon Lovvorn, Steven Nowlen, Andy Ratchford, and Gabriel Taylor for their 

technical guidance. We are grateful for the support of working group members’ organizations who 

cooperatively shared some or all of the cost and time for their participation in this study. 

We appreciate public comments on our test plan from Exelon Generation Company and Nuclear 

Energy Institute (NEI). 

Other participants of the working group who deserve a special mention include Gabriel Taylor 

(NRC Contracting Officer Representative), Mark Henry Salley, Tammie Rivera, and Mark Jeffers 

of NRC, and Ashley Lindeman of EPRI. We appreciate that the CTs tested were supplied by EPRI 

under the NRC-RES/EPRI memorandum of understanding for cooperative research. 

We thank the following BNL staff for their administrative and technical support: William Horak, 

James Higgins, Peter Kohut, Peter Wanderer, Glenn Jochen, Domenick Milidantri, Mary Chuc, 

and the safety committee members. 

Finally, we appreciate BNL’s Maria Anzaldi and Avril Woodhead for assistance in preparing and 

technical editing the report. 

xv 

ACRONYMS 

A or Amps Ampere(s) 

AC Alternating Current 

AM2CT AMRAN Fixed-Ratio CT 2000:5 

AM4CT AMRAN Multi-Ratio CT 4000:5 

ANSI American National Standards Institute 

AT Ampere-Turn 

AWG American Wire Gauge 

BIL Basic Insulation Level 

BNL Brookhaven National Laboratory 

CFR Code of Federal Regulations 

CT Current Transformer 

CD Compact Disc 

DC Direct Current 

EMF Electro-Motive Force 

EPRI Electric Power Research Institute 

EQ Equipment (or Environmental) Qualification 

FR Federal Register 

H Henry 

IEEE Institute of Electrical and Electronics Engineers 

JACQUE-FIRE Joint Assessment of Cable Damage and Quantification of Effects from Fire 

kV Kilovolt 

kW Kilowatt 

mH Millihenry 

MOU Memorandum of Understanding 

MMF Magneto-Motive Force 

MR Multi-Ratio 

NEI Nuclear Energy Institute 

NPP Nuclear Power Plant 

NRC Nuclear Regulatory Commission 

PIRT Phenomena Identification and Ranking Table 

PT Potential Transformer 

RES Office of Nuclear Regulatory Research 

RMS Root Mean Square 

STD Standard 

V Volt(s) 

VA Volt-Ampere 

VT Voltage Transformer 

1 

1 INTRODUCTION 

Current transformers (CTs), a sub-group of instrument transformers, are used throughout 

alternating current (AC) electrical distribution systems in nuclear power plants (NPPs). These 

devices monitor current levels at select locations and provide a signal from their secondary 

winding that is proportional to the current flowing through the main (primary) winding (e.g., cable, 

bus bar). CTs measure the primary current through magnetic coupling and thus do not have a 

physical connection between the primary and secondary circuits. Therefore, CTs provide isolation 

from the high voltage and high current in the primary circuit that being monitored. 

The CT’s secondary current signal is commonly used for relay protection and indication circuits 

(both local and remote). In many cases the protective relays and/or indicators associated with the 

CT are located at the same locations as the CT. For these cases, the secondary circuit is typically 

confined to the switchgear/equipment containing the CT. In other instances, the secondary circuit 

of the CT may provide a signal to remotely located protective relay(s) or indicator(s), e.g., 

differential protective relays and remote ammeters. As such, the secondary circuit may span 

numerous compartments within a NPP. These latter cases present a potential safety concern, as 

discussed below. 

Safety Concern 

Engineering principles and testing confirm that as long as current is flowing in the primary circuit, 

an open-circuit in CT’s secondary can cause high crest (or peak) voltage on the secondary circuit 

as the CT attempts to maintain the current relationship dictated by the transformer’s winding turn 

ratio. This condition, in addition to shock hazard to personnel, can result in CT damage, potentially 

can generate voltages that may exceed the dielectric strength of the CT’s insulating materials, and 

may cause arcing to connected or nearby components. 

In a letter to the NRC dated July 21, 1983 (Ref. 1), Brookhaven National Laboratory (BNL) raised 

a potential concern associated with fire-induced open-circuit in a CT’s secondary circuit. The letter 

postulated the scenario in which potentially high voltage induced on the secondary winding of a 

CT as a result of open-circuiting the CT’s secondary circuit due to a fire, ultimately causing the CT 

and/or the connected components to fail in a manner that could potentially start a secondary fire. 

A secondary fire, as used in this report, refers to a fire at a location remote from the original fire 

that is responsible for the initial open-circuit in the CT’s secondary circuit. 

From the CT’s physical location in the plant to the main control room instrument indications, the 

secondary circuit may consist of long (e.g., hundreds of feet) instrument wires whose insulation is 

susceptible to both initial and secondary fires. The resulting high voltage condition in the 

secondary from an open-circuited CT introduces a potential concern for fire protection strategies 

in NPPs. Since the post-fire safe shutdown analysis is based on postulating a fire in one fire area 

at a time, the possibility of a second fire in a separate fire area can impact the final outcome of the 

fire protection strategies. Currently NRC-endorsed (Ref. 2) industry guidance1 (Ref. 3) for 

conducting a post-fire safe shutdown circuit analysis identifies circuit failures due to an open 

circuit. An example provided in Section 3.5.2.1 of NEI 00-01, Rev. 2 (Ref. 3) includes: 

1 When used in conjunction with RG 1.189, “Fire Protection for Nuclear Power Plants,” industry guidance document NEI 

00-01, “Guidance for Post Fire Safe Shutdown Circuit Analysis,” Rev. 2, provides one acceptable method for 

conducting a post-fire safe shutdown circuit analysis. 
2 

Open circuits on a high voltage (e.g., 4.16 kV) ammeter current transformer (CT) 

circuit may result in secondary damage, possibly resulting in occurrence of an 

additional fire in the location of the CT itself. 

In October 2012, the Phenomena Identification and Ranking Table (PIRT) panel of the Joint 

Assessment of Cable Damage and Quantification of Effects from Fire (JACQUE-FIRE) program 

published NUREG/CR-7150, Volume 1 (Ref. 4) documenting an exercise conducted to better 

understand the state of knowledge related to fire-induced electrical circuit failures. As part of the 

work, the panel of technical experts reached a consensus on several long-standing fire protection 

circuit analysis issues. These recommendations were based on experts’ knowledge, experience, 

and insights from the PIRT and technical information (data) presented to the panel. One of those 

topics was the need to consider a fire-induced open-circuit on the secondary circuit of a CT that 

could result in a second fire at a different location than the initial fire that caused the damage. The 

PIRT panel recommended in Volume 1 of NUREG/CR-7150 (Ref. 4) that the secondary fires of 

those CTs with a turn ratio of 1200:5 or lower in power distribution systems are not a credible risk 

when the secondary circuit is opened. CTs with a higher turn ratio the PIRT panel felt that the risk 

to fire safety is low. However, this may experience damage to their insulation, and conceivably 

could fail. Such failure would involve the breakdown of insulation due to arcing, as is evidenced in 

higher voltage power distribution lines in a plant’s switchyard. 

Based on the experts’ judgement, the PIRT panel also determined that the concern was more 

theoretical than real for the types of CTs (i.e., open window type) found within the AC electrical 

distribution system of NPPs. However, the data available to support this recommendation did not 

cover the range commonly encountered within NPPs. Although the panel believed this to be true 

for higher than 1200:5 turn ratio CTs, the lack of data along with the concern of the phenomena 

becoming more pronounced as turn ratios increase, resulted in the panel’s conclusion that this 

concern could not be excluded for CTs with turn ratios greater than 1200:5. To resolve this 

concern, the PIRT panel recommended that additional testing be performed. 

This report describes the testing that was performed under the guidance of the JACQUE-FIRE 

Volume 3 working group and documents the results. All data from the testing program is publically 

available without restriction on the CD-ROM provided with this publication and on the NRC website 

(www.NRC.gov). The information contained in this report is expected to support a technical 

recommendation for treatment of higher turn ratio CTs in post-fire safe shutdown analysis. 
3 

2 CURRENT TRANSFORMER CHARACTERISTICS 

UNDER OPEN SECONDARY CONDITION 

Different types of current transformers (CTs) are used in nuclear power plants (NPPs), including 

wound, bar, window, bushing, auxiliary, and ground sensor types. However, the window-type 

dominates the types of CTs used in NPP’s AC power distribution system applications and is the 

focus of this research. The window-type CTs considered here have a laminated core of high 

permeability steel with a secondary winding insulated from and permanently assembled on the 

core. The window-type CTs have no primary winding as an integral part of the CT structure. The 

primary winding (bus bar or cable) is located through the window of the CT. Figure 2-1 below 

shows the different applications of window-type CTs around conductors or bus bars inside 

electrical enclosures. 

 

Figure 2-1: Application of Window-Type CTs in Nuclear Power Plants 

Under normal operating conditions, a CT reproduces a scaled-down current waveform of the 

current flowing in the primary circuit (e.g., based on the specific phase of the three-phase AC 

power distribution system being monitored) at the point of measurement. This scaled-down 

current can then be used by protective relays, metering, and other applications. The alternating 

current in the primary winding (known as excitation current) produces an alternating magnetic field 

in the core, which then induces an alternating current in the secondary winding circuit. The 

primary and secondary circuits are magnetically coupled so that the secondary current is linearly 

proportional to the primary current over an intended normal operational range. 

Since the coupling between the primary circuit and the secondary is via CT’s magnetic core, it is 

important that the operation of the CT in the excitation curve, shown in Figure 2-2, defines its 

behavior. Normally the CT operates in the linear portion (Non-saturated Zone 1) of the excitation 

curve (i.e., primary current = secondary current x turn ratio); while under open secondary 

condition it operates near or above its knee (Intermediate Zone 2 or Saturated Zone 3), the 

nonlinear portion of the excitation curve that is closely similar to that of a voltage transformer (VT). 

However, under this abnormal condition the CT still attempts to maintain the current ratio (i.e., 

primary ÷ secondary). 
4 

Figure 2-2: Excitation Curve for a Typical Current Transformer 

With current flowing in both primary and secondary circuits, the CT's exciting current (and hence 

the corresponding inductive reactance or impedance of the primary coil) typically is very small 

under the normal operating conditions (Refs. 5, 6 & 7). The secondary current’s magneto-motive 

force (mmf) keeps in check the magnetizing flux in the CT’s core. The secondary voltage depends 

upon the secondary current, and the impedance (burden) of the meter and relaying circuitry. The 

secondary circuit of the CT functions as a current source; thus, as the burden on the secondary 

side of the CT increases, the secondary voltage of the CT increases proportional to the burden. 

When the secondary circuit opens, the impedance essentially will be infinite. An ideal CT would 

have an infinite secondary voltage. In reality, normal losses (such as core loss, core saturation, 

leakage inductance, and finite insulation resistance) will limit the actual voltage. Under opencircuited secondary conditions, the total primary current becomes the exciting current, comprising 

a magnetizing current plus any additional current necessary to overcome core (iron) loss, 

consisting of losses caused by eddy currents and hysteresis. Thus, the primary current’s 

magneto-motive force (mmf) produces an exciting current “orders of magnitude” greater than that 

under normal conditions. The resultant large increase in flux density produces a high voltage in 

the secondary circuit and will likely raise the density of the core flux to saturation. As long as the 

primary current is present, the CT operates near the knee (i.e., the magnetizing saturation) of the 

excitation curve at 60 Hz, similar to a voltage transformer. 

The turn ratio of a CT was considered as one of the influencing parameters that affect the 

amplitude of the voltage spike during open circuit conditions. The higher the turn ratio the greater 

the voltage spike at a fixed AC frequency (e.g., 60 Hz line current)1. In accordance with IEEE 

Standard C57.13-2008 (Ref. 8), Section 6.7.1, Operation with Secondary Circuit Open, 

 

1 The open secondary voltage spike is increased by higher operating frequency. In all our testing, we kept this constant 

at the line frequency of 60 Hz. 
5 

“... Transformers conforming to this standard shall be capable of operating under 

emergency conditions for 1 minute with rated primary current times the rating 

factor with the secondary circuit open if the open-circuit voltage does not exceed 

3500 V crest. 

When the open circuit voltage exceeds 3500 V peak, the secondary winding 

terminals should be provided with voltage limiting devices (varistors or spark gaps). 

...” 

Once the secondary circuit’s cable conductors have opened, a high voltage in terms of voltage 

spikes is assumed to exist on the CT's secondary. Under the postulated fire event the cable’s 

insulation and conductor are assumed to have been damaged by the fire and resulted in an open 

circuit. This location where the postulated fire damage causes the open circuit, along with the 

connections at the CT’s secondary terminals, are the most likely points of failure (least resistance) 

in the circuit. Other potential failure locations in the secondary circuit may include the burden (e.g., 

the ammeter, relay) and degraded insulation locations due to the influence of aging or other 

deleterious effects2. Since any arcing that may occur as a result of the high voltage is assumed to 

occur at these points of least resistance (weak link), the most probable locations for arcing are 

assumed to occur at these points. 

 

2 Note that this testing did not consider degraded insulation or physical damage in the secondary circuit, which could 

have compounded the potential for secondary fire ignition source.

7 

3 TESTING APPROACH 

3.1 Test Objectives 

The objective of this testing was to develop data that could be used to inform technical decisions 

regarding the necessity to postulate secondary fires due to fire damage of current transformers 

(CTs). The purpose of this test program is to better understand the following scenario: 

Will fire-induced open circuiting of the secondary circuit of a wound-type CT, which is 

operating within its rated continuous primary current limits, result in an excessively high 

voltage in the secondary circuit sufficient to start a fire in the form of explosion or arcing in 

the circuit’s insulation at the location of the CT itself or at some other location in the 

secondary circuit? 

To evaluate the plausibility of this scenario, the testing focused on evaluating the transient 

characteristics1 of the core’s magnetic behavior, along with the effects of primary voltage and 

current at 60 Hz line frequency on the secondary voltage peaks (or crests) as it transitioned from 

normal conditions to an abnormal open secondary condition. 

To accomplish the objective of this research, the tests focused on characterizing the transition of 

the exciting (or magnetizing) current from the very low magnitude under normal operating 

conditions to an open secondary condition with no current in the secondary but high voltages that 

could act as a fire ignition source. The testing assumed that an open-circuit condition of an 

energized CT occurred (due to fire damage). The open-circuit is expected to cause abnormally 

high voltages in the secondary circuit, provided that the flow of the primary current continues. The 

desired outcome of this testing is to determine whether the possibility exists for the high voltage to 

cause sustained arcing or a catastrophic failure of the CT (or other secondary circuit elements) 

that is sufficiently energetic to initiate a secondary fire that damages components or equipment 

beyond the CT’s circuit and its components. 

Therefore, we postulated the following two specific conditions that could be affected by the 

saturation of the CT’s magnetic core and the high voltage in the open secondary circuit: 

 The CT itself gets overheated after being exposed to a very long core saturation period or 

an arcing occurs at the CT’s secondary taps which may need over 20-40 kV crest voltage2

for an air gap of 1-2 inches (Ref. 9). The data presented in Figure 3-1 correspond to 

ionization voltage levels of air gap distance for alternating current (AC) and direct current 

(DC) circuits. These failures are postulated to create a localized secondary fire at the CT’s 

cabinet or location.

 

1 A small portion of normal rated primary current is sufficient to saturate the core. However, there is a very short interval 

in each half cycle as the current passes through zero when the magnetic flux is very rapidly whisked from saturation 

value in one direction to saturation value in the other direction. It is this exceedingly rapid rate of change of flux during 

the short interval is responsible for the high open secondary voltage (Ref. Kaufmann and Camilli, Transactions of the 

AIEE, Vol. 62, July 1943). 

2 This voltage range is based on Figure 3-1, which is replotted from “A Basic Stun Gun Concept,” Chemlec © 2007 & 

2011. 
8 

 The open secondary crest voltage in the secondary circuit exceeds the breakdown voltage 

of the cable’s insulating system.3 

A draft test plan was prepared and the NRC requested public comments in the Federal Register 

80FR46061, dated August 3, 2015 [Docket NRC-2015-0183/FR-2015-18997]. A final version of 

this test plan after addressing all public comments received is provided in Appendix A. 

Figure 3-1: Voltage versus Air Flash Gap 

As stated in the Test Plan, to achieve the overall objective in this test program (i.e., excessively 

high voltage in the secondary circuit sufficient to start a fire in the form of explosion or arcing in the 

circuit’s insulation at the location of the CT itself or at some other location in the secondary circuit), 

the following test conditions were considered: 

 Primary Voltages4: 500V, 250V, 125V 

 Two AMRAN CT Types: Fixed-Ratio 2000:5 CT; Multi-Ratio 4000:5 CT 

 For the Fixed-Ratio of 2000:5: Varied primary current of 60A to 4000A 

 For the Multi-Ratio CT: Varied turn ratios of 500:5 to 4000:5 (or primary current ratings) 

and primary current for fixed turn ratios at 2000:5 and 4000:5 

 Open secondary mode: Fast open; Intermittent opening; Arcing simulations 

 Thermal measurements of CT surface and electrical enclosure air space. 

Note that the performance of subsequent tests, as stated in Section A.4.3 of the Test Plan were 

not undertaken due to lack of funding. 

 

3 Insulation failure (breakdown) could potentially occur at lower voltage if the cable insulation system contains a latent 

defect or has been damaged. However, no tests were performed to investigate this variable. 

4 Test voltage was limited to 500V due to 1) limitations of the power supply available, 2) test personnel safety concerns, 

and 3) logistical burden of high energy testing. The working group judged that the voltage limitation would not 

preclude obtaining representative data for higher voltage applications. 
9 

3.2 Test Configuration and Setup 

A series of tests were conducted to demonstrate the impact of an open circuit on a CT’s 

secondary has on the secondary crest voltage. This is important in determining if the crest 

voltages on the secondary are high and energetic enough to damage the insulation and pose a 

potential fire ignition source to other components. Thus, a total of 63 tests in 51 different test 

configurations involving two CTs (AM2CT – 29 configurations and AM4CT– 34 configurations) 

were completed. 

3.2.1 Testing Facility 

The CT testing was conducted in the high bay area of the Brookhaven National Laboratory 

Superconducting Magnet Division test facility. The test facility has various AC- and DC-controlled 

power sources ranging from 480 Volts, 1,600 Amps AC to 30 Volts, 30,000 Amps DC. Associated 

with these power sources, each test facility has a state-of-the-art high speed multi-channel data 

acquisition system with the sampling frequency of up to 1 MHz and 16 bit resolution. 

Figure 3-2 shows the power supply circuit for the CT tests. The primary current for the CT under 

test was supplied by a three-phase (delta/wye) source connected to a three-phase delta 

connected variable load bank. The power source was configured to a nominal voltage of 125VAC, 

250VAC or 500VAC, through a 1600 Amp switch and a remotely controlled 700 Amp circuit 

breaker. The CT being tested connected to one of the legs (Leg A) and the supply current to the 

primary circuit can be adjusted to get the required value by manipulating the resistance of the load 

bank5. Due to limitations on the available current from the power supply, the leg of the power 

supply acting as the primary circuit of the CT was wrapped around and through the CT window 10 

times. Theoretically, this configuration should accurately simulate the higher current; however it is 

not representative of a typical installation. Baseline tests of the test configuration indicated 

expected system behavior. Current in one phase will be used to generate the necessary AmpereTurns in the primary of the CT under test. For example, to test a 4000:5 CT, ten (10) turns of 

Ultraflex 500MCM cable was wrapped around the core of the CT to obtain an almost 4000 

ampere-turn in the primary. Exact current in that phase was measured and recorded using three 

in-house 800:5 CTs. 

 

5 Originally-planned load bank discussed in the test plan was replaced with a new modern portable load bank with 

resistance switches for easier manipulation of resistors. 
10 

Figure 3-2: Test Power Supply Configuration 

3.2.2 CT Test Circuit Configuration 

Two different models of AMRAN CTs were provided under the cooperative research agreement 

[Memorandum of Understanding (MOU)] between the US Nuclear Regulatory Commission (NRC) 

Office of Nuclear Regulatory Research (RES) and the Electric Power Research Institute (EPRI). 

Six units of each model were provided, for a total of 12 CTs. Figure 3-3 shows a photograph of the 

actual CTs tested. The 2000:5 CTs (identified as “AM2CT”) are of the fixed-ratio type, while the 

4000:5 CTs (identified as “AM4CT”) are multi-ratio. Both CTs meet the ANSI/IEEE C57.13 

Standard, and their outer encapsulations were enclosed in plastic-cases. Table 3-1 provides the 

specifications of the CTs tested. 

Figure 3-3: Photograph of 2000:5 CT (Left), and 4000:5 CT (Right) 
11 

Table 3-1: Specification for Current Transformers Used in Testing 

Parameter AM2CT AM4CT 

Serial # AMRAN 019591 AMRAN 605A419/F226114 

Part # CT101-202-04 CT102-402MR 

Insulation Level 600V; Basic Insulation Level 

(BIL) 10kV Full Wave 600V; BIL 10kV Full Wave 

DC Resistance 0.3329 Ω @ 24°C (measured) 

0.375 Ω @ 75°C (rated) 

0.1268 to 0.8684 Ω @ 24°C (measured) 

0.0015 Ω/Turn @ 75°C (rated) 

AC Inductance 

@ 60Hz 3.9938 henry (H) (Measured) 183.82mH to 7.621 H (Measured) 

AC Inductance 

@ 1kHz 500 mH (Measured) 26.161mH to 1.54H (Measured) 

Turn Ratio(s) 2000:5 

500:5 

1000:5 

1500:5 

2000:5 

2500:5 

3000:5 

3500:5 

4000:5 

The circuit diagram in Figure 3-4 illustrates the actual6 test setup for obtaining the characteristics of 

an open-circuited CT. As stated above, the test power supply feeds the CT’s primary side to 

represent the typical current flow in a power circuit. Approximately 100 feet of secondary cable with 

a burden resistor (i.e., an ammeter) was used to simulate an actual plant’s typical configuration. The 

CT’s secondary side was instrumented with a relay7 for intermittently opening the circuit or an 

instantaneous circuit break. The increase in the secondary voltage and decrease in secondary 

current was recorded via high voltage isolation modules. A computerized high speed data 

acquisition system was used to capture the CT secondary voltage and current. Other parameters 

monitored during testing included primary current (I1 for Harmonics and I3 for its RMS values in 

Figure 3-4) and primary voltage (V2 in Figure 3-4)8, and the surface temperature of the CT. A high 

speed video camera (D2 in Figure 3-4) also was used to capture the arcing and fire formation (if 

any) at several strategic locations. These cameras were synchronized with the high speed data 

acquisition system to get secondary circuit characteristics during the arcing process (if any). 

 

6 Figure 3-4 represents a modified circuit diagram of Figure A-7 presented in the test plan. Additional parameters V2 for 

primary voltage, I3 for RMS primary current, and D2 for digital camera were added to the data logger. 

7 This is a high voltage relay, with automatic delay but positive closing or opening after turn on, turn-off or loss of power, 

manufactured by Ross Engineering Corp (Model EV40-NC-40). 

8 V2: Primary voltage for the 4000:5 CT testing, while the same input was used for Temperature of the test cabinet 

environment for the 2000:5 CT testing. Primary voltage of 125V, 250V, or 500V and line frequency of 60 Hz were kept 

fixed and monitored from the load bank meters as well. 
12 

Figure 3-4: Secondary Circuit Layout of the CT Testing 

For safety reasons, the CTs under evaluation were placed inside a metal enclosure (i.e., test 

cabinet), as shown in Figure 3-5. The data logger used for these tests is also shown in this figure. 

The data logger used separate National Instrument Cards: for slow log (Model NIPXI-6123, S.N. 

013E2CA7, Calibration 11/3/14) and for fast log (Model NIPXI-4300, S.N. 0197E732, Calibration 

7/8/14). At BNL, the National Instrument Data Acquisition Channels for capturing signals are 

calibrated against Agilent 34410A Multimeter on a 2-year calibration cycle. The recent Certificate 

of Calibration dated 6/30/16 (on file at BNL) indicates these components within calibration. 

The test plan in Appendix A provides a complete description of the data acquisition system. 
13 

Figure 3-5: CT Testing Configuration and Data Logger 

3.2.3 Periodic Monitoring of the CT 

During each test we observed the secondary tap connections for any electrical arcing or fire 

damage and the CT’s core for its temperature rise. The high speed camera also was taking video 

clips of the CT’s secondary taps for the entire test duration. Since each video clip needed a file 

size of the order of 900 megabytes to 1 gigabyte, it was not saved unless some sort of damage 

was noted during the test. For those test cases simulating arcing video clips of shorter duration of 

significance were saved. 

In addition, the following two types of condition monitoring tests (Refs. 10, 11, 12) were performed 

periodically to assess the condition of the CT’s secondary winding after it had been subjected to 

crest voltages during several open secondary tests. 

1. DC Resistance Test 

Figure 3-6 shows the DC resistance test setup of a new CT. This test also was performed in situ 

on the CT’s secondary winding to examine if any turn of the winding was shorted or degraded due 

to high crest voltage distributed among all turns during open secondary tests. This was achieved if 

the DC resistance of the secondary winding had changed from its baseline value. 

To obtain the DC resistance of the secondary coil, one ampere of current from a precision current 

source (HP Model 6634A, S.N. 31451A02586) was injected into the secondary winding of the CT 

via its secondary taps. Voltage drop across the secondary winding was then measured with a 

digital precision volt meter (HP Model 34401A, S.N. 3146A24334). These instruments were 

calibrated on 2/8/16 and are within calibration per the BNL 2-year calibration cycle. 

The resistance value thus obtained was compared with the baseline DC resistance value included 

in Table 3-1. If no significant change in the resistance value was noted, the CT’s secondary 

winding was assumed to be in good condition. 
14 

Figure 3-6: DC Resistance Testing of CT’s Secondary Winding 

2. Impulse Test 

CT’s secondary winding was periodically impulse-tested to determine if degradation had occurred. 

This testing was performed using an ECG-Kokusai Model DWX-05 Impulse Tester (S.N. 

2004321) shown in Figure 3-7. Recent calibration dated 4/1/16 indicates that this instrument was 

within calibration. 

A capacitor charged to high voltage of 1kV (a set value) is discharged into the CT’s winding or coil 

under test. This voltage sets up a nonlinear voltage distribution in the coil that creates a turn-toturn voltage difference (Ref. 13). The voltage decay waveform is generated in response to the 

impulse, related to the Q-factor9 and inductance of the coil. The resulting waveform is captured by 

high speed data logger and displayed on the screen. This waveform is compared with an 

internally-stored baseline (or master) waveform obtained from the original condition of the coil. If 

the error between the original and the resulting test waveform is less than 5% (a set value), then it 

passes the winding test, otherwise it fails. Thus, the output of this test is either “pass” or “fail” 

indication on the screen. 

Figure 3-8 illustrates an example of a compromised winding that failed the impulse test. Spike(s) 

in the signal indicates turn-to-turn short (highlighted inside a rectangular box) compromising the 

insulation in the winding. 

In case of the AM2CT, this test was performed by connecting the Impulse Tester to the two 

secondary taps (X1-X2). For the multi-ratio AM4CT, the impulse test was performed by 

connecting to the two secondary taps (X1-X5), which includes the entire secondary winding. 

 

9 Q-factor generally is a measure of the ratio of stored versus lost energy per unit time. 

Volt Meter

1ADC Source 
15 

Figure 3-7: ECG-Kokusai Model DWX-05 Impulse Tester 

Figure 3-8: Example of Turn-To-Turn Short Indicating Compromised Insulation in the 

 Winding 

3.2.4 Periodic Monitoring of the Secondary Cable 

During each test we observed for any electrical arcing or damage to the entire length of the 

secondary cable and its connection points to the relay, ammeter, and other components. 

In addition, the secondary cable also was periodically high potential (HiPot) tested (also known as 

the dielectric withstand test) at 10 kV using HiPot Testers (Associated Research Inc. S.N. 50245 
16 

for 5kV and S.N. 2090 for 15 kV, Calibration 1/19/15), shown in Figure 3-9, and the leakage 

currents were recorded for assessing the condition of its insulating system. 

Figure 3-9: HiPot Testing of Cable Insulation 

Three different conductor configurations were included in this testing: (1) white conductor and 

black conductor/shield together, (2) shield and black/white together, and (3) shield/black/white 

together against the steel plate on which the cable was lying. The former two configurations yield 

leakage currents for the two conductors (white and black) of the secondary cable in reference to 

the shield. The third configuration provides the leakage current across the jacket of the cable in 

reference to the steel plate which was grounded. 

High voltage was applied between the two test terminals in steps and then held at that 

voltage for a minute or so until the leakage current stabilized. The leakage current was then 

recorded and compared with baseline values recorded earlier. Higher values of the leakage 

current when compared to their baseline values would have been an indication of damaged 

insulation in the cable. 

3.2.5 Test Equipment Functionality Check 

Several shakedown tests on the 2000:5 CT were conducted before the actual tests outlined in the 

test plan. This was done to ensure that the entire test set up as discussed in the test plan (see 

Appendix A) was complete and functional, and to ensure that the data logging instrumentation 

was accurately calibrated for collecting the expected information. These shakedown tests included 

evaluating the following: 

Primary circuit line’s current flow level, as appropriate, by adjusting the load bank resisters 

Secondary circuit maximum current flow of 5 amps through the ammeter 

Data logger appropriately collecting current/voltage of the primary and secondary circuits, and 

temperatures from the four thermal probes mounted on the CT or the test cabinet 

Secondary side open circuit mechanism working (i.e., high voltage relay) 
17 

Camera setup taking high speed videos of the targeted locations and synchronizing the collection 

of data by the data logger 

DC insulation resistance of the CT’s secondary winding (see Section 3.2.3) 

CTs tested for secondary winding resistance and subjected to impulse tests to determine the 

shape of the impulse voltage for comparisons. Any distortion in the wave shape is the criterion of 

a CT’s winding failure. (see Section 3.2.3) 

HiPot testing of the secondary cable’s insulations (see Section 3.2.4) 

3.3 Assumptions and Limitations 

No known test program, excluding an in-situ testing, possibly could simulate an actual real-life plant 

condition or event. In conducting the open secondary testing to characterize CT’s behavior, certain 

assumptions were made that could limit or contribute to uncertainties in the final test outcome. 

a. The two CTs tested were manufactured by AMRAN, with an insulating system rated at 600V, 

BIL of 10kV. Their construction and application represent CTs that typically are used in both 

low and medium voltage AC power distribution systems of nuclear power plants. 

b. Since the testing was focused on characterizing the propensity of an open-circuited CT 

secondary to cause a fire in a location remote to the open circuit, no effort was made to 

characterize the magnetic saturation that occurs during open secondary condition. 

c. A high voltage relay was used to cleanly open the secondary circuit. We also postulated 

that fire-induced open circuits could be intermittent. We, therefore, modified our 

programming to simulate intermittent opening of the high voltage relay for determining the 

differences in test results between clean and intermittent opening. 

d. Once the secondary circuit was open, the relay chattering or arcing between two adjacent 

conductors (i.e., intermittent opening) was simulated by using two conductors at the relay 

opening with a gap of 2 mm (0.1 inch) or smaller. 

e. CTs were fully functional and operating without any degradation from previous tests. 

Dielectric conditions of the CT under test and the secondary cabling were monitored 

periodically using dielectric strength and impulse test equipment when there was indication 

of potential degradation (e.g., high temperature rise or arcing). 

f. Because of the limitations in resister combinations in the load bank used in the primary 

circuit, the primary or line current of 200 amperes at 250V and 100 amperes at 125V could 

not be increased up to 400 amperes line current available. 

g. The determination made related to secondary fires relied primarily upon visual observation 

of the test specimens. 

3.4 Baseline CT Tests (No Secondary Open Circuit) 

The primary circuit consisting of a 60 Hz, Delta 3-Φ, AC source was connected to a Simplex 

dynaMITE 300 Portable Load Bank, rated 315 kW at 480V and 289 kW at 240V. The load bank is 

configured for 250V and 500V and therefore, was manipulated to determine the maximum line 

current achievable in each phase of the circuit at the Auto Transformer power supply voltages for 

the power supply voltage levels. For 125V, the load bank switches were manipulated to assure to 

the degree possible that the desired levels of primary current were achieved. 
18 

These tests defined the limits in the maximum AC line current in each phase as listed in Table 3-

2. With 10-loops10 of the primary cable around the magnetic core of the CT under test, this line 

current would exhibit a maximum primary current in the CT’s core equal to 10 times this maximum 

line current in ampere-turns (ATs). 

Table 3-2: Maximum Current Achievable in the CT’s Primary Circuit 

 Voltage Range Maximum Line Current Maximum CT Primary Current 

110-125 V ~100 A ~1000 AT 

220-250 V ~200 A ~2000 AT 

480-500 V ~400 A ~4000 AT 

Baseline tests were then performed using the setup of a 2000:5 CT to simulate its in-plant 

configuration. This did not involve opening the secondary circuit. In addition, the data logger, the 

camera, and tests of the insulation resistance of the secondary cable were not used during these 

tests. During each of the tests the desired voltage level and current was verified. This was 

achieved by using the resistance load combination switches in the load bank determined in an 

earlier set of tests. The actual and expected currents and voltages in both the primary and 

secondary circuits were obtained via the high speed data logger for each test set up. The multichannel fast data logger was controlled via LabVIEW real-time programming for fast-open and 

intermittent-open of the secondary. 

3.5 Open Secondary Tests 

Because of the limitations on the maximum primary current, the number of open secondary tests 

at each source voltage range was limited. Tables 3-3 and 3-4 present the test matrices for the 

AMRAN 2000:5 CT (AM2CT) and AMRAN 4000:5 CT (AM4CT), respectively. Note that all values 

indicated in these tables are the root mean square (RMS) values of their parameter harmonic 

signals. For example, 500V primary voltage is the RMS value of a voltage signal that has a peak 

value of around 707VAC (i.e., 1.414x500V). 

Table 3-3: Test Matrix for AMRAN 2000:5 CT Tests 

Test Identification Primary Voltage 

(Volts) 

Primary 

Current 

(Amps) 

CT’s Turn 

Ratio 

Secondary 

Current (Amps) 

Secondary CT 

Taps 

AM2CT01 480-500 2000 2000:5 5.00 X1-X2 

AM2CT02 480-500 1500 2000:5 3.75 X1-X2 

AM2CT03 480-500 1000 2000:5 2.50 X1-X2 

AM2CT04 480-500 500 2000:5 1.25 X1-X2 

AM2CT05 480-500 250 2000:5 0.62 X1-X2 

 

10 The test plan indicated 7-loops based on an expected 600A line current. With the limitation of 400A found in each 

phase of power supply, this was changed to 10-loops to obtain our expected 4000 amperes maximum in the primary 

circuit. Based on this, Figure 3-5 was updated to indicate 10-loops. 
19 

Table 3-3: Test Matrix for AMRAN 2000:5 CT Tests 

Test Identification Primary Voltage 

(Volts) 

Primary 

Current 

(Amps) 

CT’s Turn 

Ratio 

Secondary 

Current (Amps) 

Secondary CT 

Taps 

AM2CT06 480-500 125 2000:5 0.31 X1-X2 

AM2CT07 220-250 2000 2000:5 5.00 X1-X2 

AM2CT08 220-250 1500 2000:5 3.75 X1-X2 

AM2CT09 220-250 1000 2000:5 2.50 X1-X2 

AM2CT10 220-250 500 2000:5 1.25 X1-X2 

AM2CT11 220-250 250 2000:5 0.62 X1-X2 

AM2CT12 220-250 125 2000:5 0.31 X1-X2 

AM2CT13 220-250 62 2000:5 0.15 X1-X2 

AM2CT14 110-125 1000 2000:5 2.50 X1-X2 

AM2CT15 110-125 500 2000:5 1.25 X1-X2 

AM2CT16 110-125 250 2000:5 0.62 X1-X2 

AM2CT17 110-125 125 2000:5 0.31 X1-X2 

AM2CT18 110-125 62 2000:5 0.16 X1-X2 

AM2CT19 480-500 2500 2000:5 6.25 X1-X2 

AM2CT20 480-500 3000 2000:5 7.50 X1-X2 

AM2CT21 480-500 4000 2000:5 10.0 X1-X2 

In Table 3-4, the first 14 tests were originally planned. At the request of the working group 

members, 16 additional tests were conducted to collect data supporting comparison between the 

two different CTs tested. 

Table 3-4: Test Matrix for AMRAN 4000:5 CT Tests (Continued) 

Test Identification Primary Voltage 

(Volts) 

Primary 

Current 

(Amps) 

CT’s Turn 

Ratio 

Secondary 

Current (Amps) 

Secondary CT 

Taps 

AM4CT01 480-500 4000 4000:5 5.00 X1-X5 

AM4CT02 480-500 3500 3500:5 5.00 X2-X5 
20 

Table 3-4: Test Matrix for AMRAN 4000:5 CT Tests (Continued) 

Test Identification Primary Voltage 

(Volts) 

Primary 

Current 

(Amps) 

CT’s Turn 

Ratio 

Secondary 

Current (Amps) 

Secondary CT 

Taps 

AM4CT03 480-500 3000 3000:5 5.00 X1-X4 

AM4CT04 480-500 2500 2500:5 5.00 X2-X4 

AM4CT05 480-500 2000 2000:5 5.00 X1-X3 

AM4CT06 480-500 1500 1500:5 5.00 X2-X3 

AM4CT07 480-500 1000 1000:5 5.00 X3-X4 

AM4CT08 480-500 500 500:5 5.00 X1-X2 

AM4CT09 220-250 2000 2000:5 5.00 X1-X3 

AM4CT10 220-250 1500 1500:5 5.00 X2-X3 

AM4CT11 220-250 1000 1000:5 5.00 X3-X4 

AM4CT12 220-250 500 500:5 5.00 X1-X2 

AM4CT13 110-125 1000 1000:5 5.00 X3-X4 

AM4CT14 110-125 500 500:5 5.00 X1-X2 

AM4CT15 480-500 4000 4000:5 5.00 X1-X5 

AM4CT16 480-500 3000 4000:5 3.75 X1-X5 

AM4CT17 480-500 2000 4000:5 2.50 X1-X5 

AM4CT18 480-500 1000 4000:5 1.25 X1-X5 

AM4CT19 480-500 500 4000:5 0.625 X1-X5 

AM4CT20 480-500 250 4000:5 0.31 X1-X5 

AM4CT21 480-500 125 4000:5 0.155 X1-X5 

AM4CT22 480-500 62 4000:5 0.08 X1-X5 

AM4CT23 480-500 4000 2000:5 10.0 X1-X3 

AM4CT24 480-500 3000 2000:5 7.5 X1-X3 
21 

Table 3-4: Test Matrix for AMRAN 4000:5 CT Tests (Continued) 

Test Identification Primary Voltage 

(Volts) 

Primary 

Current 

(Amps) 

CT’s Turn 

Ratio 

Secondary 

Current (Amps) 

Secondary CT 

Taps 

AM4CT25 480-500 2000 2000:5 5.00 X1-X3 

AM4CT26 480-500 1000 2000:5 2.50 X1-X3 

AM4CT27 480-500 500 2000:5 1.25 X1-X3 

AM4CT28 480-500 250 2000:5 0.625 X1-X3 

AM4CT29 480-500 125 2000:5 0.31 X1-X3 

AM4CT30 480-500 62 2000:5 0.155 X1-X3 

Fifty-one open secondary test configurations were tested using both fixed-ratio CT 2000:5 

(AM2CT) and multi-ratio CT 4000:5 (AM4CT) as listed in the test matrices given in Tables 3-3 and 

3-4. Additional tests of certain test configurations were performed to simulate the effects of long 

duration, test repeatability, intermittent relay opening, time step optimization, and other conditions 

such as arcing. Thus, a total of 63 tests involving two CTs were performed. 

Each open secondary test typically lasted for 30 seconds. The opening relay remained open for 

about 5-6 seconds during which the data logger registered the “TRANSIENT” data for 10,000 time 

steps per second (i.e., each step being 100 microseconds). The first 1000 steps simulate the 

normal condition, followed by opening of the secondary circuit for the remaining 9000 time steps. 

As soon as the secondary circuit was opened, the secondary current becomes zero and the 

secondary voltage increases. The primary circuit remained constant for the entire 30 seconds. 

Another set of “CONTINUOUS” data also was recorded each second for the entire 30 seconds (or 

10 minutes, in a few tests) to capture the temperature rise in the CT. Figure 3-10 presents the 

timeline of a typical test cycle and the data collection sequences. All relay opening and data 

collection sequences were automated using LabVIEW real-time programming computer code. 

Several additional tests were repeated varying other test parameters, for example, with the relay 

open in the secondary for about 5 and 10 minutes to obtain the effect of the high secondary 

voltage and core saturation on the secondary cable’s insulation resistance, the temperature rise in 

the CT, and the change in the CT’s winding resistance. The sampling rate of 10,000 steps per 

second was used so that the spikes of the open secondary voltage were captured. Based on the 

shake down test results, doubling the sampling rate (from 5,000 to 10,000 steps per second) 

increased the open secondary crest voltage by a few volts (e.g., AM2CT07: 2,874V to 2,880V). 

Nevertheless, all baseline and open secondary tests were performed at 10,000 steps sampling 

rate (i.e., 100 Microsecond steps). Several other tests involved arcing simulation at the relay 

opening, intermittent opening of the relay, and examining the repeatability of each test. 
Figure 3-10: Typical Test Duration at 10,000 Steps/Sec

Secondary 

Relay

Opens

1000 frames 9906 frames

T=0 sec 0.1 sec 1.0905 sec

Pri

R

C

Da

Primary Current

Applied and 

Continuous Data

Collection Starts

Transient

Data

Collection

Ends

Transient

Data

Collection

Starts

5-6 sec 

22
23

4 TEST RESULTS 

Test results for each test setup were hand-written in a data sheet as shown in RED in Table 4-1. 

The data logger records the unique date and time of each test. In addition to the eight entries’ 

peak values from the data logger, each sheet contains any deviation from the test matrices that 

are listed in Tables 3-3 and 3-4, and any visual observations and video recordings. Also, included 

were the periodic HiPot results of the secondary cable’s insulation and the DC resistance of the 

CT’s secondary coil along with the pass/fail results of impulse testing. Arcing simulation video 

clips, and data logger files in EXCEL format involving all 63 tests are provided in the attached 

compact disc (CD). See Appendix B. 

4.1 Baseline Test Results 

Baseline tests were conducted first on a 2000:5 CT to ensure that the test setup simulated the 

plant’s normal operating condition. That is, at 2000 amps primary current the secondary current 

ammeter reading was 5 amps and the crest secondary voltages were around 20V1. Similar tests 

were made on the 4000:5 CT’s set up; they exhibited the expected results as shown in Figure 4-1 

below, indicating no dependence on the primary voltage levels. Note that since all test results 

presented in Figure 4-1 correspond to a secondary current at 5 amperes, the horizontal axis also 

represents the CT’s turn ratios (e.g., 500:5 through 4000:5). 

Figure 4-1: AM4CT - Secondary Voltage Under Normal CT Operation 

1 In the normal configuration of AM2CT and AM4CT, the secondary voltage was found to be independent of the 

primary voltage levels (i.e., 500V, 250V, and 125V), and dependent on the primary current levels. For example, at 

rated current of 2000:5 for the AM2CT, the peak secondary voltage was around 20V depending on the current and 

the impedance of the meter and the relay circuit in the secondary, while at lower secondary current level (i.e., 

<2000A) the peak secondary voltage was as low as 15V and at higher secondary current level (i.e., >2000A) the peak 

secondary voltage was as high as 25V. 
Table 4-1: Sample Data Sheet for Open Secondary Tests 

TEST DATA SHEET SH

CT TEST IDENTIFICATION: AM2CT21; CT Serial Number: AM019591; CT Winding Resistance: 0.375 Oh

DATE PERFORMED: 1-4-16; OPEN SECONDARY MODE: FAST; TURN RATIO: 2000:5

TIME TEST WAS PERFORMED: 1602.05

Nominal 

Primary 

Voltage 

Source Vp

(Volts) 

Primary

Current 

RMS/Peak 

Ip

(Amps/Turn)

Secondary 

Voltage Vs (+/-) 

Spikes 

 (Volts) 

Peak 

Secondary 

Current

Is

(Amps)*

Final Temperature Readings in oC 

Second

Curr

T1 at 

Outside 

CT (0 

degree) 

T2 at 

Outside 

CT (180 

degrees) 

T3 at 

Inside

CT (90 

degrees) 

T4 Inside 

CT 

Cabinet

500

Nominal

3808/4494

(~4000)

Nominal

3717/(-3755)

3736

Average

13.64

(~10RMS)

23.69 22.86 24.11 20.87

B+W to 

B to W

B+W+G

CT Secondary Winding Resistance (Ohms) Video Camera Observations and Frames Recorded 

0.33527

Impulse Test “PASSED”

None 

* Note that the secondary current becomes zero soon after opening the secondary

Data Input By: MSubudhi; Initials: MS Test Duration: ___1000____|_______9000_______________~1 S

Checked By: Data Logger Record Load Bank Setting: 315 kW

Reference Document/File on Data Records:

24
25 

Testing of CT’s Secondary Winding 

Figure 4-2 presents the inductance of the secondary coil in each of the tested CTs. This 

component is responsible for keeping the magnetic flux in the core in check. The DC resistance of 

each CT’s secondary coil also was measured for its normal condition and the values are included 

in Table 3-1 along with the manufacturer’s rated values for the tested CTs. The DC resistance 

values were compared with the baseline data given in Table 3-1 to periodically evaluate the 

condition of the insulation of the CT’s secondary winding. 

Figure 4-2: Inductance of Secondary Coil 

Figure 4-3 for AM2CT and Figure 4-4 for AM4CT present the baseline voltage signatures of the 

impulse tester. To monitor the condition of the secondary winding, as stated earlier in Section 

3.2.3, 1kV voltage pulse was introduced into the CT’s winding periodically and the impulse tester 

compared within 5% error in the impulse signature with the baseline signature to determine 

whether the winding was in good condition for the subsequent tests. 

Figure 4-3: CT 2000:5 (X1-X2) Impulse Test Signature 
26 

Figure 4-4: CT 4000:5 (X1-X5) Impulse Test Signature 

Secondary Cable Insulation Test Parameters 

Table 4-2 presents the HiPot testing results of three different instrument cables known to be used 

in nuclear power plants. One of them was used in the CT testing and the other two were taken 

from BNL’s storage from the environmental qualification (EQ) test on cables performed for NRC 

during the late 90’s. 

 
27 

Table 4-2: Baseline HiPot Test Results of Instrument Cables 

Cable Used in CT Testing Cable Used in EQ Testing 

Commercial-Grade, General 

Cable, PVC/Nylon/PVC, 

Shielded, AWG 16, 2/C, 600V 

(No Drain or Ground Wire) 

Samuel Moore, 20 mil Dekoron 

(EPDM)/10 mil Dekorad (CSPE) 

Bonded Jacket/45 mil Dekorad 

(CSPE) Jacket, 600V, 2/C, AWG 16, 

Shielded and Ground, Manufactured 

1982 

Rockbestos, 30mil XLPE/45 mil 

Neoprene Jacket, 600V, 2/C, AWG 

14, Firewall III, Manufactured 1979 

White (W) Wire to Black (B) 

Wire/Shield (S)* Red Wire to Violet Wire/Shield Wire to Wire 

5kV: 0.0 microampere 5kV: 0.0 microampere 5kV: 0.0 microampere 

10kV: 1.0 microampere 10kV: 0.25 microampere 10kV: 0.05 microampere 

15kV: 3.5 microampere 15kV: 6.0 microampere 15kV: 1.4 microampere 

W/B to Shield** Red/Violet Wire to Shield No Shield Wire 

5kV: 0.0 microampere 5kV: 0.0 microampere - 

10kV: 0.1 microampere 10kV: 1.5 microampere - 

15kV: 7.0 microampere 15kV: 10.0 microampere - 

W/B/S to Metal Plate*** W/B/S to Metal Plate W/B/S to Metal Plate 

5kV: 0.0 microampere 5kV: 0.0 microampere 5kV: 0.0 microampere 

10kV: 5.5 microampere 10kV: 0.5 microampere 10kV: 2.0 microampere 

15kV: 20.0 microampere 15kV: 3.0 microampere 15kV: 13.0 microampere 

* White conductor and black/shield shorted together 

** Shield and white/black shorted together 

*** Metal plate on which the cable was placed and White/black/shield shorted together for jacket 

integrity 

Leakage currents in Table 4-2 demonstrate that the instrument cables typically used in NPPs 

have very high dielectric strength even at 15 kV and can withstand higher voltage levels even 

after 40 years. In-between the open secondary tests, periodically the secondary cable used in this 

testing was disconnected from the CT and the burden ammeter followed by high potential (HiPot) 

testing of the cable’s insulating system. The DC leakage current at 10 kV was closely monitored to 

assess the condition of the cable’s insulation. 

New secondary cable was used from the start of all open secondary tests performed on each 

CT’s test configuration. The primary cable configuration remained the same for both CT 

configurations with ten turns around each CT. 

4.2 Open Secondary Tests 

Open secondary tests were performed on both 2000:5 and 4000:5 CTs as outlined in the test 

matrices listed in Tables 3-3 and 3-4. The data sheet shown in Table 4-1 was used to register 

each set of test data obtained from the instrumentation including the data logger. Also included 

were physical and visual evidences gathered during each test. Any deviations in the test, such as 

longer duration, intermediate opening of the secondary circuit, different sampling rate by the data 

logger, simulation of arcing in the secondary circuit, also were detailed in these data sheets. 
28 

Out of fifty-one test conditions, twenty-one tests on 2000:5 CT (AM2CT) and thirty test conditions 

on 4000:5 CT (AM4CT) were conducted. In each test, the primary voltage and primary current 

remained constant and independent of what was happening in the secondary circuit (i.e., from a 

closed secondary circuit to an open secondary configuration). Figures 4-5 and 4-6 present the 

typical harmonic signatures of the primary voltage and current for the open secondary Test # 

AM4CT01 in Table 3-4. Similar primary voltage and current profiles typically were seen in all 

AM2CT and AM4CT open secondary tests listed in Tables 3-3 and 3-4 for all three source voltage 

levels (i.e., 125V, 250V, and 500V). There was no change in these signatures even though the 

entire primary current became the excitation current in an open secondary configuration. The 

primary voltage remained unchanged from its set value for all of the transition period. 

Figure 4-5: AM4CT01 Test – Primary Voltage at 500V (Typical) 

Figure 4-6: AM4CT01 Test – Primary Current Approximately at 400A RMS (Typical) 
29 

Figure 4-7 presents the root mean square (RMS)2

 value of the primary current in Figure 4-6 set for 

each open secondary test. This value was used in setting the level of the primary current by 

manipulating the load bank resisters in the primary circuit. Once set at a value closer to a pre-set value 

as listed in Tables 3-3 and 3-4, this parameter was monitored for its transient behavior during the 

opening of the secondary circuit. As shown in Figure 4-7, the primary current remained unchanged 

during the transient from closed secondary configuration to open secondary one, even though under 

the latter, the entire primary current became the excitation current for the core’s magnetics. 

 

Figure 4-7: AM4CT01 Test – RMS Primary Current Approx. at 400A (Typical) 

For the first 1000 time steps, Figure 4-8 presents the typical secondary current under a normal 

operating configuration in a nuclear power plant. This represents a secondary current of 5 amperes 

RMS corresponding to a turn ratio of 4000:5. Once the relay was opened to simulate an open 

secondary condition, the secondary current dropped to zero for the remaining ~9000 time steps. 

 

2 This is the only test parameter whose RMS value was recorded by the data logger. All other test parameter data are 

peak values, unless it is noted otherwise. 
30 

Figure 4-8: AM4CT01 Test – Typical Secondary Current Harmonics for an Approximate 

 Primary Current of 400A (or 4000AT) 

Figures 4-9 and 4-10 illustrate typical secondary crest voltages for the AM2CT and AM4CT at 

their rated turn ratios (i.e., 2000:5 and 4000:5) and primary voltage of 500V, respectively. 

Figure 4-9: AM2CT01 Test – Open Secondary Crest Voltage (Typical) 
31 

Figure 4-10: AM4CT01 Test – Open Secondary Crest Voltage (Typical) 

Table 4-3 lists the results from the series of tests (21) performed on 2000:5 Fixed-Ratio CT 

(AM2CT), while Table 4-4 those from 4000:5 Multi-Ratio CT (AM4CT). Each open secondary 

crest voltage presented in these tables is the average of the plus-side peak and the minus-side 

peak values. The typical difference between these two peaks is less than 2% of the mean value. 

Out of the 30 tests on AM4CT, the first 14 were performed at their rated turn ratios and the 

remaining 16 tests were performed at constant turn ratios of 4000:5 and 2000:5, while varying the 

primary current levels. 

Table 4-3: Test Results for AM2CT Test Series Given in Table 3-3 (Continued) 

Test 

Identification 

Nominal* 

Primary 

Voltage in 

Volts 

RMS Primary 

Current in 

Ampere-Turns

CT Turn 

Ratio 

Closed 

Secondary 

Peak Current 

in Amperes 

Opened 

Secondary 

Crest 

Voltage in 

Volts 

AM2CT01 500 1953 2000:5 7.07 3612 

AM2CT02 500 1445 2000:5 5.27 3489 

AM2CT03 500 976 2000:5 3.55 3299 

AM2CT04 500 450 2000:5 2.41 2947 

AM2CT05 500 260 2000:5 1.10 2672 

AM2CT06 500 147 2000:5 0.67 2265 

AM2CT07 250 2003 2000:5 7.35 2880 

AM2CT08 250 1521 2000:5 5.57 2808 

AM2CT09 250 1010 2000:5 3.77 2684 
32 

Table 4-3: Test Results for AM2CT Test Series Given in Table 3-3 (Continued) 

Test 

Identification 

Nominal* 

Primary 

Voltage in 

Volts 

RMS Primary 

Current in 

Ampere-Turns

CT Turn 

Ratio 

Closed 

Secondary 

Peak Current 

in Amperes 

Opened 

Secondary 

Crest 

Voltage in 

Volts 

AM2CT10 250 527 2000:5 2.05 2479 

AM2CT11 250 257 2000:5 1.14 2212 

AM2CT12 250 129 2000:5 0.57 1842 

AM2CT13 250 69 2000:5 0.33 1588 

AM2CT14 125 961 2000:5 3.58 2053 

AM2CT15 125 514 2000:5 2.01 1948 

AM2CT16 125 266 2000:5 1.09 1796 

AM2CT17 125 120 2000:5 0.58 1591 

AM2CT18 125 76 2000:5 0.32 1312 

AM2CT19 500 2505 2000:5 8.97 3584 

AM2CT20 500 3049 2000:5 10.9 3628 

AM2CT21 500 3808 2000:5 13.64 3736 

*Transient data was not recorded. Load bank voltage input data recorded instead. 

Table 4-4: Test Results for AM4CT Test Series Given in Table 3-4 (Continued) 

Test 

Identification 

Peak/Nominal 

Primary 

Voltage in Volts 

RMS Primary 

Current in 

AmpereTurns 

CT Turn 

Ratio 

Closed 

Secondary 

Peak Current 

in Amperes 

Opened 

Secondary 

Crest 

Voltage in 

Volts 

AM4CT01 718/500 3848 4000:5 6.94 6138 

AM4CT02 718/500 3590 3500:5 7.17 5262 

AM4CT03 722/500 3022 3000:5 7.25 4436 

AM4CT04 722/500 2526 2500:5 7.30 3612 

AM4CT05 719/500 2016 2000:5 7.35 2745 

AM4CT06 724/500 1517 1500:5 7.41 2025 

AM4CT07 724/500 1027 1000:5 7.62 1342 

AM4CT08 724/500 513 500:5 7.98 625 
33 

Table 4-4: Test Results for AM4CT Test Series Given in Table 3-4 (Continued) 

Test 

Identification 

Peak/Nominal 

Primary 

Voltage in Volts 

RMS Primary 

Current in 

AmpereTurns 

CT Turn 

Ratio 

Closed 

Secondary 

Peak Current 

in Amperes 

Opened 

Secondary 

Crest 

Voltage in 

Volts 

AM4CT09 356/250 1948 2000:5 7.12 2200 

AM4CT10 357/250 1513 1500:5 7.43 1625 

AM4CT11 359/250 1009 1000:5 7.53 1085 

AM4CT12 360/250 528 500:5 8.13 522 

AM4CT13 179/125 955 1000:5 7.14 828 

AM4CT14 180/125 515 500:5 7.92 410 

AM4CT15 715/500 3864 4000:5 6.89 6127 

AM4CT16 720/500 3141 4000:5 5.29 5912 

AM4CT17 721/500 2047 4000:5 3.67 5611 

AM4CT18 726/500 1034 4000:5 1.90 5098 

AM4CT19 729/500 527 4000:5 1.01 4576 

AM4CT20 729/500 260 4000:5 0.55 4034 

AM4CT21 728/500 145 4000:5 0.34 3379 

AM4CT22 728/500 60 4000:5 0.12 2346 

AM4CT23 715/500 3856 2000:5 13.77 3013 

AM4CT24 717/500 3011 2000:5 10.79 2920 

AM4CT25 720/500 2027 2000:5 7.33 2773 

AM4CT26 725/500 1026 2000:5 3.80 2526 

AM4CT27 727/500 522 2000:5 2.01 2260 

AM4CT28 726/500 269 2000:5 1.10 2012 

AM4CT29 727/500 146 2000:5 0.55 1696 

AM4CT30 728/500 70 2000:5 0.16 1176 

Temperature measurements of the CT’s core were monitored throughout the entire duration of 

each test performed on AM2CT and AM4CT. In all cases the temperature remained unchanged at 

room temperature shown in Figure 4-11 and therefore, they are not included in Tables 4-3 or 4-4. 
34 

Figure 4-11: AM4CT01 Test – CT Temperature Rise Over 10 Minutes 

Several special cases involving a longer duration, intermittent opening of the secondary, and 

arcing simulation in the secondary circuit are discussed in Section 5.
35 

5 EVALUATION OF TEST RESULTS 

5.1 Effect of Primary Voltage/Current 

Unlike a voltage transformer (VT), under normal condition CT’s primary voltage has minimal effect 

on its operation (see Figure 4-1). However, the primary current level, because of the CT’s inherent 

turn ratio, has significant effect on the instrumentation readout of the secondary current. 

The primary voltage, along with the primary current and the turn ratio, has an effect on the CT’s 

behavior under an abnormal open secondary condition. Figure 5-1 illustrates the dependencies of 

the open secondary crest voltage with the primary voltage and primary current levels keeping the 

turn ratio constant. The results presented here are taken from the AM2CT testing given in Table 

4-3. This clearly indicates that the open secondary crest voltage is dependent on the primary 

current as well as the primary voltage. 

Figure 5-1: AM2CT - Open Secondary Crest Voltage Versus Primary Voltage/Current with 

Turn Ratio Fixed at 2000:5 

Figure 5-1 shows three plots for primary voltage levels of 500V, 250V, and 125V. At each primary 

voltage level, the primary current level was varied from 500 Amp-Turn (AT) through 4000 AT 

where possible. In all cases at its rated turn ratio value of 2000:5, as the secondary is opened the 

crest voltage increases instantly to a value higher than 1000V and then as the primary current 

continues to increase it starts flattening. This flattening is likely due to the core saturation effect on 

the CT’s magnetic core. 
36 

5.2 Effect of Primary Voltage and Turn Ratio 

A similar observation on the dependence of the primary voltage also is evident from the test 

results presented in Table 4-4 on AM4CT keeping turn ratio at its rated values from 500:5 through 

4000:5 where possible. Since the AM4CT has multi-ratio secondary taps, in the first 14 test cases 

presented in Table 4-4 the secondary current was maintained at 5 amperes under normal 

condition as the primary current was changed from 500 AT through 4000 AT. This also 

corresponds to changing the turn ratio from 500:5 through 4000:5. 

Interestingly, it clearly is illustrated in Figure 5-2 that the open secondary crest voltage is 

dependent on the primary voltage when the test results of the 4000:5 CT were plotted from Table 

4-4. At each primary voltage level of 500V, 250V, and 125V, the open secondary crest voltage is 

linearly dependent on their rated values for a multi-ratio CT. Because of the limitations in the 

primary line current level at 125V, only two data points are plotted and therefore, the linear 

regression has higher uncertainty. However, it does indicate linear relationship taking the zero as 

one additional data point, consistent in all three voltage classes. 

Figure 5-2: AM4CT - Open Secondary Crest Voltage with Varying Primary Voltage and 

Rated Turn Ratio 

Figure 4-1 presents the results corresponding to normal operating condition of the 4000:5 CT at 

three primary voltage levels. The same three cases corresponding to an open secondary 

condition are plotted in Figure 5-2. Comparing both figures one can conclude that primary voltage, 

while has no effect during normal operation of a CT, has significant effect on the open secondary 

crest voltage.

5.3 Effect of Primary Voltage/Current and Turn Ratio 

Figure 5-3 presents similar characteristics shown in Figure 5-1 when the CT’s turn ratios were 

kept fixed at 4000:5 and 2000:5 in the multi-ratio CT (AM4CT). When results from Figure 5-2 are 

compared for the primary voltage level of 500V, Figure 5-3 illustrates the independent behavior of 
37 

the open secondary crest voltage when operated at their rated conditions versus at a constant 

turn ratio. 

Figure 5-3: AM4CT - Open Secondary Crest Voltage with Varying Primary Current at 500V 

One additional and important conclusion observed in all tests is that we did not have any 

secondary fire or evidence for an arc occurring either at the CT’s secondary taps or within the 

secondary cable’s insulating system. The temperature probes on the CT indicated no significant 

change even under a saturated core (see Figure 4-11). As indicated in Table 4-2, the insulating 

system of the secondary cable could withstand 10kV or higher without any significant leakage 

current. Therefore, the worst cases with open secondary crest voltage of nearly 6,000V crest 

would not have broken down the cable’s insulating system. 

5.4 Impact of Arcing and Intermittent Opening 

Additional tests of the AM4CT01 were performed to examine the effects of arcing at the opening 

relay (or relay chattering) and to simulate an intermittent relay opening on the crest secondary 

voltage. Figure 5-4 presents a comparison of these results with a normal case of open secondary 

(see Top Plot). 

If electrical arcing, persistent or intermittent, occurred in the open secondary circuit then the 

secondary crest voltage diminished significantly during the sustained arcing period (see Middle 

Plot). This also was postulated by the PIRT panel1 (Ref. 4). However, the crest voltage always 

bounced back to its peak value for the duration when the secondary remained in the open 

 

1 The PIRT panel stated that as soon as an arc forms at a location in the cable, the voltage of the CT’s circuit will drop 

because of the secondary flux that develops, opposing the primary current. With this decline in the voltage, the arc will 

be extinguished, allowing the process to repeat itself. This fault condition will continue until the CT’s associated load is 

de-energized. 
38 

condition without arcing (see Figure 5-52). Similar behavior also was noted when clean (or 

controlled) intermittent opening of the secondary circuit occurred (see Bottom Plot). 

Based on the above, for a secondary location to become an ignition source for fire it would require 

a crest secondary voltage high enough to cause immediate damage to the secondary cable’s 

insulating system. There will be insufficient energy to cause a secondary fire once arcing starts at 

any location in the secondary circuit. We did not observe insulation failure of the secondary cable 

in any of our tests. 

Figure 5-4: Secondary Crest Voltage Characteristics Under Various Conditions 

 

2 Figure 5-5 represents the test data for Test AM2CT03 and the crest voltage for this case is 3,299V (see Table 4-3). 

This Figure simulates relay chattering or random arcing contacts at the opening of the secondary. 
39 

. 

Figure 5-5: AM2CT03: Secondary Crest Voltage Under Intermittent Arcing 

5.5 Other Effects 

Test Repeatability 

Several cases were repeated to understand if the same test conditions exhibit similar results. In all 

of them it was found that all tests are repeatable with results within less than 5% experimental 

bias of the crest secondary voltage. 

Test Duration Effect on CT’s Temperature 

To better understand the effect of long duration open secondary condition on the CT’s 

temperature, several tests of AM2CT01 and AM4CT01 were performed for 5 or 10 minutes. Even 

for this long duration at a secondary crest voltage of over 6 kV, no evidence of significant 

temperature change was noted. 

Effect of Magnetics 

Figure 5-6 compares the secondary crest voltage for two cases involving AM2CT and AM4CT 

from the same manufacturer when the primary voltage at 500V, the primary current at 2000 ATs, 

and the turn ratio of 2000:5 were kept the same. It clearly indicates that the open secondary crest 

voltage values are very different while the trend remains similar. This difference indicates the 

magnetics of the CT’s construction may have contributed and significantly affects the open 

secondary crest voltage. This is consistent with the difference in the inductance values of the 

secondary winding shown in Figure 4-2. This inductive component is responsible for keeping 

magnetizing flux in the core in check during CT’s operation. 
40 

Figure 5-6: Secondary Crest Voltage in Tests AM2CT01 and AM4CT05 

5.6 Evaluation of Data to Test Plan Criteria 

The test plan (Appendix A.5) identified the following three criteria to support the working groups 

assessment of test data developed here. Those criteria are: 

Neither the CT nor its secondary circuit shows any signs of arcing or explosive failure. 

Either the CT or its secondary circuit does show signs of arcing, but the duration is short 

and near the CT, or the arc is of insufficient intensity to ignite surrounding materials farther 

from the CT. 

The CT does exhibit explosive failure, but the explosive behavior is near the CT and insufficient to 

damage surrounding materials. The explosive failure is not accompanied by subsequent arcing of 

the CT or its secondary circuit. 

Based on the 63 tests performed there was not a single observance of any one of these criteria. 
41 

6 SUMMARY OF TEST RESULTS 

The primary objective of this testing was to better understand the following scenario: Will fireinduced open circuiting of the secondary circuit of a wound-type CT, which is operating within its 

rated continuous primary current limits, result in an excessively high voltage in the secondary 

circuit sufficient to start a fire in the form of explosion or arcing in the circuit’s insulation at the 

location of the CT itself or at some other location in the secondary circuit? 

The CTs tested at BNL are typically used for low and medium voltage applications. The most 

important conclusion observed in all tests is that we did not have any secondary fire or evidence 

for an arc occurring either at the CT’s secondary taps or within the secondary cable’s insulating 

system. The temperature probes on the CT indicated no significant change even under a 

saturated core (see Figure 4-11). As indicated in Table 4-2, the insulating system of the 

secondary cable could withstand 10kV or higher without any significant leakage current. 

Therefore, the worst cases with open secondary crest voltage of nearly 6,000V crest would not 

have broken down the cable’s insulating system. When an arc was deliberately initiated in the 

secondary circuit at the high voltage relay, it was observed that the crest voltage diminished 

significantly. For the configurations tested, the results from 63 individual tests indicated that an 

open circuited CT was incapable of starting a secondary fire based on the criteria established in 

the test plan. This observation applies to the CT, CT taps, cabling, and burden devices. 

Based on the results discussed in previous sections, the following findings either were observed 

or are derived from the test results. These results may be of use to the working group in 

developing guidelines on the question of secondary fires resulting from open circuits in CT’s 

secondary, including extending the limit on the 1200:5 CTs as stated in the NUREG/CR-7150, 

Volume 1 (Ref. 4). 

Parameters that affect the open secondary crest voltage include the following: 

The material and construction of the CT’s magnetic core. 

It varies between manufacturers and even between different models from the same manufacturer 

(see Figure 5-6, where the primary voltage was at 500V and the turn ratio was constant at 2000:5 

while the primary current was varied). 

Primary voltage source (see Figures 5-1 & 5-2). 

Crest voltage increases with increasing primary voltage under open secondary circuited 

conditions. This is contrary to CTs under normal operating condition, where primary voltage has 

little effect on secondary voltage (see Figure 4-1). 

Primary current level (see Figures 5-1 & 5-3). 

Crest voltage increases with increasing primary current under open circuited secondary 

conditions. 

CT’s turn ratio (see Figure 5-2). 

Crest voltage increases with increasing CT’s turn ratio. 
 

42 

The open secondary crest voltage rises to 1000V or higher immediately after opening the 

secondary circuit. 

To capture the crest voltage the minimum time step needed is above a 5,000 steps/second. Both 

test series used 10,000 steps per second in almost all open secondary tests. 

Intermittent opening/closing via a relay (i.e., clean open/close) or via arcing over a small gap does 

not affect the magnitude of the crest voltage (see Figures 5-4 & 5-5). 

Electrical arcing, however, diminishes the magnitude of the open secondary crest voltage (see 

Figures 5-4 & 5-5). 

No degradation of the CT’s secondary coil insulation was found during any tests. The temperature 

rise of the CT was insignificant (<5 oC per test). 

The insulation of the secondary cable did not indicate any degradation. The periodic HiPot tests 

indicated no leakage current at 10 kV. 

The repeatability of each test was excellent based on the open secondary crest voltage and the 

deviations noted were well below 5%. 
43 

7 REFERENCES 

[1] Brookhaven National Laboratory Letter from Mr. E. A. MacDougall to Nicholas S. Fiorante of 

NRC, dated July 21, 1983. 

[2] Regulatory Guide 1.189, Fire Protection for Nuclear Power Plants, Revision 2, October 

2009. 

[3] NEI 00-01, Guidance for Post-Fire Safe Shutdown Circuit Analysis, Revision 2, May 2009. 

[4] NUREG/CR-7150 and EPRI 1026424, “Joint Assessment of Cable Damage and 

Quantification of Effects from Fire (JACQUE-FIRE) - Phenomena Identification and Ranking 

Table (PIRT) Exercise for Nuclear Power Plant Fire-Induced Electrical Circuit Failure,” BNLNUREG-98204-2012, Volume 1, October 2012. 

[5] https://www.eiseverywhere.com/file_uploads/389f38b2dffd8bcc3c58da78777e4d71_Curre

nt_Transformers.pdf

[6] http://sound.westhost.com/xfmr2.htm , Chapter 17. 

[7] http://www.allaboutcircuits.com/textbook/alternating-current/chpt-9/mutual-inductance-andbasic-operation/, Mutual Inductance and Basic Operation, Chapter 9 - Transformers 

[8] IEEE Std. C57.13-2008, “IEEE Standard Requirements for Instrument Transformers,” IEEE 

Power Engineering Society, July 2008. 

[9] “A Basic Stun Gun Concept,” Chemlec © 2007 & 2011. 

[10] IEEE Std. C57.13.2-2005, “IEEE Standard Conformance Test Procedure for Instrument 

Transformers,” IEEE Power Engineering Society, September 2005. 

[11] NETA WORLD, Back to Basics – Current Transformer Testing, 

http://www.netaworld.org/netaworld-journal/archived-articles/468 Fall-Winter 2004. 

[12] Current Transformers: A Tester Survival Guide. http://www.docfoc.com/currenttransformers-a-tester-survival-guide-bryan-shannon-abb-inc-coral

[13] Wiedenbrug, E., Frey, G. and Wilson, J., “Impulse Testing as a Predictive Maintenance 

Tool,” http://www.reliabilityweb.com. 

A-1 

OPEN SECONDARY TESTING OF WINDOW-TYPE CURRENT 

TRANSFORMERS – TEST PLAN 

A.1 Introduction 

A.1.1 Basic Principles of Current Transformer 

The design and construction of a window-type (or donut-type) current transformer (CT) consist of 

two separate windings on a common laminated iron or steel core [Refs. A1, A2]. The primary 

winding is made out of more than one turn of heavy gauge wire capable of carrying a large 

primary current from a power distribution system, where the secondary winding uses many turns 

of smaller gauge wire with a continuous current carrying capacity of 1 amp or 5 amps, depending 

on the instrumentation design. Under normal operation, the alternating current (AC) in the primary 

winding (known as excitation current) produces an alternating magnetic field in the core, which 

then induces an alternating current in the secondary winding circuit. The primary and secondary 

circuits are magnetically coupled so that the secondary current is linearly proportional to the 

primary current over the intended range of the CT. 

The linear proportionality between the primary and secondary currents ceases when the excitation 

current increases to a certain level, depending on the magnetic saturation in the CT’s iron core. At 

this point, an increase in the excitation current produces a significantly smaller increase in the 

secondary current. 

CTs rated 1200:5 current ratio will produce 5 amps continuous secondary current when 1,200 

amps of current flow in the primary. The relationship between primary current and secondary 

current is based on the ratio of their turns in the windings of the CT’s core. In fact, the AmperesTurns (i.e., current multiplied by number of turns) in the primary winding is maintained equal to the 

Amperes-Turns in the secondary. Theoretically, the secondary voltage increases or decreases as 

required for maintaining a constant primary-to-secondary current ratio. 

CTs, a specific type of instrument transformers, are widely used in the electric power industry, 

including nuclear power plants (NPPs), to monitor the current at strategic locations in electrical 

power distribution systems. These CTs provide isolation from the high voltage primary, and stepdown the magnitude of the measured current to a value that can be safely handled by the 

monitoring instruments. Thus, they are designed to measure the current in AC power systems 

(generally three-phase systems) in their primary winding and transform this current into a 

representative low secondary current for instrumentation used for remote readout of the current. 

A.1.2 Potential Fire Due to Open-Circuited Secondary 

In a letter to the NRC dated July 21, 1983, Brookhaven National Laboratory (BNL) raised a 

potential concern for fire-induced open circuits on a CT’s secondary [Ref. A3]. The BNL letter 

postulated the scenario in which potentially high voltages induced on the secondary winding of a 

CT as a result of open circuiting the CT secondary due to fire-induced failure of the CT or its 

connected components. The postulated failure mode could potentially start a secondary fire at a 

location remote from the initiating fire area. However, the Auxiliary Systems Branch (ASB) of the 

NRC concluded at the time that the BNL’s concerns were overly conservative and recommended 

no further action on this issue [Ref. A4]. Differing opinions on the validity of the fault mode have 

persisted for decades. Although identified in regulatory and industry guidance as a potential 
 

A-2 

concern, agreement has not existed on treatment within both performance-based and 

deterministic approaches. Based on the substantial increase in knowledge of fire-induced circuit 

failures, the JACQUE-FIRE Phenomena Identification and Ranking Table (PIRT) panel elected to 

further investigate the open circuit CT issue. Upon further review of this failure mode, the PIRT 

Panel concluded that CTs with turn ratio 1200:5 or less pose no risk of a secondary fire. [Ref. A7] 

It was the PIRT Panels judgement that this conclusion could not be extended beyond this turn 

ratio limitation without further supporting data. 

It has been recognized that as long as current is flowing in the primary winding of a CT, an opencircuit in a CT’s secondary can cause high voltages on the secondary circuit as the CT attempts to 

maintain the current relationship dictated by the transformer’s winding turns ratio [Ref. A8]. This 

condition can result in CT damage and potentially generate voltages that may exceed the 

dielectric strength of the current transformer insulating materials or other CT circuit conductors, 

which may cause arcing to connected components. 

The resulting high voltage condition in the secondary circuit from an open-circuited CT introduces 

a potential failure mode that warrants further investigation as part of the final resolution of circuit 

failure issues associated with the fire protection strategies at nuclear power plants. The secondary 

circuit from the CT location in the plant to the main control room instrument indications may 

consist of long (e.g., hundreds of feet) instrument wires, whose insulations are susceptible to 

secondary fires. Since the safe shutdown analysis and the fire probability risk assessment (Fire 

PRA) are based on one fire area (safe shutdown) or one scenario at a time, the possibility of a 

second fire in a separate location fire area can significantly impact the final outcome of the 

analysis results, and subsequent fire protection strategies. 

IEEE Std. C57.13-2008 [Ref. A9], § 6.7.1, Operation with Secondary Circuit Open, states: 

“Current transformers should never be operated with the secondary circuit open because 

hazardous crest voltages may result. Transformers conforming to this standard shall be capable 

of operating under emergency conditions for 1 min with rated primary current times the rating 

factor with the secondary circuit open if the open-circuit voltage does not exceed 3500 V crest. 

When the open circuit voltage exceeds 3500 V peak, the secondary winding terminals should be 

provided with voltage limiting devices (varistors or spark gaps). The voltage limiting device should 

be able to withstand an open-circuit situation for a period of 1 min without damage to the 

secondary circuit. The voltage limiting device may need to be replaced after such an abnormal 

condition.” 

A.2 Objective 

A.2.1 Purpose and Scope 

The purpose of this test program is to better understand the following scenario: Will fire-induced 

open circuiting of the secondary circuit of a wound-type CT, which is operating within its rated 

continuous primary current limits, result in an excessively high voltage in the secondary circuit 

sufficient to start a fire in the form of explosion or arcing in the circuit’s insulation at the location of 

the CT itself or at some other location in the secondary circuit? The test results are expected to 

provide technical justifications to the working group for developing guidelines on this subject area, 

including the limit on the 1200:5 CTs as stated in the NUREG/CR-7150, Volume 1 [Ref. A7]. 
 

A-3 

Because the secondary open-circuit voltage is limited by saturation of the CT’s iron core, the rootmean-square (RMS) value measured by a voltmeter may not appear to be dangerous. However, 

as the current cyclically passes through zero (e.g., a sinusoidal AC current at a power frequency 

of 60 Hz), the rate of change of flux at current zero is not limited by saturation, and is raised to a 

higher than normal magnitude. This induces high peaks or pulses of voltage in the secondary 

circuit. These high peaks can exceed the rated dielectric strength of the instrumentation cable 

insulation (i.e., 3,500 V per IEEE Std. C57.13-2008), potentially causing break down of the 

insulation and arcing to adjacent electrical components. 

Influencing parameters, the primary voltage and the turn ratio, are expected to impact the 

secondary voltage spikes under open secondary condition of a CT. The test plan includes a set of 

preliminary testing of CTs with primary voltages 120V, 240V, and 480V, while it also includes turn 

ratios from 100 (500:5) to 800 (4000:5). If these first set of CT testing will indicate any fire, arcing 

or any degradation of the secondary cable, subsequent causal analysis would determine all other 

influencing parameters. Based on the findings, additional CTs and test configurations will be 

considered for subsequent testing. Testing of higher voltage CTs, different-manufacturer CTs, 

bush-type CTs, and other considerations such as longer secondary cabling, influence of inductive 

loads will depend on the results from this preliminary set of tests and future funding. 

A.2.2 Recent CT Test Results 

Based on a presentation given by Steve Laslo of the Bonneville Power Administration in March 2012, 

the response of a 3000:5 CT1 to an open secondary can be illustrated in Figures A-1 and A-2 below 

[Ref. A10]. Figure A-1 is a composite summary of the CT’s response in terms of flux in the core, 

primary current, and induced secondary voltage (Es) changes under open secondary condition. 

Of interest to this program is the ability to measure the magnitude and duration of the secondary 

voltage spike that is induced by opening the secondary side of an energized CT. In the same 

presentation, the secondary “peak” voltage is plotted as a function of the magnitude of the primary 

current for the 3000:5 CT (see Figure A-2). In the test program, this simplified response will be 

more accurately quantified in magnitude and time response using high-speed data acquisition 

equipment described in Section A.3.1. 

The presentation also indicates that moderate to high voltage spikes up to 2,220 volts in the 

secondary were observed for the low primary current flow below 813 amps and the CT flux 

behaved sinusoidal with some minor distortions at higher current levels. Above 813 amps up to 

2,337 amps (i.e., 80% of the 3,000 amps) of primary current the CT started making significant 

noise and the flux reached to a fully saturated waveform with 3,600 volts secondary voltage 

spikes. The testing continued to a primary current level of 6,487 amps for the 3000:5 CT, the 

secondary voltage spikes became progressively higher and narrower, and the extremely high 

secondary voltage spikes of 8,000 volts were observed with excessively high CT noise levels. 

 

1 CT used was from a retired 500kV ITE SF6 PCB – BCT 3000/5 (Full Winding). CTs used in nuclear safety systems 

are smaller and typically at low voltage ratings (600V-6kV). 
 

A-4 

Figure A-1: CT Response - Open Secondary Circuit [Ref. A10] 

Since the presentation was intended for training purposes it does not provide any test data or 

photos of arcing or damage to the secondary circuit or its components. However, it concluded that 

CTs with very high turn ratios can produce very high voltages on the open-circuited secondary 

even at low levels of primary current flow. 

Figure A-2: Secondary Peak Voltage Achieved During 

Open Circuit on the Secondary of a CT [Ref. A10] 

References A11 and A12 present similar open secondary CT testing. Unlike Reference A10 

presentation, no specific test data are available from these programs, just videos showing the test 

profiles. Both indicated no secondary wiring failure with some explanations given in Reference 

A11. Reference A12 showed that with intermittent shorting of the secondary terminals its 

insulation could be compromised with arcing at the CT connecting location. It appears that the 

size and application of CTs under the purview of nuclear power plant’s fire safety are smaller (i.e., 

typically for a line voltage up to 6 kV or less) from those used in these test programs. 

Based on the above test characteristics and many other similar test videos seen on the internet, 

high secondary voltage spikes are expected during the testing in the form of arcing or insulation 

damage in the secondary circuit. Since the insulation of the CT’s secondary is typically designed to 
 

A-5 

withstand voltages between 2,500 – 4,000 Volts [Refs. A9 & A13], it is unlikely that we will observe 

any such damage scenarios without exceeding this voltage threshold or inducing a failure by some 

other means. However, it should be noted that the sole purpose of this testing is to obtain adequate 

information that would form the technical basis for assessing the propensity of a secondary fire or 

damage to the secondary side circuit or components under an open-circuited CT secondary. 

A.3 TESTING Approach 

For this test program, we will simulate the effects of a fire that damage the secondary side 

conductors of an energized CT that are typically used in the safety and applicable non-safety 

systems of nuclear power plants, causing an open circuit on the secondary windings. As 

discussed earlier, the open circuit is expected to cause abnormally high secondary voltages when 

primary current flow is present. This testing will evaluate the potential effects of these high voltage 

spikes on an open-circuited CT secondary circuit. Specifically considered is whether the possibility 

exists for the high voltage to cause sustained arcing or a catastrophic failure of the CT (or other 

secondary circuit elements) that is sufficiently energetic to initiate a secondary fire that damages 

components or equipment beyond the CT’s circuit and its components. A secondary fire here is a 

fire at a location remote from the original fire that is responsible for the initial open-circuit in the CT 

secondary circuitry. 

A series of tests will be conducted to demonstrate the impact of the primary current values on the 

secondary peak voltage. This is important in determining if the peak voltages on the secondary 

are high enough to damage the insulation and create a potential fire source. 

A.3.1 BNL Superconducting Magnet Test Facility 

The Superconducting Magnet Division at BNL has been involved in testing various 

superconducting magnets for use in both particle accelerators and experimental facilities. 

Capabilities include: 

Superconductor materials development/test facility 

Magnetic structure design 

Superconducting magnet fabrication 

Vertical and horizontal cryogenic testing 

Magnetic field measurements 

The CT testing will be carried out in the high bay area of the Superconducting Magnet Division 

test facility. Magnet Division routinely assembles and tests very high current, high field 

superconducting electromagnets for particle accelerators like Relativistic Heavy Ion Collider 

(RHIC) at BNL and Large Hadron Collider (LHC) at the European Organization for Nuclear 

Research (CERN). The test facility has various AC- and DC-controlled power sources ranging 

from 480 Volts, 1,600 Amps AC to 30 Volts, 30,000 Amps DC. Associated with these power 

sources, each test facility has a state-of-the-art high speed multi-channel data acquisition system 

with the sampling frequency of up to 1 MHz and 16 bit resolution. Many other monitoring systems, 

sensors and equipment required to do such specialized measurements are also available for this 

test program. Available equipment includes isolation and buffer amplifiers for high voltage 

measurement, voltage and current transformers, high current controlled switches and circuit 

breakers, and deionized water supply for cooling the load banks. 

 
 

A-6 

Laboratory Space 

Figure A-3 shows an aerial view of the high bay area at the BNL Superconducting Magnet test 

facility. All CT tests will be carried out at the barricaded area (right). 

Figure A-3: Aerial View of the Test Facility 

As shown in a close-up photo in Figure A-4, its present configuration has the required AC current 

source and the spacing around it to set up an enclosed cabinet with each CT under test and to 

locate the data acquisition racks. 
 

A-7 

Figure A-4: Close-up Photo of the Proposed CT Test Area 

CT Test Setup 

At this time six CTs with 2000:5 and six other CTs with 4000:5 current ratios, as shown in Figure 

A-5, are available to BNL for testing. These twelve CTs have been supplied by the NRC and if 

needed, additional units with different current ratios will be acquired for testing. Some of these 

new ones may include different CT manufacturers. 

The 4000:5 CTs have secondary terminals for eight different turn ratios from 100 through 800. 

They include 500:5 (X1-X2), 1000:5 (X3-X4), 1500:5 (X2-X3), 2000:5 (X1-X3), 2500:5 (X2-X4), 

3000:5 (X1-X4), 3500:5 (X2-X5), and 4000:5 (X1-X5) current ratio ratings, where X’s are 

secondary taps of the CT. 
 

A-8 

Fixed-Ratio Design: 2000:5 

Multi-Ratio Design: (500, 1000, 1500, 2000, 2500, 3000, 3500, and 4000):5 

Figure A-5: AMRAN Current Transformers 

Figure A-6 shows the power supply circuit for the proposed CT tests. The primary current for the 

CT under test will be set up from a three-phase delta connected water-cooled load bank of 0.78 

ohms each leg connected to a 480V, AC power source through 700 Amps circuit breaker and 

remotely controlled contactor. These will give about 600 Amps in each phase of the circuit which 

will be measured by three 800:5 CTs. The supply current to the primary circuit can be adjusted to 

get the required value by manipulating the resistance of the load bank. As shown in Figure A-6, 

current in one phase will be used to generate the necessary Ampere-Turns in the primary of the 

CT under test. For example, to test a 4000:5 CT, seven (7) turns of Ultraflex 500MCM cable will 
 

A-9 

be wrapped around the core of the CT. Exact current in that phase will be measured and recorded 

using three in-house 800:5 CTs. 

Figure A-6: Test Power Supply Setup 

The circuit diagram in Figure A-7 illustrates the test setup for obtaining the characteristics of an 

open-circuited CT. As stated above, a test current source feeds the CT’s primary side to represent 

the normal current flow in a power circuit. Approximately 100 feet of secondary cable with a 

burden resistor (or ammeter) will be used to simulate an actual plant’s normal configuration. The 

CT secondary side will be instrumented with a relay for intermittently opening the circuit or a very 

low current fuse link (e.g. 1 amp) for an instantaneous circuit break. The increase in the 

secondary voltage and decrease in secondary current will be recorded via high voltage isolation 

modules. A computerized high speed data acquisition system will be used to capture the CT 

secondary voltage and current. Other parameters to be monitored during testing include primary 

current and voltage, magnetic flux, and the temperature of the CT magnetic elements. A high 

speed video camera and a thermal imaging infra-red camera will also be used to capture the 

arcing and fire formation (if any) at several strategic locations. These cameras will be 

synchronized with the high speed data acquisition system to get secondary circuit characteristics 

during the arcing process. 
 

A-10 

Figure A-7: Proposed Secondary Circuit Layout of the CT Testing 

Data Acquisition Equipment 

Figure A-7 above shows the instrumentation and data acquisition connections on secondary side 

of the CT under test. This setup will capture the high secondary voltage spikes and the profile of 

arcing current at the instance of opening of the secondary of a CT along with images from high 

speed and thermal cameras. Opening of the secondary of CT will also be simulated. 

Details of the instrumentation are as follows: 

High Speed Data Logger: The 8-channel high speed data logger has a sampling rate of 100K 

Samples/sec per channel, sampled simultaneously. This logger is programed using LabVIEW©

Real Time graphical programing software and uses digital to analog converter hardware from 

National Instruments. 
 

A-11 

HV Isolators: These isolator modules manufactured by VeriVolt Inc. provide fast response and 

isolation between channel to channel and channel to ground in excess of 10,000V. They have 

programmable gain to maintain output voltage within +/- 10V range for data logger. 

High Voltage Relay: A high voltage (HV) relay is used to provide clean opening (or instantaneous 

break) of the secondary circuit of the CT under test. It has contact ratings of 20 Amps at 10 kV. 

The relay ratings are intended to prevent contact fusing and excessive arcing during high voltages 

generated when the circuit is opened. 

Shunt-10A, 100mV: The voltage signal across this shunt will be used to capture the characteristic 

and duration of the arcing current at the instant of opening of the secondary of CT under test. 

Arcing current can last up to few AC cycles and its profile can give good correlation between 

arcing duration and igniting of a fire. 

The following parameters will be measured and logged using high speed data logger: 

T1, T2: Temperature of the CT’s magnetic core at two opposite ends of the outside perimeter 

T3: Temperature of the CT’s magnetic core at the inside perimeter 90 degrees off the outside ones 

V1: Secondary voltage of the CT under test. Because of the high voltage generated during opening 

of the secondary, this voltage is isolated and divided by 1000 before being fed to the data logger 

I1: Primary current of the CT under test by measuring voltage across 1ohm burden resistor 

I2: Secondary current of the CT under test by measuring voltage across 10A, 100mV shunt. 

Isolator for this signal provides the gain of 10 so that with 5A RMS flowing in the circuit, data 

logger sees 500mV RMS 

D1: Relay for opening the secondary circuit 

High speed digital camera 

All the above parameters will be captured for duration of 5-10 seconds, <5 sec before trigger and 

<5 sec after trigger. Data capture trigger and the opening of secondary through high voltage relay 

is done using isolated digital output of the data logger. 

Captured data will be logged as an excel file and its time stamp will be synchronized with that of 

camera images. 

A.3.2 Quality Control and Safety 

Existing BNL quality control procedures will be used to ensure all results are reproducible and 

accurate. Instruments will be calibrated using standards that are traceable to a national standard. 

Any new equipment will be field tested and calibrated as needed to ensure it is operating properly. 

Since the CT testing involves high voltage and potential for high energy arcing and/or explosion of 

insulations, approval (if required) from the BNL safety committee after reviewing the test plan and 

all safety requirements will be obtained. 
 

A-12 

Project reports will receive a BNL technical review and will be reviewed and approved by the 

NRC/RES Project Manager. 

A.4 Test Matrix 

A.4.1 Test Configurations 

CTs to be Tested: Amran 2000:5 and 4000:5 (with multiple turn ratios from 100, 200, 300, 400, 

500, 600, 700, 800) shown in Figure A-5 

Primary Voltages: 125V, 250V, 500V 

Primary Current Levels: 500A, 1000A, 1500A, 2000A, 2500A, 3000A, 3500A, 4000A 

Open Secondary Simulations: Fast Open using Fuse Link (for Open Circuit Fault); Intermittent 

Open using a Relay (for Arcing Fault) 

Maximum Duration of Test: 10 minutes (subject to change based on test results) 

Secondary Cable: Commercial-Grade General Cable, PVC/Nylon/PVC, shielded, 100 feet of 

AWG 16, 2 conductor 600V (no drain or ground wire) for instrument wiring application 

Insulation Resistance (Megger Test) of the secondary cable will be monitored at the beginning 

and ending of each test set up for its insulation integrity 

Locations Subject to Visual Monitoring (Camera and Thermal Imaging) for Secondary Fire (or 

Arcing): CT, 50 feet from the CT (Another Fire Area) including the open circuit simulations, and 

100 feet from the CT (Ammeter or Protective Relay Location) 

For each test set up, the following data will be obtained: 

Primary Voltage 

Primary Current 

Secondary Voltage 

Secondary Current (Prior to Open Circuit Fault) 

Temperatures at three locations (see Figure A-7) 

Magnetic flux density (if needed) 

For each test set up, the following locations will have Visual and Thermal Imaging: 

CT Location 

Open Secondary Fault Location 

Ammeter Location 

High Speed data will be obtained for the first couple of minutes, followed by slower speed data 

acquisition for remaining of test duration for capturing any arcing fault in the secondary cable. 

A.4.2 CT Preliminary Tests 

The following tests are planned to be performed first: 

 
 

A-13 

Baseline Performance 

Prior to conducting the open circuit testing of the CT, it will be operated to obtain baseline 

performance information including core temperature, and primary to secondary current and 

voltage response between 500 and 4000 amps (primary). Two separate test fixtures 

corresponding to two Amran CTs will be used to run these tests to verify all data acquisitions and 

other circuit parameters. 

Fast Open Secondary Response 

Using a time delay fuse link (not shown in Figure A-7) on the secondary circuit, the secondary 

voltage response will be obtained with primary current values ranging from 500 to 4000 amps. 

The results of this series of tests will yield information on the CT’s voltage response to a single 

initiating open circuit event as a function of primary current values. 

Intermittent Open Secondary Response 

Using a mechanical switch or relay that interrupts the secondary circuit at 1 second intervals for 

10 seconds (10 circuit interruptions), the CT’s response will be recorded to determine the 

repeatability of its response and the potential additional degradation that results to the secondary 

side cable insulation. 

Repeat this with modifications as necessary for the different sizes of CTs to be tested. 

The following relationships or conditions will be examined from this series of tests: 

Effect of primary voltage (i.e., 120V, 240V, 480V) on the secondary voltage peaks. 

Effect of turn ratio (i.e., 100 thru 800) or primary current (i.e., 500 thru 4000 amps) on the 

secondary voltage peaks. 

Effect of fast open and intermittent open conditions on the secondary voltage peaks. 

Comparison of secondary voltage response at 2000:5 current ratios for the fixed and multi-tap CTs. 

Examine the repeatability of test results. 

In addition, during each test visual and thermal imaging of the CT location, open secondary 

location and the ammeter location will be monitored. 

A.4.3 CT Subsequent Tests 

Depending on the results of the above preliminary tests, additional tests will be performed to meet 

the objectives of this test program. 

If the secondary voltage spikes demonstrate secondary fire or arcing in the secondary cabling, 

then additional tests will be performed to evaluate all influencing parameters that are important to 

this scenario. 

If the secondary voltage spikes indicate no secondary fire or arcing, then the testing will be 

extended to determine the threshold condition at which arcing in the secondary circuit could be 

observed. 
 

A-14 

Ammeters or protective relays will be added to the secondary circuit. Failures will be assessed 

based on the following: CT fails or ammeter/relay fails or cable fails. A technical basis will be 

derived from the test for developing guidelines for open secondary CT scenarios. 

If necessary, other tests will be carried out on CTs from several manufacturers. Tests at higher 

voltage levels up to 4.1 or 13 kV may require special laboratory facilities. Some of these tests will 

require support from the NRC COR. 

A.5 DATA Evaluation 

The testing will indicate the potential for a secondary fire occurring away from the CT location in a 

nuclear power plant set up. All test data will be collected directly from the Data Logger. 

If the open circuited CT secondary is considered to be incapable of starting a secondary fire, this 

testing will provide the technical basis for the working group for assessing the following conditions: 

Neither the CT nor its secondary circuit shows any signs of arcing or explosive failure. 

Either the CT or its secondary circuit does show signs of arcing, but the duration is short and near 

the CT, or the arc is of insufficient intensity to ignite surrounding materials farther from the CT. 

The CT does exhibit explosive failure, but the explosive behavior is near the CT and insufficient to 

damage surrounding materials. The explosive failure is not accompanied by subsequent arcing of 

the CT or its secondary circuit. 

If the testing does accompany the explosion or arcing failure, it will be evaluated further by the 

working group for further testing or studies. 

A.6 Documentation 

Comments and their dispositions on this test plan were documented in the Attachment 1 to this 

document. 

A technical test report (or NUREG/CR) will document the test procedures and details of the testing 

that will include all assumptions and limitations. The test results will be summarized for the 

working group so that the CT’s secondary fire issue can be technically assessed and appropriate 

guidelines are developed for circuit analysis in the fire protection program. 

A.7 References 

[A1] “Instrument Transformer Basic Technical Information and Application,” GE Digital Energy 

– ITI, www.gedigitalenergy.com/products/manuals/geh-230af.pdf 

[A2] “Instrument Transformers – Technical Information and application Guide,” ABB Field 

Engineering Services Instrument Transformers On-site Testing. 

[A3] BNL Letter from Mr. E. A. MacDougall to Nicholas S. Fiorante of NRC, dated July 21, 

1983. 

[A4] Memorandum from Olan D. Parr, Chief, Auxiliary Systems Branch to its members on “Fire 

Protection Review Guidance,” November 30, 1984. 
 

A-15 

[A5] Regulatory Guide 1.189, Fire Protection for Nuclear Power Plants, Revision 2, October 

2009. 

[A6] NEI 00-01, Guidance for Post-Fire Safe Shutdown Circuit Analysis, Revision 2, May 2009. 

[A7] “Joint Assessment of Cable Damage and Quantification of Effects from Fire (JACQUEFIRE) – Phenomena Identification and Ranking Table (PIRT) Exercise for Nuclear Power 

Plant Fire-Induced Electrical Circuit Failure,” Volume 1, NUREG/CR-7150, EPRI 1026424, 

October 2012. 

[A8] Kaufmann, R. H. and Camilli, G., “Overvoltage Protection of Current Transformer 

Secondary Windings and Associated Circuits,’ Transactions of the AIEE, Vol. 62, July 

1943, pp. 467-472. 

[A9] IEEE Std. C57.13-2008, IEEE Standard Requirements for Instrument Transformers. 

[A10] Bonneville Power Administration Presentation of CT’s Safety Hazards for Class Room 

Lecture by Steve Leslo (2/20/2013): 

https://www.eiseverywhere.com/file_uploads/dbb5ba3a17812e8cea24ddf2aeebfa2d_Cu

rrent_Transformers_for_HORS_2013-2.pdf

[A11] NGK (a Japanese Manufacturer) CT Open Circuit Test: 

https://www.youtube.com/watch?v=TGEcBCR4DV0

[A12] CT Fails When Leads Open Circuited: https://www.youtube.com/watch?v=j6v0ybdhGk0 

[A13] UL 61010B-1, “Electrical Measuring and Test Equipment; Part 1: General Requirements. 

 B-1 

DATA FROM ALL TESTS PERFORMED 

All test scoring sheets in the format shown in Table 4-1 are taken from the Data Logger records in 

its first eight columns. These data sheets were hand written and contain raw information regarding 

each test condition. Since there was no abnormal condition noted during any of the 63 tests, the 

data presented in these sheets are those taken from the data logger. Also the HiPot test data of 

the secondary cable and other information on the CT’s DC resistance test and impulse test are 

included as per Table 4-1. Since there was no abnormal measured data significantly different from 

those given in Table 3-1 and Table 4-2, we do not include them here. 

Two types of test data are included in the attached compact disc (CD): 

I. EXCEL Worksheet files contain all 63 tests in the following three folders:

1. AM2CT Test Data Files (3 Files Containing All Tests Performed Under 125V, 

250V, 500V Levels) – A Total of 29 Worksheets 

i. AM2CT01 through AM2CT21: 21 Tests per Table 4-3 

ii. 8 Additional Tests for Initial Trial Runs, Repeatability, Scanning Rate and 

Long Duration 

a. AM2CT04 and AM2CT03 Trial Tests 

b. AM2CT01 and AM2CT02 for Repeatability and AM2CT01 for Long 

Duration Tests 

c. AM2CT07 for Scanning Rate and Two Long Duration Tests 

2. AM4CT Test Data Files (3 Files Containing All Tests Performed Under 125V, 

250V, 500V Levels) – A Total of 18 Worksheets 

i. AM4CT01 through AM4CT14: 14 tests per Table 4-4 

ii. AM4CT01 for Repeatability, Arcing, Intermittent and Long Duration Tests 

3. Additional CT 4000 Test Data Worksheets – A Total of 16 Tests (16 Tests 

AM4CT15 through AM4CT30 in Table 4-4) Contains 2 Files – One for 2000:5 and 

the Other for 4000:5 Turn Ratios) 

Each EXCEL worksheet presents one out of 63 tests and contains 9 columns as marked: 

1. Time (Sampling Time in Seconds) 

2. T1 (Temperature Outside CT in oC) 

3. T2 (Temperature Outside CT – Opposite Side in oC) 

4. T3 (Temperature Inside CT – 90 degrees Off Outside Probes in oC) 

5. Phase A Sine via 800:5 CT (Primary Current in Phase A of the Power Source in 

Amperes) 

6. CT Sec Open Voltage (Secondary Voltage in Volts) 

7. Shunt in CT Sec (Secondary Current in Amperes) 

8. Phase A RMS (Primary Current RMS Value in Amperes) 

9. T4 (Temperature of the Cabinet Housing in oC for CT 2000 Tests or 

 Primary Voltage A-B (Primary Voltage in Volts for CT 4000 Tests) 

 

II. Video clip of arcing simulation at the opening relay. 

NUREG/CR-7228 

M. Subudhi, P. Joshi1, S. Dimaiuta1 and W. Gunther2

Department of Nuclear Science and Technology 

Brookhaven National Laboratory 

Upton, New York 11973 

G. Taylor, NRC Contracting Officer Representative 

Division of Risk Analysis 

Office of Nuclear Regulatory Research 

U.S. Nuclear Regulatory Commission 

Washington, D.C. 20555-0001 

Two AMRAN Current Transformers (CTs) were tested to characterize the propensity of an open-circuited CT secondary to cause a fire in a location 

remote to the open circuit. Also evaluated was the magnitude of the secondary winding peak (crest) voltage when the CT’s secondary circuit 

transitioned to an open circuit condition, and its dependencies on the primary voltage, primary current and the CT’s turn ratio. Parameters identified as 

having an effect on the CT’s open secondary crest voltage include: 

• Material and construction of the CT’s magnetic core, 

• CT’s primary voltage, 

• CT’s primary current, and, 

• CT’s turn ratio. 

The primary objective of this testing, however, was to better understand the following scenario: 

Will fire-induced open circuit in the secondary circuit of a wound-type CT, which is operating within its rated continuous primary current limits, result in 

an excessively high voltage in the secondary circuit sufficient to start a fire in the form of explosion or arcing in the circuit’s insulation at the location of 

the CT itself or at some other location in the secondary circuit? 

For the configurations tested, the results indicated that an open circuited CT was incapable of starting a secondary fire based on the criteria 

established in the test plan. This observation applies to the CT, CT taps, cabling, and burden devices. Additionally, when an arc was deliberately 

initiated in the secondary circuit, it was observed that the crest voltage diminished significantly. 

The test results provide supplemental information to the JACQUE-FIRE Volume 3 working group for developing technical recommendations to possibly 

extend the limit on the 1200:5 CT turns ratio limit as stated in the NUREG/CR-7150, Volume 1 

associated circuits instrument transformer 

circuit analysis nuclear power plant (NPP) 

crest voltage open CT secondary 

current transformer Phenomena Identification and Ranking Table (PIRT) 

fire probabilistic risk assessment power distribution system 

fire protection secondary fire 

fire safety voltage transformer 

fire safe-shutdown 

May 2017 

Technical 

Open Secondary Testing of Window-Type Current Transformers 

1 Superconducting Magnet Division 

2 Retired BNL Staff 



NUREG/CR-7228 Open Secondary Testing of Window-Type Current Transformers May 2017