Lightning Protection of Residential AC Wiring

Keith E. Crouch

Lightning Technologies *

Pittsfield, MA

François D. Martzloff

General Electric Company

Schenectady NY

f.martzloff@ieee.org

Reprint, with permission, of declassified General Electric memo report MOR-78-095

Significance:

Part 4 – Propagation and coupling of surges

Part 8 – Coordination of cascaded SPDs

Laboratory tests on the coupling of lightning current (flowing in the service drop grounded neutral

conductor) onto the phase conductors, inducing overvoltages that were limited by candidate surge

suppressors.

While the injected lightning-simulation current was unidirectional, the induced voltages in the house

wiring circuits had oscillatory components. This observation was used in support of the development 

of the “Ring Wave” concept that was adopted by IEEE 587 (now C62.41).

Three possible types of service entrance SPD of 1960-1970 vintage were investigated

• The then-commercially available silicon carbide/gap arrester

• Metal oxide varistors mounted external to the load center

• Metal oxide varistors fitted in a panel breaker housing for easy plug-in connection

The branch circuit SPD consisted of a simple MOV disc incorporated in a modified plug-and-receptacle

combination, probably the first attempt at packaging an MOV for residential surge protection.

____________________________

*The experimental work, reported by F.D. Martzloff, involved performing the tests , recording of nearly 300 Polaroid oscillograms, and

was conducted by K.E. Crouch at the General Electric High Voltage Laboratory prior to his change to Lightning Technologies, Inc.
GENERAL ELECTRIC COMPANY 

CORPORATE RESEARCH AND DEVELOPMENT 

Schenectody, N.Y. repo

Number MOR-78-0

LIGHTNING PROTECTION OF RESIDENTIAL AC WIRING 

K.E. Crouch* and F.D. Marttloff 

Automation and Control Laboratory 

ABSTRACT 

New transient suppressors using metal oxide varistors offer improved 

protection of appliances and consumer electronics against overvoltages. 

This improvement, however, could be at the risk of imposing excessive 

duty on the suppressor in case of a very severe lightning stroke near the 

house where these suppressors are installed. 

A simulated house wiring system was subjected to three levels of !Ig.h.tging 

currents injected into the ground wires (moderate, severe, extremely 

severe), with various combinations of suppressors installed alone or in 

a coordinated com bination. 

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be specified for full protection of the loads in the house. 

Allached are cep4.s a1 lecnndcal a~lorwatson sorler repons lhat you requelled lrom Dr. Bernslem Scvrtul other repoflr that wore raquastcd 

by you have no1 baon rrtl~,stLrJ to Clrrs 1. and lhsrrloro. may no1 

b releas~J. 

CLASS 3 - LIMITED AVAILABILITY INFORMATIO

"Lightning Technologies, Inc., Pittsf ield, Massachusetts 

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GENERAL @ fLCC1RIC 

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Original distribution to those individuals with specific 

information. 

Subsequent Company availability requires originating 

approval. 

~isc1osure outside General Eiectric Company requires 

of originating component. - 

1 
INTRODUCTION 

- Page 

1 

OVERVIEW 3 

1.0 ASSUMPTIONS 3 

1.1 Current Magnitudes 

1.2 Waveshape 

1.3 Lightning Current Path 

1.4 Induced Voltages 

2.0 TEST CIRCUIT AND TEST PROCEDURE 6 

2.1 Power Circuits 

2.2 Instrumentation 

2.3 Candidate Suppressors 

2.4 Test Procedure 

3.0 TEST RESULTS AND DISCUSSION 12 

3.1 Test Results 12 

Distribution Transformer Arrangement 14 

Effect of Terminal Impedance 16 

Effect of Branch Terminations 18 

Load Center Protectors 2 0 

Duty on Outlet Protectors 2 2 

Comparison of Inboard/Outboard Protectors 24 

Effect of Lead Length 2 6 

Effect of Wiring Sparkover 2 8 

3.2 Discussion of the Results 3 0 

3.3 A Coordinated Protection Scheme 3 3 

REFERENCES 3 4 

TABLE 1 3 2 

FIGURES 1-17 4-29 

APPENDIX 3 5 
LIGHTNING PROTECTION OF RESIDENTIAL AC WIRING 

K.E. Crouch* and F.D. Martzloff 

INTRODUCTION 

The development of metal oxide varistors has opened new opportunities for transient suppression in residential power circuits. The Wiring Device Department of the 

General Electric Company has introduced the VSP-1 protector, which contains a 14 mm @ 

GE-MOV varistor. The HLP (Home Lightning Protector) has been available for many 

years, but the hot-line work required for its installation has been a deterrent; and, consequently, this protector has not been very widely applied. The new 32 mm GE-MOV @ 

varistor offers higher capacity than the 14 and 20 mm discs. Prior to reassignment of 

the product scope to the Distribution Transformer Department and later the Circuit Protective Devices Department, tests made in Pittsfield by J.S. Kresge had demonstrated 

that this 32 mm disc could m e e t t h e ANSI secondary requirements. By different packaging, the hot-line work might be eliminated and performance improved, opening the 

opportunity for greater acceptance. 

Therefore, the possibility of a coordinated protection system in residential power 

circuits meeting ANSI requirements became a more likely prospect than an earlier investigation had predicted for coordination between the present design of the HLP and 

the VSP-1.") While there is little evidence that extremely high currents caused by lightning strokes enter far into the house wiring, it seemed worthy of investigation to postulate a condition of "severe" lightning discharge near the house and to attempt recording 

on a simplified model wiring system how the currents and voltages would be distributed. 

This report describes the assumptions, test procedures, results, and conclusions of such 

an investigation. 

"Lightning Technologies, Inc., Pittsfield, Mass. 

"Registered trademark of the General Electric Company 
ground conductor of the service drop, without direct injection into the phase wires, is 

sufficient to induce voltage in excess of the clearance withstand of wiring devices. The 

transmission characteristics of the model and the relative sparkover levels were such 

that internal devices (receptacles) flashed over before the watt-hour meter gaps could 

flash over. 

Coordination between a centrally located surge arrester and an outlet-connected 

protector is possible; substantial, but within rating, currents flow in the outlet protector 

(VSP-1) when coordinated with a Home Lightning Protector (silicon carbide and gap) or 

its candidate successor, the 32 mm GE-MOV " varistor. 

For extreme strokes (1 00 kA at the pole), current in excess of rating can flow in 

VSP-1 protectors located close to the service entrance without other arresters. While they 

could fail there, the protectors do not present a greater hazard than an air clearance, which 

would flash over were there no protector; and, in fact, the presence of the VSP-1 is more 

likely to reduce the hazard of a flashover with subsequent 60 Hz power-follow. 

The addition of a 32 mm varistor to the system, either in a plug-in (inboard) version 

or a s a n external addition (outboard) to the load center, will provide protection consistent 

with the ANSI requirements for secondary arresters. 

1.0 ASSUMPTIONS 

1.1 Current Magnitudes 

It was postulated that a lightning stroke attaching to the primary side of an overhead distribution system would produce a branching of the current flow into the ground, 

following sparkover of the surge arrester, which was presumed connected at the polemounted distribution transformer. Figure 1 shows the assumed circuit and the division 

of current flow. 
LA : PRIMARY ARREST 

XF DIST. TRANSFORMER 

M ' METER 

Figure 1. Division of Current Assumed for a 100 kA Stroke 

In their study of lightning environments, Cianos and indicate that only 

5% of all ground strokes exceed a peak current of 100 kA. The frequency of the strokes 

is quite dependent upon geographic location (isokeraunic levels),(3) as well as upon local 

configurations. An average expectation of a stroke involving the utility pole near a house 

with no adjacent tall trees or buildings may be in the order of one per 400 years for most 

of the U.S. Thus, for a 5% probability, the likelihood is one stroke in excess of 100 kA 

per 8,000 years. With nearby tall objects, this likelihood can be reduced I0 times; in 

areas of high lightning activities, this likelihood can be increased 10 times. The level 

of 100 kA, then, represents an expectation of being exceeded at one location only one 

time in perhaps 10,000 years (but there are millions of poles in the U.S.). 

From these assessments, the maximum current to be injected for the house model 

under discussion was selected to be 30 kA. From this maximum of 30 kA injected into 

the ground wire of the house service drop, two more values were used during the test 
series: 10 kA, corresponding to the requirement for the ANSI high

current, shortduration 

test; and 1.5 kA, corresponding to the requirement for the ANSI duty-cycle test - both 

specified by ANSI Standard C 62.1 for secondary valve arresters. (4) 

Another reason for selecting this low level (1.5 kA) is that no sparkover occurs 

in the wiring at this level. For the 10 and 30 kA levels, multiple flashovers would occur 

at variable times and locations, making exact duplication of tests impossible. By staying 

below sparkover levels, repeatability of the results was ensured, allowing comparisons 

among several alternate circuit configurations. 

1.2 Waveshape 

From ANSI Standard C 62.1, a waveshape of 8 x 20 ps would have been desirable. 

However, limitations in the test circuit required for driving 30 kA in the model loop forced 

a compromise of 10 x 25 us as the test wave. 

1.3 Lightning Current Path 

It should be noted that, in this test series, the assumption was made that the lightning current, applied first to the distribution primary (the highest wire on the pole) is 

transferred to the ground system by sparkover of an assumed surge arrester on the primary 

at the pole. In fact, if there were no arrester, an equivalent effect by direct flashover 

could be expected. 

For the secondary side, however, the assumption was made that both sides (phases) 

of the center-tapped (grounded) secondary remained uninvolved in conducting the direct 

lightning current, while the ground wire (messenger) from pole to house carried its share, 

as defined in Figure 1. 

1.4 Induced Voltages 

The generation of transient voltages in the house is attributed to electromagnetic 

coupling of the field established by the lightning current flowing in the messenger into 

the loop formed by the two phase wires encircling the messenger. In addition, there is 

some capacitive coupling between the wires (Figure 2). 
30 kA 

DISTRIBUTION 

TRANSFORMER 

BY 

= GROUND PLANE 

Figure 2. Voltages Induced in the House Wiring Systems 

2.0 TEST CIRCUIT AND TEST PROCEDURE 

2.1 Power Circuits 

The test circuit consisted of a high-current impulse generator, a distribution transformer with service drop, a simulated simplified house wiring system, and the necessary 

shielded instrumentation (Figure 3). Details of the catalog numbers, characteristics, etc., 

are given in the Appendix. 

The service drop connection between the distribution transformer and the meter 

socket was made with three AWG 416 wires, twisted at a pitch of about 5 turns/m (1.5 

turnslft), 13 m (45 ft) long. This service drop was folded in a loose "St shape, at about 

0.5 m (1.5 ft) above the ground plane serving as the return path for the lightning current, 

in order to reduce the loop inductance seen by the generator. This configuration does 

not influence the coupling between the messenger and the wires wrapped around it, 

coupling which has been identified as the voltage-inducing mechanism. 
LIGHTNING I MODEL DISTRIBUTION I 

CURRENT I TRANSFORMER - 

GENERATOR I AND SERVICE SHOP 

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I 

I \SURGE ARRESTER 

I OR JUMPER WIRE 

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MODEL 

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Dl FFERENTIAL l 

HOUSE I SHIELDED 

I INSTRUMENT 

I ROOM 

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L I GROUND I ,, 

PLANE I - 1 I I I 

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MEASUREMENTPOINTS 

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Figure 3. Schematic Representation of Test Circuit 

The simulated house wiring started at the meter socket and continued to a load 

center by a 3 m (1 0 ft) length of AWG #6 aluminum entrance cable. The meter socket, 

watt-hour meter, and load center were typical General Electric Company hardware (see 

Appendix), except as noted in the detailed procedure description. From this load center, 

four "branch circuits" connected to the load center breakers were established, each terminating at a wall receptable mounted on the same 1.2 by 2.4 m (4 by 8 ft) plywood panel 

on which the watt-hour meter and load center were also mounted. The branch circuits' 

lengths were (one each) 6, 12, 24, and 48 m (20, 40, 80, and 160 ft), the wire being loosely 

coiled between the load center and receptacles (Figure 4). 

2.2 Instrumentation 

Recordings of currents and voltages were made at several points on the wiring 

system with cathode ray oscilloscopes (CRO); differential measurements were made for 

the voltages with especially built 100:l probes. These probes were built by placing a 

5000 52 resistor in series with a terminated 50 fi coaxial cable - all of these contained 
Figure 4. Connections at Load Center 

in a shield tied to the ground plane part of the shielded instrument room. Currents flowing 

in the suppressors were measured by means of a Pearson Model 110 A wide-frequencyband current transformer. The oscilloscopes were located inside the shielded control 

room adjacent to the test area, providing satisfactory protection against spurious signals 

(see Figure 6 in Section 2.4). 

Four candidate suppressors were installed at various locations in the system, for 

various comparisons of performance: 

I. One Home Lightning Protector (HLP, GE Cat. 9L15DC BOO2) was installed 

at the bad center; when connected to the circuit, the connection was at 
the incoming lugs of the load center, as it would normally be when connected 

by an electrician. 

2. Two V250HE8O varistors were mounted near the load center and connected 

to the incoming lugs of the load center. This connection required about 

45 cm (18 in) of #I0 copper wire. The return to ground was common to 

the two discs, as it is for the HLP device. 

(The varistor package contains a 32 mm disc with characteristics suitable 

for secondary arrester duty.(4) It is the candidate metal-oxide varistor 

substitute for, or successor of, the Thyrite "- gap combination currently 

used in the HLP, and has an RMS voltage of 250 V.) 

3. Two 32 mm varistor discs of the same characteristics as (2) (above) were 

installed by the Circuit Protective Device Department in a breaker housing 

so that they could be connected to the load center bus with a minimum 

of lead (10 cm, or 4 in). This connection can be made while the load center 

is energized without requiring "hot work," in the same manner as inserting 

additional breakers on the load center. 

4. VSP-1 spike protectors, produced by the Wiring Device Department, were 

inserted in the receptacle at the end of the branch circuits. (The VSP-I 

protector contains a 14 mm GE-MOV" varistor with a voltage rating of 

170 V RMS.(~) 

5. In addition, the meter contained its standard gaps rated for a 10 kV sparkover. 

2.4 Test Procedure 

Preliminary tests indicated that flashover at the receptacles would occur with 

10 kA injected into the ground messenger, but no sparkover of the meter gaps was apparent. 

Therefore, a first test series was conducted at only 1.5 kA in order to provide consistent 

patterns of wave propagation undisturbed by flashover (Figure 5). 

It was also found that the auxiliary impulse generator used to trigger the main 

gap of the high-current generator induced voltages into the test circuit that could exceed 

those induced by the main discharge. A mechanical switch for closing the circuits was 

then substituted for the triggered gap. 

"Registered trademark of the General Electric Company for molded composite dielectric 

material 
VERTICAL - 500 A/di v 

HORIZONTAL - 5 us/div 

Figure 5. Applied Current Waveshape - 1.5 kA Crest, 10 x 25 LI s 

Noise checks were made for the voltage measurement system by shorting the probes 

together and attaching them to the neutral point on the circuit under test. Similarly, 

the center conductor of the cable to the current transformer was removed from the transformer output and connected to its sheath. No significant voltages (greater than 5% of 

measured signal) were measured. A typical noise check oscillogram is shown in Figure 6. 

VERTICAL - 5 V/div 

HORIZONTAL - 2 us/div 

Figure 6. Typical Noise Response 

of Measurement System 

with 1.5 kA Injection 

3.1 Test Results 

A large number of tests werg performed to investigate the effects of various combinations. From several hundred recorded oscillograms, a selection was made, as shown 

in this section, to illustrate these effects. The results are presented in the form of a 

matrix of oscillograms with corresponding commentary, generally providing a comparison 

of voltage and currents with or without protectors installed. First, a qualitative summary 

is presented, then some comparative oscillograms are shown to illustrate various effects. 

Figure 8 gives a qualitative summary of the effects obtained by installing a single 

protector at various locations in the system. The oscillograms are arranged in horizontal 

rows corresponding to the circuit configuration indicated in the legend. The vertical 

columns correspond to the location at which the oscillograms were recorded. From left 

to right appear Lines I and 2 of the load center, and the ends of the branch circuits at 

6, 12, 24, and 48 rn (20, 40, 80, and 160 ft), which will be referred to as B20, B40, B80, 

and B160. Quantitative information will be given in subsequent figures. 

In the first row, open-circuit voltages are shown. Note that the voltages at three 

locations of the Line 1 conductor are very similar, while there is a small difference between Line 1 and Line 2. 

The installation of a protector in Line I of the load center (second row of oscillograms) clamps the voltage on all Line 1 points, with some oscillations induced at the 

end of the B160 branch. While the initial peak of the Line 2 points is not changed, subsequent oscillations have lower frequency than in the open-circuit mode. For the oscillograms corresponding to the location where a protector is installed, the upper trace shows 

the current flowing in the protectors. 

The installation of a protector in Line 2 of the load center (third row) produces 

results analogous to the Line 1 case. Installation of a protector at the end of a branch 

rather than at the load center (last four rows) produces clamping of the voltage at the 

point of installation. At the other points of the same line, the effectiveness of the clamping decreases a s t h e protector is farther away. For the line with no protector, there 

is a minor voltage reduction and a frequency change similar to that noted in the first 

two rows. 
! ~rn ~ine *I /@ @I 

~ine*l-lZO~-ti20~ -Line 9 Outlet Box 

i 

! I - - 

Outlet Bnr 

_-( from Line *2 10 a] from Line4i 10 @I ~- . -. 

,- - 80 ft. Branch 1 I 1 1.4 from ~ine #2 1-1Outlet Box AWG "14-2W/Grd 

L------ 40 ft. Brunch 

AWG"i2-2W/Grd - - I-240~-I 20 ft. Bronch AWG *14-2W/Grd 

- - Outlel Box 160 ft. Branch 

AWG 812-2W/Grd 

A VSP-I in 

Load Center 

Line #I anly 

A VSP-l in 

Load Center 

Line *2 only 

Simuloted Household Wiring System With Measurements at Various Branches With and 

Without a VSP-I Protector Installed at a Single Paint in the System. 

1500 Amperes, 10 x 25 Microsecond Current Pulse Applied to Service Neutral. 

Figure 8. Summary of Protector Effects 

VOLTAGE AT BUS WITH H1 OF 

TRANSFORMER TIED TO 

NEUTRAL (GROUND) 

Current in HLP: 40114/div 

Voltage Across Bus: 

501) V/div 

VOLTAGE AT BUS WITH AIR GAP 

BETWEEN H1 OF TRANSFORMER 

AND NEUTRAL 

Current in HLP: 400 A/div 

Voltage Across Bus: 

500 V/div 

VOLTAGE AT BUS WITH 

DISTRIBUTION ARRESTER ON 

TRANS FORMER PRIMARY 

Current in HLP: 400 A/div 

Voltage Across Bus : 

500 V/div 

- - 

CURRENT INJECTED FOR 

TEST 157 

(The arrester affects 

the front of wave of 

the current) 

Current i n Arrester : 

2 kA/di v 

(Sweep: 5 vs/div) - 

Test Condition: 10 kA injected. 

A1 1 sweeps : 2 ps/div, except 7 571 

Figure 9. Comparison of Protector HLP Response for Various Protective Devices at the Primary of the Distribution Transformer 

15 

VOLTAGE AT OUTLET WITH 

(1) OUTBOARD PROTECTOR 

ONLY AT EACH BUS 

Voltage at Outlet: 

500 V/div I 

VOLTAGE AT OUTLET WITH 

(1) OUTBOARD PROTECTOR 

AT EACH BUS, PLUS (1) 

"100 W LOAD" AT 8-1 60 

Voltage at Outlet: 1 500 V/diu 

266/268 

VOLTAGE AT OUTLET WITH 

(1) OUTBOARD PROTECTOR 

AT EACH BUS, PLUS ( 1 ) 

VSP-1 AT EACH 8-80 AND 

8-160 OUTLET 

Current in VSP-1 : 500 A/div 

Voltage at Outlet: 

500 V/div 

(LINE 2) 

266A 

VOLTAGE AT OUTLET WITH 

(1) OUTBOARD PROTECTOR 

AT EACH BUS, PLUS (1 ) 

VSP-1 AT EACH 8-80 AND 

0-1 60 OUTLET 

Current i n VSP-1 : 500 A/div 

Voltage at Outlet: 

500 V/div -- 

Test Condition: 70 kA injected - Protectors and loads. 

All sweeps: 2 us/div, except as noted. 

Figure 10. Comparisons of Performances with Various Devices at Outlets, All with Protectors at Load Center 
visible during the first loop in each oscillogram are spaced apart at a distance which 

increases with line length. Thus, one can conjecture that these may be caused by 

reflections. 

Loading the line termination with a 130 9 resistor (Oscillograms 243A, 245, 247, 

and 249) eliminates the later oscillations and reduces the first peak to about 60% of the 

value without load. From this reduction, a Thevenin's calculation of circuit parameters 

(Figure 12), if applicable in an oversimplified form, would show that 130 nis 60% of the 

total loop impedance. Hence one can conclude that the source impedance is fourfsixths 

of 130, or about 85 n. 

When a VSP-1 protector is added to the 130 fi resistor (Oscillograms 244, 246, 248, 

and 250), the clamping action of the varistor limits the voltage at the outlets to about 

400 V, which is consistent for the currents of about 20 A flowing in the varistor. 
Voltage at Outlet 

Indicated: 500 V/div 

VOLTAGE WITH (1) "100 W" 

INSTALLED AT OUTLET 

INDICATED 

I voltage at Outlet 

Indicated: 500 V/div 

VOLTASE AND CURRENT !IITH I 

(1) vSP-1 and (I) 

"100 W" INSTALLED AT 

OUTLET INDICATED 

Current in VSP-1 : 

23 A/div 

Voltage Across VSP-1: 

-- 

Test Condition: 1 .5 kA injected - 

~rotectors and load installed at outlets as shown. 

All sweeps: 2 us/div. 

Figure 11. Effect of Load (1 00 W Light Bulb) on Voltages at Branch Outlets 

Figure 12. Thevenin's Equivalent for Oscillogram 202 
lograms 261 and 262 of Figure 13. 

With outboard protectors rather than inboard protectors, the peak voltages are 

in the 1000 to 11 00 V range (Oscillograms 263 and 264). These higher voltages are attributable to the longer leads required to connect the outboard protector, compared to the 

inboard protector. (Figure 16 shows a comparison of lead length effects, which removes 

any question that the difference between inboard and outboard protectors might have 

been the result of an intrinsic difference in the varistors.) 

While not a recommended installation location, two VSP-1 were also installed 

directly at the load center (on the bus) in an arrangement that approximates the "inboard 

protector" geometry. Oscillograms 255 and 256 show the clamping voltage at 500 to 600 V 

with current crests at 11 00 to 1200 A. Scaling up these varistor current values for higher 

lightning currents than the 10 kA injected would indicate probable excessively large currents in the 14 mm varistor used in the VSP-l protector. 

Finally, a HLP protector was installed at the center, as shown for one bus on 

Oscillogram 153. The voltage is higher and the initial rise before sparkover of the gap 

takes place at about 2.2 kV. The current crest, after the sparkover, is of the same magnitude (1 100 A) as that of other tests. 
BUS 1 

(LINE 1) 

BUS 2 

(LINE 2) 

OPEN-CIRCUIT VOLTAGE 

Voltage at Bus Indicated: 

2 kV/di v 

VOLTAGE AND CURRENT WITH 

(1 ) INBOARD PROTECTOR AT 

EACH BUS 

Current in Protector: 

500 A/div I 

Voltage Across Bus: 

500 V/div I 

VOLTAGE AND CURRENT WITH 

(1 ) OUTBOARD PROTECTOR 

AT EACH BUS 

Current in Protector: 

500 A/div 

Voltage Across Bus: 

505 V/div 

VOLTAGE AND CURRENT WITH 

(1 ) VSP-1 AT EACH BUS 

Current in VSP-1: 

500 A/div 

Voltage Across VSP-1 : 1 500 Y/div 

VOLTAGE AND CURRENT WITH 

(1) HLP ON LOAD CENTER 

Current in HLP Line 1 : 

400 A/div 

Voltage Across Bus: 

-- 

500 V/div 

Test Condition: 10 kA injected - All sweeps: 2 us/div. 

Figure 13. Comparison of Alternate Protectors at Load Center 

2 1 
those of the envelope, being at 400 to 600 V, with initial bursts at 800 to 1100 V. 

In the case of Oscillogram 284, flashover of the wiring at the load center limited 

the current impressed on the VSP-1 protector. This is a result of an unintentional wiring 

flashover, which occurs frequently. 
Test Condition: 30 kA injected 

A1 1 sweeps : 2 Us/div. 

r 

(Flashover at B-20 

limits the current.) 

VSP-1 AT OUTLET SHOWN WITH 

OUTBOARD PROTECTOR AT 

LOAC CENTER 

Current in VSP-1 : 

200 A/div 

Voltage at Outlet: 

500 V/div 

Figure 14. Comparison of Duty Imposed on VSP-1 Installed at Outlets 

for Various Load Center Protections 

NO OTHER PROTECTOR 

Current in VSP-1 : 

1000 A/div 

Voltage at 0ut:et: 

500 V/div 

Voltage Across Bus : 

500 V/div 

(1) OUTBOARD PROTECTOR ON 

EACH BUS 

Current in Protector 

1000 A/div 

Voltage Across Bus 

500 V/di v 

Test Condition: 30 kA injected - 

Protectors at load center. 

All sweeps: 2 vs/div. 

Figure 15. Comparison of Performances Between Inboard and Outboard 

Protectors Installed at Load Center 
The total lead length (60 crn, or 24 in) was approximately equal to that involved in 

separately connecting the outboard protector (45 crn, or 18 in) and the inboard protector 

(15 cm or 6 in) at the load center of the simulation circuit. Current pulses of constant 

magnitude (3.2 kA crest, lops rise time) were injected in the loop, and voltages across 

the protectors and their corresponding leads were recorded as shown in Figure 16. 

Oscillogram 1 shows a 1000 V maximum voltage across the outboard protector 

and its associated 45 cm (1 8 in) lead, compared to only 600 V for the inboard protector 

and its 15 cm (6 in) lead (Oscillogram 2). Changing the lead of the inboard device to 

45 cm (1 8 in) (Oscillogram 3) raised the voltage to 1000 V, demonstrating that the difference is attributable to lead length, not disc characteristics, and illustrating the benefits obtainable by making the protector an integral part of the load center. 
I Vol tage: 500 V/di v 

I Current: 1000 A/div 

2 1 

VOLTAGE ACROSS "INBOARD 

SUPPRESSOR" PLUS 6" 

LOOP OF WIRE 

Vol tage: 500 V/di v 

Current: 1000 A/div 

3 1 

VOLTAGE ACROSS SAME 

SUPPRESSOR AS (1 ) BUT 

18" LOOP OF WIRE 

Vol tage: 500 V/di v 

Current: 1000 A/div 

Test Condition: Laboratory bench, not house simulation. 

Current injection in the two suppressors 

connected in series: 2.7 kA. 

All sweeps: 2 us/div. 

Figure 16. Effect of Lead Inductance on Clamping Voltage 

Test Condition: Injected current 10 kA. 

A1 1 sweeps : 2 vs/di v . 

Figure 17. Voltages at Load Center with VSP-1 and "100 W load," at Remote 

Outlets, No Protector at Load Center 
3.2 Discussion of the Results 

From the oscillograms collected during the test series, a summary of maximum 

values has been compiled as shown in Table 1. The three levels of current injection are 

included in this table, illustrating a mild, severe, and extremely severe lightning incident 

near the house. For the sake of simplicity in this table, only one protector is included 

in the arrangement matrix. A subsequent discussion will address the case of a coordinated scheme involving more than one protector. 

Injection of the maximum values recorded on the protector current shows that 

no rated values are exceeded, even at 30 kA injection. Voltages observed are consistent 

with the corresponding currents, from the V-I characteristics of the varistors. 

In the first group at 1.5 kA injection, voltages that are particularly damaging to 

appliances (2500 V) are observed throughout the system. Installation of a protector (on 

both lines) at the load center eliminates the hazard and would suffice to protect all of 

the house. Installation of a VSP-1 at only one close or remote outlet provides protection 

at that outlet and moderate protection on all locations of the same line. The other line 

is not protected. 

The unsymmetrical load (diode) of the TV input circuit behaved in a predictable 

mar,ner: when the polarity of the voltage was such that a forward bias was applied, the 

diode clipped the voltage, with the series resistance limiting the current. With reverse 

bias polarity, the diode failed when the 2500 V transient occurred at that outlet. 

In the second group, representing a severe incident, flashover can be expected 

throughout the system in the absence of protection, with the associated fire hazards as 

well as damage to electronics during the initial voltage rise. Installation of a HLP at 

the load center eliminates the flashover hazard but does not lower the voltage sufficiently 

to assure protection of sensitive electronics, nor does an outboard installation of varistors 

assure protection. Installation of an inboard set of varistors is effective, for the voltage 

is limited at the load center (and consequently on the whole system) to 700 V. Installation 

of a VSP-1 at a close outlet (producing the maximum current flow, hence highest voltage) 

is effective for that outlet only; on the basis of the differential observed at 1.5 kA, one 

can presume that the voltage at the load center would be too great to consider any other 

point but that outlet as being protected. 

Current Center Outlet Current 

Protector Arrangement t k A V V A 

None 1.5 2500 2500 - Inboard Protector at Load Center 1.5 500 500 100 

Outboard Protector at Load Center 1.5 700 700 70 

VSP-1 at B-20 1.5 800 400* 30 

VSP-1 at B-160 1.5 1100 350* 20 

None 10 8800 F/O - Inboard Protector at Load Center 10 700 700 1200 

Outboard Protector at Load Center 10 1200 2000 11 00 

HLP at Load Center 10 2200 NR 1100 

VSP-1 at B-20 10 NR 700" 950 

VSP-1 at 8-160 10 NR 600" 600 

Inboard Protector at Load Center 30 950 NR 3500 

Outboard Protector at Load Center 30 2200 NR 3500 

VSP-1 at 8-20 30 NR 800 2500 

VSP-I at 8-160 30 NR 500 250 

Notes - 

NR- No Record 

* "Protected outlet" is on same line as protector. Voltages on outlets of the 

other line close to unprotected values. 

t Only one protector at a line in this table. 
continue to be difficult until a package is developed to allow connection to the load cen

ter bus bars with very short leads. 

Until such an integral package is marketed for new systems, a coordinated protection scheme can be implemented, as a retrofit, that would still provide reliable protection for millions of sensitive appliances in existing systems. 

The coordination involves a protector at the load center, either the commercially 

available HLP or a packaged 32 mm disc set (two lines) with reasonably short leads in 

a package similar to the HLP. This protector will limit the voltage at the load center 

to about 2200 V. This 2200 V level is below the flashover level of the wiring but can 

still cause damage to sensitive appliances. The currents passing through the protector 

at that location will not exceed the protector capability. In addition, VSP-1 protection 

should be installed at those outlets where a sensitive appliance is plugged. The voltages 

allowed by the VSP-1, typically 400 to 600 V, will be low enough to assure survival of 

all but excessively sensitive appliances, while the VSP-I will not be exposed to currents 

that can lead to a failure in case of frequent exposure to severe lightning incidents. 

Thus, a coordinated protection scheme is technically feasible. The cost should 

be acceptable to do-it-yourself homeowners, although it might be a deterrent to those 

owners who have to call in an electrician to install a protector at the load center. Based 

on increasing awareness in the technical and regulatory agencies community of overvoltage protection, the incorporation of protection to load centers offers the best approach 

to new installations.