BLACK BOXES, BLIND SPOTS, AND DISCONNECTORS: 

How not to test SPDs 

François Martzloff 

Surges Happen! 

Gaithersburg MD U.S.A

 

Abstract — The current standards for testing surgeprotective devices (SPDs), leave some ambiguities for their 

implementation. Field failure data show that some SPDs 

that pass standard tests can still fail in unacceptable or 

unexpected modes, perhaps because of “blind spots” in the 

test regimen. The paper shows examples of such unresolved 

issues and also suggests closer attention to the disconnector 

function. 

1. INTRODUCTION

 A long-standing philosophy, perhaps even a doctrine, in the approach to testing surge-protective devices 

(SPDs) is that test specimens should be seen as “black 

boxes” – meaning that all brands of devices should be 

treated alike, regardless of their internal design. Such an 

approach is commendable for the sake of “fairness” and 

is understandably motivated by concerns that a testing 

organization should not manipulate its test regimen to 

produce results that would favor one particular design 

while deprecating another design. 

 Meanwhile, given the practical limitations of testing 

a “reasonable” number of specimens over the wide range 

of possible stress threats, some test standards attempt to 

specify only a limited number of stress levels that appear 

“realistic,” but which can leave blind spots in characterizing the performance of the device. 

 This situation is not just an intellectual concept; this 

paper presents examples of field failures or laboratory 

tests that occur for stress conditions that were not identified in the test regimen recommended or mandated by 

the current relevant product standard(s). 

2. BLACK BOXES 

 A brief perusal of existing IEEE definitions should 

facilitate the discussions, avoid confusion, and ensure 

common understanding of the terms: 

black box – A system or component whose inp

outputs and general function are known but w

contents or implementation are unknown or irrelev

Contrast: glass box.

blind spot – A limited range within the total doma

application of a device, generally inferior to the m

mum rating. Operation of the equipment or of 

protective device might fail in that limited r

despite demonstration of satisfactory device perfo

ance at maximum ratings. 

functional testing – Testing that ignores the inte

mechanism of a system or component and foc

solely on the outputs generated in response to sele

inputs and execution conditions. Synonym: black

testing. 

glass box – A system or component whose inte

components or implementation are known. Synon

white box. Contrast: black box.

white box – See: glass box.

3. BLIND SPOTS 

3.1 Terminology

 The term “blind spot” in the context of 

testing first appeared in IEEE Std C62.45TM-1987 

as the authors of that document were keenly awar

the possibility that for some intermediate stress co

tions, the SPD might not perform as expected, in 

of demonstrated satisfactory operation at maxim

stress. One long-standing definition of that t

which appears in Webster’s as “An area in which 

fails to exercise understanding, judgment, or discr

nation,” provides a good perspective to the contex

surge testing: a lack of understanding how the de

works (as in black-box testing) can indeed lead to 

recognizing blind spots in the domain of applicatio
the SPD. Supporting the skepticism about black-box 

testing philosophy, Webster’s also mentions that lack of 

judgment (here an unquestioned application of blanket 

procedures) can also lead to blind spots. 

 This term of blind spot is now being extended to the 

arena of testing, not just for performance of the protective function, but also for failure mode testing. If a test 

regimen fails to ferret out a region where an unacceptable failure mode can occur, we now say that there is a 

“blind spot” in that test regimen, thus implying a secondary definition of that term. 

 Blind spots in surge-protective devices, the general 

subject of this paper, generally result from the transition 

of operation among internal components that respond in 

a different manner, depending on the parameters of the 

applied stress. An example of blind spot in the SPD 

performance can occur in the transition of operation from 

a voltage-limiting device to a voltage-switching device 

(in the same package or in a combination of two separate 

packages). An example of blind spot in the test regimen 

is the occurrence of an unacceptable failure mode at the 

transition from a fast-acting to a slow-acting overcurrent 

disconnector, allowing higher energy deposition during a 

mid-range fault that endures, compared to the potentially 

higher energy deposition for a large fault but which is 

promptly cleared by the SPD disconnector. 

 With knowledge of the principle of operation of the 

SPD, as well as details on the component characteristics, 

a test laboratory and the agency requesting the tests have 

a much greater likelihood of successfully anticipating 

where in the range of possible stresses a test blind spot 

might occur, and therefore focus on that range of stress. 

 In a non-adversarial test program, the manufacturer 

is also in a good position to share the experience gained 

in the design stages and thus offer recommendations to 

the laboratory for test levels that would indeed focus on 

the crucial parts of the range where transitions might 

occur between the operation of one internal component 

to another. During the design stages, a manufacturer can 

be expected to thoroughly explore the range of stresses to 

which an SPD might be exposed, and make the appropriate design adjustments to prevent the occurrence of a 

blind spot, which might otherwise have been overlooked 

if only the test regimen mandated by the present standards had been applied. 

3.2 The quest for blind spots 

 As a result of the two meanings of “blind spot” we 

need to differentiate between the two: 

3⁄4 A blind spot in the surge-protective function; 

3⁄4 A blind spot in the demonstration(s) of acceptable failure modes. 

 Blind spots in the protective function have by now 

been recognized and are not the subject of much debate. 

For instance, the protective function of a hybrid SPD that 

includes a voltage-switching device, a decoupling inductance, and a voltage-limiting device might not work at 

some intermediate ranges of surge current, or with 

rising surges because the inductive drop expected f

the decoupling inductance is insufficient to spark

the voltage-switching device. 

 As this type of test is nondestructive, it can 

easily repeated on the same specimen over the w

range (matrix) of waveforms and amplitude le

Hence, the phenomenon is well recognized and 

been described at some length in the IEEE Rec

mended Practice on Surge Testing [1]. 

 In recent years, recognition of temporary overv

ages (TOV) as the most likely cause of SPD failu

rather than excessive surges, has considerably 

creased. This recognition is quite apparent in 

inclusion of guidance on the occurrence of TOVs 

recent standards whose primary scope is describing 

occurrence of surges (and thus were expected by s

members to exclude consideration of TOV issues)

instance IEEE C62.41.1TM-2002 [2]; IEC 62066:2

[3], and UIE Guide Part VI-2001 [4]). However, t

new standards do not include much guidance on tes

the performance of SPDs under TOV conditions. 

be meaningful and realistic, a test scenario intende

produce failure of the test specimen under such a T

condition must stipulate a well-defined level of 

available fault current to be delivered by the po

distribution system to the failed specimen. 

 That is where the issue of blind spots in the 

regimen, and blind spots in the performance of 

disconnector (if any) becomes the subject of the 

sent debate in the intense quest for selecting suit

available fault current values that have a chanc

ferreting out the blind spots in what might other

appear to be reassuring set of acceptable failure m

demonstrations. 

 Some SPDs returned from the field as in-ser

failures have an appearance quite different from 

obtained by laboratory testing under TOV condit

suggested or mandated by the present SPD standa

Examples of such discrepancies are given later in 

paper. Such a discrepancy then raises the old ques

of how a test regimen is expected to “duplicate” 

conditions by “realistic” testing, or whether a 

should be performed on the basis of arbitrarily (but 

intelligently) stipulated stresses. This problem 

caused, and still causes intense debates in the fiel

surge testing: waveforms and amplitudes have been 

in standards by a consensus process based someti

on very limited field data. 

 An example of that debate occurred in the de

opment of the so-called “SPD Trilogy” [1], [2], 

when addressing the case of a direct lightning flas

the building of interest A similar debate has 

emerged on the subject of realistic TOV testing – 

the added complication that the position of blind s

within the range of available fault currents is prese

more a matter of consensus than of hard facts. 
4. EXAMPLES OF BLIND SPOTS 

 The following nine photographs showing examples 

of failure modes of cord-connected or hard-wired SPDs, 

(communicated to me as anecdotal information for the 

purposes of this paper) illustrate the concerns about blind 

spots in earlier as well as in the present standardized test 

procedures. Five of these examples deal with consumertype plug-in or cord-connected SPDs, the other four with 

hard-wired SPDs. These four sources of data are gratefully acknowledged for providing me with real-world 

examples, but please note that I have no intention to pin 

blame on a particular product, only to illustrate that the 

problem is real. For this reason, the photographs of the 

cord-commected SPDs shown here were selected from 

many available ones, so as not to be readily identifiable 

as a particular brand. 

4.1 Blind spots in performance and testing of cordconnected SPDs – EPRI PEAC tests 

 Figure 1a shows an example of a UL-listed SPD 

(UL 1449 First Edition) [6] that failed in the field with an 

unacceptable manner after exposure to a TOV caused by 

loss of the neutral connection. The available fault current 

at that site was not known. Figure 1b shows an example 

of TOV failure induced in the laboratory during exposure 

to 170 % of normal voltage (a typical lost-neutral scenario) with an available fault current of 10 A – a level 

that would not trip a typical 15 A breaker. 

(a) 

(b) 

Figure 1 – Examples of TOV-induced failure modes 

on SPDs listed per UL 1449 First Edition 

(Courtesy EPRI PEAC Corporation) 

 Growing awareness of this type of failure m

was one of the motivations for the development o

Second Edition of UL 1449 that now includes a r

men of tests with a stipulated range of limited avail

fault currents for abnormal voltage conditions. 

 As a result of the new edition of UL 1449, s

design changes were made in existing products 

were found in laboratory tests to fail the new requ

ments. Figure 2a shows an example of a pre-1

Second Edition SPD that failed in an unaccept

manner when subjected to the new limited-current 

per UL 1449 Second Edition. Figure 2b shows 

same but redesigned SPD model in which the add

of a thermal cut-out made the failure mode accepta

Thus, evolution of UL 1449 into specifying additi

available fault current values did prove to be effec

in enhancing the product safety, but there are still s

blind spots in some 2002-vintage SPDs, as Figu

will show. 

(a) Before redesign 

(b) After redesign 

Figure 2 – Failure modes before and after redesi

(Courtesy EPRI PEAC Corporation) 
4.2 Blind spots in performance and testing of 

 cord-connected SPDs – CPSC data 

 Figure 3 shows one example of the many cordconnected SPDs that failed in the field and were then 

submitted to the Consumer Product Safety Commission 

(CPSC) for evaluation. The photograph shows the back 

of the enclosure of a seven-year old SPD (pre-Second 

Edition UL 1449) which was reported as having occurred 

upon recovery from a power system outage – a classic 

scenario of temporary overvoltage occurrences. 

 Thus, real-world situations can occur in the field 

where some UL-listed SPDs might still be failing in an 

unacceptable manner when exposed to TOVs for which 

the actual available fault current might not have been 

included in the present mandated test regimen. 

Figure 3 – Field failure submitted for assessment 

(Courtesy U.S. Consumer Product Safety Commission) 

 Ongoing laboratory tests at the CPSC indicate that 

commercially available, UL-listed SPDs can experience 

unacceptable failure modes for TOV-induced failure in 

ranges not included in the present UL 1449 requirements 

for the available fault current. This situation indicates 

that there are some blind spots left in the available fault 

current range of the present test regimen prescribed by 

UL 1449, Second Edition. 

4.2 Blind spots in performance and testing of 

 hard-wired SPDs – CH data 

 A combination of the Cutler-Hammer (CH) data on 

field failure returns and laboratory tests provides some 

further insights in the problems of black-box testing 

SPDs under an arbitrarily set range of available fault 

currents. 

 Figure 4a shows an example of in-field failure. In 

that example, there was no practical after-the-fact possibility of performing measurements of available fault 

current at that particular site, but a comparison with 

results of hundreds of in-house laboratory tests le

the conclusion that the available fault current at 

site must have been in a range of 5 A to 500 A bec

laboratory test samples tested outside this range (be

5 A fault currents and above 500 A fault currents) h

totally different level and type of damage. To simu

in-field conditions in the laboratory and obtain sim

results, the operator had to provide a power su

with an available fault current of 100 A. This sh

that the range for this test procedure, which did r

cate the in-field conditions, is not covered in an

present UL 1449 standards tests. 

(a) 

(b) 

Figure 4 

(Courtesy Cutler-Hammer) 

 Figure 4b shows an example of an SPD failed 

5 A fault current. Comparing the two figures (4a 

4b) does show the differences between a failure m

test with 5 A available fault current – the prese

prescribed UL 1449 test – and a failure mode 

performed with 100 A available fault. This exam

adds further strength to the point that some med

available fault currents must be included in SPD 

requirements. 
4.4 Blind spots in performance testing of hard-wired 

SPDs – Schneider Electric data 

 Figure 5a shows the results of a laboratory test 

performed on a new product under development to 

simulate an abnormal overvoltage test with an available 

short circuit current of 500 A. These hard-wired SPDs 

under test were designed and intended to be installed on 

the side of enclosures used with 120/240 Vac service 

entrance load centers. The value of 500 A was intended 

to replicate conditions when the SPD would be employed 

in remote locations where the available fault current 

from the local electric service provider is limited by the 

impedance of the long distribution circuits. This SPD 

might also be employed in locations where there would 

not be any secondary overcurrent protective devices. 

The only local overcurrent protective device might then 

likely to be the fused cutout on the primary of the local 

distribution class transformer. The use of a 500 A value 

represents an available fault current that is not presently 

specified as a standard test in any edition of UL 1449. 

 Figure 5b shows an example of one of several infield failures in remote areas of the same rural coastal 

county. The available fault currents at these locations 

were greater than 100 A but less than 1000 A. This 

specific SPD was protected by 15 A fuses. The 15 A 

fuses were found intact and functional. However, the 

phenolic cover of the SPD had been overheated and was 

deformed. It was suspected that exposure to TOV conditions were major contributing factors in the in-field 

failures. The factory designs and construction of that 

specific model SPD was UL witness-tested to the Second 

Edition of UL 1449. and all the UL tests had been passed 

satisfactorily. 

5. WHERE IS THE DISCONNECTOR ? 

 The critical function of an SPD disconnector has not 

yet been sufficiently recognized although – with some 

hindsight – it should have been obvious. The seminal 

1976 publication on metal-oxide varistors (“GE-MOV”) 

applications (GE Transient Manual [7]) includes several 

pages discussing the need and details of “fusing protection” and yet some present standards (IEC 61643 series 

[8]; [9]) do not mandate the application of a disconnector 

(optional as internal or external) while others (IEEE Std 

C62.34TM-1996 [11]) do not provide any guidance on 

testing the disconnector function. 

It is now urgent that this ambiguity on disconnectors 

be replaced by appropriate guidance or, better, clear 

mandates in SPD application standards. 

 Furthermore, the parameters cited for Class I test 

leave some ambiguity on the issue of what waveform is 

appropriate for applying the specified charge transfer, Q, 

to the test specimen. Emphasis given on charge transfer 

being the critical parameter can lead to performing the 

test with relatively short waveforms, which results in 

peak impulse current values in hundreds of kiloamperes. 

While such peak values do provide the stipulated values 

(a) 

(b) 

Figure 5 

(Courtesy Schneider Electric) 

for the charge transfer, Q, they should be care

reviewed for consistency with reality. 

 The high values of di/dt associated with a 

peak of a short waveform would result in a l

voltage drop in a cable postulated to bring the 

world surge to the targeted SPD. That means tha

driving voltage at the “sending end” of that s

would be so high that a flashover of clearances w

occur at that sending end, throttling further applica

of the surge current to the SPD at the receiving 

(Martzloff, 1997, [12]). Thus, one must conclude 

in the real world, SPDs will not be exposed to that 

of high peak, short stress, although it may be a v

approach to demonstrate the capability of the 

component for that charge transfer (⌠idt), but only 

 Notwithstanding this somewhat reserved acqu

cence to accepting the simple Q parameter as 

significant one, a paper by Bartkowiak et al.[13] sh

that the relationship between charge transfer 

withstand capability is not that simple. Diffe

stresses – and therefore different failure modes – in 

case of distribution arresters are described in 

paper, such as edge flashover related to the rate of 

di/dt, mechanical stresses related to i2

 and the
effects in the zinc oxide grains that are related to an 

adiabatic or non-adiabatic heat deposition, depending on 

surge duration and ratio of the boundary-layer-to-grain 

volumes, as well as uneven current density related to the 

“skin effect” that increases the current carried near the 

periphery of the discs. I am not aware of a similar study 

having been performed on the typical 20 mm discs used 

in low-voltage SPDs. These smaller discs might not be 

as sensitive to the rate of energy deposition as the larger 

disks used in distribution arresters. 

 If now the SPD is associated with a disconnector (a 

feature that needs a clear mandate in SPD application 

standards) the fusible element of the disconnector responds to⌠i

2

dt, not⌠idt. By applying an “equivalent” 

high peak (with a linear relation to the duration of the 

waveform) aimed at obtaining the stipulated Q, the 

disconnector is exposed to the thermal effect of the

current, with a square-law relation to the duration of the 

waveform. Under these conditions, the disconnector is 

likely to open and thus provoke a judgment that the 

specimen SPD has failed the test. Worse yet, if the 

disconnector opens during the “equivalent” surge, the 

behavior of the package becomes unpredictable. 

 A more insidious consequence of demanding that 

the disconnector not open during the charge transfer test 

might be that the designers, anxious to met that criterion, 

might erroneously provide a disconnector with a timecurrent characteristic that would leave a blind spot in the 

functionality of the disconnector in the range of medium 

fault currents, as demonstrated by the examples provided 

in this paper. 

 One solution, perhaps the only one, is to by-pass the 

disconnector when performing the charge transfer test, 

but that might be seen as opening the black box and 

therefore be deemed unacceptable – one more reason 

why SPD standards should avoid the well-meaning but 

misleading black-box test philosophy and procedure. 

6. CONCLUSIONS

 The present situation in the guidance – or unclear 

mandates – for application of surge-protective devices is 

leaving gaps – blind spots – in test procedures and 

qualification tests that cry for redemption, as illustrated 

by the five examples cited in this paper. Greater cooperation, coordination, and candid consultations among 

standards-developing organizations, manufacturers, and 

end-users is one approach that would bring positive 

results. While some progress has been made in that 

direction, the present situation still leaves several gaps 

that urgently call for action, as listed below: 

6.1. The present situation 

3⁄4 Lingering perception that “black-box testing” is 

the desirable and “fair” test method. 

3⁄4 Discrepancies between field failures and labinduced failures observed under standard specifications. 

3⁄4 Lack of consensus on what is a reasonable 

number of tests in ferreting out blind spots

3⁄4 Insufficient knowledge of the range and 

values of real-world TOV scenarios. 

3⁄4 Insufficient recognition of the function and 

location of SPD disconnector 

6.1. Recommended action items 

3⁄4 In a few words, but with much work imp

address all the concerns listed above ! 

7. ACKNOWLEDGEMENTS 

 The contributions of anecdotal but real-world 

by the four organizations identified with the ph

graphs and narratives (EPRI PEAC, US Consu

Product Safety Commission, Eaton/ Cutler-Ham

and Schneider Electric) provided for me and – m

important, for readers of this paper – valuable sup

for the “How not to test SPDs” theme, and all 

gratefully acknowledged. 

8. REFERENCES 

[1] IEEE Std C62.45TM-2002, IEEE Recommended Prac

on Surge Testing for Equipment Connected to LowVoltage (1000V and Less) AC Power Circuits. 

[2] IEEE Std C62.41.1TM-2002, IEEE Guide on the Surg

Environment in Low-Voltage (1000 V and less) AC 

Power Circuits. 

[3] IEC 62066: 2002, Surge overvoltages and surge prot

tion in low-voltage a.c. power systems 

[4] Guide to Power Quality of Electrical Supply for Indu

trial Installations, Part VI – Transient and Temporary 

Overvoltages and Currents, UIE, 2001. 

[5] IEEE Std C62.41.2TM-2002, IEEE Recommended 

Practice on Characterization of Surges in Low-Voltag

(1000 V and less) AC Power Circuits. 

[6] UL 1449, Second Edition, Transient Voltage Surge 

Suppressors, 1998. 

[7] Transient Voltage Suppression Manual, General Elec

Company, 1976. 

[9] IEC 61643-1: 2002, Surge protective devices connec

to low-voltage power distribution systems – Perform

requirements and testing methods. 

[10] IEC 61643-12: 2002, Surge protective devices conne

to low-voltage power distribution systems – Selection 

and application principles. 

[11] IEEE Std C62.34TM-1996, IEEE Standard for Perfor

ance of Low-Voltage Surge-Protective Devices. 

[12] Mansoor, A. and Martzloff, F.D., “Driving High Sur

Currents into Long Cables: More Begets Less,” IEEE 

Transactions PWRD-12, July 1997. 

[13] Bartkoviak, M., Comber, M.G., and Mahan, G.D, 

“Failure modes and energy absorption capability of Z

varistors,” IEEE Transactions PWRD-14, January 19