A trade magazine describing the confusion that exists on the market for the meaning of visible indications

(“lights”) of the condition of a consumer-type surge-protective device equipped with a built-in disconnector

that will operate in case of failure of the SPD component(s) in the package.

Presents a brief history of the recognition of the need for a disconnector function, and makes an appeal

for more consistent practices – short of standards – on the meaning of these status indicating lights.

Also underlines the basic difference between a disconnector that only disconnects within the package the

failed SPD component but leaves the load powered but unprotected and packages where the

disconnector function provides maintained protection by also disconnecting the load.
What Are the Lights on Your 

Surge Protector Telling You? 

Fran~ois Marlzloff, National Institute of Standards and Technology, Gaitherburg, Maryland 

E specially in a free market, sensible standards and reasonable regulations have a role to play toward providing 

useful, reliable and safe consumer products, such as the 

now familiar transient voltage surge suppressors (TVSS). 

However, the definition of what is sensible and reasonable 

has not been stipulated in any TVSS standard. This article 

provides a historical perspective of the evolution of how 

TVSS standards address the issue of a disconnector, an 

emerging concept in TVSS design. 

A sampling of how this concept has 

been implemented by the TVSS industry, including overcurrent protective devices (fuses) and thermal runaway protection (thermal cut-out), shows a variety of rather confusing approaches. To 

~llustrate this point, let's consider three 

familiar products: fruit cakes, automobiles, and TVSSs. 

We will all agree that our military 

personnel serving in some desolate foreign country are entitled to a high-quality product when the traditional fruit 

cake is sent to them at holiday time. To 

ensure that this happens, as reported 

tongue-in-cheek by the media a few 

years ago, a lengthy specification had 

been developed for quality-control in 

the procurement of fruit cakes. 

Consider now the plight of the tired 

traveler, arriving at night in an unfamiliar airport and renting a car. The agent 

behind the counter gives the traveler the 

keys and the location pf-the car in the 

parking lot, and smiles with good wishes 

thrown into the deal. Arriving at the 

designated spot in the dark pnrking lot, 

everyone expects to find that the steering is located on the left, controlled by 

means of a wheel, the engine is started 

with an ignition key, generally near the 

steering column, and gas and brakes are 

controlled by the right foot But what 

about finding the headlight control after you have clxed the door (it's cold 

and windy out)? Worse yet, two miles 

out of the airport, in heavy traffic, it 

suddenly starts ro rain and the traveler 

fumbles to find the windshield wiper 

control? Now would it not be lovely if 

there were standards mandating the location of these control;'? 

No Common Meaning to Lights 

Coming now to the point of TVSS, 

consider the plight ofthe consumer who 

bought a TVSS, and discarded the 

bubble package or misplaced the instruction sheet. This consumer is now 

trying to find the meaning of the pilot 

light, or pilot lights, indicating the state 

of the device - specifically whether protection is in effect or protection is lost. 

The original 1985 version of UL Standard 1449 stipulated that a visible or 

other indication of the protection status 

must be provided when an overcurrent 

protection has operated. But that is no 

help to the computer user who has installed a plug-in suppressor sandwiched 

between the wall and the desk. Even if 

the TVSS is visible, there might be one, 

two or even three pilot lights, with or 

without clear and reliable indication of 

their meaning. 

TO place this undesirable situation in 

perspective, I visited three local electronics and do-it-yourself stores and 

bought 17 different brands of TVSSs. 

The question was, "How would the user 

be informed on the status of protection 

after a temporary overvoltage caused 

failure of the protective element in the 

TVSS package?" temporary overvoltage was selected as the method of failure because there is growing evidence 

that they cause a large number of the 

failures - perhaps the majority -rather 

than "large" surges. Although that statement could be the subject of one entire 

issue of this magazine, it is only mentioned here to describe how the failures 

were obtained. 

The point of the inquiry was not to 

describe how the failure occurred (in 

particular whether there was a potential 

hazard, since the devices were manufactured before the 1996 Second Edition of UL 1449 became effective). The 

point of the inquiry is to find what information is clearly conveyed to the user 

after the TVSS has failed, and it? 

68 POWER QUALITY ASSURANCE. JULY/AUGUST 1998 
disconnector has operated. Neither the 

new UL 1449, nor any [EEE or IEC 

standards address that practical question. In fact, the new 1996 version of 

UL 1449 no longer stipulates inclusion 

of a visible indication. As it turned out. 

several of the packages among the 17 

specimens had very little information on 

the significance of the indicating light 

or how the indications should be interpreted. Furthermore, as mentioned earlier, once the bubble package is discarded, the information is lost, because 

it is unlikely the typical user will safeguard the back of the bubble package 

where the details - ~f any - might have 

appeared. 

SPD Failure Modes 

Surge-protective devices (SPDs) 

components - switchmg away from the 

jargon 'TVSS' to the official term of 

'SPD' approved by the IEEE and IEC - 

like any electrical component, are subject to fallure tor a val-lcr) 01 cau\cb Including severe overstl-c.;\ hcyond their 

--- . -- -. 

rating, natural aging, premature Pallure 

caused by misapplication, etc. The outcome of such failures ranges from an 

open circuit to a very low impedance. 

but ral-ely a zero-impedance short circuit. A final open circuit may be difficult to achieve in an environment of high 

available fault current. If achieved. it 

implies loss of protection against subsequent surges. A zero-impedance condition would mean no energy dissipation (no heating) in the failed SPD component and thus a relatively benign failure mode from the point of view of the 

SPD environment, but that would require interruption by an upstream 

overcurrcnt protection device. 

In reality, the nature of the Pallure 

mechanism, in particular for rnetal-oxide varistors, (MOVs) is that the impedance of the failed SPD, which is resistive, is low, but not zero. Consequently. 

the power circuit will deliver to this 

medtum-range resistance a cut-rent 

wh~ch ~v~ll involvc signif~c;tnt cncrgy. 

depcndtrig upon the actual resistiuicc of 

the failed dev~cc and the upstream impedance of the power system. 

If left unabated. this deposited energy will produce severe overheating 

with attendant risk of fire or other objectionable I'ailure modes. It is to 

prevent such objectionable scenarios 

that the concept of an SPD discor~r~ecror 

emerged, a\ seen in this historical perspective. 

The term "disconnector" is a recent 

addition to the vocabulary of the SPD 

industry, and the spell-checker of most 

word processors flags it as unknown (as 

if your processor had never disconnected on you?). Even the definition of 

the tcrrn has golie through several iterations in recent years. 

According to emerging standards, the 

disconnector function is to "disconnect 

an SPD from the power system in case 

of SPD failure." This clear and simple 

concept, however, has not been consistently ~rnplcnlcnted by industry, in particular t)! the low-voltage SPD industry. In p~~~-t~cul;~r. two different devices 

CIRCLE 149 

POWER QUALITY ASSURANCE. JULYIAUGUST 1998 69 
SPD Indicator lights 

GE Transient Suppression Manual 

First Edition 

Text Remarks - --- . . -- - .. 

First mention that a fuse should 

be used in conjunction with an 

SPD. 

There are two possible sequences of events leading 

to failure ... Accordingly, and in keeping with good 

safety practices, the varistor should be placed 

following the equipment fuse ... 

1978 

GE Transient Suppression Manual 

Second Edition 

... the careful designer may wish to plan for potential 

failure modes ... mechanical rupture of the package 

accompanied by expulsion ... minimize this potential 

hazard by fusing to limit high fault currents and 

locating varistor away from other components ... 

The guidance is becoming mort 

specific, but this was still a 

recommendation, not a mandat, 

I985 

UL 1449 Transient Voltage Surge 

Suppressors, First Edition 

12. Supplementary Protection 

'A transient voltage surge suppressor that is 

provided with supplementary overcurrent protection 

in series with the suppressor shall also be provided 

with visual or audible indication or both of the 

opening of the protection device." 

Exception: not required if in series with the load 

terminals 

Arrester Disconnector 

A means for disconnecting an arrester in anticipation 

of, or after, a failure in order to prevent a permanent 

fault on the circuit and to give indication of a failed 

arrester 

Note- Clearing of the fault current through the 

arrester during disconnection is generally done by 

the nearest source side overcurrent protective 

device. - .- ~ ~~ - . - - 

SPD Disconnector 

A Device for disconnecting an SPD from the power 

system in case of SPD failure. 

At least three functions are needed ... They may be 

integrated in the SPD ... or integrated to another part 

... They may be used in the SPD circuit or in the 

mains 

Implies the existence of a 

disconnector function but does 

not mandate it. 

1987 

C62.11 - IEEE Standard for metaloxide surge arresters for 

alternating current power circuits. 

The word 'arrester" in the title is 

unlikely to attract the attention c 

low-voltage SPD users. 

The note allows shifting the 

function to a separate device 

(presumed to be existing). 

- 

1994 

IEC 37A LV SPDs 

Working Group Draft - SPD 

Selectton and Application 

Prmaples 

Many options offered 

(Too many to be useful) 

-. -~- 

UL 1449 

Second Edition 

3.24 - Supplementary protection device definition: 'A 

device intended for overcurrent protections ... Has to be replaceable or 

resettable unless the whole unit 

replaceable. 

12 - Supplementary Protection Visible or audible indication of 

operation, stipulated in the 198! 

edition, is no longer required. - 

1996-1997 

IEC 61 643 SPDs for LV power 

dlstrlbutlon systems Performance 

requrements and testmg methods 

37A/44/CDV, 37Al63lFDIS 

Visible indication of failure is 

implied as a requirement within 

this definition. 

SPD Disconnector 

'A device for disconnecting an SPD from the system 

in the event of SPD failure. It is to prevent a 

persistent fault on the system and to give visible 

indication of the SPD failure." -~ -- 

SPD Disconnector 

"An internal or external device required for 

disconnecting an SPD from the system in the event 

of SPD failure. It is to prevent a persistent fault on 

the system and may give visible indication of the 

SPD failure." -- -- 

Consensus at last 

I 

Allows separation of the -- 1 

disconnector function into a 

separate device. Note the 

insertion of 'may" in the provision 

of a visible indication. 

1997 

Proposed modification for 

37Al63lFDIS 

Hopefully, before the turn of the 

Table 1. Brie/ history of the need for disconnector function as stated in standards and other references 

70 POWER QUALITY ASSURANCE JULY/AUGUST 1998 
-a fuse or a thermal cut-oft' - can serve 

toward that end, each covering one type 

of SPD failure. In case of SPD fa~lure 

resulting in a low impedance mode (a 

few ohms) an ovcl-current occurs, with 

amplitudes in excess of the SPD rating, 

so that a fuse can clear the circuit. In 

case of a thermal runaway. the beginning of SPD failure. the current is st111 

too low to operate the fuse, but the consequences of failure (overheating) can 

be prevented by the operation of a thermal cut-out in close proximity to the 

SPD experiencing the beginning of 

overheating. 

This ambigu~ty In rhc type ol 

disconnector des~gn and thc Ii~ch ofconsistency or consensus in the ~nterprctation of the funct~on is apparent in the 

wide variety of designs found on the 

market, while standard-writing bodies 

are still working on developing an acceptable consensus uvcr a hroad rangc 

ofappl~cations. Onc unrcolved ~siuc I\ 

whcthcr the usct- cxpccrs 1mrccr101i alter hilure - tluougll a co~~~l~lc~i. d~scoti- 

- - 

nett of the load - or would rather take a 

chance by maintaining continuity of 

operation, but without protection. 

Standards development generally 

lags the development of products in a 

competitive, fast-moving technology. 

Low-voltage SPDs are no exception. To 

illustrate this point, Table I presents a 

chronological tabulation of the evolution of the disconnector concept, from 

the introduction of metal-oxide varistors 

in the mid-seventies, to the latest amendment of the definition of a disconnector 

in the late nineties. 

The tabulation shows the reference 

document and cites, verbatim or in summary, the implied need - and eventually the specification - for a suitable 

disconnector function. Inspection of the 

chronology shows a clear trend toward 

mandating a reliable disconnector function, but also some concessions to existing product practices. Nevertheless. 

the disconnector remains the ultimate 

protection against objectionable failure 

modes. As recognitton of this essential 

role of a disconnector increases, one can 

expect that standards are more likely to 

mandate provision of a reliable and 

clearly defined disconnector function 

lor all low-volrage SPDs. It may beeasy 

for an outsidcr to make judgments on 

the apparent chaos of the marketplace 

and the present standards, but there are 

extenuating circumstances because the 

concept as well as the implementation 

of a satisfactory disconnector are not 

trivial. Some of the options, advantages 

and drawbacks are briefly enumerated 

below. 

Disconnector Provisions 

lntegral discortr~ec~or 

f o is connects only the shunt-connected 

SPD 

A Design challenge: requirement of 

not opcrattng lor the expected 

\urge\ but of' operating promptly 

upon L~tlurc ol rllc 51'13 element. 

A LJxl ~r~tor~liat~orr: necessity to 

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iPD Indicator lights 

SPD Status lndication - - Continuity Type of Indication 

1 Protection off Maintained Not recommended - -- ~ 

2 Protection off Interrupted - - - - Desirable - 

3 Protectron off Interrupted Desr rable -- 

4 Protection off Interrupted Desirable - - -- -- - 

5 Protection off Maintained Not recommended - -. 

6 

- Protection off Interrupted Desirable - - -. 

7 

--- Protection off - 

Mamtained Not recommended . 

8 Protection off Interrupted Desirable - 

9 Protection off Maintained Not recommended - -- -- - 

10 Protectioc off Ma~ntained Not recommended -- - - 

11 Protection off Interrupted Desirable -- 

12 Protection off Mamtained Not recommended - -- -- - 

13 

- Protection ON - - Maintained Misleading 

14 Protection off Maintained Not recommended -- 

15 Protection off Maintained Not recommended -- -- - - - - - - . 

16 Protection off Maintained --- Not recommended - - 

17 Protection ON Maintained - --- Misleading . - 1 

This comment IS based on the postulate that the user would prefer to have costly 1 

equlpment protected, albeit disconnected, rather than have the power supply rnalntalned 1 

L. at the risk of not protecting the equipment -- - 

Table 2. Summary of status mdrcations 

i.li.,~r I! \lgn;il that PI-otection 

ity.1111\1c~l~\ecluent sul-ges is lost. 

f D~\co~~r~c.ct\ the SPD and the pro 

tccrctl Io;~tl 

A I~L-\I:II challenge: respond to ab111)1111~1l Sl'L)c~nclition (tlicrmal 

I c~ti~\i ,I\ I ,I[ current aniplitudes 

Iwlo\\ tI1c I-ati11.g of the SPD. 

A I'IK ilccc\\aly \tci~dy-state current 

C'III-LIII~ capacity for the fuse 

ci~sct~nncc~or allows persistence 

i11 I~ni~icd current faults in the 

Y1'1). rc\~tlt~ng ~n hazardous overIIC,I[III~, IIII!C\\ an additional pro1i.cr1011 I\ p~ovided by a thermal 

Clll OLtl. 

A Usel- ~rrfor~nation: some users 

m~ght prefer to maintain operation 

of the load. albeit with no surge 

prurection. However, if this is the 

modc provided by the design, it 

riiust hc clearly identified on the 

prodi~ct. and ~ndication of proteetlon loss must be signaled - a 

merc indicating light on an SPD 

installed in a manner th;u the uwr 

will not inspect seem.; :I poor 

choice for an uninformed user. 

External disconrzecfor designed for the 

circltil 

f Protection of Jer S) \lCl1l I\ the pou - 

achieved, hut (he disconnector-I\ 1111 

likely to respond to limited I:iull CIII 

rents below the normal rating ol'ihc 

circuit supplying the SPD. Furthermore, the thermal runaway protect~on 

cannot be performed by an cxtcrrwl 

device which is not in closc corltai.1 

with the SPD conrponent 

From the poss~bilities Ils~ecl aho\i.. 

it is clear that a reliable disconnccror 

function is imperative for the successful and safe application of SPDs in 

power systems. However, market pressures and the resourcefulness of designers result in a wide range of design approaches, which can be overwhelming 

for the end user, just like our tired.[raveler arriving at the airport. Beyond the 

basic function of a low-voltage 

CIRCLE 152 
SPD disconnector, the details of the 

\operation and the interpretation of indications provided by all the different 

manufacturers do not offer a systematic, 

user-friendly and standardized situation. 

To illustrate this point, Table 2 presents a summary of the visible indications on the 17 blind-purchase of 

TVSSs, after producing their failure by 

applying a temporary overvoltage. In 

this table, it is postulated that for the 

typical consumer, ensuring protection of 

expensive equipment is preferable, even 

if it means that power is cut off for the 

equipment; in other words, maintaining 

continuity is undesirable. There may be 

some applications where maintaining 

continuity would be the first choice. Ln 

either case, a clear indication of which 

state prevails should be clearly stated. 

From this table, it is apparent that the 

two possibilities - maintained continuity o~. maintained protection - are ofI'crcd by different rnanufac~urcrs. Howcvcl-. the implications ot'rhis choice were 

not clearly stated on the package, and 

in two instances, the indications were 

in fact misleading. Hopefully, industry 

and standards-writing bodies will rem- 3. 

edy this situation. 

Without invoking a bureaucratic system of regulations, it would seem that a 

clear consensus among the manufacturers would make the informed end-user 

much more appreciative of the industry 

offerings, and provide enhanced safety 

for the public. 

Conclusions 4. 

A survey of the low-voltage SPD 

(TVSS) products currently on the 

marker shows not only a variety of 

disconnector responses after a failure, but, in some cases, false indication of the protection status. 

Information provided by several 

manufacturers, on the product packxgc (soon to be discarded after purchase by the end-user) or on the prodtection after failure of the SPD con, 

ponent. 

Anecdotal information indicates tha 

devices currently on the markc. 

might not be provided w~th , 

disconnector that meets the undefined but expected safety in failurmodes. The second edition of U1 

1449 is a very constructive step to 

ward a remedy, although it does nvaddress the question of statu 

indication. 

This uncoordinated - not to say chii 

otic - situation clearly points to th-. 

need for accelerating the develor. 

rnent of an industry-wide conwnilon the design, operation, anti inre 

pretation of the indications of lob 

voltage SPD disconnectors. Hop;: 

fully, this article will help tt,. 

process.