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The Short Circuit Current Ratings of

Surge Protective Devices (SPDs)

B.R. Cole,IEEE Member, K. Brown,IEEE Member, P.S. McCurdy,IEEE Member,

T.E. Phipps,IEEE Member, and R. Hotchkiss,IEEE Member

Abstract--This document examines the standards

associated with the short circuit current ratings (SCCRs) for

surge protective devices (SPDs). A review of the IEEE, UL, and

NFPA documents is used to determine why SCCRs are required

for SPDs and what is the correct methodology for determining

the coordination of the SPD and the electrical distribution. An

example is provided detailing the proper and improper applica-

tion of the SPD as it relates to the SCCR. For improper applica-

tions of an SPD, remedial actions are presented.

Keywords AC circuit breakers, overvoltage protection, short

circuit currents, surge protection

I. INTRODUCTION

Transient Voltage Surge Suppressors (TVSS), more

commonly referred to as surge protective devices (SPDs),

have been applied as part of an overall power quality strategy

for more than three decades. As power and control systems

become more complex through the utilization of semiconduc-

tors used in electronic devices to control, process, and com-

municate information, SPDs will continue to be recognized as

an essential component of power quality. However, a large

knowledge base regarding the proper application of SPDs

does not exist; therefore, many issues cloud the safety, per-

formance, and the correct application of the device.

In particular, the short circuit current rating (SCCR)

of the SPD is discussed within this paper. The SCCR of an

SPD is defined as the suitability of the SPD for use on an AC

power circuit that is capable of delivering not more than a

declared rms symmetrical current at a declared voltage during

a short circuit condition1. This paper will examine the

standards, electrical codes, and application guidelines associ-

ated with the installation of SPDs. Detail will be provided

determining sufficient testing methodologies associated with

obtaining the SCCR and the pass/fail criteria as determined by

the listing agency. In addition, two examples will be provided

that detail specific applications of an SPD, as related to

coordinating the SCCR of the SPD to the power distribution

system of the facility. One example will be a correct applica-

tion. The second example will be an incorrect application of

1UL 1449 3rdedition Draft, Underwriters Laboratories, Incorporated, dated

2005 December 2005.

This report has been developed by a task force of the Surge Protective

Devices Committee Working Group 3.6.6 on the Applications of Low

Voltage Surge Protective Devices and recommended for presentation at the

PES General Meeting, 2006 July. Members and contributors of the task force

included: B. Cole, K. Brown, P. McCurdy, T. Phipps, and R. Hotchkiss,

the SPD, but will provide specific methods that will allow the

application to be corrected.

II. STANDARDS AND GUIDES

In the United States, there are a small number of

documents that have been produced to evaluate both the

performance and safety attributes of a properly installed and

coordinated SPD with the electrical distribution. These

documents have been produced by telecommunication com-

panies, by regulatory bodies, and independent organizations.

The primary documents developed and addressed in this paper

include those by the National Fire Protection Association(NFPA), Underwriters Laboratories, Incorporated (UL), and

the Institute of Electrical and Electronic Engineers (IEEE).

A. The NFPA

The National Fire Protection Association (NFPA) is

the primary sponsor of the National Electric Code (NEC), a

top-level document detailing the guidelines for application of

electrical and/or electronic equipment connected to low-

voltage power distribution. The NEC is primarily concerned

with the safety aspects of all electrical wiring and electri-

cal/electronic devices, including SPDs. As an essential

component of ensuring the safe application of an SPD, the

NEC requires that virtually every component of the buildingwiring or device connected to the building wiring, including

SPDs, be provided with a means of over-current protection

[1]. In particular, Article 110-10 of the NEC states: The

over-current protective devices, the total impedance, the

component short-circuit current rating, and other characteris-

tics of the circuit to be protected shall be selected and coordi-

nated to permit the circuit-protective devices used to clear a

fault to do so without extensive damage to the electrical

components of the circuit. This fault shall be assumed to be

either between two or more circuit conductors. Listed prod-

ucts applied in accordance with their listing shall be consid-

ered to meet the requirements of this section.2

The NFPA 70 has added a new article to the 2002

codebook. Article 285 Transient Voltage Surge Suppressors

was added to address general, installation and connection

requirements for permanently installed Transient Voltage

Surge Suppressors. Prior to Article 285, the installation of

SPD systems was generally referenced under NEC Article 280

Surge Arresters. In the 2002 NEC, Article 280 continues to

address the general installation and connection requirements

2NEC 2002, National Fire Protection Association, Incorporated (NFPA) 70,2002.

1-4244-0493-2/06/$20.00 2006 IEEE.

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for Surge Arresters; however, this is beyond the scope of this

technical report.

Items in the 2002 NEC, the short circuit current rat-

ing of the SPD are detailed in Article 285.5, and Article

285.6. Article 285.5 states that the SPD shall be a Listed

device [1]. Article 285.6 states that the SPD shall be marked

with a short circuit current rating, which is obtained through

an independent safety organization, and that the SPD shall beinstalled at point on the electrical distribution that does not

exceed the rating of the SPD [1]. Listed products are those

products that have been evaluated by an independent safety

organization in accordance with OSHA 29 CFR 1910.7 [2].

The predominate safety agency that provides product listings

in the US is Underwriters Laboratories, Incorporated (UL).

UL provides product Listing of SPDs in accordance with the

Standard for Safety, Transient Voltage Surge Suppressor,

number 1449, Second Edition, dated 1996 August. This

standard will be discussed in greater detail later in this article.

As detailed in Article 285.6, the code allows SPDs to

be used at points within the electrical distribution system

where the short circuit current rating of the SPD is greater

than that of the system, but not where the short circuit current

of the system is greater than that of the SPD SCCR. What the

code does not address is the type of over-current protection

that is required for the SPD to comply with these require-

ments. It is the responsibility of the facility engineer to detail

what the proper over-current protective device is and how the

SPD is evaluated and determined to comply.

B. UL Standards

Within the United States, the UL Standards for

Safety have long been considered the primary documents in

assessing the safety of products. In Canada, the Canadian

Standards Association is the primary safety assessment

organization. There are also many other organizations that

have been approved as independent safety assessors by

OSHA.

The UL Standard for Safety, Transient Voltage Surge

Suppressors requires that the devices be subjected to many

tests that determine the overall safety of the product. These

tests include, but are not limited to dielectric withstand, surge

testing, and the testing of the product to two-types of abnor-

mal over-voltage scenarios: high current and limited-current.

To meet the requirements of Article 285.6, UL util-

izes the test procedure associated with abnormal over-voltages

and high available fault currents. This is achieved by per-

forming the full-phase voltage, high-current abnormal over-

voltage test described in section 37.3 of UL 1449 [3]. The

testing consists of applying an over-voltage with a specified

available short circuit current to production representative

SPDs. The applied over-voltage is the maximum phase

voltage or twice the conductor pair voltage. For 120/240 Vac,

single-phase, 3W+G system, the phase-to-neutral and phase-

to-ground test voltages are 240 Vac. For a 120/208 Vac,

three-phase wye, 4W+G system, the phase-to-neutral, and

phase-to-ground test voltages are 208 Vac. In a 240 Vac,

three-phase delta, 3W+G system, the phase-to-phase, and

phase-to-ground test voltages are up to 480 Vac.

To either trip or open the over-current protective

device during the testing of the SPD, it is necessary that the

surge components conduct a significant amount of current.

The SPD may use either internal or external over-current

protective components. During and following the full-phasevoltage, high-current test, the SPD shall not create openings in

the enclosure, exhibit emission of flame, charring, glowing or

flaming of the cheesecloth wrapped around the product during

the test, or compromise the enclosure grounding of the SPD.

Once the SPD has successfully completed the testing,

the device shall be marked with its short circuit current

rating. The marking states that the SPD is suitable for use on

a circuit capable of delivering not more than (the nominal)

rms symmetrical amperes, (at tested) volts maximum3. The

short circuit current rating shall be one of the values indicated

in Table I, but not less than the amount detailed in Table II,

which is a reproduction of Table 61.3 and 12.1 respectively in

[3].TABLE I

LISTING OF AVAILABLE FAULT CURRENT RATINGS

FROM UL 1449, SECTION 61

Available Fault Current Rating RMS

Symmetrical Current

(Amperes)

5,000

10,000

14,000

18,000

22,000

25,00030,000

42,000

50,000

65,000

85,000

100,000

125,000

150,000

200,000

If the SPD requires an external fuse or circuit breaker

to pass the test at a specified SCCR, the device shall have an

additional marking as described: when protected by __A__class fuses and/or when protected by a __B__ circuit breaker

rated __C__ volts maximum. The interrupting rating of the

fuse or circuit breaker shall not be less than the available fault

current, whereAis Class CC, CD, G, H, J, L, R, T or K fuse,

Reference to Class H or Class K fuses shall not appear in the

marking if the indicated rms symmetrical fault current is

3Standard for Safety, Transient Voltage Surge Suppressors, UnderwritersLaboratories, Incorporated, UL 1449, 1996 August.

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greater than 10,000A,Bis the current rating of circuit breaker,

and Cis the nominal system voltage.

TABLE II

AVAILABLE FAULT CURRENT FROM SOURCE OF SUPPLY

FROM UL 1449, SECTION 12

TVSS (SPD) Rating Available Current in

Amperes

100A or less 5,000101 400A 10,000

Over 400A 25,000

UL 1449 states that an external circuit interrupting

device may be used and if one is employed in the test, then it

is required at installation [3]. When an SPD is marked for use

with an external over-current protective device, the test circuit

is to include that device: e.g. circuit breaker or fuse. Installing

an over-current protective device with an adequate interrupt-

ing rating in series with the SPD is required at installation.

With UL providing the test and marking requirements of the

SPD and the NEC providing the marking requirement of the

electrical system, an installation with the correct coordinationof the SPD and the power system available fault current is

obtainable. The coordination of power system, over-current

protective device and SPD are made easier as each is marked

with its appropriate short circuit current rating.

C. IEEE Guides

The IEEE is active in providing recommendations

and guidelines that help engineers correctly describe transient

environments, and help to identify the many attributes of a

correctly applied SPD. The most notable document on surge

environments and the anticipated waveforms at specific

locations within a facility are IEEE Guide on Surge Voltages

in Low Voltage AC Power Circuits, IEEE C62.41.1-2002,and IEEE Recommended Practice on Characterization of

Surges in Low Voltage AC Power Circuits, C62.41.2-2002.

While the aforementioned documents do provide an

overview of the surge environment, they do not provide any

input into how the SPD can, or should be coordinated to the

electrical distribution. Therefore, recommendations on the

performance, suitability, or best engineering practice for using

an over-current protective device in series with an SPD is

relegated to other industry standards.

III. APPLICATION EXAMPLE

Two main factors should be considered when select-ing over-current devices to be used with SPDs. First, the

ability to limit the available short-circuit energy to levels that

are compatible with an SPD during an end-of-service life

event. Second, the over-current device should not signifi-

cantly affect the ability of an SPD to divert transients within

the electrical distribution system. If an over-current protec-

tion device connected in line with an SPD disconnects during

a transient, the SPD is removed from the circuit when it is

needed most. In addition, it could be some time until this is

noticed, thereby making the SPD ineffective. Therefore,

careful attention should be made when coordinating series

connected over-current devices with SPDs to ensure not only

the safe end-of-service scenario, but additionally the maxi-

mum performance and reliability of the SPD.

An example is the best way to illustrate the correct

and incorrect application of an SPD in accordance with the

codes, conditions of acceptability, and guidelines of the NEC,

the UL, and the IEEE, respectively. In addition to providingan example of an incorrect application, the authors will also

present remedial methods, which engineers and installers can

use to bring the installation into compliance with the afore-

mentioned recommendations and guidelines.

As an example of a correct application, assume that

an SPD is going to be installed on the power distribution

system of a facility at a branch panel location. At this point in

the power distribution system, the available fault current

capability is 42,000 A. The available fault current at this

location, or any other within the power distribution system of

the facility, must be determined through a fault current analy-

sis by the supervising engineer. Assume that this SPD is

placed behind a circuit breaker with an SCCR of 42,000 A,

shown as in Figure 1. In addition, the SPD has obtained a

SCCR of 65,000 A. In this example, the SCCR of the SPD is

coordinated with that of the facilitys power distribution

system, resulting in correct and coordinated application. In

fact, as long as the SCCR of the SPD is equal to or exceeds

the point of application within the power distribution and the

preceding over-current protective device, the SCCR of the

SPD is correctly coordinated.

As an example of an incorrect application, assume

that a SPD has obtained a SCCR of 25,000 A from UL.

Assume that this particular SPD was placed behind a circuit

breaker with an SCCR of 65,000 A, as shown in Fig. 1.

Additionally, assume that the maximum SCCR available at

this particular location on the power distribution system is

42,000 A. As with the previous example, the supervising

engineer is responsible for determining the available fault

current at this particular point within the power distribution

system of the facility. In this particular example, the SPD is

incorrectly applied. Because the SPD has obtained an SCCR

rating of only 25,000 A, it might not be capable of withstand-

ing an interrupt current of 42,000 A.

A few different approaches can be considered to

rectify this particular example so that the SPD has been

installed in accordance with the NEC, the IEEE recommenda-

tions, and UL conditions of acceptability. The first approach

would be to apply an SPD with an SCCR of 42,000 A or

greater. Even though the circuit breaker preceding the SPD is

greater than the SPD, the maximum short circuit current

available at the panel is 42,000 A. Therefore, the SPD has an

SCCR equal to the power distribution system at that point.

The second approach is to utilize a current limiting

circuit breaker that will limit the SCCR to the SPD to a value

of 25,000 A or less. This option presents some complications.

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In accordance with the NEC, the device must be applied in

accordance with its safety agency listing requirements. The

SPD must have been tested with a specified circuit breaker at

a specified current rating. Using a circuit breaker produced

by a different manufacturer, at a different SCCR, or even a

different part number is not acceptable and is outside the

scope of its safety agency listing. If the SPD is installed

utilizing an untested or improperly tested current limiting

circuit breaker, then it is the same as not utilizing a productevaluated by an independent safety agency, such as UL.

The third approach would be to install a finite

amount of impedance in the line prior to the panel. The

amount of impedance would have to be high enough to limit

the amount of SCCR to 25,000 A or less. One possible

method to accomplish this is by installing a transformer in the

power distribution system to lower the overall SCCR rating at

that particular point. This option can be very expensive and

might require additional space, but has some advantages in

mitigating other power quality problems that might need to be

addressed.

Fig. 2 represents a SPD that includes the over-current

protection within the device itself. In this case, the SPD does

not require that the device be preceded with a circuit breaker.

However, the same rules apply. The SPD has a specified

SCCR, which has been evaluated by UL; it is marked either

on the product or in the accompanying installation instruc-

tions, and must be connected appropriately.

IV. RECOMMENDATIONS

To ensure that a short circuit current rating (SCCR)

of the SPD has been properly coordinated with the power

distribution system of the facility, the authors recommend the

following:

1. A fault current analysis of the power distribution sys-

tem be performed and documented at the time of new

construction,

2. A fault current analysis of the power distribution sys-

tem be updated and documented every time new dis-

tribution equipment is installed, e.g. power class

transformer, increase in size of electrical conductors,

etc.,

3. A fault current analysis of the power distribution sys-

tem be updated whenever there is a change in the

utility service,

4. A fault current analysis of the power distribution sys-

tem be updated and documented whenever an SPD is

installed or replaced and,

5. That the available fault current, as determined by the

engineer conducting the fault current analysis, be

marked on the distribution panel.

As part of these recommendations, it is imperative that

the SCCR of any SPD be coordinated with the power distribu-

tion system of the facility. This includes examining all

existing SPDs and ensuring that the SCCR of the SPD is

coordinated to the available fault current at these specific

application points.

V. CONCLUSION

As SPDs have become a common device in the over-all picture of a complete power quality strategy, their per-

formance and safety attributes have continued to be examined

and tested. One of the top attributes is the safe and correct

application, and coordination of an SPD to a power distribu-

tion system of the facility. Analysis is required for the short-

circuit current rating of the power distribution system, for any

over-current protective devices, and for the SPD itself.

The NEC has established rules determining the rela-

tionship between the interrupting rating of a device and the

over-current protective device required. An added clarifica-

tion was implemented by the second edition of UL 1449 by

testing SPDs to a particular set of conditions and providing

SPDs with a short circuit current rating. Additionally, contri-

butions made by the members of the IEEE on guidelines and

recommendations of a correctly installed SPD will help ensure

the correct application of an SPD. Responsibility for the

coordination of the SPD to the application rests on the speci-

fying engineer, the contractor or end-user installing the SPD

and the inspecting official.

In conclusion, the only set rule for determining the

short circuit current rating of an SPD is to examine the label-

ing, paperwork and installation instructions of that particular

SPD. Once the short circuit current rating of the SPD has

been determined, and a fault current analysis has been per-

formed on the power distribution system, the appropriate SPD

can be installed. Failure to ensure the proper application of

the SPD with the development of specifications, guidelines,

and standards can result in a potentially dangerous condition.

The inappropriate application of an SPD can no longer be

tolerated or excused.

V. REFERENCES1. National Fire Protection Association, National Electrical Code, 2002,

Ninth Edition, Quincy, MA.

2. US Department of Labor, Program Regulation (29 CFR 1910.7), [On-

line]. Available: http://www.osha.gov/dts/otpca/nrtl/nrtlregs.html.

3. Underwriters Laboratories, Incorporated, Standard for Safety, Transient

Voltage Surge Suppressors, Second Edition, 1996 August 15, Melville,

NY.

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Fig. 1. Connection of the SPD utilizing a dedicated circuit breaker Fig. 2. Connection of the SPD utilizing the main circuit breaker of the

panel.

Bryan Cole, NCE (M, 1993) is the President of

Technology Research Council, LLC, and former

Director of Engineering & Product Safety Officer for

Control Concepts Corporation. Bryan has been

involved with the R&D of one-port and two-port surgeprotection devices, electromagnetic interference (EMI)

filters, and harmonic filters, and product safety for

eighteen years. He a member of the IEEEs Power Engineering, Engineer-

ing Management, Product Safety, and EMC Societies, a member of UL

Standard Technical Panel for TVSS, temporary power taps, and Switchgear

and Panelboards, and member of the US National Committee to the IEC on

SPDs. Bryan is a NARTE Certified Engineer in product safety, has a

Bachelors degree in Electrical Engineering from Binghamton University, a

MBA from the University of Phoenix, and is a PhD student at Northcentral

University.

Kenneth Brown (M', 1996) is the Director of

Engineering for Power Quality Products at Leviton

Manufacturing Company. Ken has been involved

with the design and development of surge protection

devices for 8 years. Ken is presently chairman of thetechnical committee for the NEMA 5VS Section.

Ken is a member of the US National Committee to

the International Electro-technical Commission on

SPDs (37A) and he is a member of the IEEE Working Group's 3.6.4,

3.6.6, 3.6.9, and 3.6.10. Ken received an undergraduate degree in

Electrical Engineering from the Ohio Institute of Technology, and an

MBA from West Coast University.

Pat McCurdy (M, 1996) is the Product Marketing

Manager and technical specialist for Phoenix Con-

tacts North American Surge Protection division.

Patrick has worked for Phoenix Contact in various

technical, application, and management roles foreleven years. Patrick has a Bachelor of Science

Degree in Electrical Engineering Technology from

Penn State University and a Master in Business

Administration from Lebanon Valley College. Active

professional associations include: International Society of Measurements

and Control (ISA), Institute of Electrical and Electronic Engineers (IEEE),

American Gas Association (AGA), and National Electrical Manufacturers

Association (NEMA).

Thomas Phipps (M 2000) received the Manasseh

Cutler Scholarship at Ohio University in Athens,

Ohio while majoring in Electrical Engineering.

Tom has been involved in power quality engineer-

ing over the past seventeen years working in areasof Production, Applications, Test, and Research &

Development Engineering. His employment

experience includes the areas of power condition-

ing, power distribution, Rotary and Static UPS and for the past eleven

years TVSS. Tom has worked for several of the power conditioning

companies within the industry including Power Distribution, Incorpo-

rated, United Power, Incorporated, Power Systems & Controls and is

presently the Vice President of Engineering for Thor Systems, Incorpo-

rated located in Richmond, Virginia.

Ron Hotchkiss is the Vice President of Engineering

for Surge Technology. Ron has been involved in the

design, development and certification testing of one-

port and two-port surge protection devices for thirteen

years. Ron also manages engineering, safety agency,

compliance and quality operations. He is a member of

IEEEs Power Engineering Society and is an active

participant and contributor to several IEEE SPD

working groups and the Underwriters Laboratories

Standard Technical Panel for Transient Voltage Surge Suppressors. He is a

member of the balloting group for IEEE. Ron received his Electrical

Engineering degree with Honors from the University of South Florida in

Tampa.

Branch

Panel

Main

Circuit

Breaker

(Service

Entrance)

SPD

L1

L2

L3

N

G

L1

L2

L3

N

G

Ground Bar Neutral Bar

3-Phase

Circuit Breaker

Branch Panel

Main Circuit Breaker

Conductors to Other

Branch Panels

Branch

Panel

Main

Circuit

Breaker

(Service

Entrance)

SPD

L1

L2

L3

N

G

L1

L2

L3

N

G

Ground Bar Neutral Bar

Branch Panel

Main Circuit Breaker

Conductors to Other

Branch Panels