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A C O M P A R I S O N O F S P D S L I G H T N I N G & T R A N S I E N T S

KENNETH BROWN

Leviton Mfg. Company, Inc.

Chula Vista, CA

PUUUUURCRCRCRCRCHHHHHAAAAASESESESESE DECISIONS USUALLY INCLUD�

weighing the pros and cons of the pur-chase and performing benefits versus

cost analyses. The results are then factoredinto the decision-making process to ensurethat a capital equipment investment is in thebest long-term interest of the corporation.A comparison of advantages and disadvan-tages can help you decide whether your surgeprotection device (SPD) should be integratedinto service equipment and panel boards orwhether it should be attached to a separatediscrete panel on the load side of a discon-necting means.

DEFINITIONS

For purposes of this article, an integral SPDis defined as “an SPD mounted internal toservice equipment and panel boards that con-nect directly to the energized bus.”1 A discreteSPD is defined as “an SPD mounted in a sepa-rate enclosure and connected (wired) thoughthe service equipment and panel boards tothe load side of a dedicated breaker, i.e., 30-A, 3-pole circuit breaker.”1

The National Electrical Code (NEC) defi-nition of Service Equipment is: “The neces-sary equipment, usually consisting of a cir-cuit breaker(s) or switch(es) and fuse(s), andtheir accessories, connected to the load endof service conductors to a building or otherstructure, or an otherwise designated area,and intended to constitute the main control

and cutoff of the supply.”1

Service equipment may consist of circuitbreakers or fused switches provided to dis-connect all ungrounded conductors in abuilding or other structure from the serviceentrance conductors. The disconnectingmeans at any one location must not consistof more than six circuit breakers or sixswitches, and it must be readily accessibleeither outside the building or structure orinside nearest the entrance point of the ser-vice entrance conductors.

The introduction of an SPD as an inte-gral part of the service entrance equipmentis one alternative to the incorporation ofsurge suppression. Service entrance equip-ment with built-in surge protection shouldbe considered when weighing surge suppres-sion options within a building or structure.Still, an integrated SPD does have both ad-vantages and disadvantages.

INTEGRAL PANEL ADVANTAGES

One advantage of integrating an SPD into theservice equipment is the convenience of pur-chasing a single integrated unit to meet twoneeds. The company saves money when cre-ating the purchase order (less paperwork),and the service equipment installation costsare reduced because the SPD is already a partof the equipment. A second benefit of inte-gral surge protection is that there is “mini-mal wire length due to the fact that the inputof the SPD is typically connected directly tothe service entrance conductors.”1 The resultis lower overall let-through voltage or clamp-ing. Another notable benefit is that the in-

A comparison of integral SPDs anddiscrete panel SPDs

Too often, the easy-way-out purchase decision has long-term negative and costly

results.

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put is connected directly to the ener-gized bus bypassing the circuit breaker,thus improving overall clamping.

INTEGRATED PANEL

CONCERNS

In the National Electrical Code Hand-book, Article 240 “Overcurrent Protec-tion” Section 1,2 it is stated that “Mount-ing on open-type switchboards, panel-boards, or control boards that are inrooms or enclosures should be free fromdampness and easily ignitable mate-rial and should be accessible only toqualified personnel.” It is also noted inthe NEC Handbook (2002), 9th Edition,under Article 230-70, General (A) Lo-cation, that “The length of service-en-trance conductors should be kept to aminimum inside buildings, since powerutilities provide limited overcurrent pro-tection; and, in the event of a fault, theservice conductors could ignite nearbycombustible material.” Metal oxidevaristors (MOVs) are common compo-nents incorporated into SPD products,and they contain easily ignitable com-bustible materials (Figure 1). It is notrecommended engineering practice toplace a combustible material within theservice entrance where a fault currentcondition or a large surge current couldcause significant internal damage to theservice entrance panel. During a light-ning strike, a significant amount of en-ergy can cause the MOVs within an SPDto turn on and conduct current. If theSPD is not sized appropriately, the en-ergy contained in a lightning strike or afault current condition could cause acombustible event.

The necessity of keeping overcurrentprotection away from combustible ma-terials is mentioned throughout theNEC code. Some additional referencesare found in the NEC Handbook (2002),Article 230-70, Article 230.11, Article408.7, and Article 490.21. Even in caseswhere a combustible event does notoccur, a large surge event could causethe SPD to emit smoke. In the NECHandbook (2002) Article 230.82, it isnoted that “Hazardous arcing couldburn down the service equipment.”When a MOV emits smoke, it spreadsa carbon residue throughout the inside

of its enclosure; and the carbon mate-rial may increase the chance of arcing.

Service entrance equipment requiresperiodic maintenance. If the SPD is in-tegrated into the service entranceequipment, can maintenance be carriedout easily? Service entrance enclosuresare designed to enclose or guard againstaccess to the service conductor andother energized parts. By definition,servicing an integrated SPD causes a“forced” access to the energized partswithin the service entrance equipment.The NEC Handbook (2002) Article408.2 states that “Where it can be dem-onstrated that it is unfeasible to discon-nect or de-energize the service conduc-tors supplying a service switchboard,qualified electricians may be requiredto work on these switchboards with theload terminals de-energized but with theservice bus energized. Barriers are re-quired in all service switchboards toproved physical separation (adequatedistance or obstacle) between load ter-minals and the service busbars and ter-minals, thus providing some measure ofsafety against inadvertent contact withline-energized parts during mainte-nance and installation of new feeders orbranch circuits. In most multi-sectionswitchboards, barriers are not required,

because the line-side conductors andbusbars are not in the same switchboardsections that contain the load terminals.It must be clearly understood that de-energizing the load side of a switch-board, by operation of a disconnectingmeans, does not de-energize the un-grounded service conductors. Every ef-fort should be made to make arrange-ments to completely disconnect powerfrom the equipment before performingany work inside. For complete disconnec-tion that is not feasible, the installershould become familiar with NFPA 70E,Electrical Safety Requirements for Em-ployee Workplaces. This industry recog-nized document provides the guidancefor protective equipment and appropri-ate work rules that must be followed forworking on energized equipment.” 2

It is sometimes easy to overlook theliability concerns related to this access.Facilities’ personnel may not be experi-enced with service entrance equipmentmaintenance and its associated safety-related issues. Many electricians’ workexperience is limited to low voltage elec-trical equipment or HVAC equipment.Given this limited expertise, even elec-tricians who recognize the hazardsposed by electrical service equipmentmay not be prepared to maintain andrepair equipment when exposure toenergized service entrance conductorsis a factor.

Another disadvantage associatedwith integrated SPDs is the limitedspace that is available within a serviceentrance panel or panelboard. Theremay not be enough room within theservice entrance for adequate housingof both an SPD and the essential ser-vice disconnecting means. When anSPD is installed within service equip-ment, the NEC “readily accessible” pre-scriptive for the disconnecting meansis somewhat compromised. Service en-trance cabinets and panel boards aredesigned to house overcurrent protec-tive devices and their energized inputand output conductors. If the samecabinet is used to house an SPD withits own input conductors, the spaceavailable for the SPD tends to be lim-ited.

Also, because of the inherent space

Figure 1. Ignition of combustible material

within an MOV.

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limitations, the electrical contractor haslittle choice in where to locate the ser-vice conductor feeds because of theirsize. However, in a discrete SPD panelthe service conductor can feed from anylocation around the panel, includingfrom the top, from the bottom, or fromeither side.

The last concern related to an inte-gral SPD is heating caused by inductiveeffects. The NEC Article 408.3(B),Overheating and Inductive Effectsstates, “The arrangement of busbars andconductors shall be such as to avoidoverheating due to inductive effects.”1 Ifan SPD is integrated into a switchboard,a thermal analysis should be performedto ensure that the inductive effects arenot compromised.

DISCRETE PANEL

ADVANTAGES

SPD products are specifically designedto protect sensitive down-line electron-ics from power surges. The correct in-stallation of a discrete SPD panel re-quires a separate service entrance withdedicated disconnecting means to com-plete the infrastructure of the electri-cal system. In a discrete SPD installa-tion, the down-line load does not re-quire the SPD to complete the network.In the presence of a fault current con-dition or MOV failure, the dedicateddisconnect will open and the serviceentrance equipment will not be dam-aged. If the same fault current condi-tion or MOV failure occurs in a serviceentrance installed with an integratedSPD, NFPA standards recommend re-placement of the damaged or compro-mised service entrance equipment.

A dedicated circuit breaker (Figure 2)mounted prior to a discrete SPD pro-vides many advantages. Circuit breakersare safe, and specifically designed-in iso-lation characteristics prevent exposureto live parts. Circuit breakers are also de-signed to open during an over-currentcondition (i.e., a swell or temporary over-voltage). Circuit breakers open the cir-cuit in the presence of a fault currentcondition. Additionally, the circuitbreaker serves as a disconnect for thedown-line SPD. Circuit breakers can bereset, and they have current limiting

characteristics that are beneficial to thefunctionality of a well-designed andimplemented SPD product.

Maintenance of a discrete SPD panelcan be accomplished by de-energizingthe circuit breaker disconnect for theSPD and performing the necessarycleaning or repair on a safe and sepa-rate de-energized panel. Figure 3 showsan example of an SPD installation fea-turing a discrete panel SPD.

CLAMPING VOLTAGE

Low clamping voltage is often overem-phasized within the SPD engineeringcommunity. In the IEEE C62.72, Draft11, Section 16.10 it is noted that “SPDswith lower clamping voltage rating causefrequent interventions of the protectivedevices and accelerate their aging. Thebest approach or product is not necessar-ily the one claiming to suppress surges tothe lowest levels.”4 After reviewing the

Figure 2. Quicklag 3-pole circuit breakers, 15–100 A trip current vs. time.3

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integrated SPD advantage of lower clamping because ofshorter lead length, it was found that within a typical serviceentrance the direct connection to the energized conductorand the neutral allows a distance of approximately 20 inchesbetween conductors and neutral. In many discrete SPD ap-plications, the panel can be mounted in a configuration wherethe total length of energized conductors is about the same.

CONCLUSION

After reviewing the advantages and disadvantages of bothintegrated SPDs and discrete SPDs, it appears that the dis-crete SPD is both the safest and most effective method ofproviding comprehensive surge protection to a building orstructure. Incorporating an integrated SPD in service en-trance equipment lowers installation cost, but that optionmay not provide a cost-effective solution for the long-termuse of the product. Too often, the easy-way-out in a pur-chase decision can have long-term ramifications and costlyconsequences. Making a purchase decision without ad-equate review of in-service concerns can be a costly lessonthat need not be learned.

REFERENCES:

1. National Electrical Code, 2002 Edition. National Fire Protection Asso-ciation Inc., Quincy, MA 02269-9101.

2. National Electrical Code Handbook, Ninth Edition (2002). National FireProtection Association Inc. Quincy, MA 02269-9101.

Figure 3. Note #1 reference from the NEC Article 384-3.

3. Cutler Hammer Technical Data (April 1998). TD. 19.02.TE.4. IEEE WG 3.6.6 Task Force – Draft No. 11 (April, 2001).

KENNETH BROWN is the Director of Engineering (Power Quality) for LevitonMfg. Co. in Chula Vista, CA. Ken holds an undergraduate degree in ElectronicsEngineering from the Ohio Institute of Technology and an MBA from WestCoast University. Ken can be reached by e-mail at kbrown@leviton.com. ■