Surge protector in switchboards > 50kA ?

I would like to share a problem I face when I have to install a surge protection device type 1 in high power electrical switchboard. I already asked this question on another blog, and I’m trying to get more feedbacks from electrical engineers.

The situation is very “unconfortable” for me as a quality provider, because if a short-circuit of high value happens, though luckily probability is low, it would mean very serious safety risks …

I am the manager of a panel building company, and we receive more and more specifications requiring us to install surge protection devices type 1: they are requested for industrial sites (chemical, water station, also airport …) where they always install a lightning protection system (lightning rod, mesh cage).

The specifications I receive are not very detailed about the choice of surge protector to install, and I found out that surge protection devices available on the market cannot be connected directly to high power busbar: they cannot withstand short-circuit level higher than 50kA, but I regularly have specifications for switchboards of 70kA or more.

The photo below is an example of surge protection device (in the circle) connected to high short-circuit level busbars.

How to solve this problem?

Is there a way to reduce the short-circuit level to be compatible with the limitations of the surge protection device?

Or wouldn’t it be better to have surge protection devices with higher short-circuit level available on the market?

Thanks for your opinion



  • Dear Boby,

    In Circuit Breakers (CB) when it shows 50kA, it means the highest short-circuit current intensity which the circuit breaker is able to interrupt. However, in Surge Protective Device (SPD) you mentioned above, the value I(imp): 50kA has other meaning, it means Maximum discharge surge transient current of Type 1 SPD 10/350 microsecond. Although the magnitudes between the breaking capacities of the CB and the maximum discharge current of the SPD are similar, these are different faults with different type of protection.

    IEC 62305 assumes 99% of lighting discharge currents usely 10-40kA, so manufactures provide to maket SPDs with I(imp) under 50kA only to optimize a product.

    For installation: You should connect in order: Busbar + Disconector/ Circuit Breaker (eg: 70kA, short-circuit protection) + SPD (eg: 50kA, surge protection: protect the equipment against transient overvoltages mainly due to the lightning).

    I hope that my idea is clear. :)

    Best regards,

    • We maybe co-mingling IEC and UL terms. A Type 1 IEC SPD is not required to incorporate safety controls within the SPD. In most instances, these IEC based SPDs are spark gap arrestors having very high let-through voltages.

      UL 1449 3rd Ed. Type 1 SPDs must include fully coordinated safety controls. Fault current protection can not be achieved through the use of external safety dis-connectors (i.e.. breakers or fuses). Since UL Type 1 SPDs are constructed in this manner, they can be directly connected to either the “line” or “load” side of the main service without the need of a fuse or breaker disconnect.

      Siemens Industry, Schneider Electric, and APC private label or labeled our patented SPDs, and they connected them within their distribution equipment without the need to use a fuse or breaker. Since our SPDs incorporate fully coordinated safety controls, we’ve never had an incident compromising the integrity of electrical system.

      If you want to learn more about UL 1449 3rd Edition, you can download our white paper by copying and pasting the following URL within your browser:

  • Iwould recommend to install a SPD with SCC of 100KA, which uyou can find in the US market. I THINK THAT DEVICES rhat meet UL 1449 3rd. edition are rated 100 KA of SC.

    • Not all UL 1449 3rd Edition SPDs have a Short Circuit Current Rating or SCCR of 100kA. When installing SPDs within electrical distribution equipment, you should incorporate SPDs having the highest recognized fault withstand capability, which is 200kA.

      This doesn’t mean the cost of the SPD is greater, but it means that the surge protector has demonstrated a higher degree of safety. When SPDs disengage from the electrical system benignly, the electrical system will not be compromised.

      All of our SPDs are Type 1 rated carrying the highest UL 1449 3rd Edition SCCR of 200kA.

      Our web site is,

  • I am also a director of a switchboard manufacturing company and have found most consultants have very little idea of what is required in the way of installing SPD’s.

    1. For the Main Switchboard of a high current demand installation, one would use Example:-
    Erico TDS Movtec TDS-MT-277 1PH 1 MODE 277V 100kA thats at MSB point of entry.
    Keep you feeder cables tom these AS SHORT AS POSSIBLE photo example cables are to long.
    Also keep the return to earth as short as possible 600mm max if possible ( which in most cases
    hard to achieve) these units have LED’s to show status of life left.
    2. For Dist Boards and sub Boards Erico TDS150-1SR-277 1 PH 1 Mode 50kA with Alarm contact
    series back to BMS if available.

    A similar Novaris unit could be used, keep these away from busbars as much as possible and add FCL’s if you wish however who is going to check these on a regular basis to see if an FCL has blown?

    I personally believe let them do their job and blow the small DIN type 20kA units are good for
    8 * 20 uS 20kA Strikes if a board cops that you in trouble anyway.

    Best Regards

    • Have you considered using SPDs manufactured by Advanced Protection Technologies ( I checked on, and our SPDs provide better VPR’s and higher I-n’s at a lower cost than what you are currently purchasing from ERICO.

      Siemens Industry has private labeled our SPDs for the last 14 years, and they are integrally mounted within their electrical distribution equipment. Also, not long ago, Schneider Electric and American Power Conversion private labeled our integral and wall mounted style SPDs.

      • We can solve this problem. protecting equipments very well without grounding and it passed the IS09001 international quality system authentication, until to now it is still qualified. all arresters can still protect equipments very well without grounding whether thunder strong or not, which called “Equipotential” SPD, covering Communication, CATV, Security monitoring, Computer, Industrial control, Office automation, Remote education etc. Using international lead-edge technologies that Protection effect has nothing to do with ground resistance.

  • We are investigation engineers that specialise in power quality and surge protection.

    Short circuit current is not the same as surge current nor lightning current. All are different and not to be used equally. This has been expanded in prior blogs. Basically lightning current flows via earthing and only some energy gets through to conductors (NOT all the strike energy). Power poles and transformers already have MOV’s to prevent transformers and cable insulation from failure. Rarely will a surge of 20kA reach your MSB surge arrestor, which is why IEC61643-1 only tests to 20kA. MOVs of higher kA only give longevity, as the kA rating is for a SINGLE hit. Higher kA means more hits at lower kA. This does not assure the let through voltage though, so MOVs on board are just to prevent dielectric (cable and winding insulation) breakdown.

    The fuse protecting the MOV should be rated to clear at the MSB fault level, that is the key.
    Having a MOV within the same chamber is risky in case the MOV fails and escalates into the chamber.
    If you are actually trying to protect equipment then the MOV is only 2% of the energy capture solution.
    To protect for the other 98% of energy, typical of mains surges from grid and building, use SASD at DB’s.

    Typically MSB should have I2RLSA230-150/20 (with a 63A fuse/breaker rated according to fault level).
    Typically DB should have I2R240W10K (with 20A breaker) for best equipment protection. The overload protection provides a limit of the let through energy and its steady rating is not the key but its i2t rating. As you can see we use the Transtector MOV and SASD products to suit the application, rather than one device does all.

    The energy protection sequence is thus 1. Earthing, 2. MOV at point of entry only, 3. SASD at DBs.
    Coordinated voltage clamping is the priority, matching what it needs to protect.

    Hope this helps.

  • Please come back to me on
    We have extensive information that I can send you via email on SPDs. XXXXXXXXXX. (Comment edited by Moderator Team)
    I think that Paul is correct.

    I can take up an example for the ERICO TDS MPM 277 SP which is design for MLVP. When installing this product you need to add one MCB 4P/100A to feed the SP. This what we call Back up over current protection.

    Thanks and regards,

    Jhamsheed OOZEERALLY
    MD, Scantech Co. Ltd

  • The fuse protecting the MOV should be rated to clear at the MSB fault level, that is the key.
    Having a MOV within the same chamber is risky in case the MOV fails and escalates into the chamber.
    If you are actually trying to protect equipment then the MOV is only 2% of the energy capture solution.
    To protect for the other 98% of energy, typical of mains surges from grid and building, use SASD at DB’s.

    • Kumar,

      You are absolutely right! Most SPDs do not incorporate fully coordinated safety controls. These devices contain blind spots within their fault current protection controls that will not clear during certain extended overvoltage or fault conditions. This means the SPD will short out the electrical system compromising safety and reliability.

      This is not the case for APT SPDs. We’ve been manufacturing integrally mounted SPDs for Siemens Industry for nearly 14 years and until the early 2000’s Schneider Electric. Since our SPDs incorporated patented safety controls, the 10’s of thousands of SPDs they’ve mounted integral within their equipment have never compromised electrical system safety or reliability.

      Check us out at

  • We seem to have gone off topic and onto which product is for what purpose (huge topic).
    The original question was “what to do in switchboards rated for fault currents >50kA” (not lightning).
    Any suppressor will briefly pull current to reduce voltage spikes. During the surge conduction, if the suppressor fails to a short circuit, which both Silicon and MOV will do, then you will get the prospective fault rating of the switchboard flowing into the suppressor. This was the core question.

    If so, what do you do? You always must have a fault/overload clearing device, to isolate the suppressor when it fails, otherwise you will escalate the fault (we have investigated fires to prove this). Suppressors DO fail. When they fail, how do you isolate them to replace them?
    If no isolation is installed then the supply must be powered off to be serviced safely. In a critical site, this power down is unacceptable.

    When installing a suppressor, whether MOV for short pulse (lightning) or Silicon for long pulse (grid), you need an isolator whether by a fuse or by a breaker. That isolator/overload device needs to be rated to the fault capacity of the input supply. For example if the supply transformer feeding a switchboard can produce 75kA then you must use a 75kA fuse or breaker to protect the SPD from the failure mode event. The rating of the SPD is NOT the same kA. A 150kA MOV is only rated at this short pulse for one time. The actual reason an MOV has a high kA is that it will survive many smaller surges. Short pulse surges do not make it into a building due to the inductance of the cabling impeding high speed pulses (high frequencies). Trying to pass even a few kA (less than 5kA) will destroy the cable insulation first and not the MOV. In effect the cable inductance saves the MOV from the lightning.

    The reason why an MOV fails, is that it is exposed to long wave pulses from the grid. Long wave pulses degrade a 150kA MOV into approximately a 3kA single event suppressor! This data is real and published by the MOV manufacturers like Siemens. Look for “Surge current rating curves” in the MOV data sheets and compare rating and life for short versus long pulses. It is very clear.

    Regards, Paul

    • I apologize. I just remembered that you are the Transtector rep. Don’t you have access to the LEA branded SPDs? The PV and SP SPDs incorproate TPMOV technology. This would be good for Boby’s application.


      • Yes, we do import Transtector. Our prime business however is fault investigations and solutions.

        After years of poor reliability findings in critical power systems like UPSs (AC) and rectifiers (DC), we were introduced to the Transtector line of Silicon products, which we coordinated with the existing MOV installations. Failure rates reduced with Silicon, so we continued with Transtector.

        We do already use MOVs at the point of entry applications to stop short pulse lightning entry. We use only Silicon to protect critical equipment within a building from grid and internal reflected long pulse surges.

        In both cases, we use a breaker rated to the supply fault current for isolation and overload/fault protection. The reason is that the thermal isolation in the SPD is not rated for the short circuit current. It must use an upstream fuse or breaker to cascade with so that the energy let through does not blow the SPD off the wall during a failure of the SPD.

        An SPD does not show or have a fault rating (clearance) it only shows a conduction rating. The clearance rating has to equal or be more than the available supply fault rating.

        Regards, Paul

  • Paul,

    Boby’s original posting raises concerns not about the surge current capacity of an SPD, but of it’s SCCR. It looks like the SPDs he has been using have limited to no SCCR. This is a big issue for IEC style SPDs. Most of these style of SPDs rely on spark gap technology to surge protect an electrical system, and in most instances, they have limited to no internal fault current protection. When they short circuit, they definitely will compromise the electrical system.

    The dilemma is to provide a SPD that safely disengages from the electrical system while allowing the client to achieve lightning protection certification for or similar to UL 96A. All of our SPDs were designed using input from engineers at Siemens and Schneider, and this resulted in the development of our safety control circuit that incorporates thermal protection with surge rated fuses. This circuit allows allows our SPDs provide excess surge current capacity up to a 1000kA while safely disengaging from the electrical system during a fault condition. I think some of the confusion is based on how an MOV fails.

    During end of life, SPDs tend to short circuit. The common assumption is that when short circuiting occurs, the SPD will draw the full available fault current. If this was true, fault current protection would be rather simple. However, SPDs comprised of either MOVs or SADs or both act like voltage sensitive resistors. Depending upon the applied voltage the MOVs/SADs will short circuit soft or hard. In a majority of instances, SPDs will draw a small amount of fault current. If this fault current is in the range of a safety control coordination blind spot, the SPD could fail as Boby described.

    How do we know this scenario occurs? Because, Underwriters Laboratories keeps receiving reports of field failures, and they keep updating their fault current testing to root out these blind spots. With UL 1449 3rd edition, SPDs need to pass low, intermediate, and high fault current testing in order to pass 1449. IEC style SPDs can not pass this test and meet the the Type 1 I-n requirements without the use of external safety controls. This them limits their Type rating to “Type 2” as well as limited I-n ratings.

    I agree with you assessment concerning long pulses versus lighting pulses. Long wave pulses coming from the grid tend to be multi-cycle, which is an eternity and outside the scope of a transient SPDs are designed to mitigate. Thus, fault current protection needs to be coordinated to take the SPD off line when exposed to electrical disturbances outside the scope of electrical surges or transients. Since 1998, our SPDs incorporated coordinated full spectrum fault current protection, which makes our SPDs unique in the industry.

    I coved this topic in greater detail in Siemens Step 2000 – Basics of Surge protection training module that I authored. Check it out and give me your thoughts.


    • Thanks Ken, a very good course.

      We do see type 1 SPDs (MOV) installed as shown in the course. The typical type 1 MOV installation is in parallel with the HV primary of an external transformer or its associated HV cable and without a fuse or breaker. The reason a type 1 installation is acceptable without protection is that when the MOV fails it does so in open air and presents little risk to the supply. An outage however is required to replace these lightning arrestors when they do fail. The MOVs are chosen with a high enough voltage rating so that they do not conduct at the grid surge level of voltage spikes. These MOVs consequently do a great job and rarely fail as they are being used correctly to make the grid more robust. The grid robustness comes from protecting the insulation of the transformers and the cables insulation from punch through failure due to lightning.

      These external events are exactly where MOVs are best installed and without the need for a fuse or breaker.

      Regards, Paul

  • I am currently working of my college thesis regarding surge protection device used on high powered electrical switchboard and google directed me here. Some of the information I need are all spread out here and I’m very thankful for that.

  • We can solve this problem. protecting equipments very well without grounding and it passed the IS09001 international quality system authentication, until to now it is still qualified. all arresters can still protect equipments very well without grounding whether thunder strong or not, which called “Equipotential” SPD, covering Communication, CATV, Security monitoring, Computer, Industrial control, Office automation, Remote education etc. Using international lead-edge technologies that Protection effect has nothing to do with ground resistance.

  • Each type 1 or combined Type 1+2 or type 1+2+3 SPD has a max allowed prefuse !
    If you have such a high short circuit current you have a pre-fuse with a rating much higher than the max allowed SPD pre-fuse.
    For that reason you have to install a SPD back-up fuse with highest possible Ampere ratings….usually a fuse breaker, in industrial installations necessary for maintenance reasons and to avoid that line fuse will trip.
    The back up fuse (a melting fuse with signal contact) will limit the prospective short circuit current (follow-on current….transformer short circuit capabiliy + line impedance until installation place of SPD.
    Look also on the cross sectional of busbar or round wires

  • Surges are the voltages which are a great deal more intense than the normal voltage and which appear in system, for instance, a neighbourhood for a period and thusly are in like manner once in a while suggested less briefly as “transient over-voltages”. These surges can rise up out of trading of close-by electrical equipment or from the flexibility of an electrical short out accuse (e.g. by a breaker blowing), yet the most exceptional source is lightning. The usage of surge defenders is to shield fragile apparatus from lightning hits and surges on Ethernet information lines.

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