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Regulation 534.4.4.6

Greetings to the forum.

Can someone clarify this for me. Regulation 534.4.4.6 says:
"In general, the short-circuit current rating ISCCR of an SPD, as stated by the manufacturer, shall not be lower than the maximum prospective short-circuit current at the connection points of the SPD assembly"

Then Note 1 below the regulation says:
"The SPD alone, or as an assembly in conjunction with its disconnector and/or overcurrent protective device (OCPD), is required to withstand the short-circuit current rating ISCCR as stated by the manufacturer. This value is defined and tested according to BS EN 61643-11, in conjunction with the disconnector and/or OCPD as stated by the manufacturer"


I understand that ISCCR (in kA) is a short circuit withstand capability value given by the manufacturer on the condition the maximum upstream protective device rating is not exceeded.


If the upstream SPD protective breaker/fuse has a breaking capacity higher than the system fault level, can an SPD with a ISCCR lower than the system fault level be used? I am confused because Note 1 says either the SPD alone or SPD+Disconnector as an assembly has to withstand the system fault level.


Reason for all this scenario is because I have come across an SPD with short circuit withstand capability of 25kA protected upstream by a breaker with Icu of 50kA is specified on a system with 37kA fault level.


Thanks,

M.


  • Protection afforded by a B.S. 88-3 80kA H.R.C. fuse perhaps.

    https://www.youtube.com/watch?v=Ya_Zz1J0aYA


    If the S.P.D. shorts in use in extreme conditions its bits may fly about with the explosion due to the potentially large fault current within the device, so perhaps it is best to enclose it in a sturdy metal enclosure as well as having a H.R.C. fuse to protect the enclosure wiring.


    Z.
  • Then there's this as well.

    https://www.youtube.com/watch?v=r3glwiMcmQk


    Z.
  • Also, we must not forget that a B.S. 88-3 H.R.C. fuse will interrupt a short circuit current long before its maximum value is reached. The cut off effect greatly reduces both thermal and magnetic stresses on the equipment being protected. The fuse clears a short circuit fault well within the first quarter of a cycle.


    Z.
  • I presume the 37kA is the fault level upstream of the circuit breaker. Depending on the breaker type it may or may not provide some degree of energy limiting - by cutting the prospective fault current off quickly enough, a fast acting fuse or breaker can reduce the effect of the fault current downstream of it, although a normal PSSC meter which uses the voltage drop on  a small test current will not see this effect and will over estimate the potential for damage.

    Without 'let through' or PSSC curves for the breaker this will be hard to verify.

    If the limiting is not enough, or cannot be verified then the simplest fix will probably be a fuse with a suitable rupture capacity in the live side and in close proximity to the SPD.

    As above, SPDs can fail dead short, especially if over-zapped during a thunderstorm or similar, and it is important that a blown SPD is disconnected gracefully, and does not generate a more serious problem than the original.

    This is for the wrong sort of fuse, but it shows an example of the sort of limiting action clearly enough, where the let through can be quite a bit 

    less than the fault current upstream at higher fault currents.
    80f71a102ad3ee3430a7397cfcd26516-huge-fuse_limiting.png

    Note that these lines  are more or less straight on log-log axes, but more nearly  proportional to 1/ (I squared t) to the right and proportional to I to the left.
  • A Siemens' fuse and short circuit demo. Very quick disconnection of supply and arc quenching.

    https://www.youtube.com/watch?v=Uj0oHUSSW_8


    Z.


  • H.R.C. fuses are very mechanically strong.

    https://www.youtube.com/watch?v=U3Sil-xFzvU


    Z.

  • Protection afforded by a B.S. 88-3 80kA H.R.C. fuse perhaps.



    OP seemed to say it was a breaker...

      - Andy.

  • MG:

    ...protected upstream by a breaker with Icu of 50kA...




    No 'seemed to say' about it. I can see two possible explanations.

    1. The SPD is at the end of a significant cable length so the fault level at the SPD is significantly lower than the 37kA at the upstream breaker (rather unlikely), or

    2. The designer got it wrong.

    (There is a possibility of 3. The upstream breaker provides limitation of let through energy, but while I have seen breakers of that type they generally have a much higher Icu so I have discounted this)

    I am hoping that someone can come up with a further alternative which I haven't thought of.

    Alasdair

  • "Reason for all this scenario is because I have come across an SPD with short circuit withstand capability of 25kA protected upstream by a breaker with Icu of 50kA is specified on a system with 37kA fault level."


    O.K. it is time to have a chat with the maker of the S.P.D. and get its opinion.


    Z.
  • I think this question should have some serious discussion with the manufacturers, because it points to a potential difficulty with SPDs fitted on larger systems. Whilst I think that failure dead short of a device is very unlikely, it seems to be a serious snag and cost to have to fit a series fuse to limit potential fault levels. A simple design feature such as a fusible connection, would fail open at extreme fault level and even if an arc is formed would reduce current to manageable levels. In fact I would be more worried that short circuit failure would result in perhaps a few 10s of milliohms and the resulting dissipation start a serious fire, but with little possibility of an upstream CPD operating. In many ways I am far from convinced that SPDs at the supply end of an installation are necessary, and the idea that this may prevent damage to appliances, such as VFDs will work. Semiconductor designs already contain features to limit surge damage, and are in general very reliable unless there is a direct lightning strike, and in that case an SPD is unlikely to make much difference anyway.