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Regulation 534.4.4.6
MG
23 Posts
Question
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.

 
11 Replies
Zoomup
1762 Posts
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.
Zoomup
1762 Posts
Then there's this as well.

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

Z.
Zoomup
1762 Posts
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.
mapj1
2355 Posts
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-fu
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.
Zoomup
1762 Posts
A Siemens' fuse and short circuit demo. Very quick disconnection of supply and arc quenching.

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

Z.

 
Zoomup
1762 Posts
H.R.C. fuses are very mechanically strong.

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

Z.
AJJewsbury
1766 Posts

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
Zoomup
1762 Posts
"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.
 
mapj1
2355 Posts
In this case it is not clear what is between the 50KA breaker and the substation - I suspect there may be a 'death or glory' fuse that will limit the damage to some extent.
However, even assuming there is,  it is not a sensible design to have a system where when the SPD operates for the last time and becomes a glowing lump of low impedance semiconductor, for the whole site to be disconnected and put out of service until the wiring to the SPD can be unbolted and then the main fuse changed, it is far more sense for the SPD to have its own local fuse, and possibly an alarm that operates if it blows, so when the SPD succumbs to the inevitable, power can be restored as soon as reasonably practical, and the SPD repair scheduled when convenient.
It is worth noting that the really big surge protectors that you get on the 11kV OH lines are exactly like this, and have something akin to an expulsion fuse in concept to blow the wiring off so that the line power can be restored as soon as possible. To be fair compared to the 230V devices,  each SPD has a string of several chunks of rock all of which have to fail, and are sized to put the sort of thing that goes in a CU to shame, so they last many years.
The additional advantage of the explosive disconnector is that a pair of binoculars can reveal the 'tripped' SPD by the obvious dangling link wire, from the safety of the landrover at the roadside, and changing it can be left to a time when the power can be routed around or outage scheduled. I'm not really suggesting that for indoors.

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