Ze and distance from substation

[ian.settle1]

Are you refering to the 80Ka on bs88's or did i miss something ?

Clue

MCBs 6Ka rating PFC reading 19.1Ka

 

[plugsandsparks]

If it was o.o1 i would expect to see the sub in the car park to be honest or even closer, a look at the size of the incoming cable might help a bit

 

[ringer]

Customer is not rushing to sort this out, but wants to speak with me next week about it. Assuming for the moment that the measured value of Ze=0.01 ohms is correct, I am trying to establish what can potentially be done about this. To recap: TN-S, the DB circuit breakers are 6kA SCC, the DNO fuse is BS1361 type II 100A 33kA SCC, PFC is 19.9kA

So the DNO fuse will withstand the PFC, but the MCBs will not. Can we rely on the DNO's fuse to protect the installation? What about the potential for harm to the MCBs (and persons, livestock and property) at that level of fault current?

 

[ian.settle1]

Customer is not rushing to sort this out, but wants to speak with me next week about it. Assuming for the moment that the measured value of Ze=0.01 ohms is correct, I am trying to establish what can potentially be done about this. To recap: TN-S, the DB circuit breakers are 6kA SCC, the DNO fuse is BS1361 type II 100A 33kA SCC, PFC is 19.9kA

So the DNO fuse will withstand the PFC, but the MCBs will not. Can we rely on the DNO's fuse to protect the installation? What about the potential for harm to the MCBs (and persons, livestock and property) at that level of fault current?

I would read the warning that all MCB manufacturers give regarding the PFC and MCBs

 

[ringer]

I would read the warning that all MCB manufacturers give regarding the PFC and MCBs

Where would I find that warning then? These are MEM 3871 M6 type 2 MCBs. Have looked on the Eaton website, and found their MCB literature but unable to find any specific warning about excessive PFC.

 

[ringer]

And it seems the Memera 2000 HRC fuse carriers have been discontinued, and they were only rated for fuses up to 16kA PFC.

HRC fuse carrier at RS

 

[Engineer54]

I've just re-read this thread, and it seems that the distance to the distribution TX is some 250 metres. We don't know the size of the TX though!! I would have to say that the supply cables would have to be sizable to allow a Ze of 0.01 ohm and a PFC of 19+KA. I would suggest that measurements taken have a largish error factor within them... You would only expect to see around 16KA in central London's DNO extensive networks.

250 metres is a fair distance, and when taking into account all the resistances/impedances of connections, switch gear, and cables etc, it's going to be highly unlikely that the given figures are anywhere near correct. Going by those figures the TX would have to be 1000 KVA, which would normally be capable of providing around 20KA into a fault at it's LV terminals, but very doubtful 250 metres distant, passing through all those said impedances...

To be totally honest, you would only find those figures within the actual sub-station itself or very, very close to it.

 

[ringer]

Took a while to get an answer to my original question, so thank you E54 for your last post! I was hoping someone was going to say that.

I hope to get the opportunity to take my own measurements sometime soon - I hate it when I suspect something is wrong, and cannot rest until i get to the bottom of it. My previous musings about SSC and HRC carriers were me trying to establish a plan B, just in case. Hopefully it won't come to that.