Can anyone tell me if you are allowed PFC of 10kA on a domestic TNCS system with a Ze of 0.03 Cheers
Maximum PFC (domestic)
- Thread starter bandit22
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Of course it's allowed, after all, what could you possibly do about it?
Request a longer supply cable from the DNO?
Request that the substation is moved further away?
Request a longer supply cable from the DNO?
Request that the substation is moved further away?
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Eh? If the PFC if 10kA then the PFC is 10kA. That's like asking if it is allowed to rain on a Sunday.
The question is can your OPDs as installed handle that PFC....
GN3 mentions BS1361 type 2 fuses up to 100A (not BS88) supplying consumer units to BS 5486-13 and BS EN 60439-3. In this case the short circuit capacity of the OPD's may be taken to be 16kA.
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Where the PFC is high it can be accepted by working on the "Let through" value of a fuse. However when I came across this problem I found the resistance of the fuse dropped the PFC to within acceptable values anyway so I never did work out the "let through" values. It seems the I²t (Let through value) is it seems published by manufactures which is all well and good if you fit the fuse but where the DNO fits the fuse I don't know if you can rely on them fitting the same type of fuse I will watch with interest to see what others say.
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Found this http://www.cooperbussmann.com/pdf/a03b2dad-1686-4d32-8a96-86142b1684f2.pdf which if I am reading right means with a 100A BS88 fuse from CooperBussmann with a 240v system you could use MCB's marked 7500 if I am reading the chart correct.
It would seem then those marked 10,000 are unlikely to have a problem?
I would like to see what others say about this as I will admit I have never really had to worry about it.
It would seem then those marked 10,000 are unlikely to have a problem?
I would like to see what others say about this as I will admit I have never really had to worry about it.
The 16kA rating is correct, so in pretty much all domestic situations, the fault level is not an issue. It is somewhat unlikely it would be 10kA anyway, as the measurement won't be particularly accurate due to the measurement device itself.
The let though energy is more significant, but as ericmark already points out, this will be fairly small due to the suppliers fuse. While this could be 100A, many are 80A or even 60A.
Fuses limit the let through energy anyway, since when a fault occurs, the fuse element heats up, significantly increasing the resistance.
Not forgetting that the cable to your house won't be connected directly to the substation transformer anyway, there will be other fuses and various joints in the cable.
The let though energy is more significant, but as ericmark already points out, this will be fairly small due to the suppliers fuse. While this could be 100A, many are 80A or even 60A.
Fuses limit the let through energy anyway, since when a fault occurs, the fuse element heats up, significantly increasing the resistance.
Not forgetting that the cable to your house won't be connected directly to the substation transformer anyway, there will be other fuses and various joints in the cable.
230/0.03 = ??
Didn't spot it was TN-C-S sorry.
Still it's probablly well within the bounds of measurement error to get those figures together especially if the voltage is at the higher end of acceptable and/or the Ze was measured at a different current to the PSCC (e.g. the tester was in "dont trip RCDs" mode).
Still it's probablly well within the bounds of measurement error to get those figures together especially if the voltage is at the higher end of acceptable and/or the Ze was measured at a different current to the PSCC (e.g. the tester was in "dont trip RCDs" mode).
Could be to do with the amount of digits a tester can display 
If it can display another one it might have displayed 0.024 ohms albeit it should have really rounded it down - but in loop measurements I can see the advantage of rounding up.
If it can display another one it might have displayed 0.024 ohms albeit it should have really rounded it down - but in loop measurements I can see the advantage of rounding up.
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