If you verify that the PSSC is enough to meet disconnection time you also have, in a sense, verified the volt drop is reasonable. Not necessarily < 5% (which would imply > x20 short to nominal current, enough for a type D MCB) but also it might not need to be that as the volt drop applies at the design current which could be a lot less than the MCB.
Meeting disconnection times comes under chapter 41 - Protection against electric shock, which I don't think would be a concern for L-N faults (or perhaps it might be, on a TN-C-S?). So to disconnect in <= 0.2s for L-N fault on TT is not necessary.
There are of course other considerations - adiabatic and cable rating.
There are of course other considerations - adiabatic and cable rating.
Correct, there isn't a requirement to operate for L-N or L-L faults within any set time but 434.x does say that protection against fault current is required, to me that does mean it has to operate in a timely manner, not necessary 0.2/0.4 sec - or even 5 sec but not several mins.
My concern wouldn't really be the cable, but the consequences of the fault itself - if this is a water/rodent/whatever issue causing the fault, the likelihood is a strong arc persisting for a long time before finally being cleared some time down the line. This could cause thermal damage/fire.
Perhaps this is why the 18th amendment 2 is looking for AFD devices - just like the premature wiring collapse, caused by bad practices - instead of fitting trunking properly Mr 'Bodgeit and Scarper' (Ltd of course) just stick the double sided tape on a dusty flaking paint surface, so by the time they leave site it's already fallen off! - Result, we have to use metal cable clips et al. same here - rather than people thinking about the consequences of high loop impedance, they just assume an RCD will sort it - hence down the line we have to start adding additional devices:
Fuse/MCB - Primary protection
RCD - Additional protection
AFDD - Additional Additional protection
????? (TBA) - Additional Additional Additional protection
Not very convincing, I don't think anyone has answered the OP original question!
If its not so 'grey' it should be pretty obvious!
L
loz2754
Telectrix answered correctly, 1667 ohms.
I agreed, but also said there would be nothing wrong with putting 200 ohms.
Then as usual, the thread took an interesting and most thought provoking turn, thanks mainly to @Julie.
I'm not sure it would ever take anywhere near that long to disconnect in any healthy circuit. The OP's measured Zs of 3.79ohms has to be the earth loop, not L-N loop, or it would be ~370m long at 4mm².
There is a mention somewhere in the OSG saying that the disconnection times should be met for L-N faults, but it doesn't link to a regulation. I'll try to find where it says this. Some of the final circuits in chapter 7 are limited by short circuit, but very few. I can't understand why this is, if it is not required by regs.
Here it is. Chapter 7, P77. Talks about 5s for some reason:
No, I strongly suspect that the actual L-N loop would be much less than Zs in a TT as you say.
But really my point is that we shouldn't be guessing - we should be checking.
On a TNx usually the Zs is higher than the L-N loop purely because the cpc portion is 2.5/1.5× or 4/2.5x the neutral resistance, which isn't a great difference; however in TT they are vastly different, - could be orders of magnitude.
So actually using 1667 or 200 is totally irrelevant, you wouldn't be cross checking this anyway, you would be using the rcd actual tests to prove it's OK.
As for the osg saying it should meet the same disconnection times, I am not sure it does, but I am more familiar with the regs, than the osg as I don’t need it that often, so happy to be wrong.
But that is sort of what I am saying, there is no MUST disconnect, but it should disconnect within a reasonable time frame. (if that's within the same times, great)
Exactly my point! ....... Is it 1667 ohms or 200 ???
OR Something else??
Confusing for the OP!
Cheers,yea it’s the earth loop.
Agree, it should disconnect sooner rather than later. But it is a struggle to find a circuit where SC loop impedance would the limiting factor, at least for the type of work I do. Voltage drop almost always gets there first. Very lightly loaded, very long lighting circuits perhaps.
The OSG paragraph is in my previous post #24, but doesn't link to a regulation, and I can't find anything in the regs that directly back it up.
On TT yes, I believe so.
In my opinion this should replace the earth fault loop test/calculations, as earth fault is covered by testing the rcd.
On TNx, not required, the earth fault loop covers it, if the rcd is additional protection
BUT, if the rcd is applied to the TNx because the earth fault loop cannot meet the disconnection time, then I think both sets of loops/calculations are needed.
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