by ericmark »
Tue Aug 15, 2023 12:23 pm
In theroy the loop impedance needs to be low enough so the volt drop with not exceed 5% and the prospective short circuit current will be high enough to ensure the over load device will trip within the required time if there is a short circuit.
Main limiting factor is volt drop, it was 4% which was 88 meters of 2.5 mm sq in a ring final, now 106 meters or 5% with 2.5 mm sq. But to be frank volt drop is unlikely to be a problem, so looking at tripping time.
The MCB is two devices in one, the thermal part will trip at current stated on the device, in the fullness of time, But to trip fast enough we need the magnetic part, which will disconnect on a short circuit in a fraction of a second, however this part of the device is set to a much higher current, B = 3 to 5 times, C = 5 to 10 times, and D = 10 to 20 times the current that the thermal bit requires.
We also now allow 5% for safety so for a B32 MCB the earth loop impedance needs to be 230/160 = 1.44 ohms and with the 5% 1.38 ohms. This is a problem as the cheap (£50) testers with loop, tend to pass at 1.8 ohms, clearly designed for Europe which don't use ring finals, even when it has a 13 amp plug on the tester, so it needs a tester costing around £200 to test it.
One can hire the tester, around the £50 mark for a week, the reason so expensive is they need calibrating again on return, the problem is you do all the work, and then it fails, so what than? You want to know the readings before you start, so you have a good idea if it will pass, if you have the installation certificate you can look it up. But other wise one is crossings ones fingers.
The point is a fault to earth will trip the RCD, and a fault between the lives (neutral is regarded as a live we call the other wire line) not so important to disconnect quickly. I am sure there are many installations which would not pass, and I am talking about TN earthing, with TT we rely on the RCD.
So if you do use too much cable, likely nothing will go wrong, and know one will know until you get an EICR done, if rental it must pass the EICR, but if owner occupied up to you.
So you can go down the notify LABC route, and DIY or use a scheme member to do the work, or cross ones fingers and hope nothing goes wrong. To tell you to do latter is like saying you can drive down that 30 MPH speed limit road at 60 as the police here never check. On a forum we clearly can't tell you to break the rules.
Driving at 35 MPH is like adding one socket and taking a chance, but with a full refurbishment that is like driving everywhere at 70 MPH, clearly you need to decide, Likely the best compromise is to do the work, then get an EICR done. Can't pick up all faults, like wiring not in safe zones, but should pick up most.
I hope I don't get kicked off the forum again for saying that, what I am aware of is if you simply say you should not do that, people think we are just looking for work, which I am not, so that's why I have tried to explain why it may be better with major work to get a scheme member to do the work.
