Testing

[Hayden@2468]

I’m currently testing some site temps I’ve got the main three phase head with the metre tails coming out of that going into a 100 Amp RCD with a 25mm armoured going out to feed a board when trying to test my PFC with a megger MFT1741 it comes up with 2 lead high however this will trip the RCD what is the best way round this I tried on 2 lead low but was coming up with 20 amps it’s a TT system

 

[Barx]

Test before the RCD? What is the Ze reading. 20 amps could be a reasonable reading on a TT. Hence the requirement for upfront RCD on TT systems as the fault current won't always be high enough to trip / blow fault protection.
If you're doing a Zdb / Zs reading, going to have to be on lo current.

 

[Hayden@2468]

Test before the RCD? What is the Ze reading. 20 amps could be a reasonable reading on a TT. Hence the requirement for upfront RCD on TT systems as the fault current won't always be high enough to trip / blow fault protection.
If you're doing a Zdb / Zs reading, going to have to be on lo current.

My ZE was coming up as 31.3 but when I went to do my PFC on 2 lead low it was giving me funny readings like 21 Amps would it make it much difference doing the PFC before the RCD ?

 

[Barx]

My ZE was coming up as 31.3 but when I went to do my PFC on 2 lead low it was giving me funny readings like 21 Amps would it make it much difference doing the PFC before the RCD ?

out of curiosity, what PFC reading would you expect?

 

[Hayden@2468]

out of curiosity, what PFC reading would you expect?

Tbh this is my first TT system I’ve tested it’s thrown me out a bit with some of the readings but I know the PFC will be considerably low as the ZE is 31.3 wouldn't it be around 7 - 8 amps on a TT based on the ZE reading

out of curiosity, what PFC reading would you expect

 

[Barx]

Tbh this is my first TT system I’ve tested it’s thrown me out a bit with some of the readings but I know the PFC will be considerably low as the ZE is 31.3 wouldn't it be around 7 - 8 amps on a TT based on the ZE reading

At least you're on the right understanding mate. Just making sure you weren't expecting results in the thousands of amps like those typical of other earthing systems. Not 100% sure how the megger conducts it's tests. Low current results can be a bit flakey at times anyway.

 

[Phoenix]

Your PFC is going to be a lot higher than your PEFC on a TT system, remember that PFC is the highest of PEFC and PSCC, seen many certs for TT installs witha comically small PFC filled in!

For PSCC, 2 lead high, test between neutral and each phase (3 tests) and take the highest and double it to acocunt for a ph-ph fault.

I expect the reason you weren't getting 7-8A for PEFC is chances are with your earthing conductor connected back in, you have parrallel paths, and the zdb is probably quite a bit lower than Ze/Ra

OOI, whats the overcurrent protetcion for the SWA?