63A 3 phase socket

[Rob.]

Hi Rob i was thinking if it could be due to the armour being earthed at both ends, but i understood it was terminated to a plastic socket and the charger was a free standing thing..

Anyway, you are not going to get Zs that low unless the DNO transformer is next door to you.. [or you have like a 200A supply.]

I quite commonly see Zs readings that low. In fact the loop tester I have will measure down to 0.001ohm however I'm only ever on industrial sites, (I was presuming Binky  is too) that have their own Tx supply. 

 

[apprentice87]

I've always been of the opinion that the live test is more important as this is what the cct will see in operation - but that's my opinion! R1+R2 is more interesting to me when there are multiple joins along cct eg 10 sockets, where there is far higher chance of a dodgy connection and high resistance at some point along it's length.

Yes, but it should all be designed as a "stand alone" type thing, otherwise, what happens if the other bits and bobs are removed?? This is why you use the DNO quoted Ze figures and not the measured ones [which might well be lower] [as you have no idea what will happen to the LV network in future] and then add R1 + R2 to these for design purposes..

This is why you disconnect bonding conductors before doing a Ze test, as otherwise, you are including other items that may well be removed one day, [like say metal pipes being changed for plastic ones..

john..

I quite commonly see Zs readings that low. In fact the loop tester I have will measure down to 0.001ohm however I'm only ever on industrial sites, (I was presuming Binky  is too) that have their own Tx supply. 

You might with them, but i doubt you will with a 100A supply..!!

john..

 

[Rob.]

You might with them, but i doubt you will with a 100A supply..!!

john..

Of course not. Unless, as you say, the circuit is sat on the supply transformer. 

The Op is connecting a 63amp circuit though. It's not a domestic application. The supply is going to be greater than the 100amp domestic average. 

He also mentioned that the SWA is terminated into steel trunking, the steel trunking contains other circuits from around the site. 

 

[apprentice87]

Ahh, then that will be it then as you say!!

john..

 

[binky]

small industrial unit on an industrial estate, not sure what the incoming supply actually is as I didn't care to open the Ryefield board ( not allowed to anyway). The site has several DBs from the main incomer, so at least 200A per phase, can't remember what the MCCBs were, but SWA was terminated at steel trunking above 1 DB 125A main switch, trunking directly connected to board and steel frame  bonded in 16mm right next to board. So parallel earth paths in all directions, I was still abit surprised by the Zs values, but it was even across all 3 phases and armour of SWA is far greater cross sectional area than any CPC.

 

[Tony S]

I thought Ryefield boards only went up to 100A per way?