Low End to End R2 value Help

[Alan Marriott]

Hi
I’m still learning so forgive my stupid question. I’ve been testing a ring main off 4mm/1.5mm singles in metal trunking and conduit. My reading are:
L-L 0.91
N-N 0.90
E-E 0.80

I was expecting my R2 value to be 2.66 x 0.91=2.42

I this purely parallel path related??

many thanks

 

[westward10]

Almost certainly the only way you can establish the correct value for the conductors would be to isolate all accessories from the containment.

 

[pc1966]

If you are wondering why the E-E is lower than the live values I think you will find the conductive trunking & conduit is a factor there!

 

[Julie.]

if each socket outlet etc is metal clad then in each socket the CPC is connected to the conduit via the front screws, this is in parallel to the cpc to the next socket etc.

 

[Alan Marriott]

Thanks for the replies guys. Problem is that the singles go into dado trunking drops and no metal clad sockets in the ring. Some have metal conduits bent into the plastic vertical drops and some have metal kopex but none are made of into plastic obviously. Which made me question the R2 value more as if it was parallel earth paths I’d understand the low reading

 

[westward10]

Is the dado trunking pvc or steel.

 

[pc1966]

Also remember you only need a few of sockets to have near-zero ohms from the metallic surround to bring the E-E value down. You don't need all to be connecting.

And yes, that does make interpreting the E-E resistance difficult or even pointless. But at least your L-L and N-N and close enough for comfort!

 

[timhoward]

If you are saying there is pvc trunking separating the steel trunking and the sockets, and all the sockets are plastic backed, I'd be inclined to do a continuity test between each end of cpc and the steel trunking.
Making assumption this is EICR not EIC is there a possibility someone has 'bonded' the steel trunking using the circuits cpc, or joined the bonding conductor to the cpc at least twice?

 

[pc1966]

In a normal RFC (insulated surround) the E-E continuity gives you very high confidence all sockets have CPC continuity. Not absolute, as the CPCs could be twisted and have fallen out of one socket's E terminal, but still good confidence.

In the typical industrial situation (all metal trunking/conduit) you often don't need the CPC as the metalwork is doing the job and, short of major corrosion or stupid plastic-replacement work, is also a very reliable CPC.

But if you have the CPC used for multiple bonds and a mix of plastic socket supports there is really no guarantee that a low E-E reading is verifying (more or less) all sockets' CPCs. The good readings on L-L and N-N suggest things are probably OK, but if concerned you might want to do the figure-of-eight test and check a decent portion of the sockets have good low L-E results on that.