Disconnecting the functional earth on an rcbo

[Adam_92]

I know it’s good practice to disconnect the functional earth when doing an insulation resistance test but my assessor has told me that he thinks you should disconnect the functional earth when you measure Zs?
Which I don’t understand does anyone else do this? And is he correct?

 

[gazdkw82]

You don't just remove the functional earth when IR testing with an RCBO in the circuit, you disconnect the load side of the circuit so you don't test through the RCBO.

 

[Adam_92]

Yeah sorry forgot to mention that, I’m just wondering about disconnecting the functional earth when measuring Zs

 

[gazdkw82]

Not sure the functional earth would have an impact. Might be worth doing 2 test, 1 with it connected and one with it disconnected.

I'd be interested to know the difference in results

 

[Deleted member 26818]

I have never known that to be advised before.
Some manufacturer’s advise disconnecting L & N as well as both the Neutral and FE flying leads, before conducting an IR test.

 

[Adam_92]

Very interesting I’ll check and see if it makes a difference and let you know well my assessor also told me you could measure Zs at the distribution board! Which is insane that’s why I thought I would check with you guys

 

[Deleted member 26818]

You can measure Zs at the distribution board.
It is commonly known as Zdb.

 

[Adam_92]

Surely that would be if it was fed from another distribution board? Because otherwise that’s just a Ze? No?

 

[Deleted member 26818]

No.
Ze would be measured at the DB/CU by disconnecting the Installation Earth conductor from the MET.
Zdb would be measured at the DB/CU with the Installation Earth conductor, bonding conductors and CPCs connected.

 

[Adam_92]

So you would just clamp on to the Earth bar? You’d get exactly the same reading no? What would this prove?

 

[Deleted member 26818]

You might get the same reading.
I would expect it to be slightly to quite a lot lower depending on the type of earthing.
Don’t know that it proves anything, other than there may be parallel earths.

 

[gazdkw82]

Ze is a measurement with just the suppliers earthing conductor performed at the main incoming point (usually the first DB, C/U)

Zs is a measurement usually at the end of a circuit, fitting, accessory to confirm compliance with Zs values.

Zdb is performed at a sub board. This will measure the earth path prior to the board your measuring at. This could be through numerous other boards.

I think that's right? :-/

 

[Adam_92]

Yes gaz this is what I was thinking