High Zs readings

markc17 · Mar 22, 2012

Hi all, some advise please...
I have a building full of type 4 3871 FE breakers, that have recently undergone a PIR. Many of the ring circuits have come back as having high Zs readings and have thus failed, with each only failing by like 0.04 of an ohm. There are no earths in the circuits, with the conduit and trunking system being used. Without going into reasons why, it is unlikely i am going to be granted the time or resources to pull new earth wires down to each of the affected circuits. Was just wondering my options... I have managed to procure some 30ma Federal Electric RCBO's off Ebay, and was wondering if by fitting these it would mean my readings would comply? Ill be honest, im a bit confused by the 1,667 ohm allowance on a 30ms rcd. Any clarification would be greatly recieved.
Just so you know, i cannot gain access to alot of the conduit system, so am unable to tighten much, i could possibly increase some of the bonding?
Thanks, Mark

VoltzElectrical · Mar 22, 2012

you need to check the earthing arrangement and size of main earth. may be possible to upgrade main earth to get zs down.

a1guvner · Mar 22, 2012

I would start at the begining and check your external arrangement and readings

Silly Sausage · Mar 22, 2012

Are these readings close to the MAX Zs or the adjusted values?
What's the Ze?

Richard Burns · Mar 22, 2012

markc17 said:
Hi all, some advise please...
I have managed to procure some 30ma Federal Electric RCBO's off Ebay, and was wondering if by fitting these it would mean my readings would comply? Ill be honest, im a bit confused by the 1,667 ohm allowance on a 30mA rcd. Any clarification would be greatly received.

The 1667 ohm is just the Ra x Idn <=50v so with a current of 0.03A and 50 V resitance (Zs0 must be less than 1667 ohms.
This is a way of overcoming the issue but relies in the RCD, however you would still disconnect the breaker just not in the required time.

Amp David · Mar 22, 2012

The use of and RCD/RCBO is one way around your problem, but as already said, it would be a better job to try and find why your readings appear high.

Its all to easy to say bang in a RCBO and be done IMO

Engineer54 · Mar 22, 2012

Federal Electric breakers, I take it, these are those black Bakelite ones?? ...I wouldn't give you tuppence for them!!
These should be treated the same as the old VOELB's, with a recommendation to replace!! lol!!

markc17 · Mar 22, 2012

Thanks all... basically its a building that is in constant use, to upgrade main earthing and the like is, at the moment atleast, an impossibility. All readings at the intake etc, fall within regs, and the high Zs readings i am getting fall within the regs in BS7671, but are outside the rule of thumb readings in the OSG..... when i say outside i mean literally by a few points of an ohm... still got a code 2 tho. Am assuming that its the path of the conduit and trunking that is the issue.. to pull into the rooms that are an issue is again fairly difficult to do at the moment so was just wondering about adding extra bonding to the conduits, or trunking.. and whether this would be seen as bead practice or acceptable? I would like to get the readings under if poss, rather than juhst add an rcd...
thanks again guys
mark