Zs on rcbo

[spidersparks]

Hi guys had a conversation with the boss in the office after work today about a zs I got on a 32A c type rcbo bs 61009, the earthing system is TN-C-S, he said he wasn't worried I got a zs reading of 0.90 even though a type c 32A rcbo has a max zs of 0.72 his words were "it's just like a TT system, when you have a high zs because of the electrode" my understanding was that as its on a TN system it would still have to meet disconnection times in table 41.1 in the BGB, I wanted the change it to a type B then I can go up to 1.44 which I'm well in. what are your thoughts? Many thanks
David

 

[Sharpend]

Unfortunately Dave you've come up against the rcbo/rcd one size fits all group. We have to remember that rcd protection is classed as Additional Protection. As you say the regs require us to meet disconnection times for MCB (over current) on TN systems.
Just because he's not bothered doesn't mean he's right, if you are signing your name to the certificate then you have to be satisfied its correct.

 

[Brightspark2]

Technically correct as max Zs would be 1667 (IIRC). Although shouldn't be designed this way. Is this for EICR?.

 

[Brightspark2]

He should of gave you a better reply too...

 

[IQ Electrical]

I agree with Brightspark2, you are meeting disconnection times with the RCD.
Obviously if you were looking at a much higher Zs then that would suggest a fault that should be investigated.

A popular 2391 question asks for remedies to an exceeded Zs on an MCB and among the acceptable answers is the incorporation of RCD protection to meet disconnection times.

 

[somersetsparks]

Agree with all of above, I don't like the application of an rcd to compensate for poor circuit design and certainly not to cover up fault, however if the circuit length tied up with the readings and a L-N loop was satisfactory then it complies. The readings you have don't suggest a fault really, however only you know the circuits length.

 

[Engineer54]

It'll not be long before MCB max Zs values will be a thing of the past, and you'll then be totally relying on RCD values. It seems that it's almost that way now!! lol!!

My rule for exceeded Zs values for MCB's is to fix/correct the problem, and not the get out of jail route of the cover-all RCD device...

 

[spidersparks]

Thanks guys, I totally agree that technically I can have a higher zs due to the fact it's an rcbo, my dead test came back text book so I know there's no fault on the circuit, I just didn't want to solely rely on the rcd and for the sake of changing to rcbo to a B type it would fine, the only reason its a c type is because its a three phase board and the supply c types standard rather than b type
Thanks again

 

[somersetsparks]

It'll not be long before MCB max Zs values will be a thing of the past, and you'll then be totally relying on RCD values. It seems that it's almost that way now!! lol!!

My rule for exceeded Zs values for MCB's is to fix/correct the problem, and not the get out of jail route of the cover-all RCD device...

Exactly I have seen RCD's fitted to equipoment for which there not suitable and as such nuisance trip, sometimes an extra bit of bonding is all thats needed to get Zs values down a bit, I did one where there was a plastic isiolater bolted to a unistrut framework and it just exceed the 0.72 value (about 0.80) With a bit of 6mm lugged onto the strut it achieved 0.5 ish.