What would you put on the cert?

[Dave 85]

Did a job today where I had a 6mm T&E on a 32amp retrofit MCB (re used old circuit) going into a RCD spur (13amp) then a 2.5mm going off to a shower pump. There is also an unswitched 3amp fused spur halfway down the 2.5mm with a 1mm going to a mirror light.

Sounds a bit of a muddle I know but was the only way I could do it. 3036 Fuseboard full and hard to get cable into, existing lighting circuit impossible to extend as it was buried in concrete ceiling.

My question is, on the cert, in the overcurrent protection box do I state the 32amp 60898, 13 amp 1361 or 3 amp 1361. And in the cable box do I put the 6mm, 2.5mm or 1mm.
Im guessing I'd mark it down as a 32 amp 6mm circuit as the other bits are essentially fused spurs but just wondered if anyone thought different.

 

[Dave 85]

The funny thing is the bit of 6mm is only about a foot long. I was gonna stick a 16amp MCB in the board, but didnt see much point. I'm pretty sure, if I remember rightly, that the zs at the end of the 1mm was around 1 ohm anyway so it dont really matter, but its an interesting debate and shows that test certficates can quite often carry quite misleading information.
I mean by all rights its a 13 amp circuit with a 3 amp spur

 

[Guitarist]

Not quite Dave.
Anything that can be disconnected via a plug is not considered as fixed wiring , and can be ommited from the tests.

Connection via a plug and socket no longer allows fixed wiring to be omitted from testing.

 

[malcolmsanford]

Does no one else disagree with a max. Zs of 1667Ω for a 32A mcb?

The max. Zs should still be 1.44Ω even when the circuit is RCD protected.

Agreed , poor design otherwise.

So what is the point of an RCD then gentlemen if your not going use the additional protection it gives you and therefore the values for the device.

So if you have a TT system that the Zs on a given circuit could be 5 ohms lets say that is what your going to certificate and therefore your certification is going to show a fail as your going to put as the maximum value 1.44ohms??

The RCD on your circuit gives you the opportunity to allow that circuit to be safe and and comply, the RCD will carry the fault protection part of the circuit, otherwise why install one in the first place leaving the MCB to carry the overload part.

Biff your comment to be honest was inane. I doubt any proper electrician would design a circuit that relied solely on an RCD to provide complete fault protection. The very concept of the RCD in the UK is for additional protection, not sole protection and the majority of circuit design would still be for a Zs to be sufficiently low enough to trip a non RCD protection device.

Because the 17th edition as now deemed virtually every circuit in a domestic situation needs this additional protection our certification will have to reflect this. All circuit values will be 1667. I'm sure that most of us will still want to achieve disconnection times for a non RCD protection device, and to be honest expect that on a TN system, and if it wasn't achieved then you would want to find out why it wasn't

Now gentleman you can put whatever you wish on your certification, but the facts are if you have an RCD on a circuit for additional protection then your Zs value for that circuit is 1667 if you don'r believe me then have a look at table 41.5 in the regulations, it comes after all those other tables that we get our values from and we seem to believe them

 

[Guitarist]

I think that you summed that up quite nicely Malcolm. Of course we aim for non-RCD protected figures, but if you think about it, we accept quite happily a Zs of say 70 ohms on a TT system because it has an RCD, yet get hung-up if it's 0.1 over on our RCD-protected TN system.

 

[Guest55]

So what is the point of an RCD then gentlemen if your not going use the additional protection it gives you and therefore the values for the device.

So if you have a TT system that the Zs on a given circuit could be 5 ohms lets say that is what your going to certificate and therefore your certification is going to show a fail as your going to put as the maximum value 1.44ohms??

The RCD on your circuit gives you the opportunity to allow that circuit to be safe and and comply, the RCD will carry the fault protection part of the circuit, otherwise why install one in the first place leaving the MCB to carry the overload part.

Biff your comment to be honest was inane. I doubt any proper electrician would design a circuit that relied solely on an RCD to provide complete fault protection. The very concept of the RCD in the UK is for additional protection, not sole protection and the majority of circuit design would still be for a Zs to be sufficiently low enough to trip a non RCD protection device.

Because the 17th edition as now deemed virtually every circuit in a domestic situation needs this additional protection our certification will have to reflect this. All circuit values will be 1667. I'm sure that most of us will still want to achieve disconnection times for a non RCD protection device, and to be honest expect that on a TN system, and if it wasn't achieved then you would want to find out why it wasn't

Now gentleman you can put whatever you wish on your certification, but the facts are if you have an RCD on a circuit for additional protection then your Zs value for that circuit is 1667 if you don'r believe me then have a look at table 41.5 in the regulations, it comes after all those other tables that we get our values from and we seem to believe them

Inane ? maybe , but correct all the same.
And how is any of this relevent to Daves' original question / topic ?
Dont you think a new thread should be started if you want to explore this issue ?


To be honest i dont think i want to be part of this forum any more , every topic discussed is like banging your head against the wall and i can think of more rewarding avenues for sharing my knowledge.
So i'm done here , all the best.