what max zs on certs do i use

[ohlins]

hi guys been a while since ive worked on a tt system.We are changing some boards and are going to be installing wylex dual rcd ones.on the certs what do i put for max zs?.do i put 1667 because of rcd or do i use cb max?.The max tested zs on ring was 10.5 and the ra is 101.Thanks

 

[Des 56]

1667 for 30m/amp

 

[Plonker 3]

Dual rcd boards, so the main switch is a RCD. Surely you have to record the max Zs for the MCB then?

 

[Amp David]

I always place the figure for the MCB on my certs

 

[Deleted member 9648]

I always place the figure for the MCB on my certs

Cant see it mate...it's a TT....you are going to put on the cert a max Zs for a 32a/B mcb of 1.15 ohms....and perhaps record a measured Zs of up to 200 ohms????.........it doesnt add up

The maximum Zs allowed where a 30ma RCD is used for earth fault protection is 1667 ohms...so that is the value that should be used IMO.
It could also be argued that as it is recommended that the resistance of the electrode should not exceed 200 ohms...the max Zs should be 200 + the highest R1R2.

 

[sjm]

Des 56 spot on 1667 for 30ma RCD.

 

[ohlins]

Thanks for replys chaps.Yes i know 1667 is max for the rcd's but on page 3 of certs in maximum zs permitted by bs7671 box would you record the cb max or rcd?.Obvously my maximum measured is going to be well over the cb max but im still covered by rcd.We are not using rcbo's because the client threw a bluey about possible N-E faults and wanting D-P protection.

 

[Deleted member 9648]

You would use the RCD max....on a TT the RCD provides earth fault protection NOT the mcb.

 

[malcolmsanford]

You could have given him RCBO protection by using double pole RCBO's but RCD/MCB protection for TT is just as suitable.

As was said previous the maximum Zs for a BS 61008-1 and a BS 61009-1 is 1667ohms for any circuit on any earthing system that has these protection devices. The use of RCD as now virtually left the Zs readings obsolete. Though you of course will still need Zs when using just MCbs and Fuses

 

[Spudnik]

Personally, on TN systems i always right the Max Zs for the MCB.

TT would be 1667.

 

[Amp David]

Personally, on TN systems i always right the Max Zs for the MCB.

TT would be 1667.

Thank the lord i'm not on my own then

Saying that, my first post made it sound as though i woud for tt too.

 

[IQ Electrical]

You could have given him RCBO protection by using double pole RCBO's but RCD/MCB protection for TT is just as suitable.

As was said previous the maximum Zs for a BS 61008-1 and a BS 61009-1 is 1667ohms for any circuit on any earthing system that has these protection devices. The use of RCD as now virtually left the Zs readings obsolete. Though you of course will still need Zs when using just MCbs and Fuses

I fully agree with Malcolm here, as long as continuity of protective conductors has been confirmed.

 

[Deleted member 9648]

I think it's worth pointing out here that the 1667 ohms figure is a figure only.....no one is suggesting that a Zs reading anything like that would be acceptable.On a TT you would expect the highest Zs to be around 200 ohms max.....and on a Tn it should be within the limits of the mcb/fuse.....
On a TN there may be occasions when additions to an existing circuit are being installed for example,and a type C mcb is in use,and the measured Zs slightly exceeds the max for the mcb...IMO where RCD protection is included the 1667 value can be used,subject to other tests being satisfactory.