high zs reading

[graham123]

Testing a new terrapin buliding at a primary school today. The supply to the terapin is 90metres of 35mm 4core. Swa as CPC. At the terapin db i get 0.59 ohms as Zs. All the circuits in the terapin are 30ma RCD protected. Is this acceptable as the zs of the individual circuits does not meet the disconnection times of the RBCO. I heard somwere that if the circuit was 30ma rcd protected Max zs did not matter. Thanks Graham

 

[telectrix]

with RCD, max. Zs is 1667 ohms ( 30mA RCD . the |RCD will then act aS FAult protection .

 

[yellowvanman]

For a 30mA RCD then Zs must be below 1667ohms, so yes your RCDs gets you out of the jam for high Zs measurements.

 

[robbie18]

is this the same for all types of earthing systems including tt??

 

[Deleted member 26818]

is this the same for all types of earthing systems including tt??

Most certainly, even more so with TT, as in most cases the only method of earth fault protection with TT installations is by means of an RCD.

 

[robbie18]

so RCBO is protecting a circuit which has high ZS but is below 1667 ohms so thats covered by that and then we have to have RCD tests below 20mseconds for TN systems and 7mseconds for TT system at 1x test. Is this correct??

 

[Engineer54]

Ha,ha!!!! And once again, bang goes the ''Additional protection'' status on a TN system, as the additional protection has now become your primary protection, ...and before the installation is even put in to service What's more it's an electronic means of protection, that can go belly up at any time.

The fact is, that your 35mm cable can't and/or isn't delivering a suitable means for ADS operation, so it doesn't conform.
Bit different if you lost a working ADS due to a loose connection or whatever during service, but on a new installation?? As a customer, i'd expect everything about that new installation to be A1, not fundamentally flawed from the very start....

I really don't know why you guy's even bother checking circuit Zs any more, as your never going to exceed 1667 ohms!!!

 

[somersetsparks]

If you are intending on relying on RCBO's I Would at least do a loop test from L-N at each of the circuits though, as it may be a dodgy live connection giving the high readings and under short circuit conditions may not operate the RCBO's.

 

[Guest55]

I'm definately with E54 in his view on this issue.
Why bother with earthing at all if your just gonna rely on rcd's , just smacks of poor design IMO.
The dustbin in my back yard probably has an earth reading below 1667 lol.

 

[Guitarist]

Also agree with Eng and Biff here. Can't add any more than they have. Why design a system properly when you can just pop in a "magic RCD" and all your worries float away....

 

[ackbarthestar]

I would have thought that a Zdb of 0.59 ohms was sufficient for the type of circuits run in a terrapin.
What circuits are installed?
half a dozen sockets, half a dozen lights, a couple of panel heaters?
Why not use 16A radials for the power circuits that will increase your minimum allowable Zs....