time delay RCD

[JD6400]

there's no way he's got a Ze of 0.18 on a TT system. someone's telling porkies.either that or his pi$$ is marketable.

It is possible , I have personally seen it on an ex military site that we look after and is now a farm . I can not remember the exact Ra now as it was a few years ago , but I have posted about it before ( I can only go back as far as the date of my user name change , for some reason ) in a post about a 100 Ma time delayed RCD going out before the 30Ma RCD down stream , and it was because of the exceptionally low Ra at source .

 

[JD6400]

I can't understand why people do not believe what's written in BS7671. If its a TT system with a Ze of 0.18 Ohms after the MPB have been disconnected then 411.5.2 (ii) is sufficient. We used to work to that all the time 20 odd years ago.

However, I've never seen a TT system as low as that. Maybe 0.5 - 1.5 Ohms.

Maybe so , but for the price of them I would rather err on the side of caution and have it as a secondary safety net anyway . No matter what others will no doubt preach , I have seen more than enough Ra's rise and fall , ( Not just the twigs ) far more than I would like to see .

 

[ackbarthestar]

be realistic. who'd give a trusted badge to a zoo inmate?

Get it right - still in the primate classification, there are many out there who walk on all fours......

 

[Engineer54]

I agree with all here, that an Ra of 0.18 ohm on a typical domestic TT system is going to be extremely hard to believe!! lol!! You'd have to be driving a deep rod into a salt marsh to stand any chance of seeing that sort of value!!

As for the Very low Ra on an ex military site, that i can believe, there are probably multiple linked deep driven rod positions around the site, or a full dedicated earth field in place...

On the majority of standard domestic, small/medium commercial systems it is Always prudent to provide the extra back-up protection of an up-front 100/300mA S type RCD device. No-way would i rely on a single RCD device as my sole means of earth fault protection. They are just not that reliable!!

 

[D Skelton]

Me thinks someone has forgotten to disconnect the main earth to carry out Ra test!