Earthing question

RCDs don't sense in relation to earth, they sense by imbalance between live and neutral. Some RCDs *do* have a "functional earth" connection wired in cream rather than green/yellow, to allow operation in the event of a lost neutral.

No, as mentioned - the RCD just senses magnetically a current imbalance between a pair of wires, it doesn't care what voltage those wires are at. If it's a 30mA RCD then it just needs enough current to flow to create a

Reasons to have a low earth impedance:

1. To allow enough current to flow to trip the MCB / blow the main cutout fuse were the MCB to fail closed

1. To avoid the earth floating to live before the RCD trips. If the earth is high impedance then a small fault current means a substantial voltage could appear on the earthing terminals. The RCD should trip within a specified time, but until that point all your exposed metalwork would be near live potential.

No.2 is obviously bad, but not as bad as if there was no RCD. If your RCD is faulty then you end up in pre-RCD situation no.1

(if it's an RCBO and both parts are faulty, I suppose you fall back on the cutout)

So a high earth impedance would be a problem because you start losing the layers of protection.

But note that TT systems are fine up to 200 ohms earth impedance. That's

Theo

They are fine up to 1667 ohms with a 30mA RCD. The only reason 200 ohms is quoted is that above that the earth connection may not be stable, but, in some places in the UK, above 200 is all they can get, some areas of Cornwall are like that, very rocky ground, where it is near impossible to fit a deep earth rod, so buried earth mats may have to be fitted to get an acceptable earth reading.