Does an RCD need an earth?

[Stuthehandyman]

I know how an RCD works (at least I thought I did); a toroid with 3 windings, one connected to the live, one to the neutral and the other, which detects an imbalance between the first two, connected to the tripping circuit. My question is, if the RCD works purely on the principle of imbalance between the live and the neutral conductors, what is the point of having an earth connection in the house?
I live in a house supplied via overhead cables and therefore we have a TT earthing system. I suspect the resistance of the earth rod is probably too high (must test it one of these days) but I've never felt any urgency to do so as I have a 100mA RCD on the main incomer and 3 separate 30mA RCDs for the 3 final ring circuits so I felt reasonably well protected. Please could someone explain the necessity for an earth if you've got RCDs fitted and they work like I think they do?

Thanks in anticipation.

 

[Big_Spark]

The most basic of RCD's do indeed operate in this manner, however there are several types of RCD and two of them certainly need a connection to Earth in order to operate correctly. They will operate, just not in the manner designed by the manufacturer.

Some have small white flyleads which connect to Earth, these are either DC sensing or AC sensing. The DC type simply monitor the earth for DC leakage on the earth, if this rises to a particular level they will disconnect the supply. The AC sensing type uses another coil on the toroid which generates a seperate magnetic field, if this rises above a given thresh-hold the RCD will trip.

RCD protection, regardless of type, is not a substitute ofr a properly installed and maintained earthing system, one does not replace the other, they simply compliment each other to improve safety.

 

[Stuthehandyman]

Thanks Big Spark, but I'm still confused. Is the white flying lead type the DC sensing type? Is this what I call the "old" type that relies on a good connection to earth and measures the current flowing to earth? (This is the type that was originally fitted in my house, 80mA I think in a Bakalite enclosure!) But now I have the more modern type (AC sensing?) that do not require an earth connection and work on the "imbalance" principle. Therefore, is the earthing system only there as a back-up system in case the RCD(s) go faulty?

 

[Softus]

Please could someone explain the necessity for an earth if you've got RCDs fitted and they work like I think they do?

How can you guarantee that they will work like you think? In other words, what happens if you have a faulty RCD and no earth?

 

[Spark123]

A modern RCD may/may not have a cream wire which connects to earth known as a functional earth. Generally the ones with a functional earth use electronics to trip the contacts as opposed to just using coils.
The one you mention may be the voltage operated ELCB. These were removed from the regulations in the early 1980s as there are parallel paths (due to main eq bonding) which cause voltage operated ELCB to fail to operate.
These are not to be confused with current operated ELCB which provides the same type of protection as an RCD.
The earthing system is not there to back up an RCD, a (30mA or less)RCD is used to provide additional protection against electric shock and the earthing system is there to keep all exposed/extraneous surfaces at or about the same potential in the event of a fault and to clear the fault quickly by operating the protective device. Where the earth fault loop impedance is too high for a fuse/mcb to disconnect the supply quickly in the event of a fault, an RCD is used (such as in a TT installation.)
As Big_Spark put it very well, they compliment each other. It is better the earthing system disconnect a fault rather than your body becoming part of the circuit to trip an RCD.
Also, an RCD greater than 30mA does not provide personal protection against fatal electric shock.

 

[JohnD]

I've seen a Memera RCD with the thin white earth lead, if this is detecting a current, is it between the Neutral and the earth, or what? I'm familiar with the idea that if some of the phase runs through me to earth, instead of back down the Neutral, there will be a current imbalance between the phase and neutral conductors; so what is the white lead detecting?