Why do some CU RCDs have earth tails and others don't?

[Seb101]

Just wondered.

 

[JohnW2]

Just wondered.

Do you mean RCBOs (most, but not all, have 'flying earth leads')? There might also be some RCDs which do have such tails, but I've personally never seen one.

Under normal circumstances, an RCBO does not need an earth connection in order to work. They almost invariably contain electronics circuitry, and that is normally powered from the supply connected to the device.

However, in the (very rare) event of a fault in the incoming neutral to the installation, the electronics of the RCBO would not get power, and so the device would not work as intended. Some/most manufacturers therefore provide a 'functional earth' lead which enables the electronics to be powered even if the supply neutral fails. Many people (and some manufacturers) believe that it is so incredibly unlikely that an earth leakage fault (which should trip the RCBO) would happen at the same time as a (very rare) supply neutral fault that this eventuality does not really need to be catered for (i.e. don't believe that the 'functional earth' is really necessary).

As above, the same might apply to some RCDs, even though I have personally not seen one with a 'functional earth' lead.

Kind Regards, John

 

[Seb101]

Well funny you should say, because the pretty much the only RCDs I can find with a functional earth is the brand for my CU:

Hence why I asked the question.

 

[JohnW2]

Well funny you should say, because the pretty much the only RCDs I can find with a functional earth is the brand for my CU: .... Hence why I asked the question.

Interesting - as I said, for what it's worth, I've never seen one. However, as far as I can see the argument for having a functional earth is exactly the same for RCDs as for RCBOs, but it was only when RCBOs appeared that we (at least, I) started seeing them.

Kind Regards, John

 

[Iggifer]

AFAIK it's just the old MEM RCDs that had FE connections. Which is ironic because Eaton are now one of the only (reputable!) manufactures that don't use a FE on their RCBOs.

Eaton's reasoning is

Another often-made point is that, without a separate earth connection, an RCBO cannot detect an earth fault if the neutral becomes disconnected. This is true, but in reality it is not an issue worthy of consideration.

Losing the neutral connection is a rare event in itself, especially in the TN-C and TN-C-S systems that are almost invariably used today. The probability of losing the neutral and having an earth fault at the same time, a double fault condition, is vanishingly small – certainly so small that there is no reason to take it into account when specifying protection devices.

Edit: Coincidental unrelated bit of bit of research has just led me to discover the new Hager reduced height RCBOs also do not have an FE connection

 

[securespark]

I thought it was there for DC current-sensing, but I may be wrong.

 

[JohnW2]

Eaton's reasoning is ....

Thanks. That sounds remarkably similar to what I wrote above.

As I hinted above, the thing I find most difficult to understand is why most manufacturers suddenly started including 'functional earths' when they started making RCBOs (and, even then, only for RCBOs), whereas the (incredibly weak) argument for having them had been there since they first started making RCDs.

Kind Regards, John

 

[Seb101]

I don’t really understand the logic of putting them on RCBOs and not on RCDs. Surely the probability/protection argument is the same regardless of form-factor.

But having worked in a couple of product design departments in my time I suspect the “well theirs has it so ours should otherwise people will think ours is not as good” logic plays a large part.

 

[JohnW2]

I don’t really understand the logic of putting them on RCBOs and not on RCDs. Surely the probability/protection argument is the same ....

Indeed. That's what I've said, more than once. I can't see that there can be any true logic in it.

The earliest RCDs (and maybe even some modern ones, although I haven't seen any myself) contained only passive components. Current from the imbalance sense coil directly operated the trip solenoid. Since there were no electronics which needed to be powered, there was no conceivable reason for needing a functional earth. However, when the electronics started appearing in them, most/all continued not to have a functional earth, maybe because no-one even thought of the potential 'need'. When something 'new' (RCBOs) came along, maybe they were 'designed from scratch' by someone who did perceive a 'need'? Hardly logical, but perhaps a possible 'explanation'.

However, as discussed, the 'reason' for having a functional earth relates to the situation in which there is a combination of faults, the likelihood of which occurring simultaneous is 'vanishingly small', so there isn't much point in the FE. I imagine that it is far more likely that an RCD or RCBO will become faulty (unbeknown to anyone) and then fail to trip when a fault arises which should cause it to trip - but we are not told to 'double up' on the devices for that reason.

Kind Regards, John