Do only electronic RCD's/RCBO's have functional earths?

[eveares]

Just wondering if the only reason some RCD's/RCBO's have functional earths is to allow the electronics inside the RCD/RCBO to remain active in the event the incoming neutral is lost or impaired casing a low/absent voltage between L & N.

Any other reasons for functional earths? Are RCBO's/RCD's which do not require a functional earth always maniacal/non-electrical based devices?

Regards: Elliott.

 

[JohnW2]

Just wondering if the only reason some RCD's/RCBO's have functional earths is to allow the electronics inside the RCD/RCBO to remain active in the event the incoming neutral is lost or impaired casing a low/absent voltage between L & N.

Yes, AFAIAA.

Any other reasons for functional earths?

Not that I am personally aware of, but there may be things I don't know.

Are RCBO's/RCD's which do not require a functional earth always maniacal/non-electrical based devices?

As often discussed, I'm not aware of any 'consumer' RCDs on sale having been non-electronic for a long time, and I'm not sure that non-electronic 'consumer' RCBOs have ever existed. However, I think that there are some RCBOs without functional earth leads, but that could well be because some manufacturers feel that it is "vanshingly improbable" that an L/N imbalance (e.g. due to an L-E fault), requiring the device to operate, will ever happen at the same time as there is a lost supply neutral (itself extremely rare).

The odd thing is that there is very rarely, if ever, provision of a functional earth for an (electronic) RCD, even though the argument for 'needing' it is, AFAICS, exactly the same as with an RCBO.

Kind Regards, John

 

[bernardgreen]

provision of a functional earth for an (electronic) RCD

Maybe the humble domestic RCD is still not electronic.

It may be that the RCD is shared over several MCBs and loss of Neutral would be apparent from loss of power to those circuits where as the loss of Neutral to an RCBO would affect only that one circuit and could go un-noticed by the people in the house.

 

[eveares]

The odd thing is that there is very rarely, if ever, provision of a functional earth for an (electronic) RCD, even though the argument for 'needing' it is, AFAICS, exactly the same as with an RCBO.

Quite so, maybe as bern said, a RCD losing the incoming neutral would be more noticeable as more circuits would be affected.

 

[Iggifer]

This is why Eaton don't use a FE on their RCBOs

http://www.eaton.com/Eaton/OurCompany/NewsEvents/NewsReleases/PCT_255273

 

[bernardgreen]

This is why Eaton don't use a FE on their RCBOs

http://www.eaton.com/Eaton/OurCompany/NewsEvents/NewsReleases/PCT_255273

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

I am never happy when probabilities are dismissed simply because they are vanishly small.

 

[JohnD]

This is why Eaton don't use a FE on their RCBOs

They used to, when did they stop?

 

[EFLImpudence]

I am never happy when probabilities are dismissed simply because they are vanishly small.

Surely that which is vanishingly small cannot be a probability.

 

[JohnD]

It can, but you need to multiply the very small probability by the severity of the potential impact to judge how important it is.

Let me think.

You have lost the N and the E at the same time, for example due to a digger mishap which has entirely severed the E and N conductors or the PEN while leaving the L live, and has happened so close to your home that there is no earth connection beyond the damage. So your installation appears to be dead, but is not. Suppose you touch a L conductor. You might or might not be leaning against a metal water pipe at the time. The water pipe is/is not main/supplementary bonded. The circuit is protected by an RCBO. What will happen? How severe is it?

Multiply that by the probability that it will happen to your installation.

Compare that to the risk that you will be injured in a sock-related accident (10,773 hospital admissions p.a. in the latest UK statistics I found).

Which should you be more worried about?

How much of your time and money do you spend on precautions against sock accidents?

 

[JohnW2]

Maybe the humble domestic RCD is still not electronic.

All I can say, is that I have dissected a good few RCDs, of a variety of brands, over the last 20 years or so, many of which were at least 10 years old at the time of examination, and I have yet to come across one which was not electronic. In fact, the only non-electronic one I can recall seeing originated in the 70s.

It may be that the RCD is shared over several MCBs and loss of Neutral would be apparent from loss of power to those circuits where as the loss of Neutral to an RCBO would affect only that one circuit and could go un-noticed by the people in the house.

I suppose that's conceivable, but I think it more likely that no-one had thought of it (or consdiered it necessary) for RCDs in the pre-RCBO days, so they just carried on doing what they have already done.

Kind Regards, John

 

[JohnW2]

I am never happy when probabilities are dismissed simply because they are vanishly small.

You must be unhappy most of the time, then! Perhaps you don't 'dismiss' extremely small probabilities, but you clearly must 'accept' them. If you didn't, you would never step outside your home, never cross a road, never drive a car or travel in any sort of mechanised transport, never go up/down stairs, never use any tools etc. etc. etc.

Kind Regards, John

 

[EFLImpudence]

It can, but you need to multiply the very small probability by the severity of the potential impact to judge how important it is.

Let me think.

You have lost the N and the E at the same time, for example due to a digger mishap which has entirely severed the E and N conductors or the PEN while leaving the L live, and has happened so close to your home that there is no earth connection beyond the damage. So your installation appears to be dead, but is not. Suppose you touch a L conductor. You might or might not be leaning against a metal water pipe at the time. The water pipe is/is not main/supplementary bonded. The circuit is protected by an RCBO. What will happen? How severe is it?

Multiply that by the probability that it will happen to your installation.

Compare that to the risk that you will be injured in a sock-related accident (10,773 hospital admissions p.a. in the latest UK statistics I found).

Which should you be more worried about?

How much of your time and money do you spend on precautions against sock accidents?

It, therefore, is only a possibility.

 

[JohnW2]

Surely that which is vanishingly small cannot be a probability.

A probability can, in theory, even be zero - although that's pretty rare (and most examples are theoretical and 'silly'). However, you know what is meant - "vanishingly small" has become a fashionable expression to refer to probabilities that are extremely small - usually so small that they are "not worth thinking about".

Kind Regards, John

 

[EFLImpudence]

As I said, only a possibility; not a probability.

 

[JohnW2]

It, therefore, is only a possibility.

You appear to be talking 'English' now - and I suspect that, in that sense, you feel that a 'probability' is more likely than a 'possibility'.

However, we are (well, at least, I am) talking about the statistical concept of "probability", which is still called probability (never 'possibility') whether it has a value of 0.00000001 or 0.999999999.

Kind Regards, John