RCD TD & Single pole RCBO

[ericmark]

You can get single modular width RBCO's which switch neutral although the current in the neutral conductor is not monitored.

But even with double pole discrimination is rather hit and miss. So likely still would have tripped all but would allowed you to reset.

 

[Risteard]

If all of your final circuits have RCD or RCBO protection then, AIUI, you will not usually** need to up-front Type S RCD, so you can rid yourself of that problem.

I would ALWAYS advise the use of an S-type RCCB protecting the entire TT installation in order to mitigate against failure of a single device (which is around 7%).

But there is nothing to prevent the use of RCBOs which switch the neutral - it does not have to be an RCCB and overcurrent protective device.

 

[Risteard]

You can get single modular width RBCO's which switch neutral although the current in the neutral conductor is not monitored.

But even with double pole discrimination is rather hit and miss. So likely still would have tripped all but would allowed you to reset.

It's not hit-and-miss with a Selective (S-type, i.e. time-delayed) RCD. That is why S-types must be used upstream.

 

[JohnW2]

If all of your final circuits have RCD or RCBO protection then, AIUI, you will not usually** need to up-front Type S RCD, so you can rid yourself of that problem.

I would ALWAYS advise the use of an S-type RCCB protecting the entire TT installation in order to mitigate against failure of a single device (which is around 7%).

In general, I'm a great believer in redundancy ('belt and braces'), so cannot knock the concept of what you are saying. However, I suspect you say it only because of one, small and far-from-perfect study which suggested an 7% in-service failure rate of RCDs (hence possibly the 'RCD part' of an RCBO). However, since they effectively cannot be tested, we haven't got a clue as to what the in-service failure rate of MCBs might be, but I suspect you do not similarly advocate 'doubling up' on over-current protection when fault protection is reliant upon MCBs (i.e. in TN installations) "in order to mitigate against the failure of a single device", do you?

But there is nothing to prevent the use of RCBOs which switch the neutral - it does not have to be an RCCB and overcurrent protective device.

I obviously agree with the first part of that (although they're not that easy to find, and many/most are 2-modules wide), but I don't really understand the second part - what is an RCBO if not "an RCCB and overcurrent protective device"?

Kind Regards, John

 

[JohnW2]

You can get single modular width RBCO's which switch neutral although the current in the neutral conductor is not monitored.

I don't understand what significance you are attaching to that (lack of monitoring neutral current). If there is an excessive current in the N, it is will also be flowing through the L, in which case the L overcurrent part will detect it. The device obviously does also 'monitor' the L-N current difference.

But even with double pole discrimination is rather hit and miss. So likely still would have tripped all but would allowed you to reset.

There is not really any 'hit and miss', and a TD RCD will almost inevitably discriminate against a non-TD one, regardless of their respective IΔns - the minimum operating time for TD one is considerably greater than the maximum operating time for a non-TD one.

Kind Regards, John

 

[Risteard]

what is an RCBO if not "an RCCB and overcurrent protective device"?

Kind Regards, John

An RCCB is a standalone device, so either an RCBO or an RCCB plus an overcurrent protective device is required. An RCBO has INTEGRAL overcurrent protection.

 

[JohnW2]

what is an RCBO if not "an RCCB and overcurrent protective device"?

An RCCB is a standalone device, so either an RCBO or an RCCB plus an overcurrent protective device is required. An RCBO has INTEGRAL overcurrent protection.

Needless to say, I know all that - but I still don't understand what you meant by:

But there is nothing to prevent the use of RCBOs which switch the neutral - it does not have to be an RCCB and overcurrent protective device.

Did you perhaps mean to type "RCDs" (or "RCCBs"), rather than "RCBOs"?

Kind Regards. John

 

[Risteard]

Did you perhaps mean to type "RCDs" (or "RCCBs"), rather than "RCBOs"?

RCD is a generic term. An RCBO is an RCD, as is an SRCD, as is a PRCD, as is an RCCB etc.

 

[JohnW2]

RCD is a generic term. An RCBO is an RCD, as is an SRCD, as is a PRCD, as is an RCCB etc.

Mainly true, although I would personally have said that "RCCB" was more-or-less as generic as "RCD". Be that as it may, maybe I'm just being dim, but I still don't understand what you meant by:

But there is nothing to prevent the use of RCBOs which switch the neutral - it does not have to be an RCCB and overcurrent protective device.

Can you (or, failing that, someone else) help me understand?

Kind Regards, John

 

[AndyPRK]

Maybe the word seperate is missing.

 

[JohnW2]

Maybe the word seperate is missing.

Where in the sentence would you add that word in order to make it make sense?

Kind Regards, John

 

[stillp]

It's the overload part of an RCBO (i.e. the 'MCB' bit'), not the 'earth leakage' part, which can operate in response to the very high current which can flow for a very brief period (fraction of a second) whilst some types of lamps/bulbs are dying.

Not 100% correct. RCDs can trip on rapidly-rising fault currents with no fault to earth.

 

[JohnW2]

It's the overload part of an RCBO (i.e. the 'MCB' bit'), not the 'earth leakage' part, which can operate in response to the very high current which can flow for a very brief period (fraction of a second) whilst some types of lamps/bulbs are dying.

Not 100% correct. RCDs can trip on rapidly-rising fault currents with no fault to earth.

I think we may be getting a bit 'esoteric' here - and I suspect of little practical relevance, at least in domestic installations. In the days of incandescent lighting, like most other people I not infrequently experienced an MCB tripping in response to the death of a lamp/bulb, but have never experienced (or heard of) an RCD operating in such circumstances. Have you?

Kind Regards, John

 

[stillp]

Yes John. There have been several threads on here about this phenomenon.

 

[JohnW2]

Yes John. There have been several threads on here about this phenomenon.

Interesting. I certainly haven't experienced it and, frankly, don't recall any such discussions here (blame the effects of age on memory!). There have been some fascinating discussions, without any firm conclusions about mechanism, about RCDs operating in response to switch-off transients (including 'loose connections'), but I don't recall having discussed this particular issue.

I presume that a 'perfect' RCD would not do this, certainly not with a resistive load, and that we are therefore talking about 'unintended imperfections' in the RCD functionality. Is that correct?

Kind Regards, John