"17th Edition CUs" & "Amd3 CUs"

[John D v2.0]

overcurrent is what an MCB detects. i.e. either an overload or fault current. Given it's probably not TT an earth fault would be sufficient to trip an MCB.
I didnt mean to imply anything else, or that they were necessarily tripped by different events. Just a concrete example of what EFLI was mentioning.

 

[JohnW2]

overcurrent is what an MCB detects. i.e. either an overload or fault current. Given it's probably not TT an earth fault would be sufficient to trip an MCB. I didnt mean to imply anything else, or that they were necessarily tripped by different events.

Fair enough.

Just a concrete example of what EFLI was mentioning.

I'm not so sure about that. In the scenario you mention, even one of those fancy ultra-expensive RCBOs which indicates the type of trip would presumably indicate that both had happened. I presumed that EFLI was talking about the much more common situation in which an RCBO has tripped because of either residual current or overcurrent (but not both) - and that, in the absence of an 'indicator', one does not know what sort of event caused the trip.

Kind Regards, John

 

[ban-all-sheds]

Plus - you cannot tell if a trip was caused by overload or fault.

So?

 

[EFLImpudence]

Just stating a disadvantage.

 

[ban-all-sheds]

Why is it a disadvantage?

 

[John D v2.0]

I presumed that EFLI was talking about the much more common situation in which an RCBO has tripped because of either residual current or overcurrent (but not both) - and that, in the absence of an 'indicator', one does not know what sort of event caused the trip.

I presumed that too, not sure why the confusion.
That flat had separate rcd and mcb units so it could be confirmed both tripped during the early stages of the fire. With a standard rcbo, they wouldn't have had that level of information available to the investigation.

They could have fitted a fancy rcbo with indicators but that's not likely in social housing, and even less likely in private housing unless owned by a techie.

 

[davelx]

Probably now in all installations and circuits ...

... in the April 2018 draft, the first clause of that draft reg ["Except in certain special installations or locations (Part 7),"] has disappeared - with that change, the reg then (and probably 'finally') simply reads "For protection against electric shock, there is no requirement to disconnect / switch the neutral in TT or TN systems."

... which appears to be a blanket statement which encompasses all circuits in all installations.

Kind Regards, John

Indeed. In fact I wonder why 531.3.1 still says: "Except where Regulation 531.3.1.201 applies, an RCD shall disconnect all live conductors of the circuit protected."

Under what circumstances would 531.3.1.201 not apply, in the context of the subject of reg 531?

 

[ban-all-sheds]

I presumed that too, not sure why the confusion.

I am confused over what the real-world practical disadvantages are.

That flat had separate rcd and mcb units so it could be confirmed both tripped during the early stages of the fire. With a standard rcbo, they wouldn't have had that level of information available to the investigation.

What was the real-world practical benefit of having that information?

 

[JohnW2]

Indeed. In fact I wonder why 531.3.1 still says: "Except where Regulation 531.3.1.201 applies, an RCD shall disconnect all live conductors of the circuit protected." .... Under what circumstances would 531.3.1.201 not apply, in the context of the subject of reg 531?

Interesting question.

It could just be oversight - i.e. they forgot to remove that bit from 531.3.1 when they remove the first clause of the previous draft of 531.3.1.201.

However, I note that 531.3.1.201 start "For protection against electric shock ....". Hence, strictly speaking, it does not apply when an RCD is being used for reasons other than 'protection against electric shock' - and that is at least partially true when, for an example, an RCD is used to provide fault protection in general - either in TT installations or when ADS via OPDs is 'not practical' (however one interprets that).

Kind Regards, John

 

[JohnW2]

I presumed that too, not sure why the confusion. That flat had separate rcd and mcb units so it could be confirmed both tripped during the early stages of the fire. With a standard rcbo, they wouldn't have had that level of information available to the investigation.

Fair enough. We are obviously all agreed, and maybe I read too much into what you initially wrote.

Kind Regards, John

 

[EFLImpudence]

Why is it a disadvantage?

Well, let me see.

In view of the numerous posts asking "My RCD (RCCB) occasionally trips. What could it be?" and the numerous replies stating the various causes and that it might be very time consuming to find this intermittent fault.
Whereas if only an MCB has tripped then presumably these various causes and time consumption can be discounted and only faults which cause MCBs to trip need be investigated.

 

[JohnW2]

The disadvantage of not knowing what 'type' of trip has occurred with a standard RCBO is that it can frustrate at least the early stages of fault finding.

A while ago, a neighbour of mine suffered a number of RCBO trips on an outdoor circuit, following a period of heavy and persistent rain. We spent a fair bit of time hunting high and low for water ingress, to no avail. The circuit IRd satisfactorily and the (residual current part of the) RCBO tested OK. Nevertheless, I tried changing the RCBO, but the trips continued. As a last experiment, I tried changing the RCBO to an MCB, and that then tripped. It transpired that a pond pump had an intermittent L-N fault.

Had we know that the trips were due to overcurrent, we would probably have wasted a lot less time with the fault-finding.
Edit: typed a bit too slow again!

 

[ban-all-sheds]

In view of the numerous posts asking "My RCD (RCCB) occasionally trips. What could it be?" and the numerous replies stating the various causes and that it might be very time consuming to find this intermittent fault.

And do those causes ever include cumulative leakage from a number of electronic devices? If so then multiple RCBOs vs one RCD for several circuits could well eliminate that cause, which seems like an advantage to me.

But be that as it may, can you explain why it is a disadvantage to lose one circuit rather than several, and how the process of finding the cause is made more time consuming or difficult by having one circuit rather than several to investigate?

Whereas if only an MCB has tripped then presumably these various causes and time consumption can be discounted and only faults which cause MCBs to trip need be investigated.

If an appliance is tripping an RCBO, how is identifying it made harder by not knowing whether it's overcurrent or an earth fault/leakage?

Having found a faulty appliance, how does not knowing what type of fault it has make it harder to decide what to do with, or about, the appliance?

If a device is tripping, how does having fewer places to look for a cause make it harder to deal with than having more cables and more appliances?


In your experience, indeed in the experience of everyone here, how common are nuisance trips of MCBs, unaccompanied by any obvious clues such as noises/flashes/smoke/smells from an appliance or an accessory, compared to nuisance trips of residual current devices?


You seem to be postulating a scenario where a device trips, and there are no indications such as flashes and bangs etc to help identify what/where the problem is, so it would be interesting to know how likely it is, in practice, for it not to be an earth fault.

Because if it is an earth fault then I am confused about why it is a disadvantage to have fewer places to start looking and to lose fewer circuits to the fault.

 

[EFLImpudence]

I have told you (although you already knew) where an RCBO would be a disadvantage.

That there may also be advantages is irrelevant unless you can foresee the types of faults and quantify them.

 

[aptsys]

Why is it a disadvantage?

Nuisance tripping will almost always make the lack of discrimination a disadvantage, especially in the case of intermittent faults.

Fault finding always relies upon a series of steps to eliminate factors. By limiting visibility of the result of a fault you're reducing the chances of finding the fault in a timely manner.