"17th Edition CUs" & "Amd3 CUs"

[JohnW2]

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.

Indeed - as illustrated by the example I recently described.

However, there obviously is another side to the story, as mentioned by BAS - in that an RCBO will nearly always narrow down the search to one final circuit. However, that is an argument for having separate MCBs and RCDs for each circuit (or an RCBO with a 'type of trip' indicator).

I would not have thought that it would be particularly difficult or costly to build an 'indicator' into RCBOs, but perhaps I am wrong. It would really only have to detect and identify one sort of trip (residual current probably easier, since that already involves electronics) - since the absence of that indication (after a trip) would imply the other type of trip. Maybe it will come!

Kind Regards, John

 

[ban-all-sheds]

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

You've made that claim, but so far have been unable to explain the practical disadvantages. I replied to you "telling me", but you've not taken any notice.

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

Not many people refuse to consider the advantages of something and weigh them against disadvantages. Quite what they think whenever they have heard the phrase "the advantages outweigh the disadvantages" I don't know.

I wonder if it's along the lines of this:

"Fools. Everybody knows that this is the only way -

 

[ban-all-sheds]

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

In your experience, what percentage of nuisance trips is of MCBs tripping, and what is that of residual current devices? This is very important.

Fault finding always relies upon a series of steps to eliminate factors. By reducing the number of places to carry out those steps you're increasing the time it will take to carry them out.

Really?

 

[EFLImpudence]

Out of interest, would a CU with RCBOs on every circuit be a better solution than the current dual option.
From a laymans point of view, nuisance tripping would, I guess, then be restricted to 1 circuit.

Yes, but buying lots of them is expensive - so most people don't have them.
Plus - you cannot tell if a trip was caused by overload or fault.

So?

Just stating A disadvantage.

Why is it A disadvantage?

An explanation of THE disadvantage.

 

[John D v2.0]

Interestingly, to extend the debate if i dare, at present there is no indication whether an mcb tripped due to a fault or an overload.
If we still had fuses for overload and also included a separate shared magnetic trip unit in newer cus, we might be having the same debate about that.

 

[ban-all-sheds]

Are you able to answer these questions:

What is the percentage of MCB trips for unknown reasons vs the percentage of residual current device trips for unknown reasons?

If investigating an RCD trip, what do you initially do which is different to what you would do when initially investigating an RCBO trip?

 

[JohnW2]

Interestingly, to extend the debate if i dare, at present there is no indication whether an mcb tripped due to a fault or an overload.

True - but I would suggest that it is very unusual for significant 'fault finding' to be required in the (pretty rare) event of OPDs operating as a result of overload - since the reason is usually fairly obvious from the circumstantial evidence.

Kind Regards, John

 

[JohnW2]

To give one example - if, as in the example I gave, one knew that a protective device had operated as a result of overcurrent, I would not waste my time hunting for water ingress.

 

[ban-all-sheds]

True - but I would suggest that it is very unusual for significant 'fault finding' to be required in the (pretty rare) event of OPDs operating as a result of overload - since the reason is usually fairly obvious from the circumstantial evidence.

I've asked about that.

It appears that we must not concern ourselves with how unusual it would be, but must instead only consider scenarios where an RCBO has operated and there is no circumstantial evidence to indicate why.

To give one example - if, as in the example I gave, one knew that a protective device had operated as a result of overcurrent, I would not waste my time hunting for water ingress.

With an RCD trip, would hunting for water ingress be the first thing that you did?

How much time would you spend on it before looking for other causes?

 

[EFLImpudence]

Are you able to answer these questions:

To your satisfaction; probably not.

What is the percentage of MCB trips for unknown reasons vs the percentage of residual current device trips for unknown reasons?

I don't know. That is why I said "unless you are able to quantify...".

If investigating an RCD trip, what do you initially do which is different to what you would do when initially investigating an RCBO trip?

Discount overload and short-circuit causes.


Are you saying that, when confronted with an RCBO trip, it would not be an advantage to rule out one group of possible causes?

 

[davelx]

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

But... 531 is all about "devices for protection against electric shock" ☺

 

[JohnW2]

Just to be clear, I'm not arguing against RCBOs (the main 'con' of which is the cost) - rather, I am agreeing that it would be advantageous if they provided an indication of the type of trip which had occurred.

 

[JohnW2]

But... 531 is all about "devices for protection against electric shock" ☺

Not really - 531 is about "Devices for fault protection by automatic disconnection of supply" and, indeed, the whole of Chapter 53 is about "Protection, Isolation, Switching, Control and Monitoring". It is Chapter 41 that is about protection from electric shock.

As I said, particularly in the case of TT installations, RCDs can be required for reasons (e.g. fire prevention) other than (in addition to) protection against electric shock - so I think it could be argued that 531.3.1.201 does not apply in such circumstances.

The distribution circuits in my (TT) installation are provided with fault protection by RCDs. I wouldn't say that protection against electric shock was the 'primary' (certainly not the only) reason for this, and I therefore suspect that a SP RCD/RCBO would, strictly speaking, not conform with a combination of 531.1 and 531.3.1.201.

Kind Regards, John

 

[ban-all-sheds]

To your satisfaction; probably not.


I don't know. That is why I said "unless you are able to quantify...".

That seems like a firm claim that you have no idea how common MCB trips for unknown reasons are compared to RCD trips, and yet it is only the situation where what would have been an MCB trip for unknown reasons becomes an RCBO trip for non RCD reasons where any disadvantage arises.

In short, you have no idea how likely it is that this disadvantage which exercises you is purely theoretical, never actually arising in practice.

Discount overload and short-circuit causes.

Firstly, why do you discount L-E and N-E short circuits?

Secondly, that's more a case of what you don't do (i.e. think about possible causes), so I'll rephrase my question:

What positive testing/inspecting/other-physical actions do you initially do which is different to what you would do when initially investigating an RCBO trip?

Are you saying that, when confronted with an RCBO trip, it would not be an advantage to rule out one group of possible causes?

I have no information on how likely it would be that there would be an overcurrent cause not flagged by phenomena such as flashes/bangs/smells of burning or the powering on of a specific item to judge whether it would create a real practical disadvantage worth considering.

And neither do you - all we do know is that

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

might be a "so what, that is of no consequence in practice".

 

[ban-all-sheds]

Just to be clear, I'm not arguing against RCBOs (the main 'con' of which is the cost) - rather, I am agreeing that it would be advantageous if they provided an indication of the type of trip which had occurred.

But how often in practice would it be advantageous?