Ring Final question - not DIY

[JohnW2]

The upstream device only helps us if it trips rapidly.

Sure, but you seem to be concerned about 'breaking capacities' - and if the fault current is anywhere near that, it will trip very rapidly. Even if (as you have in places) one talks about a downstream device of only 1 kA breaking capacity, a fault current approaching 1 kA would trip your hypothesised C50 MCB (let alone a B50) very rapidly.

What we want to avoid is leaving a "gap" where the fault current is high enough to overwhelm the downstream protective device, but not high enough to rapidly trip the upstream one..

See above.

In any event, there is surely only an issue to discuss if the plug fuse has a lower breaking capacity than some upstream protective device (which, is rarely, if ever, going to be the case in domestic installations when the fuse is {as it nearly always will be} a BS1362 one). If the breaking capacity of the plug fuse is at least as great as the PFC, that alone presumably satisfies protection requirements downstream of the socket, the upstream device only being relevant to the protection of the cable between CU and socket?

It's weird that BS1363 apparently specified a high breaking capacity fuse for plugs, but allowed a much lower breaking capacity one for adapters, and afaict says nothing at all about protection inside wall-warts leaving those up to IEC standards (which I suspect are based on the assumption of some reasonable upstream fuse, given that most of the world doesn't have fused plugs). ... But I guess that is how the standard's sausage is made.

In terms of BS1363, I'm only familiar with BS1363-1 and BS1363-2, and BS1363-1 only refers to use of BS1362 fuses. I would therefore have to do a bit of reading to determine what it has to say about things other than BS1363-1 plugs (with BS1362 fuses) plugged into a BS1363-2 socket.

 

[plugwash]

Sure, but you seem to be concerned about 'breaking capacities' - and if the fault current is anywhere near that, it will trip very rapidly. Even if (as you have in places) one talks about a downstream device of only 1 kA breaking capacity, a fault current approaching 1 kA would trip your hypothesised C50 MCB (let alone a B50) very rapidly.

Yup. A C50 has a fast-trip of 500A max giving a substantial overlap with the 1000A breaking capacity of a BS646, A D32 or B63 would give a smaller overlap. A D50 OTOH would be right on the margin of leaving a gap, and a D63 would leave a substantial gap.

 

[JohnW2]

Yup. A C50 has a fast-trip of 500A max giving a substantial overlap with the 1000A breaking capacity of a BS646

Quite so.

A D32 or B63 would give a smaller overlap. A D50 OTOH would be right on the margin of leaving a gap, and a D63 would leave a substantial gap.

Agreed - but either a D32 or B63 would be incredibly improbable in a domestic installation, wouldn't it?

... and don't forget that even 50A OPDs only got into this discussion because you noted that "13A sockets on 50A cooker circuits are OK". Such circuits are presumably pretty rare, given that a 32A circuits adequate for any credible domestic cooker).

...and, of course, as I've said, a PFC (at, or downstream of, a socket) of even 1 kA or more would (at least in my experience) probably be rare in a domestic property - and the only reason for concern about something with a BS646 fuse would exist if the PFC were above 1 kA. However, as you've said, goodness only knows what (explicit) 'protection' exists in wall-warts, so I can't comment about that!

 

[plugwash]

Agreed - but either a D32 or B63 would be incredibly improbable in a domestic installation, wouldn't it?

This thread was not about a domestic installation.

but yes, having done the research and sums it seems almost any MCB you are likely to find in a domestic environment will provide adequate upstream protection for a BS 646 fuse in a shaver adapter. Wall-warts are still a mystery though.

... and don't forget that even 50A OPDs only got into this discussion

Sunray was talking about replacing a 45A fuse with a C45 breaker, I rounded up to 50A since IIRC 40A and 50A are more common ratings than 45A nowadays.

given that a 32A circuits adequate for any credible domestic cooker

I think you overestimate electricians faith in the diversity guidelines for cookers and underestimate their desire to avoid callbacks. OTOH I've just done some searching and it seems most cooker control units with socket are only rated at 45A, though scholmore's ones are rated at 50A.

 

[JohnW2]

This thread was not about a domestic installation.

That is true.

... but yes, having done the research and sums it seems almost any MCB you are likely to find in a domestic environment will provide adequate upstream protection for a BS 646 fuse in a shaver adapter. Wall-warts are still a mystery though.

Yes, we are agreed about that.

Sunray was talking about replacing a 45A fuse with a C45 breaker, I rounded up to 50A since IIRC 40A and 50A are more common ratings than 45A nowadays.

He was. In fact, this recent part of the discussion (rather than the original 5-year old bits) all really started when you wrote *** it's hard to see how a ring with 40A cable and a 45A (or even 50A ....) breaker would not be ok." - and I immediately agreed with you about that from the very start. It was your caveats about 'breaking capacities' that led to most of the subsequent discussion.

..... I think you overestimate electricians faith in the diversity guidelines for cookers and underestimate their desire to avoid callbacks. OTOH I've just done some searching and it seems most cooker control units with socket are only rated at 45A, though scholmore's ones are rated at 50A.

Maybe - although in my (admittedly limited) experience, new cooker circuits seem to very commonly be 32A these days. In any event, even a B32 will allow about 46A to flow for an hour (or appreciably higher currents for shorter periods) without tripping, so I doubt it's very likely that there will be problems with a B32, even if one does not have a lot of faith in the standard diversity guidelines.