Cable Colours

[bernardgreen]

How does, say, a portable radio, start to draw 20A?

Melt down of the mains transformer maybe ?

Yes, but unless you were barking mad you would not put flex that size on a table lamp,

But if the circuit protection is going to allow 20 amps continuous current then the flex has to be rated at 20 amps.

Far from being "the safest on several counts" our plugs are integral components of a system which is unsound, and which the Wiring Regulations should deprecate.

I have met many electricians in many countries ( factory control systems ) and while their first comments about the UK system are often a bit negative on the size of the plug they almost always see the advantages of the fuse per appliance for safety and conveniance to user and installer once the system is explained to them.

 

[JohnW2]

John - this has been discussed before, including recently, and a B16 does indeed provide fault protection for any size flex.

With any real-world size of flex, I agree with that, but only if one makes the assumption (as BS7671 generally does) that all faults are necessarily of negligible impedance. As you know, I am uneasy about that notion. Although undoubtedly rare, there are possible failure modes of some equipment that will result in faults which are not of negligible impedance, hence currents which, although excessive, are far less than that required for operation of the magnetic trip of a B16.

Many people here are quick to criticise/condemn (as dangerous) imported adapters, plugs, leads etc. which contain no fuse, even though a B32 probably provides adequate protection for any size of flex against faults of negligible impedance.

Kind Regards, John

 

[ban-all-sheds]

Melt down of the mains transformer maybe ?

Appliances designed with internal protection measures which stop them drawing too much current if they suffer an internal failure maybe?

But if the circuit protection is going to allow 20 amps continuous current then the flex has to be rated at 20 amps.

No it doesn't - read the regulation on omission of devices for protection against overload.

 

[bernardgreen]

But if the circuit protection is going to allow 20 amps continuous current then the flex has to be rated at 20 amps.

No it doesn't - read the regulation on omission of devices for protection against overload.

So are you saying there is a regulation which allows a circuit to have cable rated at lower amperage than the amperage at which the circuit protection operates ?.


I fully accept that an idiot ( or ignorant person ) could wire a 2 kilowatt heater with 5 amp flex but no system is fool proof when in the un-supervised public domain ( aka domestic DIY electrical work ).

I have seen photos of a house fire where the cause was determined to have been thin flex from a 15 amp ( un-fused ) plug. The flex was under a carpet igniting the carpet. Had that had a five amp fuse in a 13 amp plug then the chance of the carpet being set on fire would have been reduced.

Plugs should be sold with out fuses fitted or with a 3 amp fitted.

 

[JohnW2]

Melt down of the mains transformer maybe ?

Appliances designed with internal protection measures which stop them drawing too much current if they suffer an internal failure maybe?

Indeed - but, as we know, such internal protection measures are very far from being universal.

But if the circuit protection is going to allow 20 amps continuous current then the flex has to be rated at 20 amps.

No it doesn't - read the regulation on omission of devices for protection against overload.

In terms of regulations, you're probably right - but regs are not everything. I wouldn't mind betting that a good, quite probably high, proportion of casualties and fires attributed to electricity arise in relation to installations which are essentially compliant with the regulations.

Kind Regards, John

 

[JohnW2]

So are you saying there is a regulation which allows a circuit to have cable rated at lower amperage than the amperage at which the circuit protection operates ?.

Effectively there is, but only because the regs essentially regard anything other than a dead short as being an 'overload', rather than a 'fault', and do allow protection against overload (but not against 'fault') to be omitted in certain circumstances. However,a s both you and I have pointed out, it is possible to have a 'fault' (everyday sense) which is not of negligible impedance.

Kind Regards, John

 

[ban-all-sheds]

So are you saying there is a regulation which allows a circuit to have cable rated at lower amperage than the amperage at which the circuit protection operates ?.

Have you never heard of spurs wired from 32A ring finals using 2.5mm² T/E?

 

[JohnW2]

So are you saying there is a regulation which allows a circuit to have cable rated at lower amperage than the amperage at which the circuit protection operates ?.

Have you never heard of spurs wired from 32A ring finals using 2.5mm² T/E?

Indeed - but this line of thought can be taken to apparently absurd lengths. What about a 1.5mm² dedicated circuit supply just one socket and protected by, say, a B50 MCB?

Kind Regards, John

 

[EFLImpudence]

Indeed - but this line of thought can be taken to . What about a 1.5mm² dedicated circuit supply just one socket and protected by, say, a B50 MCB?

That only seems unreasonable because you have written "apparently absurd lengths".

The socket will have a 13A fused plug inserted into it therefore the cable is protected - at a point other than the point of reduction in csa.

However, That is not really what we are discussing.
It is not the same as an unfused plug with a small flex.

The recent thread to which Simon referred was one where he asked about the continental practice of 16A circuits and unfused plugs and small flex.
As long as the flex can cope with the fault current until the opd operates then it would comply here - if unfused plugs were allowed.

A very small flex is adequate for this.

 

[ban-all-sheds]

Indeed - but this line of thought can be taken to apparently absurd lengths. What about a 1.5mm² dedicated circuit supply just one socket and protected by, say, a B50 MCB?

That was not the point of my post.

Read what Bernard asked, again. It really does seem as if he is unaware (and possibly finds it hard to credit) that there is a regulation which allows a circuit to have cable rated at lower amperage than the amperage at which the circuit protection operates. It really does seem as if he is unaware of that basic principle ever being allowed.

I wanted to know if he had ever heard of spurs wired from 32A ring finals using 2.5mm² T/E.

 

[JohnW2]

Indeed - but this line of thought can be taken to apparently absurd lengths. What about a 1.5mm² dedicated circuit supply just one socket and protected by, say, a B50 MCB?

That was not the point of my post.....I wanted to know if he had ever heard of spurs wired from 32A ring finals using 2.5mm² T/E.

Yes, I realise that, but it doesn't alter my point (whether one calls it 'absurd' or not).

Kind Regards, John

 

[JohnW2]

Indeed - but this line of thought can be taken to . What about a 1.5mm² dedicated circuit supply just one socket and protected by, say, a B50 MCB?

That only seems unreasonable because you have written "apparently absurd lengths". The socket will have a 13A fused plug inserted into it therefore the cable is protected - at a point other than the point of reduction in csa. However, That is not really what we are discussing. It is not the same as an unfused plug with a small flex.

Fair enough - I perhaps should not have used the word 'absurd'; maybe something like 'surprising' would have been better. However, I do think it's essentially the same concept that we're discussing - namely using a device to protect cables only against (negligible impedence) 'faults', and relying on the fact that the fixed/single load precludes any 'overload' situation.

It is, at least to me, 'surprising' because it flies in the face of normal practices. If I understand correctly what's being said, then we might as well use 50A MCBs for all circuits (including lighting, immersions etc.) other than those supplying multiple sockets (and maybe get a good price for bulk-buying B50s!). Are you saying that (given an appropriate Ze) you would have no problem (e.g. when doing an EICR) with a lighting or immersion circuit being protected by a B50?

Kind Regards, John

 

[bernardgreen]

I wanted to know if he had ever heard of spurs wired from 32A ring finals using 2.5mm² T/E.

Yes I have heard of them and have installed such to single sockets where the maximum load that could be taken from the socket is 13 amp and thus less than the safe continuous current that 2.5 mm² can carry.

A spur to a double socket is ( in my opinion ) at a maximum of 26 amps is border line.

 

[EFLImpudence]

Fair enough - I perhaps should not have used the word 'absurd'; maybe something like 'surprising' would have been better. However, I do think it's essentially the same concept that we're discussing - namely using a device to protect cables only against (negligible impedence) 'faults', and relying on the fact that the fixed/single load precludes any 'overload' situation.

It's not the same, essentially or otherwise, because in your example the cable is protected against overload.

It is, at least to me, 'surprising' because it flies in the face of normal practices.

But it is normal practice. 2.5 spurs on rings, plus the fact that some think the situation is different when wired directly to the mcb.

If I understand correctly what's being said, then we might as well use 50A MCBs for all circuits (including lighting, immersions etc.)

A lighting circuit could quite easily be overloaded - especially these days - and the Zs would be a problem with 1mm² cable.
So, not lighting.

Immersions, I suppose, would be alright if it were allowed (see below) but what would be the point?

You are forgetting a couple of things.
There is the three metre rule and the reason for the omission of opd protection. Namely, it is only advantageous when the csa of a larger circuit is to be reduced, for whatever reason, and fitting an opd at that point would be a nuisance and pointless because the smaller cable cannot be overloaded.

 

[JohnW2]

It's not the same, essentially or otherwise, because in your example the cable is protected against overload.

You mean by the fuse(s) in the plug(s)? If so, then change the socket (on a 1.5mm² dedicated circuit protected by a B50) to an immersian heater.

But it is normal practice. 2.5 spurs on rings ...

Is that not because such spurs are specifically 'dispensated'? If one relied on the regs in general, wouldn't the length of an unfused spur be limited to 3m?

A lighting circuit could quite easily be overloaded - especially these days ...

Surely not if it were designed satisfactorily in the first place? ... and as for 'especially these days', haven't we been moving in the direction of much smaller lighting loads?

... and the Zs would be a problem with 1mm² cable. So, not lighting.

Possibly/probably. I did qualify my statement by saying that one would need an acceptable Zs. With a small circuit and 1.5mm² cable, I would have thought that it could well be possible, though.

You are forgetting a couple of things. ... There is the three metre rule ...

There is, indeed - so, as above, how does that fit in with 2.5mm² spurs?

...and the reason for the omission of opd protection. Namely, it is only advantageous when the csa of a larger circuit is to be reduced, for whatever reason, and fitting an opd at that point would be a nuisance and pointless because the smaller cable cannot be overloaded.

Yes, but what started all this was the discussion about a situation in which there was no fuse in the plug, hence a thin flex from plug to appliance would not be benefitting from adequate overload protection from anything.

Kind Regards, John