Protection against overload current

[SUNRAY]

I have seen red 13 amp plugs clearly marked NHS which do not have fuses, but that is the only time I have seen it were there is no overload protection, and they are only fitted to machines where failure would mean loss of life.

Calculations for volt drop use design current, so for example the standard for a ring final is to consider 20 amp drawn at centre of ring, and 12 amp even spaced, so for volt drop we use 26 amp not 32 amp but it is still 32 amp for over load protection.

Years ago, about 1974 to 80 I was involved with entertainment and lighting. I managed to obtain a license to use type approved unfused 13A plugs which had to be protected within 1m by a 13A fuse or 15A MCB. The plugs were made to a high quality and in reality the only way of getting them was as a manufactured unit with plug, 1m 2.5mm² flex and enclosure with 15A MCB and reyrole, 15A 546 or couple of 13A socket in exchange for part of the license document, ie only one per license. Then we discovered the Xray plug [which was orange in those days] and marked "XRAY MACHINE" nice chunky plugs with terrific grip to pull it out but hard to obtain without being in the know.

 

[JohnW2]

Years ago, about 1974 to 80 I was involved with entertainment and lighting. I managed to obtain a license to use type approved unfused 13A plugs which had to be protected within 1m by a 13A fuse or 15A MCB.

You obviously did not have access to a ready supply of ¼" (or whatever) copper rod

As I previously wrote in response to eric's comment, those red NHS plugs (usually seen plugged into 'maintained' sockets) seem to be of very limited usefulness, since, even if the equipment supplied was essentially to keeping patients alive, it's very unlikley that it would still be 'keeping them alive' after it had developed a fault which had blown a 13A fuse!

Kind Regards, John

 

[ericmark]

We all know how to make a non fused 13 amp plug, but I am not saying how on the forum.

 

[SimonH2]

Apart from which, that doesn't negate the downsides of having the fuseholder there in the first place.
As an anecdote, someone trying to run a big welder (e.g. those oil filled Pickhill Bantam jobs) off a 13A plug soom learns alternatives to a fuse ... and eventually the effects it can have on plugs & sockets. I wasn't there when it happened, but for a while we kept the remains to show people ...
Rubber plug and socket (from the days when they were good quality, welder plugged into an extension lead), not much left of the plug, the socket opened up like a peeled banana with some blobs of copper on the ends of the wires

 

[SUNRAY]

We all know how to make a non fused 13 amp plug, but I am not saying how on the forum.

Yes indeed but the mouldings were never quite right for the job.

 

[SUNRAY]

You obviously did not have access to a ready supply of ¼" (or whatever) copper rod

As I previously wrote in response to eric's comment, those red NHS plugs (usually seen plugged into 'maintained' sockets) seem to be of very limited usefulness, since, even if the equipment supplied was essentially to keeping patients alive, it's very unlikley that it would still be 'keeping them alive' after it had developed a fault which had blown a 13A fuse!

Kind Regards, John

99% of the time the big problem was the pathetic arrangements for the fuse holder and/or the movement in the L pin that caused the problems.

And those red plugs tend to get used for all sorts of applications in hospitals, somewhere I should have a pic of one on a fan during the summer last year.

 

[ericmark]

I think some of the life dependent equipment really needed 16 amp, likely designed for rest of EU, and there was a worry that under normal use the 13 amp fuse could rupture, plus a fuse works by metal melting so has to produce heat, in my dad's house (1954) the immersion heater used a 15 amp plug on its own circuit as with no fuse it did not get hot, so having no fuse reduces heat on the line pin so reduces faults from over heated line pin, it really would have been better to have provided 15 amp outlets in the hospital, but suppose that may have required a rewire. And it also means the equipment can be used in the home.

I would not fit a 13 amp socket in an airing cupboard to supply an immersion heater due to heat from fuse with a load for an extended time, in a FCU they seem to be able to cool better.

 

[JohnW2]

Apart from which, that doesn't negate the downsides of having the fuseholder there in the first place.....

99% of the time the big problem was the pathetic arrangements for the fuse holder and/or the movement in the L pin that caused the problems. And those red plugs tend to get used for all sorts of applications in hospitals ...

Interesting.

I must say that, given the countless occasions over the years/decades on which I have been in proximity to and/or handled those red plugs, I am surprised that I have never given into my inevitable curiosity and opened one up (if that is possible) to see what is inside! However, I think I had always 'assumed' that they did have fuseholders, but with some sort of 'solid link' in them.

Is it being said that they do not have a fuseholder at all (i.e. they simply have an L-pin just like the N one - which I suppose would make total sense) - and, if so, does that, in turn, mean that the reason for their existence is because of the possibility of 'plug failures' (rather than that a fuse within them might blow)?

Kind Regards, John

 

[JohnW2]

I would not fit a 13 amp socket in an airing cupboard to supply an immersion heater due to heat from fuse with a load for an extended time....

How do you feel about 13A plugs/sockets immediately behind (often virtually in contact with), and supplying, electric ovens?

... in a FCU they seem to be able to cool better.

Maybe, but intuitively I think I would have tended to expect that the opposite would be the case.

Kind Regards, John

 

[SUNRAY]

How do you feel about 13A plugs/sockets immediately behind (often virtually in contact with), and supplying, electric ovens?

I would not fit a 13 amp socket in an airing cupboard to supply an immersion heater due to heat from fuse with a load for an extended time, in a FCU they seem to be able to cool better.

Maybe, but intuitively I think I would have tended to expect that the opposite would be the case.

Kind Regards, John

Personally I would try to avoid a 13A socket/plug for an oven behind an oven, particularly if the rating is close to 3KW. But that is only my opinion.

Tha thing about 13A plugs is the movement they suffer, the fuse is usually clipped with some rather flimsy copper or brass directly to the live pin and every time the the pin moves, the spring action of the clip is stressed. That movement is not present in a FCU.

I see 2 points about the heating effect in a FCU:
1. the fuse being deeply burried in the plastic FCU has no air movement to cool it.
2. the fuse is surrounded by a mass of plastic which will have some heat conducting properties to take the heat from the fuse.

I can't make up my mind which of the 2 points has the most relevance.

Personally I'd try to avoid a 13A fuse at all for an immersion heater.

 

[JohnW2]

Personally I would try to avoid a 13A socket/plug for an oven behind an oven, particularly if the rating is close to 3KW. But that is only my opinion.

So would I, but it seems to be a very common situation - also with other 'white goods'.

Tha thing about 13A plugs is the movement they suffer, the fuse is usually clipped with some rather flimsy copper or brass directly to the live pin and every time the the pin moves, the spring action of the clip is stressed. That movement is not present in a FCU.

That's true, but only really relevant with plugs that are fairly often plugged/unplugged, which is not true of immersions, ovens or any other essentially 'fixed' appliance.

I see 2 points about the heating effect in a FCU:
1. the fuse being deeply burried in the plastic FCU has no air movement to cool it.
2. the fuse is surrounded by a mass of plastic which will have some heat conducting properties to take the heat from the fuse.
I can't make up my mind which of the 2 points has the most relevance.

Indeed, but I would add the fact that, in the case of a plug, there is appreciably more thermal mass (not the least the plug pins) dissipating, and conducting away, the heat. As I said, my inclination would be to suspect that, in combination with your (1), that means that the fuse would stay cooler in a plug than in an FCU - but I wouldn't put any money on that!

Personally I'd try to avoid a 13A fuse at all for an immersion heater.

Indeed, and it's hard to see that there would ever be a need. There's certainly no point in a 13A fuse for a 16A circuit (which is what they commonly are) and, at least theoretically, no need even if it were fed directly from a 32A ring (contrary to the guidance of Appendix 15!) or radial circuit. Ironically, probably the most common reason for an FCU feeding an immersion is that they are usually cheaper than 20A switches!

Kind Regards, John

 

[ericmark]

Personally I would try to avoid a 13A socket/plug for an oven behind an oven, particularly if the rating is close to 3KW. But that is only my opinion.

Where the oven is fan cooled likely the fan also cools the plug, and the power soon starts to switch off/on so time at 3 kW is not that long, more important is the supply to tumble drier or washer/drier, where it can draw 3 kW for an hour.

The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). --------- This can generally be achieved by:------- (iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit.

Sounds good, but how often it is actually done?

 

[ericmark]

duplicate

 

[SimonH2]

That's true, but only really relevant with plugs that are fairly often plugged/unplugged, which is not true of immersions, ovens or any other essentially 'fixed' appliance.

On the other hand, plugs that are regularly removed/inserted will keep the contact points "bright". I doubt the contact pressure in most plug/socket combinations is sufficient to make them "gas tight" - which for those who don't know, means it's tight enough to keep atmospheric oxygen out and so prevent corrosion/tarnishing of the contact surfaces.

The load current in any part of the circuit should be unlikely to exceed for long periods the current-carrying capacity of the cable (Regulation 433.1.5 refers). --------- This can generally be achieved by:------- (iii) connecting cookers, ovens and hobs with a rated power exceeding 2 kW on their own dedicated radial circuit.

Sounds good, but how often it is actually done?

Not as often as it should be.
As I think I've probably mentioned previously, a new build my mother was looking at a few years ago did not have a "cooker circuit" on the basis that "it's gas" and "they plug in anyway these days". As I was querying the state of the stairs (already sagging and propped up with bits of timber underneath ) with the company that would be doing the warranty, I also queried this as their build manual specifies a 32A dedicated supply for a cooker.
I didn't hear the outcome as my mother pulled out shortly afterwards - having got fed up after over 6 months of "when will it be ready ?", "ask me in a few weeks, I've just found out I need to <something>" exchanges. Also, mum had finally realised it wasn't going to be very suitable for her - especially the stairs that made me (as someone fully mobile) a little uncomfortable. But I do know that the vendor had told mum that I'd "caused him no end of trouble". Had I had the opportunity I'd have been able to explain to him that I hadn't caused him any trouble - he'd caused himself trouble by cutting corners and not knowing what he was doing, and fixing both issues was going to be cheaper to do before someone was living in the property than after
If she had gone ahead and bought it, I was going to have "some fun" reporting the electricians for providing false declarations of conformance to BS7671

 

[JohnW2]

On the other hand, plugs that are regularly removed/inserted will keep the contact points "bright". I doubt the contact pressure in most plug/socket combinations is sufficient to make them "gas tight" - which for those who don't know, means it's tight enough to keep atmospheric oxygen out and so prevent corrosion/tarnishing of the contact surfaces.

Yes, I did think of that 'other side of the coin', but I was responding to the comment which I took to be referring to repeated unplugging/re-plugging 'loosening' things.

As for that 'other side of the coin', although what you say is undoubtedly true, it's quite often (literally) years between occasions when some of my kitchen/utility room appliances get unplugged (often probably only a handful of times in the entire lifetime of the appliance) and, although the pins don't stay 'bright';, I've never experienced any electrical issues as a consequence.

Kind Regards, John