Illegal mains plug?.....possibly lethal

[Andymsa]

13 minutes ago, Michael Hodgson said:

 

Quite so. 

 

My only doubt about ring mains is that they work on the principle that a fully loaded 30A ring carries 15A on one lead from the fusebox and 15A on the other so that it won't exceed the rating of 2.5mm T&E cable.  But you have no way of knowing if the ring somehow becomes broken (eg one wire accidentally disconnected from one of the sockets) unless there is also some other fault which causes the fuse to blow.  Surely you can't even prove that the ring is complete when you first connected it to the fuse box?  If 30A of appliances happens to be running all on one side of the break, that cable is then carrying double its rated load but still won't bother the 30A fuse.  So why isn't there a fire risk in the cables buried in the wall?  Are we relying on a theoretical safety margin in the rating of cables?

 

this is not correct about 15 amps one way and 15 amps the other way, a ring main when being designed has demand and diversity applied to the circuit. In other words it is predicted what the potential load on that circuit will be.  Many factors will determine the max loading of a 2.5mm T&E cable from enclosure to environment. 

[smokebox]

Post withdrawn as info not correct

Edited December 8, 2020 by smokebox

[Nearholmer]

1 hour ago, Michael Hodgson said:

Are we relying on a theoretical safety margin in the rating of cables?

 

There is some margin in the cable-sizing, but more to the point you are reliant upon the competence and diligence of the installer, as you are with a great deal else in your electrical system, and indeed everything else in your home, workplace, car etc etc. Proving the continuity of a ring is a basic check as part of installation.

 

It is very rare indeed for a professionally installed domestic electrical system to be in a dangerous state after completion and testing (not completely unknown, but very rare), or to fall into a dangerous state in decades, even given minimal to absent routine testing and checking.

 

It is an unfortunate truth that most dangerous domestic electrical systems have been made that way by ill-advised alterations carried out by the resident or someone else who isn't competent. 

[stewartingram]

I have over the years come across quite a number of loose screws on the back of the 13A sockets which have meant that joint is either open -circuit or high resistance. And as I had diligently installed many of those in earlier years, I know they were originally tight!

 

Stewart

Edited December 8, 2020 by stewartingram

[Crosland]

1 hour ago, Michael Hodgson said:

 

Quite so. 

 

My only doubt about ring mains is that they work on the principle that a fully loaded 30A ring carries 15A on one lead from the fusebox and 15A on the other so that it won't exceed the rating of 2.5mm T&E cable. 

 

I'm pretty sure that 2.5mm T&E is rated at least 26A when buried in an insulating wall, more when run in conduit.

[stewartingram]

All this talk about fuses and them blowing reminds me of my time in the Pye factory as an apprentice. This gets a little long-winded, but I have to get the detail right to explain it.

I was working as a test engineer on 2-way car radios. (Pye Westminster etc if you know them). Tested at 13.8v (for a car 12v system use). As with all apprentices, boredom set in....

The test bench had a variable voltage psu, which I set to 13.8v. The o/p of the psu was taken to a little wooden board with 2 croc clips attached, for easy attachment of the radios. The psu was quite sophisticated; you could also control its max current o/p, though this was normally turned right up to 10A, as the radios were fused at 5A when installed, and one of the tests was to check current max was <4A anyway.

All fine so far? OK then, boredom....

Taking a glass fuse, I think it was probably around 2A rated I (and a number of the other apprentices) started playing by putting the fuse across the croc clips. With the psu normal, it blew more or less instantly. However, by turning the max current o/p right down (effectively "off"), the fuse could be laid across the clips before slowly winding up the current. I can't remember the current now (it was over 50 years ago!), but I could wind the current up so that the fuse would heat up, the whole thing would bend to 90 degrees, and slowly cook for many minutes; I think the record was 20-30 minutes. It proved to me that a fuse is not an accurately rated thing, and electrickery is not something to play with anyway. I've always had respect for it, that idle boredom did in fact teach me something.

 

Stewart

Edited December 8, 2020 by stewartingram

[Nearholmer]

Decent modern fuses are very accurately rated, but the rating is a curve, not a point or a straight line, and some of the curve shapes are quite complex, to suit particular uses.

 

That's why very detailed protection coordination studies consider the time-vs-current responses of the devices, not simply the "nominal rating".

 

But, I suspect I'm teaching grandmother to suck eggs here.

[TheQ]

Of course a proper PAT test or more properly "In-service Inspection & Testing of Electrical Equipment" includes taking the back off all mains plugs.. Checking the tightness of connections and the correct fuse is fitted.

 

The PYE of Cambridge, Westminster Radio.. Now that's a blast from the past.. worked on a few of those..

 

Definitely, not a fast blow fuse then.. yes there are fuses designed to be  fast and slow blow .

[Grovenor]

1 hour ago, TheQ said:

Of course a proper PAT test or more properly "In-service Inspection & Testing of Electrical Equipment" includes taking the back off all mains plugs.. Checking the tightness of connections and the correct fuse is fitted.

A large percentage of such plugs being moulded onto the cable these days! But at least you can check the fuse.

But PAT testing is not required on domestic stuff anyway.

[Crosland]

1 hour ago, stewartingram said:

It proved to me that a fuse is not an accurately rated thing, and electrickery is not something to play with anyway. I've always had respect for it, that idle boredom did in fact teach me something.

 

This is the difference between an "overload" and a "fault".

 

An overload can be very dangerous, especially with devices such as old fashioned wired fuses, that can take a long time to trip until they heat up enough to go open circuit.

 

A fault (a short circuit) will trip the circuit instantaneously.

 

 

[MarkC]

On 17/11/2020 at 18:16, Clive Mortimore said:

A side story nothing to do with Gordon's unsafe plug.

 

As an apprentice with the GEGB I had a short period of time working in the control room of the power station I was working in. A generating set was going to be added to the grid and they asked me to switch it on. Generators are not switched on to the grid until the frequency equals that of the grid and as Phil says as close to 50Hz as possible. Switching involved turning two handles at the same time and the control room staff told me if I got it wrong I blow the whole grid, well at sixteen you believe people. I was so nervous as the two needles on the dials reached the same frequency.

 

Today I imagine the fun of scaring an apprentice has been taken out of this procedure by a computer, and they call it progress.

Ah, the joys of winding up first trip Engineer Cadets, the first time they were allowed to put an extra genset "On the board". Warnings about blackouts & what "The Chief" would do to them if they did black the ship out, followed by what the "Old Man" might do to what was left after Chiefy had finished... 

 

Mind you, I had my ex-wife with me on a trip, back in the mid-80s. We did suffer a blackout when a diode in a genset decided that it wasn't going to play any more. 10 minutes of everyone running round getting the job going again, no problem. So, we all go up for a brew, & herself appears, looking very sheepish. She thought she'd caused the lights to go out - everything went dark the moment she had plugged her hair dryer in...

 

...the crowd had her life for that! 

 

Mark

[AndyID]

7 hours ago, Nearholmer said:

Andy

 

not sure why you have such reservations about ring mains. They’re electrically very efficient, and in the U.K. context if standards are properly followed the whole protection arrangement works perfectly safely.

 

I can appreciate that in a commercial/industrial setting where some quite small loads can actually have high importance then fully radial can be helpful to minimise nuisance tripping, but not really necessary in a domestic setting.

 

Kevin

 

Hi Kevin,

 

I used to think it was a clever system but having come to appreciate the simplicity of radial systems I think it's a bit too clever. Relying on parallel conduction paths and assumptions about how the user will distribute the load just does not seem like a good idea to me.

 

Cheers!

Andy

 

[roythebus]

3 hours ago, TheQ said:

Of course a proper PAT test or more properly "In-service Inspection & Testing of Electrical Equipment" includes taking the back off all mains plugs.. Checking the tightness of connections and the correct fuse is fitted.

 

The PYE of Cambridge, Westminster Radio.. Now that's a blast from the past.. worked on a few of those..

 

Definitely, not a fast blow fuse then.. yes there are fuses designed to be  fast and slow blow .

Some have no-blow fuses installed, usually loads of silver paper, a nail, or suchlike.

 

[roythebus]

Ring mains and radial systems both seem to work perfectly well here and abroad from what I've seen in Europe. The British fetish to earth everything and PAT test everything amuses those on the continent.

 

I used to own a Wurlitzer jukebox that kept blowing fuses when i bought it. It turned out that the previous owner had a problem with it blowing fuses because I discovered the change-over relay that made something inside go the other way and flipped its contacts so they had jammed. He replaced the slow blow fuse with a no-blow fuse, (see above) with a nail which then blew not only the 13A in the plug but the house RCD as well!

 

In my old workshop we had a set of Prolift vehicle lifts, 3 phase 5 tonne jobbies. We had a problem with those, so someone decided to try to get them to work by holding in a relay. that was the one that reversed the phases to reverse the lift motors. Not only did that blow the internal fuses but the 63A fuses in the 3-phase input! It was quite a spectacular BANG!

 

My previous house was built by Colt, a novel timber frame design from the late 1920s, but built in 1971. I was installing some double sockets and found several loose connections in the backs of the old single sockets. I've learnt that these can work loose just with the 50hz frequency as the cause.

[Joseph_Pestell]

35 minutes ago, AndyID said:

 

Hi Kevin,

 

I used to think it was a clever system but having come to appreciate the simplicity of radial systems I think it's a bit too clever. Relying on parallel conduction paths and assumptions about how the user will distribute the load just does not seem like a good idea to me.

 

Cheers!

Andy

 

 

I like radial systems for greater flexibility when doing work. With ring mains, you usually need to turn off half the house.

 

The radial means a greater number of circuits, breakers, etc but these have become a lot cheaper than they used to be.

[melmerby]

2 minutes ago, Joseph_Pestell said:

 

I like radial systems for greater flexibility when doing work. With ring mains, you usually need to turn off half the house.

 

The radial means a greater number of circuits, breakers, etc but these have become a lot cheaper than they used to be.

Our first house was built in 1956 and had 13A sockets all on radial circuits.

I converted it to ring mains.

[melmerby]

2 hours ago, Grovenor said:

 

But PAT testing is not required on domestic stuff anyway.

Unless it is sold on through a commercial channel, such as an auction house.

[Joseph_Pestell]

2 hours ago, Grovenor said:

But PAT testing is not required on domestic stuff anyway.

 

Even on commercial equipment, PAT testing is not required. I only found that out recently and was rather surprised. It would have saved me a lot of money not to have it done.

Edited December 8, 2020 by Joseph_Pestell

[AndyID]

17 minutes ago, melmerby said:

Our first house was built in 1956 and had 13A sockets all on radial circuits.

I converted it to ring mains.

 

Did you pull out the wire and replace it with thinner stuff? 

[TheQ]

16 minutes ago, Joseph_Pestell said:

 

Even on commercial equipment, PAT testing is not required. I only found that out recently and was rather surprised. It would have saved me a lot of money not to have it done.

pat testing is not a legal requirement,

The legal requirement is that any equipment you are responsible for, is safe for use.

So how else can you prove it is safe?

Many insurance companies won't issue commercial insurance unless you have it pat tested. Certainly it's a requirement for use of many public halls for exhibitions.

 

Even if it requires for some purpose PAT testing it doesn't necessarily require that it is done every year. A computer that is never unplugged then you'd probably only need it done every five years,. But a kettle in a working environment  once  a year is probably not enough.

At my work some our equipment is never tested.. pat testing would blow it up!!! They found out the expensive way....

[Crosland]

3 hours ago, Grovenor said:

But PAT testing is not required on domestic stuff anyway.

 

It is (or can be depending on their rules) if you take it into a care home with you. The only exception, in my fathers case, being anything that was brand new at the time.

 

39 minutes ago, Joseph_Pestell said:

Even on commercial equipment, PAT testing is not required. 

 

Even in public places such as hotels there are other ways to demonstrate that equipment is safe.

[melmerby]

2 hours ago, AndyID said:

 

Did you pull out the wire and replace it with thinner stuff? 

 

Why? As it was thinner than 2.5mm as it was

Sorry, don't understand what you are getting at.

 

 

[AndyID]

22 minutes ago, melmerby said:

 

Why? As it was thinner than 2.5mm as it was

Sorry, don't understand what you are getting at.

 

 

 

Sorry about that! It was supposed to be a joke. Rings can use thinner wire than radial systems but obviously if it worked as a radial system it would work as well or better as a ring using the same wiring.

 

BTW, when you say thinner than 2.5 mm do you mean the wire diameter or the cross section area? The wire diameter in 2.5 T&E is 1.8 mm.

 

When I wired my workshop I used 12 AWG copper which has a CSA of 3.31 mm. Each branch is protected by a 20 amp breaker and a GFI (Ground Fault Interrupter also known as a RCD). It's all in steel conduit too. To be on the safe side I had it inspected and tested by a licensed electrician.

[melmerby]

33 minutes ago, AndyID said:

 

 

 

BTW, when you say thinner than 2.5 mm do you mean the wire diameter or the cross section area? The wire diameter in 2.5 T&E is 1.8 mm.

 

 

3/0.036 (IIRC) with 15A wire fuses, each to two sockets per radial, which AFAIK is on it's limit of normal use but in those days there was very little electrical equipment around (no cooker or kettle supply was provided. I think there were 6 or 7 sockets in total in a 3 bed house, no room had more than one single outlet. There was an immersion heater with thicker (7/0.029?) wire.

Interestingly the light switches were fitted into wooden plaster depth boxes and there were no earths at all on the lighting wiring, just L & N (red & black) single wires.

The 13A sockets did have metal back boxes but they didn't have the customary earth terminal of today.

Standards were very different then.

[AndyID]

13 hours ago, Michael Hodgson said:

 

Quite so. 

 

My only doubt about ring mains is that they work on the principle that a fully loaded 30A ring carries 15A on one lead from the fusebox and 15A on the other so that it won't exceed the rating of 2.5mm T&E cable.  But you have no way of knowing if the ring somehow becomes broken (eg one wire accidentally disconnected from one of the sockets) unless there is also some other fault which causes the fuse to blow.  Surely you can't even prove that the ring is complete when you first connected it to the fuse box?  If 30A of appliances happens to be running all on one side of the break, that cable is then carrying double its rated load but still won't bother the 30A fuse.  So why isn't there a fire risk in the cables buried in the wall?  Are we relying on a theoretical safety margin in the rating of cables?

 

It's actually slightly worse than that

 

Admittedly It is pretty unlikely (and probably not even allowed) but it is possible to exceed the cable rating without any wiring fault. e.g. if the ring was 21 meters long and someone was clever enough to pull 30A from it 1 meter from the feed point there would be 28.6A flowing through the 1 meter length of cable. The other 20 meters would only be passing 1.4A

 

It's a resistors in parallel problem.