This extension was found feeding a 2kW heater (240V) which apparently had been running on it for several hours per day for the last two years or so:






Although not obvious from this photo, the PVC insulation on the hot conductor has gone quite brittle with the heat:

I'd say, poor connection where the line conductor goes to the terminal. Was the screw loose?

I'd be more inclined to suspect poor spring tension in the plug contact. That looks like a single-wipe contact.

I'd say there was a bad connection between the pin on the heater plug and the contact on the cord connector.
Paul,
Is that a shuttering device just above your thumb in the 2nd pic?.

The left part of the burnt contact looks a bit thinner than the opposing contact in the same place.. I'm going with Yaktx on this one as overheating/arcing from a worn contact in the cord cap.. (If thats what you call the ends over there ) Is there a photo of the plug prongs on the heater that was in use with this?

Yeah, that seems like a really cheap and bad-quality connector.

You can just tell by the cardboard strip that's being used as a cord grip.

Most of those only have single-wipe contacts.

It would be called a trailing socket.


Yes, all BS1363 sockets, including trailing types for extensions have a shutter. My thumb was keeping it open in order to get the picture.


No, it was quite tight.


Your wish is my command....... You can see that the line pin has certainly been getting warm:


You'll quite often find marks around the live pin on BS1363 plugs due to the heat buildup around the fuse and clips, especially on the cheap types where they've been run on higher currents. For those unfamiliar with BS1363, here's the inside of the plug where you can see that the top of the prong goes straight to the fuse:



A closer look at that prong:


The bright mark does line up with the contact on the socket (the plug was being withdrawn regularly to turn off the heater).

The socket looks like the type which would have been intended for garden tool extensions originally -- The orange color being a giveaway. Although BS1363 is normally rated for 13 amps, this one being part of such a low-power set was clearly never intended for such use:




I did run the heater on a known good socket for a while and noted the normal heat around the live pin after a half hour. It didn't seem unusually warm though, so I think given the evidence the best bet is poor prong/socket contact on a socket which was never really intended to run on 8 amps for extended periods.

The usual backtracking of heat down from the fuse could well have contributed to the problem after a few hours use though.



[This message has been edited by pauluk (edited 11-19-2005).]

Oh, now I see the "max 3 amp load" label!

We have 3A in-line switches, but not plugs! I thought those contacts looked flimsy!

The cord-grip is a sort of fibre composition, not cardboard, and was the usual material used for cord-grips before plastic became commonplace.

AIUI the socket *must* be rated to 13A to comply with BS1363 but it's possible it was originally manufactured with the 3A MAX marking for use on extension leads rated at 3A only.

Is the heater plug cracked or was that a hair on the lens?

A 3A Extension lead??!!.
Why such a small rating?.
Most lighting circuits have a higher current rating than that.
Is the other end (that plugs into the wall socket) fused at 3A?.
Is this a case of wrong connector or wrong sized ext. lead all round?.
I would have thought that a UK ext. lead would have to have a current rating of 13A, to match the connector?.
Or are there different ratings for different things?.

Fibreboard composition, [or 'compo' as it was known], is a pressed resin/woodfibre material of immense strength when dry. It far outperforms crummy plastic grips- just try breaking a piece with your fingers!
It was much used in the pre-plastic era for electrical insulation in transformers, switchgear and motors.
One modern use is for seals in domestic water connections here in France, because it swells when wetted, effecting a reliable joint.
Historical note: The British Army issued our squaddies with 'compo' ID dog-tags during WW2 for economy. The poor blighters who fell on the Normandy beaches became unidentifiable when the tags disintegrated in the seawater.

Alan

That's the impression I was under as well, but I'm really not sure about the contacts on that socket being able to take that current continuously. There were no clues as to the manufacturer or country of origin on this socket.


It wasn't cracked -- Must have been a hair.


As I said, my guess would be that this socket started out life on an extension cord sold for garden tools. It would probably have had orange flex, and most likely an orange plug too, fitted with a 3A fuse. You can still buy extension cords using 0.75 sq. mm flex fused at 3 or 5A.

The plug I found on the other end of this extension was just a generic white type, fused at 13A.


Yep, the old fiber cord grips are actually much stronger than they look. The usual failure is where someone has been a little over-enthusiastic about tightening them and stripped out the threads.


I've read that in the U.S. a couple of states issued fiber license plates for cars during WWII as well to save metal. Apparently people in one area had problems with wild goats deciding that these made a tasty snack!

"No license plate officer? Gee, a wild animal must have eaten it!"