Burnt neutral wire on 100A main switch...

[Boberto]

Is the most common cause of this problem a loose connection? I came across a burnt out neutral wire on a 100A DP main switch earlier. It was hard to tell if it was loose as it was charred and burnt so bad I couldnt tell! The burnt wire was the 16mm incoming neutral.

The establishment is a fast food take away. There are a number of 100A DP switches supplying flats above, the basement and the main shop area. The isolator where the problem occured was supplying the shop which is in use 7 days a week 10-12 hours aday. Current = 25A approx when checked with clamp meter however when the deep fat fryers kick in the current drawn was showing around 60A. That is the only appliance pulling current of any size. The rest of the shop consumes of grid lighting and a couple of ring finals which supply juice to chest freezers, the till, fly exocutor etc.

From the isolators 16mm tails supply the consumer units located in basement, flats and shop (10-15m distance approx). The main fuse is 80A and the 16mm tails are installed in MT4 trunking to the sub mains with main earth.

Am I missing something? Could this merely be a loose cable connection or is there a chance this is a loading issue. I was under the impression 16mm tails fine protected by a 80A main fuse providind the tails are clipped direct of installed in correct spacing factor trunking?

Advice....?

 

[Marvo]

In my experience, loose connections are by far the most common cause of burned terminations.

 

[GLENNSPARK]

could be a loose termination here that caused this.....

 

[GLENNSPARK]

do you think eddy currents appearing in the neutral could be an issue here n all?......

 

[topquark]

Ditto.

Edit: to the loose neutral.

 

[Boberto]

Just to mention the burnt parts were the incoming neutral terminal on the main switch and about 4 to 5 inches of the incoming 16mm neutral. All other parts of the cable are fine including the outgoing 16mm neutral. I wanted a 2nd opinion because with the fryer switching itself on when the temperature of the oil in it drops the current drawn to the shop increases to 60A. Just wanted to make sure that the size tails were sufficient on a 80A main fuse. As I said, loose connection is always my first evaluation on these sort of faults too.

Thanks.

 

[Engineer54]

The only other cause, is bad contact on the DP switch, but then i would of expected to see extensive heat damage to the housing of the switch. As Marvo states ....By far the most common cause for these types of failures is a loose connection at the cable terminal point...

 

[Boberto]

Eddy currents? No I dont.

 

[GLENNSPARK]

OK loose neutral then.....it was just a speculative one....post 4..lol.....

 

[Boberto]

Cheers all for putting my mind at rest. As I said there are flats above shop etc so wanted to be sure. Thanks.

 

[Knobhead]

As to the remedial work for it, replace the switch as well as remaking the cable. You may have to cut back beyond the burnt area of insulation, heat softens copper so will not make a good connection. Copper that has been heated has a darker colour when you cut it, so you should be able to see how far back the go. If needs be you’ll have to put in a through crimp and then sleeve it. Likewise any springs in the switch will be weakened by heat.

 

[Garz]

I would hedge the bets on the neutral being loose also, what loads are the flats etc pulling?

 

[Boberto]

Flats unoccupied for the time being so nothing being used there.

 

[Knobhead]

Your in the best situation to do a good repair then.
You don’t have Mrs. Brown from flat four breathing down your neck.
BUT there again you’ve got to make your own tea…. Bugger, there’s a down side to everything!

Going back to what I said, anything that shows sign of heat damage, sort it now while you can do it in your own sweet time.