dangerous wiring!

[Hawk]

Got called to a job to investigate smoke coming from an industrial single phase socket. Met with the client and he said there was a bang, sparks and smoke coming from the socket. He showed me to it and I started investigating. The circuit was still live so I isolated it and noticed the circuit was fed from a 50A type D mcb. The circuit was feeding a forklift charger which was rated at 44A, however the cable supplying the socket was only 6mm and the length of run was around 30mtrs! The cable was completely melted. See picture attached. I made safe the circuit and left it isolated so it can not be turned on until I get back to upgrade the cable.
I was wondering why the circuit didn't trip if the live and earth touched, which I assume due to the sparks and a bang that was seen and heard. I haven't carried out any tests yet although I was getting 230v between live and earth. I take it the disconnection times have not been met?

 

[Brightspark2]

Zs too high?...Needs to be lower than 0.18 for D50.


On the picture it looks like the cables been trapped with lid screw and shorted out...

 

[telectrix]

yep. looks far more likely that the insulation has been damaged and you've had a short circuit, not an overload. repair the existing cable, using crimps, wagos or whatever, grab a cash repair job, go pub.

 

[Hawk]

The cables in the picture aren't the one, sorry should have said that. I had it out of the trunking. When I first seen it it had melted up into the trunking. I was thinking of upgrading the cable to 10mm

 

[Lucien Nunes]

I was wondering why the circuit didn't trip if the live and earth touched

A D50 will let through a large enough amount of energy to blow a piece off whatever is causing the short, without reaching tripping point, even if the loop impedance is OK.

Suppose one of the socket terminals isn't tight - the cable end will overheat and the insulation melt or crumble, but the conductors will only tend to touch together gently as the insulation disintegrates from between them. The arc that forms will remove a bit of the metal, maybe without reaching the 1000A that a D50 might need to trip rapidly, leaving the circuit live until further melting has taken place. This kind of thing can carry on for a long time,especially if the loop impedance is high and further reducing the chances of an instantaneous trip.

 

[Knobhead]

Bet you’re popular. Leaving a job overnight when the FLT should be on charge.

 

[Darkwood]

The cables in the picture aren't the one, sorry should have said that. I had it out of the trunking. When I first seen it it had melted up into the trunking. I was thinking of upgrading the cable to 10mm

Thinking!.... I'd prefer you to say I've done the cable calcs and found replacing with 10mm with suffice

 

[Hawk]

They have other forklift charging points Tony! I'm not that mean! Lol. And yes darkwood after doing the calculations it can be seen that 10mm cable is required.

 

[Engineer54]

They have other forklift charging points Tony! I'm not that mean! Lol. And yes darkwood after doing the calculations it can be seen that 10mm cable is required.

So how have these other charging points been wired?? I take it using 6mm cable also. Are any
of these circuit cables showing signs of similar overheating/burning?? If not, ...why not?? lol!!!