Orange Unilab Power Supply Decided it wanted a Viking Funeral

[Peter Dale]

Teacher was doing electromagnets with their class this morning and rang me in a panic about one of the power supplies catching fire and smoking. They were able to turn it off at the mains very quickly (within a few seconds) due to the TA's proximity, and I've removed it from the trolley to inspect it. I've never opened it up to mess inside, and it isn't one that I've sent for repair. The fire came came from the power lead at the back of the supply (Photo below). A visual and conductivity test on the fuse in the plug shows that it hasn't blown, and the plug internals are all fine, so current doesn't seem to have been the problem.

It looks to me like the cable might have been cut or sliced and caused a short, but I'm not certain it's not just down to melting from the 2 seconds the flames were active before the power was cut, or years of wear and tear from wrapping the cable. Asking your opinions based on the photo.

 

[Nick Mitchener]

Those cable glands are very harsh on the cables. Sometimes a PSU of that design will stop working because a conductor has been cut through. in this case it seems two were.. If the cable gland is still ok you can usually remake it after pulling more cable through and reconnecting inside.

 

[ThePFoJ]

100% the cable clamp - I have replaced all mine with glands (like that one below).
The problem is that it gets tighter when its pulled and it eventually starts eating into the cable, breaking through the insulation and causing a short.

 

[Blackadder]

To quote Dr. Mcoy from Star Trek "Its worse then that its dead JIm"

 

[Wobbly John]

Those cable glands are very harsh on the cables. Sometimes a PSU of that design will stop working because a conductor has been cut through. in this case it seems two were.. If the cable gland is still ok you can usually remake it after pulling more cable through and reconnecting inside.

Even before I scrolled to the photo, I guessed it would be the cable gland (I missed this post, originally).
Although that type of cable gland doed not usually give issues as much as the 'wedged in' ones, the cable gland is one of the biggest issues with the older Unilab power supplies. I once repaired and inspected14 from another local school. All had issues with the cable grip, a few with other faults too, and about half failled PAT testing because the copper wires had fractured at the cable gland.
It's one of the reasons I do annual PAT testing - I know what to look for.
I retire at the end of the week, and the school will be using an outside contractor for 2-yearly testing (often ends up nearer 4 yearly...)

 

[HatMan]

It's one of the reasons I do annual PAT testing - I know what to look for.
I retire at the end of the week, and the school will be using an outside contractor for 2-yearly testing (often ends up nearer 4 yearly...)

The problem there is it's often just people who have done a course in how to press a button ... I still load test (often supplies are not enabled internally until the power is applied) but they don't even do that any more ... It was better in the old days when I were a kid

 

[cephid]

Looks like there's no strain relief where the cord goes into the power supply. I would probably put it down to flexing overtime resulting in the failure. Oops just s Every single seen Wobbly Johns comments.