Mains cable blown out on two products

[Jimmy Jones]

Hello,

Could anyone please identify what has caused this kind of blow-out on a 3-core flex? One is from an electric blanket and one from a heated pillow type thing. I think both cables are the same though, probably the same factory. The 500mA fast-blow fuses inside the controllers and the 3A plug fuse were not blown.

The customer had the electric blanket for 6 months before this happened.

 

[Wilko]

Hi - there's seems to be a sharp cut in the conductor, that can't help with its current carrying capacity. It looks like a deliberate mechanical cut rather than failure due to flexing, but you never know.

 

[Jimmy Jones]

Two customers similar products, damage in the same place bar 1-2 inches it's really odd. We've sold many thousands of these and no other problems at all. I've just checked, the top one, the Line was severered, and the bottom one the neutral, so less likely to be a cable fault?

 

[snowhead]

Possibly been trapped under the bed leg /castor or behind the headboard against the wall.
I doubt it's a manufacturing fault.

It should be possible to trace exactly where the blown bits were, by laying them back onto the bed.

 

[The Ghost]

Apart from sabotage I suspect maybe the customer has folded the cable acutely to get it out of the way or not appear unsightly and perhaps it has been against a part of the bed chafing and them getting in and out of bed has done the dirty deed. Have you examined the bed or got photos of it. Would like to see where exactly the damage is in relation to the whole cord/product. It is a bit worrying the fuses had not blown. Something must have popped with what looks like a fault of neglible impedance.

 

[Shoei]

Could it have been damaged in manufacture? Is the fault fairly close to one end of the cable? Long shot but it could have been damaged by a cable stripping machine used to prepare the cable for fitment.

 

[SparkyChick]

I'm no expert, but I have had one or two cables give way in a similar fashion and if it's a short sharp failure, in my experience it has only ever created a puncture wound if you will.

These both look like they have been subjected to heating caused by a high resistance section/break (and resulting arcing) for some time... this is based solely on the level of damage to the surrounding area.

If the 3A fuse in the plug has not failed, then I would be asking questions of the fuse manufacturer/supplier of said products. Is it safe to assume they are manufactured in China? Chinese products have a tendency to use crappy cables (check out John Ward on YouTube for some good examples).

I'm noticing some deformation of cables on floodlights that I believe is being caused by them being tightly packed for shipping, with no regard for the minimum bend radius of the cable. Could be these cables have been subjected to similar packaging.

 

[Jimmy Jones]

Could it have been damaged in manufacture? Is the fault fairly close to one end of the cable? Long shot but it could have been damaged by a cable stripping machine used to prepare the cable for fitment.

It's in the middle of the section between the controller and the plug which slots into the blanket.

 

[SparkyChick]

It's in the middle of the section between the controller and the plug which slots into the blanket.

And with the blanket on the bed, where does that section sit in relation to the edge of the bed?

 

[Jimmy Jones]

It would hang down to about level with the bottom of the mattress. Where it sits on the bedframe.

 

[telectrix]

bet it's been tucked under mattress.

 

[static zap]

Trapped , or nibbled (any pet rabbits ?)

 

[Marvo]

Looks to me like internal arcing rather than external trauma. If it was crush damage or excessive wear/abuse or even rodent/pets I'd expect to see at least some signs of it in the area surrounding the failure but there isn't.

I've never encountered this mode of failure with a cabtyre trailing cable before, my best guess would be latent manufacturing fault.

 

[Jimmy Jones]

Would it be an issue that the fuse it connected to the neutral and not the line? The fuse in this position was part of the testing of EN 60335-2-17 but it tested against short-circuit only and not perhaps someone/something like a metal bedframe cutting into the cable.

 

[Toneyz]

I have two questions
1 Is it the same product each time and
2 Is it the same manufacturer?

 

[telectrix]

Would it be an issue that the fuse it connected to the neutral and not the line? The fuse in this position was part of the testing of EN 60335-2-17 but it tested against short-circuit only and not perhaps someone/something like a metal bedframe cutting into the cable.

how do you know the fuse is in the N? if you look at the reverse side of the PCB, you can follow the tracks. might find it's fused in the L.

 

[Jimmy Jones]

how do you know the fuse is in the N? if you look at the reverse side of the PCB, you can follow the tracks. might find it's fused in the L.

If you look on the left where the N wire goes in, on the other side it's soldered directly onto the left side of the fuse.

 

[picturevalve]

As the fuse hasn't blown it looks like the cable has been bent causing some (or all) of the strands to break leading to a hot spot/arcing whilst in use. The fact that it hasn't been noticed before it failed means it was possibly tucked under the mattress (if so very lucky there wasn't a fire) or behind a bed leg/castor as others have said.

 

[ruston]

Possibly been trapped under the bed leg /castor or behind the headboard against the wall.
I doubt it's a manufacturing fault.

It should be possible to trace exactly where the blown bits were, by laying them back onto the bed.

Possibly an adjustable bed?