Mystery Burn-out!!!

[croydoncorgi]

Someone explain this:

House under refurb but still occupied. Two fridges connected to extension lead, along with a 230/110 transformer, for site tools. Tranny NOT powering anything at time of 'incident'.

Someone plugs a standard kettle into a spare socket on the connector strip. Kettle starts to heat normally. Then BOTH fridges start to smoke from the back and one actually catches fire in the motor area!

After everything disconnected, fire put out and dust has settled, the kettle, tranny and extension lead all worked perfectly. No problem found with house consumer unit or with any other electrical kit. Only BOTH fridge motors were fried.

WTF happened????

 

[electronicsuk]

For a start you were trying to run far too much off a single extension lead, so an overload of some sort was always going to be likely. I can't, however, offer a good explanation of why this should have caused any issues with the fridges themselves - only the extension lead and multiway adaptor. If you're sure the smoke came from the fridges then I'd have to guess at some kind of issue with P.D. between N & E causing the problem.

 

[Lectrician]

Kettle could have caused a very large undervoltage the freezers if they where all on an excessivly long extension lead...

Wouldn't usually result in a burn out though.

 

[jj4091]

Were you running them off a generator?

 

[Space cat]

I can only offer one explanation. It's exceedingly tenuous but it's all I've got.

Fridges use induction motors and these have a rather low start-up torque. There are various ways round this, one of which is to just accept it, but there can be an extra start-up winding that is switched out when the motor gets up speed.

If the kettle dropped the voltage far enough, the motors might have slowed to the point where this start-up winding stayed in until it overheated. Alternatively the motors might have slowed below that critical speed at which the torque drops off rapidly but the start-up circuits didn't operate. The stalled motors wouldn't last long.

 

[TicklyT]

I'm inclined to agree with Space cat.

Jist thinking - If you were to 'look down the cable' towards the 'fridges, and see a capacitive load ('cos the motors have stalled?) and a nice big inductor in the site transformer connected in parallel with the capacitors, you've got something closely resembling a tuned circuit that may start to resonate, driven by what's left of the mains voltage at the end of the extension lead.

 

[croydoncorgi]

I must point out first off that the wiring arrangement of the fridges, etc. was NOTHING to do with me! My job was to install a new boiler, cylinders, plant-room plumbing & solar panels. NOT electrics!

Since both fridges went bang at the same time, the voltage drop explanation could be it: too much resistance / impedance in the wiring, therefore motors slow down into the 'starting zone', which puts even more capacitance onto the system, probably dropping the volts still further. Motors stall and burn out.

The 'temporary' setup was perfectly OK for a couple of weeks at least. The change was the introduction of the kettle (at 2Kw plus, that's a big change!) so maybe the voltage drop idea is right.

Nevertheless, I'm still intrigued by the thought of tuned circuits made up of 'everyday appliances' resonating at 50Hz. What happens once resonance starts? Would each appliance suddenly see an increased voltage? Or what?

 

[mattylad]

The problem now comes to who's to blame & who pays for the new fridges!

 

[Space cat]

What happens once resonance starts? Would each appliance suddenly see an increased voltage? Or what?

The voltage would rise slightly because the parallel tuned circuit would present a higher impedance than either component in isolation. If you pay for your electricity by the volt amp, as heavy consumers do, you take steps to tune out reactive (generally inductive) loads and so minimize your amps. This tuning can take the form of some large economy size capacitors. Alternatively, over-excited synchronous motors are capacitive. In this case, everything was OK until the resistive kettle was switched on so we can discount resonances as the cause.

The problem now comes to who's to blame & who pays for the new fridges!

Apportioning blame would be a nightmare. Who provided the extension lead? Who plugged the kettle in? Wasn't it reasonable for a non-technical person to plug a kettle into a spare socket? Can you prove that this caused the damage?

If you were to take this to the small claims court you would most likely need an expert witness and maybe a test under identical conditions too. Is it worth it?