One for the theorist amongst you...........................

[itselectric]

Could you try loosening the socket from the wall completely. plug in the microwave and see if it trips, I have had a similar problem in the past and it turned out although the insulation had not been damaged,with the socket screwed back the cables were pressing on he back box and breaking down to inrush load.

 

[SirKit Breaker]

Another thought , could you carry out a ramp test at that particular socket , see what that shows , compare the readings with everything plugged in and without ...

Thats worth a try, and another idea i have had is to replace the plug on the microwave, don't know why but you never know.

 

[SirKit Breaker]

Could you try loosening the socket from the wall completely. plug in the microwave and see if it trips, I have had a similar problem in the past and it turned out although the insulation had not been damaged,with the socket screwed back the cables were pressing on he back box and breaking down to inrush load.

Should have mentioned, the back box is a 40mm, due to the overboarding of the wall.

 

[SirKit Breaker]

Could you confirm these please Howard?

You are correct on all points.

 

[SirKit Breaker]

Out of interest, how close is the microwave to the consumer unit?

About 25 feet

 

[SirKit Breaker]

Just had a phone call, the mystery is solved, his wife has spilt a kettle full of hot water over it, and it fizzed and died, and tripped the RCD, so the microwave is in the bin!

 

[ImpededLoop]

Assuming this is a theoretical scenario?

My thoughs are:

The microwave only has sufficient leakage to earth to trip the RCD when it starts to cook so it doesn't show when PAT tested.

The offending socket is nearest to the consumer unit, so it has a lower Zs than the others.

The leakage of the microwave is only just over the operating current of the RCD. Therefore it operates when plugged into the nearest socket as the Zs is low, but the slightly higher Zs at the other sockets is enough to prevent the RCD operating.


I'm not convinced by my answer, as I suspect the fault on the upstair lights is connected, despite it seemingly having been rectified.

As I say, I'm taking this as a theoretical question to test us, rather than a real situation, so I'm assuming you've given us enough details to solve the problem...

 

[R G]

Well the microwave deffiniatelly has some leakage now ...FFS...RIP microwave ..

 

[SirKit Breaker]

Assuming this is a theoretical scenario?

My thoughs are:

The microwave only has sufficient leakage to earth to trip the RCD when it starts to cook so it doesn't show when PAT tested.

The offending socket is nearest to the consumer unit, so it has a lower Zs than the others.

The leakage of the microwave is only just over the operating current of the RCD. Therefore it operates when plugged into the nearest socket as the Zs is low, but the slightly higher Zs at the other sockets is enough to prevent the RCD operating.


I'm not convinced by my answer, as I suspect the fault on the upstair lights is connected, despite it seemingly having been rectified.

As I say, I'm taking this as a theoretical question to test us, rather than a real situation, so I'm assuming you've given us enough details to solve the problem...

This all seems to make sense, the socket is the second nearest by about 4 feet from the nearest, and the microwave wasn't tried in that socket, as it is a cupboard for the washing machine. but we will never know now, but it must have been the oven at fault, the new one works fine!

 

[Octopus]

This all seems to make sense, the socket is the second nearest by about 4 feet from the nearest, and the microwave wasn't tried in that socket, as it is a cupboard for the washing machine. but we will never know now, but it must have been the oven at fault, the new one works fine!

Take your own microwave to site and see what happens when that is plugged in the same socket?

 

[Engineer54]

What was the leakage shown on the load test when you PAT tested the now deaceased MW oven??

 

[Lucien Nunes]

The leakage of the microwave is only just over the operating current of the RCD. Therefore it operates when plugged into the nearest socket as the Zs is low, but the slightly higher Zs at the other sockets is enough to prevent the RCD operating.

I like this but only if the leakage is caused by an N-E fault in the microwave, that is not connected to the plug unless cooking, or lies in a critical band of resistance.

Zs is only going to vary by a fraction of an ohm from socket to socket, but if the source of the leakage is faulty insulation L-E the leakage impedance must be in the order of kilohms to produce a current at the tripping threshold of the RCD. So changing socket will vary the leakage current by tenths or hundredths of one percent, which is too small to produce repeatable behaviour.

If the fault is N-E, it could be low enough to be of the same order of magnitude as the ring cable, yet not cause a trip unless the circuit is loaded. With an N-E fault one would expect the kettle to cause a trip too when plugged in near the microwave, unless the fault were downstream of the cook relay, which would either have to be DP or switch the neutral (we've seen in another thread that some appliances have internal controls that do this).

Microwave ovens produce heavy inrush currents every time the cook relay closes, exactly how heavy depends on the size and design of transformer. For example, the one on my boat would often trip the B10 feeding my shoreline but another of the same wattage didn't (I cured this by installing a soft-start resistance of a few ohms in the TX primary).
It is possible that this inrush being dumped into a rather higher resistance N-E fault might be enough to trip the RCD while other nearby loads (and indeed the steady running current of the oven) would not.

But we'll never know now.

 

[Adam W]

Just had a phone call, the mystery is solved, his wife has spilt a kettle full of hot water over it, and it fizzed and died, and tripped the RCD, so the microwave is in the bin!

So why did the microwave work fine in other sockets? And why tell us it PAT tested OK?
There are too many red herrings in this scenario.

 

[telectrix]

red herrings take 2 minutes on full power. then you have brown herrings.