UK Fridge tripping RCD via one socket but not another

[TomHombre]

Hi,

We have a ten year old fridge-freezer plugged into an older socket (20 yrs plus I should think). It started tripping the RCD on the consumer unit. I assumed I needed a new fridge and, to avoid repeatedly rebooting all equipment on the circuit while I sourced one, I plugged it in via a plug-in RCD. For space reasons this was into a different socket. It does not blow in the new socket...

Is it a fault in the fridge or the socket?

For space reasons I can only use the the plug-in RCD via an extension lead. (The "old" socket is awkward to get at because a kitchen unit has been installed in front of it.)

With fridge plugged into RCD into extension lead into "old" socket it trips the plug-in RCD between 1 and 5 times per week.
With fridge plugged into RCD into extension lead into "new" socket it works fine for at least 4 weeks.
The only difference between the two set ups is the routs of the extenssion lead from the RCD to the socket.

I don't want to buy a new fridge if it won't solve the problem (and using the "new"socket is not a long term solution).

Any advice gratefully recieved.

Tom.

 

[fairlight]

It would be best to get an electrician to test the installation, sockets and fridge. If there is a N-E fault anywhere on the system the RCD can trip when any load is switched on, leading to the false assumption that the particular load which tripped the RCD is at fault. There's many an appliance been replaced with new only to find the problem persists. A competent local electrician will be able to test and locate any issues for about an hours labour.

 

[Lucien Nunes]

As the fridge is tripping the plug-in RCD, and the probability of a trip changes along with the socket to which it is connected, a likely cause is a neutral-earth fault in the fridge. Although a fault in the fixed wiring could cause the RCD in the consumer unit to trip, it is unlikely to trip the plug-in RCD.

Because the voltage between N & E is small and dependent on the load on the circuit (and on other parts of the installation) it is quite common for an N-E fault to manifest as occasional tripping when an appliance is switched on. Whether the load current of a particular appliance is sufficient to drive enough leakage through the fault to cause a trip, depends on the resistance of the circuit all the way back to the supply where N & E are connected together, and whether anything earthed by another route is connected to the circuit.

Your 'new' and 'old' sockets may have different resistances to the consumer unit due to different cable lengths, and different parallel earth resistances due to earthed loads fed from other points on the circuit. This may affect whether enough current diverts to earth through the fault in the fridge to cause the plug-in RCD to trip.

Unfortunately this is not the only possible cause, so without testing the insulation of the fridge and the circuits, it may be impossible to draw any hard and fast conclusions. Another possibility is that there is a line-earth fault in the fridge, that caused leakage in the 'old' socket, but the earthing of the 'new' socket is high-resistance so the fault is simply making the fridge casing live without causing leakage, resulting in risk of electric shock.

If you have a multimeter you might be able to make simple resistance tests between the pins of the fridge plug that would have a modest chance of locating the fault, although really a more sophisticated set of tests would be needed to do so convincingly.

 

[Deleted member 105166]

Also worth being aware that, although the plugtop RCD adaptor appears to have been the device that has tripped, this is not guaranteed to trip first, where you have two similarly rated, 30mA devices in series.

 

[TomHombre]

Thank you Lucien
[automerge]1571479127[/automerge]
Thank you Fairlight.

 

[Lucien Nunes]

If there is a N-E fault anywhere on the system the RCD can trip when any load is switched on

I am not sure this is a good generalisation. The load that triggers the trip event can be anywhere on the system, but the fault will normally be downstream of the RCD that trips since the current imbalance occurs at the location of the fault. Hence, if an RCD trips that supplies only the fridge, it is likely that fault is within the fridge.

Some plug-in RCD adaptors have other properties such as undervoltage trip that could possibly confuse the results, hence my caution in stating that we can't be certain of the N-E fault diagnosis.

 

[Deleted member 105166]

Some plug-in RCD adaptors have other properties such as undervoltage trip that could possibly confuse the results

Indeed. Some also have a functional reference to the CPC and won't engage if the CPC is not present. I dislike these when doing ISITEE as they cannot be energised for the RCD test using an isolation transformer.

 

[Kidsolo]

There is a good chance that you have moisture shorting out wires. Sonetimes you can give your fridge a second life if you turn it off for several days and let it dry right out. I am not saying it is not faulty, but if it is going 4 weeks without tripping, it can't be anything majour