Fuse Box RCD vs RCD Adapter - which should trip first?

[Benjohnson20]

In an attempt to isolate for certain whether my fridge freezer is causing my main RCD to trip or something else, I used an RCD adapter in the socket that the fridge is plugged into. (Note - The tripping had stopped after unplugging the fridge freezer, but because it’s the second appliance to do it in a short period I’m trying to work out whether it’s something else).

Within 1 hour of turning the appliance back on, the main RCD tripped again and NOT the RCD adapter. If the fridge freezer is faulty, would the RCD adapter which is nearest the appliance be expected to trip first or am I making an assumption?

Cheers!

 

[MJPD29]

If they are both 30ma it is a lottery as to which will trip first

 

[brianmoooore]

Do you have an electrical test meter of any kind? The type that measures volts, amps and ohms.

 

[Benjohnson20]

Do you have an electrical test meter of any kind? The type that measures volts, amps and ohms.

No I don’t, are they easy enough to get hold of and use?

 

[brianmoooore]

Cheap ones start at about £5.
WITH THE POWER TURNED OFF AT THE MAIN SWITCH it might throw some light on the matter if you measure the resistance between the neutral and earth wires at the socket.
The correct instrument to use for this purpose (IR testing) costs a lot more than £5, and applies 500 volts between the wires, more than double the normal mains voltage, which will tend to exaggerate any leakage, making it more obvious. The cheap meter will only apply about 3 volts between the wires, so may fail to show anything amiss, but on the other hand, it might.

 

[pc1966]

As above, it may or may not show the fault:

• If your appliance is causing a 'hard' fault then usually both would fire, but that is not guaranteed.
• If it is a weak fault causing only a small amount of leakage then it is a lottery as to which will trip first as both the trip threshold and the response times are overlapping in general specification.
• If it is an accumulation of leakage currents on multiple circuits off one RCD then typically the adaptor will see nothing and the main RCD will be triggered.

Even if it appears that an appliance is triggering the fault on a common RCD it is not necessarily on the same circuit if it is a N-E fault somewhere. The current from something switching on can be the trigger for enough current to divert N-E for the RCD to see a fault.

If it was a smaller appliance I would suggest checking to see if you have another socket circut off a different RCD and plug it in there. You might be able to do that with an extension lead but take care not to cause a trip hazard or to allow the cable to be crushed by doors closing, etc, which can be a fire hazard.