We've several times had the RCD in our fuse box[1] trip when a workman (or bouncy castle operator) has had an extension cable with its own RCD attached. The RCD on the extension cable tripped too. Last time it happened, he switched to an RCD-less cable and said that using two RCDs in this way made them likely to trip unnecessarily.
Is this true, and if so why?
[1] I'm sure there's some new-fangled term for this, but I don't recall it.-- Richard
no, he just removed a safety feature that sounds like it wasnt needed.
consumer unit
No, it's not true.
RCDs have three basic characteristics:
Usually, RCDs for the protection of final circuits are rated at 30mA trip current and "instant" trip.
They are not supposed to trip at 1/2 the rated trip current, and they are supposed to trip within 40ms (milliseconds) at five times the rated trip current.
If you have two such RCDs "in series" as you would appear to do in this case, then you have no discrimination, which means that in the event of a fault, either or both of the RCDs may trip.
To introduce discrimination, you would normally have the upstream RCD trip at a higher nominal current (e.g. 100mA) and have a built-in time delay (e.g. 100ms), so that in the event of a small fault, the
30mA/instant RCD would trip and the 100mA/delayed RCD would not.Without further knowledge of the operating conditions in your case one can not comment further on your particular situation. Suffice to say that there is no good reason why reducing the number of RCDs from two to one would make the arrangement less likely to trip, unless there was in fact a fault on the RCD-protected extension cable.
Regards, Rumble
Or the test button was pressed on the downstream RCD.
John
Both RCDs reliably tripped when the appliance was turned on, and the house RCD didn't trip when the appliance was turned on connected through another (RCD-less) extension. Presumably there was enough leakage in the extension that in combination with the leakage in the appliance it was sufficient to trip the RCDs.
-- Richard
I think that would depend on the RCD. Certainly the circuit diagram on my plug-in RCD here shows that the Test button merely activates the trip coil as opposed to causing any L/N inbalance (and leaving the coil to detect/activate itself). Hence, the operation of my test button could not trip an upstream RCD.
Mathew
Second thoughts, I think I've got this wrong.
Feel free to disregard.
Mathew
You were right first time. The test button connects a resistor between one pole on the input side and the other pole on the output side. This causes an internal imbalance and should trip the device under test, but it doesn't cause any imbalance on the supply side - there's no earth leakage - so an upstream RCD should be unaware of the test.
Dammit! Makes sense now... Thanks for the clarification.
Mathew
Sounds like the extension was faulty... which end was its RCD?
If at the socket end it seems unlikely that the RCD there would detect a fault in its supply lead. It could be the socket end was damp though - that could place enough leakage at the far end to trip both or at least sensitise them. The switch on transient and/or any leakage in the appliance would possibly then be enough to cause a trip.
At the plug end, so any leakage in the cable would affect it. I suspect it was often used in damp conditions - it was connecting the fan of a bouncy castle.
-- Richard
It would be aware if it was a non latching RCD that was upstream of the test. Most plug RCDs (if the extension used an RCD plug) are of this type AFAIK.
So if the house RCD trips faster than the extension RCD both RCDs would trip.
Adam
You've lost me. Why should the house RCD tend to trip at all? Are you saying that the test button on the plug-in / non-latching / NVR type RCDs works differently and does divert current down the earth wire?
Depends on the make of your RCD. Some (Powerbreaker are crap) are horribly sensitive to short-duration spikes, spikes so short that they just shouldn't register with anything like an RCD, but they do.
When you break a circuit carrying current, there's a spike. This can easily pop a 2nd RCD.
No. Only that if the house RCD trips then a non latching RCD running off the house RCD would also trip due to lack of power to it. It would not be an imbalance that tripped the non latching RCD just a lack of supply.
Adam
Yes, but that's downstream.
One might argue that the house one is more likely to trip since it will probably be seeing a cumulative leakage that is greater than that seen by the downstream one. So if the downstream one has a NVR function it will always appear to trip along with the house.
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