RCD Spur tripping consumer unit RCD when pressing 'test'

[watertight]

Hi guys,

I wonder if any of you could give me a little advice, I've installed an IP66 rated socket in an outhouse adjoining my house. This is fed from a Safetysure RCD spur, wired as a spur from a double socket next to it.

When I turned my electric back on at the consumer unit the external socket worked, I tested this with a socket tester, all LEDs lit to show it was wired correctly, this issues arise when I press the 'test' button on the RCD, this trips not only itself but also the RCD on the consumer unit. As a test I wired this as a spur to another socket on the ring main but got exactly the same issue. Am I overlooking something or is it more likely I have issues with the electrics in my home?

Many thanks in advance.

Mark

 

[EFLImpudence]

It will.

Two RCDs on the same circuit.

 

[EFLImpudence]

Having said that, do I now remember a recent post where it was stated this should not happen?
Or did that not include FCU RCDs?

Await further replies.

 

[watertight]

Thanks for the advice.

Possibly as simple as that too!!! With an RCD on my consumer unit does that mean I can just use a fused switch instead and still have the protection I want?

Out of curiosity, if I plug an extension lead with an RCD plug into a plug socket and press the test button why would this not trip my consumer unit's RCD, is that not in theory the same as a fused RCD spur?


Mark

 

[bernardgreen]

Testing an RCD with its test button should NOT trip other RCDs on the circuit

The test button does not create an earth leakage ( it cannot as there is no earth connected to the RCD )

The test button connects a resistor from Live before the sense coil to Neutral after the sense coil. The current in this resistor passes through the sense coil on the Neutral but it does NOT pass through the sense coil on the Live. This creates the unbalance to trip the RCD when tested.

 

[Robin0577]

Thanks for the advice.


Out of curiosity, if I plug an extension lead with an RCD plug into a plug socket and press the test button why would this not trip my consumer unit's RCD, is that not in theory the same as a fused RCD spur?


Mark

It depends on the type of RCD and in particular, how the manufacturer has chosen to implement the test function. There are ways of doing it which will trip the upstream RCD and ways that wont.

In theory there is no point having 2 RCDs protecting the same circuit, as they will both trip in the event of a fault. Some would argue that having 2 guards against the possibility of one failing to trip when it should.

 

[EFLImpudence]

Possibly as simple as that too!!! With an RCD on my consumer unit does that mean I can just use a fused switch instead and still have the protection I want?

It does but some are coming to think that two may not be a bad thing.

Out of curiosity, if I plug an extension lead with an RCD plug into a plug socket and press the test button why would this not trip my consumer unit's RCD, is that not in theory the same as a fused RCD spur?

It is but I seem to remember that there is something different about FCU RCDs.

Sometimes RCDs (the one in the consumer unit) do trip unexpectedly when some switches (the FCU RCD) are switched off.

I am not sure about this so wait and see what others say.

 

[Robin0577]

Testing an RCD with its test button should NOT trip other RCDs on the circuit

The test button does not create an earth leakage ( it cannot as there is no earth connected to the RCD )

The test button connects a resistor from Live before the sense coil to Neutral after the sense coil. The current in this resistor passes through the sense coil on the Neutral but it does NOT pass through the sense coil on the Live. This creates the unbalance to trip the RCD when tested.

Absolutely true for the usual DIN rail type inside a CU, but there are RCD spur some units which DO use an earth based test for reasons best known to the manufacturer.

I can't remember offhand which ones do it that way.

 

[bernardgreen]

but there are RCD spur some units which DO use an earth based test for reasons best known to the manufacturer.

It would be worth knowing which ones are built that way in order to avoid them.

If the polarity was incorrect then the resistor would be Neutral to Earth and would need to be a low value to create the 30mA unbalance since the potential difference between Neutral and Earth is very close to zero.

But that low value would allow a very high "test" current when the device was connected correctly and would therefor need to be very large wattage,

 

[Robin0577]

Wish I could remember who made it but I remember finding one after a board upgrade and thinking "what effin genius came up with that design"

Must have been some time back, I've not done a domestic board change since pre Part P days.

 

[EFLImpudence]

If the polarity was incorrect then the resistor would be Neutral to Earth

If the polarity was reversed in a 'normal' RCD it would make no difference.
If Neutral and Earth were reversed then the trip test would be Line to Earth.
(or Line to Neutral in the FCUs being mentioned)

The socket testers used by the OP cannot distinguish between Neutral and Earth so that could be the cause.

 

[JohnW2]

Testing an RCD with its test button should NOT trip other RCDs on the circuit ... The test button does not create an earth leakage ( it cannot as there is no earth connected to the RCD )

Absolutely true for the usual DIN rail type inside a CU, but there are RCD spur some units which DO use an earth based test for reasons best known to the manufacturer.

Quite so. In fact, in my limited experience, quite a lot of RCDs which have a earth connection and therefore 'can' use an L-E 'test leak' (RCD FCUs, sockets, adapters etc.) do do it that way. As you say, goodness knows why - it's not necessary, and creates the nuisance being discussed!

Kind Regards, John

 

[JohnW2]

but there are RCD spur some units which DO use an earth based test for reasons best known to the manufacturer.

It would be worth knowing which ones are built that way in order to avoid them. ... If the polarity was incorrect then the resistor would be Neutral to Earth ....

I suppose that's arguably one 'useful' consequence of this very annoying (and unnecessary) design practice - since failure of the test button to result in the device operating may stimulate some investigation that might reveal the polarity reversal.

...and would need to be a low value to create the 30mA unbalance since the potential difference between Neutral and Earth is very close to zero. ... But that low value would allow a very high "test" current when the device was connected correctly and would therefor need to be very large wattage,

True, but (with a large wattage, low value test resistor) the test facility would obviously then be 'unfit for purpose' when L/N polarity was correct (as it nearly always will be)!

Kind Regards, John

 

[ericmark]

I would agree a test button on a RCD socket should not trip a RCD further down the line, but I know they do.

I had a 10mA RCD socket on the bench and the test button tripped the 100ma in the main board. Using the proper RCD tester it was OK just something to do with how test button is wired.

 

[watertight]

Many thanks to all you guys.

Just to clarify would you therefore suggest that a fused switch is ample for my needs?

Mark