RCD Spur tripping consumer unit RCD when pressing 'test'

[JohnW2]

Many thanks to all you guys. Just to clarify would you therefore suggest that a fused switch is ample for my needs?

Provided the RCD in your CU functions correctly, there is, indeed, nothing gained by having the RCD FCU - an 'ordinary' FCU would be adequate.

However, as has been said, some people believe that the in-service failure rate of RCDs is quite high (the figure of 7% of in-service ones being faulty is often quoted), in which case there is an argument for the 'belt and braces' approach of having a second one in the circuit. However, that applies to all situations in which RCDs are used, and it is very rare to see people routinely installing a second one 'just in case'.

In your situation, since you already have the RCD FCU, I would personally be inclined to leave it there. The only 'downside' is the nuisance value of what happens when you press its test button (which is not an everyday event).

Kind Regards, John

 

[JohnW2]

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.

If that happened, it surely must mean that the 10mA RCD was badly designed (or it's test circuit faulty)? Even if the test button introduced an L-E leakage current, if the purpose was to test the functioning of the 10mA RCD, the leakge current it created did not ought to be remotely high enough to trip a 100mA one. If it were, that would be a bit frightening!

Kind Regards, John

 

[AndyPRK]

Its interesting and reminds me of when I had 2 wylex RCBO's in series.
(which have a functional earth lead connection)

Pressing the Test button tripped both.

The RCBO's can also generate a flash/spark inside, even when there is no load conencted!

 

[JohnW2]

Its interesting and reminds me of when I had 2 wylex RCBO's in series. (which have a functional earth lead connection) ... Pressing the Test button tripped both.

That seems even more bizarre/unnecessary, particularly given that (I believe) some people don't bother connecting the functional earth. Mind you, maybe it's a cunning plot to alert people (when pressing the test button does nothing) to the fact that the functional earth is not connected!

IThe RCBO's can also generate a flash/spark inside, even when there is no load conencted!

I'm not sure how one could explain that!

Kind Regards, John

 

[333rocky333]

With fcus and sockets i think the reason for the L to E test is because they recomend testing before each use, in fact they label them not to use if the test fails.

I reckon its a safety back up so if the unit had lost its earth for some reason, failure of the test would alert the user not to use.

Some go one step further and the unit wont even turn on if theres missing or inadequate earth

 

[JohnW2]

With fcus and sockets i think the reason for the L to E test is because they recomend testing before each use, in fact they label them not to use if the test fails. ... I reckon its a safety back up so if the unit had lost its earth for some reason, failure of the test would alert the user not to use.

That's possible - and very similar to what I just suggested about RCBOs (to alert one to the fact that the functional earth has not been connected).

Some go one step further and the unit wont even turn on if theres missing or inadequate earth

Interesting - maybe I haven't thought deeply enough, but I can't, off the top of my head, think of any particularly easy way of engineering that!

Kind Regards, John

 

[ericmark]

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.

If that happened, it surely must mean that the 10mA RCD was badly designed (or it's test circuit faulty)? Even if the test button introduced an L-E leakage current, if the purpose was to test the functioning of the 10mA RCD, the leakge current it created did not ought to be remotely high enough to trip a 100mA one. If it were, that would be a bit frightening!

Kind Regards, John

I never stripped down the RCD socket but know it always tripped the 100ma trip (S type) when the test button was pressed. I agree it shouldn't happen but it did. Since the test button is internal not much one can do. Using a robin RCD tester both the 10ma and 100ma were within spec.

 

[JohnW2]

I never stripped down the RCD socket but know it always tripped the 100ma trip (S type) when the test button was pressed. I agree it shouldn't happen but it did. Since the test button is internal not much one can do. Using a robin RCD tester both the 10ma and 100ma were within spec.

Fair enough - but, as I said, if the test button on the 10mA RCD is resulting in a L-E leakage current ≥100mA, then it's a totally unsatisfactory (and potentially dangerous) test of whether the 10mA one is functioning reasonably satisfactorily!

Kind Regards, John