Passive RCD unit

[dor]

Not a boat issue but this is on the supply to a powershower at home.   Unit is similar to this:  http://www.screwfix.com/p/13a-30ma-double-pole-passive-rcd-connection-unit/40097

When the test button is pressed, the unit buzzes but doesn't disconnect and the reset button doesn't move.  L-N & L-E show 237V  N-E shows 0V.

As far as I can make out the wiring is correct and the shower pump works normally.

I think the RCD unit is faulty;  is there anything I am missing?

Your expert guidance appreciated.

[mross]

I would say, don't use the shower and get an electrician in to fit a new RCD.

[Chewbacka]

If the shower works ok that would indicate the supply is fine and if the T button does not work , then for me. that indicates the RCD unit is broken.  It needs replacing and the shower is almost certainly not adequately protected, and as said above the only sensible advice you can be given is NOT to use the shower until it is fixed.

[system 4-50]

Ok, here's the non-sensible advice. Use the shower as normal and replace the RCD as soon as possible. Why have you got the powershower on a separate RCD from the rest of your domestic stuff? 

[WotEver]

3 minutes ago, system 4-50 said:

Why have you got the powershower on a separate RCD from the rest of your domestic stuff? 

That's very common. 

To answer the OP's question, the RCD is faulty and should be replaced. 

[system 4-50]

3 minutes ago, WotEver said:

That's very common. 

To answer the OP's question, the RCD is faulty and should be replaced. 

Ok, its common. But why? If there is a good reason then I'll modify mine to match.

[Chewbacka]

9 minutes ago, system 4-50 said:

Ok, here's the non-sensible advice. Use the shower as normal and replace the RCD as soon as possible. Why have you got the powershower on a separate RCD from the rest of your domestic stuff? 

If one one is hurt, then fine, if someone were to be electrocuted and as you are well aware that the RCD does not work having discussed it in a public forum, I would be interested in hearing your proposed explanation as to why you were not grossly negligent in allowing it to be used.

[system 4-50]

8 minutes ago, Chewbacka said:

If one one is hurt, then fine, if someone were to be electrocuted and as you are well aware that the RCD does not work having discussed it in a public forum, I would be interested in hearing your proposed explanation as to why you were not grossly negligent in allowing it to be used.

Simple, I am not in a position to allow or disallow anything. That responsibility lies elsewhere no matter what I say. By the way, don't use wheels or fire in any form - they are dangerous. Also beware, the sky may fall on your head. Now where is my bucket of postumus smileys...

Oh and I did say it was non-sensible. I just invented that word so I'm not sure what it means.

Edited May 16, 2017 by system 4-50

[Giant]

4 minutes ago, system 4-50 said:

Ok, its common. But why? If there is a good reason then I'll modify mine to match.

I would guess that it's because there is a higher risk of a fault in a shower environment with water everywhere - and if a fault occurs in the shower unit, having all the lights go out in the bathroom when the master RCD trips is not that helpful or safe!

I speak from experience...

 

[system 4-50]

Just now, Giant said:

I would guess that it's because there is a higher risk of a fault in a shower environment with water everywhere - and if a fault occurs in the shower unit, having all the lights go out in the bathroom when the master RCD trips is not that helpful or safe!

I speak from experience...

 

Thats an argument I can understand, but good grief, a little bit of dark. I can remember power failures - and having been employed by a big company, I am used to working in the dark.

[WotEver]

Not every house has a whole house RCD (ours doesn't). Therefore it's simpler to just make it standard practice when wiring in a power shower to always fit it with its own RCD. 

[system 4-50]

1 minute ago, WotEver said:

Not every house has a whole house RCD (ours doesn't). Therefore it's simpler to just make it standard practice when wiring in a power shower to always fit it with its own RCD. 

Um, I believe the latest edition calls for 2 RCDs per household, in readiness for when divorce proceedings start.

[dor]

OK thanks for the replies.  The consensus is that the unit is faulty so I will replace it asap.  The shower is in a guest bedroom so not used all the time.

Just wanted confirmation of my thoughts and that I hadn't missed something.  I know that a faulty earth connection can affect their operation, but the voltages on the supply suggest the earth is fine.

[nicknorman]

9 minutes ago, dor said:

OK thanks for the replies.  The consensus is that the unit is faulty so I will replace it asap.  The shower is in a guest bedroom so not used all the time.

Just wanted confirmation of my thoughts and that I hadn't missed something.  I know that a faulty earth connection can affect their operation, but the voltages on the supply suggest the earth is fine.

The test function doesn't require an earth.

[tomsk]

The 'Test' function on these does not simulate fault conditions as I recall, merely that the mechanism is functional should fault conditions arise.

If it doesn't trip when you press the button it is knackered and should be replaced.

[dor]

36 minutes ago, nicknorman said:

The test function doesn't require an earth.

Interesting Nick.  I haven't gone into it that closely, but I thought they detected an imbalance between live and neutral currents, resulting from a leak through something to earth.  I though the test function would simulate this by introducing a link to earth via a suitable resistor.  I understand that you can make a plugtop work as a test unit by wiring a resistor between the live and earth pins. Presumably then the RCD unit does it differently.

Edited May 16, 2017 by dor
crossed with tomsk

[nicknorman]

11 minutes ago, dor said:

Interesting Nick.  I haven't gone into it that closely, but I thought they detected an imbalance between live and neutral currents, resulting from a leak through something to earth.  I though the test function would simulate this by introducing a link to earth via a suitable resistor.  I understand that you can make a plugtop work as a test unit by wiring a resistor between the live and earth pins. Presumably then the RCD unit does it differently.

The live and neutral are both wound round a torroid (or core, anyway) such that their currents flow in opposite directions and thus there is zero magnetic field produced. If there is an imbalance, there is an alternating magnetic field and a small auxiliary coil has some current induced in it, which trips the breaker. There are a few ways of doing the test but a common way is to have yet another small coil which, when the test button is pressed, is put across L and N (via a resistor) and thus inducing a magnetic field which the aux coil picks up and the breaker trips. The earth can't figure in this because there is normally no earth connection to an RCD. An American ELCB is different, I think.

if you want to test an RCD remotely e.g. using a test plug, this is done as you suggest by connecting a resistor between L and E.

 

Edited May 16, 2017 by nicknorman

[mross]

On a modern RCD, the test button puts a resistance in the circuit to simulate a fault.  It is not just a mechanical test, it is a functional test.  If you let someone use this shower you are highly irresponsible and, I would say, criminally negligent.

[dor]

4 minutes ago, mross said:

On a modern RCD, the test button puts a resistance in the circuit to simulate a fault.  It is not just a mechanical test, it is a functional test.  If you let someone use this shower you are highly irresponsible and, I would say, criminally negligent.

Don't worry, I have no intention of!!!

What do you take me for?  The unit will be replaced before the weekend.

[mross]

1 hour ago, dor said:

The consensus is that the unit is faulty so I will replace it asap.  The shower is in a guest bedroom so not used all the time.

My apologies, I read this as you not taking it very seriously.

[dor]

2 minutes ago, mross said:

My apologies, I read this as you not taking it very seriously.

"I will replace as soon as possible",   "not used all the time"

Where is the frivolity in that?

Anyway, let's move on. No point in discussing morality when it was the technical aspect I was checking on.

[Chewbacka]

59 minutes ago, dor said:

Interesting Nick.  I haven't gone into it that closely, but I thought they detected an imbalance between live and neutral currents, resulting from a leak through something to earth.  I though the test function would simulate this by introducing a link to earth via a suitable resistor.  I understand that you can make a plugtop work as a test unit by wiring a resistor between the live and earth pins. Presumably then the RCD unit does it differently.

If the test button were to put an actual current between L and E - which would be the 'best' way to test - has one very big disadvantage, and that is any RCD upstream would be at a high risk of tripping as it would also sense the in-balance current and that could switch off lights, or other 'safety' systems which means quite quickly no one would press the test button.

[mross]

The upstream RCD can be set with discrimination - ie, trips at higher current and/or a time delay.

[Chewbacka]

1 minute ago, mross said:

The upstream RCD can be set with discrimination - ie, trips at higher current and/or a time delay.

But domestic ones are all 30mA, would not want to reduce the sensitivity of the whole house trip to 50mA or a much slower response time, in which case the downstream would probably have to be 10 or 15mA to discriminate, but that could give more nuisance trips.  So though not ideal the current test button set-up is probably the best compromise. 

[BEngo]

Does this apply to all earth types? I thought a TT earthing system had to be protected in its entirety by a 30 ma RCD.  I have not seen an adjustable RCD of this trip current.

The other problem with trying to discriminate between trips is the presence of stray and deliberate earth leakage ( wall wart power supplies are bad in this regard, as are immersion heaters.) The extra leakage uses up the discrimination margin so the wrong trip goes anyway.

 

N