Consumer unit replacement cost

[JohnW2]

Not if it was tested in isolation....

I'm not sure what that is a response to, but if it relates to the final copy of sentences of my last post, what Rocky was talking about was ...

... due to this like securespark i opted to go test at the Rcbo, as usual like most pubs, the Db was a mess and jammed pack, it was unsafe to disconnect the load due to the wires buried deep at the rear....

... and if the loads were not disconnected, I do not see that as any sort of 'isolation' - as I implied, I cannot see why testing there should be any different from testing 'at the socket' (without invoking 'extraordinary faults' ).

Kind Regards, Joihn

 

[JohnW2]

I would not of thought the leads are totally isolated as from memory isn't there circuitry that indicates if the leads are connected wrong ie wrong polarity, or no earth prior to pushing the GO button.

Sure - I shouldn't have used the word 'isolated'. What I should have said was "extremely high impedance" (such as, for example, one would expect to see with a DVM on a voltage range) - and certainly not remotely as low an impedance as would be needed to create enough 'leakage' to trip an RCD.

As for the last bit, sorry to confuse you, a few times it has failed only one of the 5 x times tests, but then passed when tested at the Rcbo mainly with it "disconnected", which I tend to do if safe enough.

Fair enough - but, yes, you did confuse me, because you had written (and I quoted) ...

...it passed the first tests but failed the 5 times test twice when tested at the socket .... due to this like securespark i opted to go test at the Rcbo, as usual like most pubs, the Db was a mess and jammed pack, it was unsafe to disconnect the load due to the wires buried deep at the rear.

With a load still connected, which is rarer, I too cannot explain why it could be any different, but i am sure sometimes it has been, maybe its because the Rcd has become saturated through the previous tests, maybe due to the type of loads connected further down the line, often Electronics or Refridge

Yes, they are all possibilities, but I don't think it makes any difference whether one tests at the RCBO terminals or at a socket connected to those terminals by 50 metres of cable - in other words, if it were possible to test at both those places, with the same tester, at exactly the same moment in time, then I think you would get identical results. If not, something strange must have happened to the Laws of Physics (or some extraordiunarlky weird fault was at play!).

Kind Regards, John

 

[RF Lighting]

When I was doing loads of commercial PIRs as they were then known it was a regular occurrence to have RCDs exceed the permitted trip time. If this happened as others have said, it was standard practice to disconnect the load and test in isolation and the vast majority would then pass.

I was taught that this happened due to the circuit having some capacitance so it holds a charge and appears live to the tester for an amount of time after the RCD has actually tripped giving a higher than is actually true reading.

Perhaps this explanation was wrong and it was actually “noise” on the circuit preventing an incorrect type of RCD from operating properly.

 

[JohnW2]

When I was doing loads of commercial PIRs as they were then known it was a regular occurrence to have RCDs exceed the permitted trip time. If this happened as others have said, it was standard practice to disconnect the load and test in isolation and the vast majority would then pass.

Fair enough - but, as you will realise, I'm a little nervous/concerned about that.

I was taught that this happened due to the circuit having some capacitance so it holds a charge and appears live to the tester for an amount of time after the RCD has actually tripped giving a higher than is actually true reading.

That explanation seems a little hard to believe, given that we are talking about AC current - although I suppose that within one half-cycle (10 ms) it is effectively DC. I certainly wouldn't see that as a credible explanation if the reported trip time was more than about 10 ms more than the figure obtained when one disconnected the load.

However, I don't think that alters the generality of my concerns. There clearly are some uncertainties about how the results produced by RCD testers may be influenced by various factors, be they known or unknown - but they are the best way of testing the function of an RCD that we have.

If a tester which simulates an L-E fault indicates, on a circuit 'as is' (with whatever connected loads), performance which does not meet our 'minimum requirements', then I cannot see how we could be confident that it would perform satisfactorily [again with the circuit 'as is' (with whatever connected loads)] should an L-E leakage (maybe through a human being) subsequently arise - even if the device meets the requirements when 'tested in isolation'.

I recently mentioned an analogy in relation to car brakes. Imagine that the brake on one wheel of a car failed the standard ('rolling road') test - a test which I'm sure is in some senses 'imperfect' and subject to distortion by all sorts of factors. If they then removed the brake from the car and 'bench tested' it ('in isolation') and found it to perform satisfactorily, would you be happy for the car's braking system then to be 'passed', despite the 'in-service test' (analogous with testing an RCD in situ) failure?

Kind Regards, John

 

[333rocky333]

Rcds appear to be becoming a bit of a pain and I think future Electricians will spend half there life with work relating tp Rcds.
I dot recall in all my career so many issues relating to them like there is now, then again testing although compulsory i feel was never really done like it is nowadays.
I bought Rcbos for my house and before I had even fitted them all, they were obsolete and redesigned, by Wylex and now upon fitting, I find one not even operating on the Test button, let alone with a Tester and that was straight out the box, so what hope have we ten years down the line.

 

[JohnW2]

Rcds appear to be becoming a bit of a pain and I think future Electricians will spend half there life with work relating tp Rcds. I dot recall in all my career so many issues relating to them like there is now, then again testing although compulsory i feel was never really done like it is nowadays.

They (and testing thereof) are certainly generating a lot of discussion, and I don't see that reducing.

One thing I've never really understood is why there has not been a requirement to test them whilst the circuit(s) was under at least a moderate degree of load, particularly given that the current flowing through an RCD in, say, a dual-RCD board could be very substantial in service. If one tests and RCD 'in isolation', what the RCD sees is, say, 30mA passing through the L and zero current passing through the N - but I don't find it hard to believe that its in-service performance might be different (from that) if the situation were, say 30,030 mA passing thorough the L and 30,000 mA passing through the N.

I bought Rcbos for my house and before I had even fitted them all, they were obsolete and redesigned, by Wylex and now upon fitting, I find one not even operating on the Test button, let alone with a Tester and that was straight out the box, so what hope have we ten years down the line.

That's obviously a different matter. Things which are faulty 'out of the box' are not anything like as uncommon as they should be - but I suppose that ultimately comes down to the desire of users/consumers to have 'cheap' products.

Kind Regards, John

 

[securespark]

Fair enough - but, as you will realise, I'm a little nervous/concerned about that.

Don't hold back, John......say it like it is:

That's surely rubbish, and dangerous rubbish at that, isn't it?

 

[JohnW2]

Don't hold back, John......say it like it is:

In view of some of the things you've subsequently said, I may have partially misunderstood what you were talking about, but I would still be inclined to describe as "dangerous rubbish" a situation in which an installation (and one of its RCDs) had been 'passed', and one had 'walked away', despite the fact that, as the installation was in-service, a test procedure which did its best to emulate a current of 30 mA leakage through, say, a human being either did not result in the RCD tripping at all or else did not result in the RCD tripping quickly enough. That's what I initially thought you were talking about.

However, as I said, if (as I think you're now saying is far more common in your experience), you are talking about an RCD 'failing' when tested in situ (but not when tested 'in isolation') only because it tripped (rapidly enough) at a current ≤0.5 x IΔn, then that's more of a potential 'inconvenience' than a 'safety risk' issue, so it's probably OK to call that a 'pass' - although, as I've said, in that situation I would personally be inclined to advise the owner of the installation about redistribution of loads etc. in order to reduce the risk of 'nuisance' trips.

Kind Regards, John

 

[JohnD]

When I was doing loads of commercial PIRs as they were then known it was a regular occurrence to have RCDs exceed the permitted trip time. If this happened as others have said, it was standard practice to disconnect the load and test in isolation and the vast majority would then pass.

I was taught that this happened due to the circuit having some capacitance so it holds a charge and appears live to the tester for an amount of time after the RCD has actually tripped giving a higher than is actually true reading.

Perhaps this explanation was wrong and it was actually “noise” on the circuit preventing an incorrect type of RCD from operating properly.

Looking back, do you remember if any of them were designed to handle DC or pulsing currents? MEMshield, have been for many years, and are quite common.

 

[RF Lighting]

Honestly I have no idea. I didn’t even know there were different types of RCD back in those days!

 

[JohnW2]

Looking back, do you remember if any of them were designed to handle DC or pulsing currents? MEMshield, have been for many years, and are quite common.

"That" topic again

However, on the assumption that "that issue" does sometimes show itself in practice, does it not underline and support what I am saying? - i.e. that if an RCD 'fails' (because of too high a trip threshold or too long a trip time) when tested 'in situ', but 'passes' when tested 'in isolation', one should NOT regard that as a 'pass' - since the implication is that something (be it current waveforms in connected loads or whatever) which is at play when the RCD is in service is, or may be, impairing the ability of the device to operate satisfactory/safely in its in-service environment?

As far as pass/fail of the testing is concerned, the reason for failing in-situ ('on load') testing is surely irrelevant - if (for any reason) it cannot function satisfactory when 'in situ', that means that it is not safe, and therefore should not 'pass', even if it "works fine on the bench"?

It has been suggested that the 'in-situ' test failures may not be a true representation of the actual performance but, rather, an erroneous test result (maybe as a consequence of connected wiring or loads). That's obviously not impossible, but it is surely not safe to simply guess that such is happening. If the 'best (but not perfect) means of testing' reports a 'fail', then we surely have to assume that it may be (probably is) doing so correctly?

Kind Regards, John