Whats this mean on a rcd test

[davesparks]

You can join Derek in the naughty corner too.

I hope he's got plenty of cider in!

 

[Maxstone89]

Yes but why would there be fault voltage?

 

[Midwest]

I would like to add my morsel.

When I did my C&G course on initial testing & inspection, I was taught to use an MFT with plug & lead at a socket to conduct RCD testing. I can see when there perhaps is no other option to test at the RCD terminals, i.e. RCBO protected radial circuit with just FCU at the end. Electricity at Work Regs does permit working live, but only if ‘unreasonable in all circumstances to be dead’ etc.

I have looked at several references on how to carry out the functional test of an RCD.

GN3 states; ‘the test is made on the load side of the RCD between line conductor of the protected circuit and associated cpc’.

OSG goes on to add; ‘as near as practicable to its point of installation’.

I find that somewhat ambiguous. If I was trying to describe the test being conducted at the RCD terminals only, I would not use the phrase ‘load side of the RCD between line conductor of the protected circuit and associated cpc, and as near as practicable to its point of installation’.
I would have written something like ‘on the load side terminals of the device being tested and not on the circuit being protected’. But that’s my opinion.

Unfortunately neither GN3 nor OSG have a pretty set of diagrams (unlike most other tests) to reinforce what they are actually describing.

BEAMA RCD Handbook; All tests must be performed with all loads disconnected, making use of an appropriate calibrated test instrument connected as close to the RCD as possible for convenience.

Advanced Electrical Installation Work publication (by Trevor Linsley); ‘…using standard lead supplied with MFT, disconnect all loads and plug in test lead into socket at the centre of the ring (i.e. socket at the furthest point from the source of supply).

I have spent some time web engine searching to find anything to suggest that the test ‘MUST’ be carried out on the RCD terminals ‘ONLY’, with a negative result.

I’ve emailed some of the RCD manufacturers, putting forward both sides of the argument (citing, poor IR, other leakage currents and possibly cable capacitance reasons for not carrying out test at socket etc), some of them responded;

MK: Unfortunately we are unable to comment on site specific testing of any devices including RCDs; however we would advise that all testing must be carried out strictly in accordance with the requirements of BS7671 and associated guidance notes.

Hager: The correct way to test an rccb is at the terminals with the load disconnected in accordance with the on site guide.

Wylex: The test should be carried out as near as practicable to its point of installation. This may vary from install to install and it may be that a test using a plug in lead at the first socket is that nearest practicable point, just depends upon the install and any accessibility issues that may exist.

Then others;

Elecsa: The issue here is being able to test the RCD in a safe manner. Whilst the OSG and GN3 state “load side of the RCD with loads disconnected” which suggests purely testing the RCD at the board, it is quite an acceptable option to test the RCD by plugging the tester into a socket as long as your tester has that facility to allow you to do that.

Electrical Safety First: Usually we would refer you to the (scheme helpline), but I have scanned in this page (attached in .pdf format) from our “snags and solutions” guide, which will hopefully be helpful. This refers to the NICEIC booklet part 3 Inspection & testing; Snag 49 RCD testing – when current using equipment is connected. Answer: disconnect equipment, measure at any convenient point in the circuit, such as socket outlet. There is also a diagram that shows test being done at the RCD terminals with circuit still connected, but loads disconnected.

I then asked my training establishment: 'It does not matter where on the circuit you carry out the test as the tester uses a set amount of current i.e. for a 30 mA rcd at 1x the tester will use 30 mA thus the resistance of the circuit cable is compensated for. If the circuit resistance was a factor so would the Ze value be. It is convenient and safer to carry out the test at a convenient socket.' (There is a longer response from them, which I haven’t included here).

Kewtech: Both are correct. You have to look at practicality and accessibility. What happens if you test at a socket and the RCD does not trip due to an earth fault and you then go to the board and test the RCD and it trips? You have proved the RCD at the board as opposed to the socket.

Lastly I spoke to my MFT manufacturer (Megger). Who said; neither is the wrong way to carry out the test, but it would be preferable to test on the circuit being tested. Poor I/R should have already been tested and investigated, earth leakage should not be present (as loads disconnect) & the circuit would have to be extremely long for cable capacitance to have an effect (also loads disconnect) (not verbatim conversation). Also said ‘we are aware of an RCBO – MEM MR30 – which will not function test with the tester at its terminals, but will when tested on the circuit being protected (I can provide the citation if required).

 

[Lucien Nunes]

I'm suprised that MK don't feel able to advise on the safe use of their products. Hopefully they themselves know how to test an RCD.

I agree that it should not matter where on the circuit you test as far as test validity or performance are concerned, unless the circuit parameters are exceptional. I would avoid disconnecting conductors unless essential, and I know of RCDs that can't be tested from the terminal screw heads as the outgoing cables themselves pass through the transformer core. Other than that I'd go with the safest / most convenient location. One advantage of testing at the device in a CU is that the test path can be from load L to supply N, avoiding reliance on the CPC. At a socket outlet, the test must use the CPC. as the supply side is inaccessible. OTOH one is then working with higher PFC on the test leads and greater chance of arc flash or shock injury in the event of damage.

Whether the circuit is connected or not will only make a significant difference if the cable runs are extreme or the IR very poor. 1 megohm L-E only contributes 0.23mA of resistive leakage and 100m of 2.5 T+E another 0.8mA of capacitive leakage (at 110pF/m), possibly a bit more if run in contact with earthed structure. Normal domestic circuits in good condition should not materially affect any but the most borderline RCD tests. Likewise the effect of r2 should be negligible if continuity has already been proven.

 

[NDG Elecs]

Good work Mid. So no definitive answer then! As near as possible to terminals seems to be the main guidance gleaned from that lot.

 

[NDG Elecs]

I would like to add my morsel.

That's a morsel and a half Mid!!

 

[Midwest]

You wouldn't believe it with my flipping browser, gave up in the end, borrowed a laptop and cut and pasted it!

 

[Deleted member 26818]

I think consideration should first be given as to why the RCD test is being conducted.
Is the purpose of an RCD test, to prove that the RCD operates, or that it provides the intended protection?
All very well proving an RCD operates at the board, but not much use in proving that the RCD provides protection for circuits of a location containing a bath or shower, socket-outlets, equipment used outdoors or cables concealed in walls.

 

[Midwest]

The other side of the argument, sorry discussion, is that having completed all the other tests, when testing the RCD it is just a function test of the device itself, which could be carried out at it's terminals. That said, you have to consider conducting that test inside a live CU. We are all skilled persons (and grown up enough to know how careful to be), but I don't know why take the risk, when it can be carried out otherwise? The suggestion to dismantle bus bars or disconnect circuit conductors to carry out a device test only doesn't seem necessary, to me anyway. Unnecessary disturbance of a CU. Perhaps we shall see further debate shortly?

 

[davesparks]

I think consideration should first be given as to why the RCD test is being conducted.
Is the purpose of an RCD test, to prove that the RCD operates, or that it provides the intended protection?
All very well proving an RCD operates at the board, but not much use in proving that the RCD provides protection for circuits of a location containing a bath or shower, socket-outlets, equipment used outdoors or cables concealed in walls.

Nonsense, if it works it works. The outgoing wiring will not make it trip slower or be less sensitive if it has passed the other tests, what it can do is make it trip faster.

 

[Deleted member 9648]

Still splitting hairs then fella's lol.

 

[Deleted member 26818]

Nonsense, if it works it works. The outgoing wiring will not make it trip slower or be less sensitive if it has passed the other tests, what it can do is make it trip faster.

Why assume it has passed the other tests?
In what way could the wiring make the RCD trip faster?

 

[telectrix]

OK. consensus of opinion is to rest with circuits dissed. seen a few RCDs test fine. then connect the circuits, 1 of which has a N-E fault. in certain cases, the RCD will not trip. so, a perfectly functional RCD will not operate in the real world and maybe death or injury results.

 

[Deleted member 26818]

What would be nice would be a consensus of reasons as to why the RCD should be tested with the circuits dissed.
Would it be considered Satisfactory if an RCD operated when tested at the terminals, but failed to operate when tested at a socket-outlet?

 

[telectrix]

What would be nice would be a consensus of reasons as to why the RCD should be tested with the circuits dissed.
Would it be considered Satisfactory if an RCD operated when tested at the terminals, but failed to operate when tested at a socket-outlet?

exactly my point.

 

[Midwest]

what it can do is make it trip faster.

What evidence do you have for that?

 

[Deleted member 9648]

OK. consensus of opinion is to rest with circuits dissed. seen a few RCDs test fine. then connect the circuits, 1 of which has a N-E fault. in certain cases, the RCD will not trip. so, a perfectly functional RCD will not operate in the real world and maybe death or injury results.

Shouldnt the circuit with the N-E fault be identified before it is energised?

 

[Dave OCD]

Exactly right.

 

[VoltzElectrical]

Interesting thread. Did we reach a consensus on this or not? My own opinion still stands by the way, the RCD still needs to operate within the parameters as close to the point of use as possible.

Respect

 

[marc8]

Hi All
just pause for thought here about the RCD test & what you have said, it's says disconnect LOAD.
but if you think about it if you have unplugged everything then really & truthfully you are just using long test leads to do the job as there is no LOAD on the circuit, unless you are taking into account the resistance of the wire in the circuit.
but from a LOAD point of view there is no LOAD, just a pair of long test leads.
we also a while ago answered this & have had RCD units that would not test until we installed a LOAD on them which was bizarre