RCD testing at 5I on a TT supply

[pcboffinuk]

Whilst adding a small CU to a job today (to stop the garden lighting tripping the whole house) I ran into an issue with testing the rcd.

The Ze of the house (TT installation - quite a lot of them round here) was rather high at 200 Ohm but since the main CU had a 30ma main incomer could be considered compliant.

The problem came when testing the 30ma RCD in the new CU (Fed via henley block from the meter tails).

It passed the usual tests at 1/2I & 1I, but when it came to the 5I test my Megger MFT1552 would not test, because it detected the protective conductor voltage rising about 50V.

Note that it was the 50V limit that was stopping it, rather then the >40ms that I might have expected.

Since the RCD tripped at I in 33ms or so, I had no reason to suspect a dodgy rcd, but it appears that the poor earth was causing it to reach the limit and abort the test.

In fact there was an earth problem in the lighting circuit, which I will be returning to fix, but am I missing something here? I got the same results testing directly at the CU, where the Zs was 200 or less.

It appears from my reading that the 50V limit cannot be turned off on the Megger, although it seems that the regs only see it as an issue AFTER the 40ms time has expired?

Has anyone run into this before and found a work around, or is it a 'feature' of my Megger that I may run in to again.

Its late, but quick calcs suggest that an earth of > (50V / (5*150ma)) = 333 Ohm would always give this problem, and yet with rcds an earth of that value could still be considered compliant.

I realise that ideally fitting a new earth rod would be the answer, but since this job was just moving the garden lighting circuit to a separate rcd and not altering the existing setup the client may well not be prepared to consider if what is there is technically 'compliant'.

Any views welcome, and if I've missed something stupid I claim the late hour as an excuse

Gavin

 

[THRIPSTER]

Boff,



I suppose the manufacturer do not want to be held responsible for raising the voltage on the MET to >50v for fear of being held accountable. Another example of the pansified world we are now living in.



Regards

 

[Spark123]

It does say in the guidance notes that an earth electrode resistance exceeding 200ohms should not be considered (iirc for reliability). This may even be lowered to 100ohms if what is written in the 17th DPC is adopted.
As you are making an addition/alteration you should ensure the earthing and bonding is adequate.
The RCD is required to protect to the 50v rule for protection against indirect contact. Is the RCD also providing supplementary protection against direct contact?

 

[pcboffinuk]

I realise that the earth is not ideal, but it could still be seen as compliant, and I would argue therefore 'adequate'. The reading I took for Ze was just under 200 Ohm, although the fact that my Megger wouldn't test suggests that it considered it higher at the time of test.

In fact in this house there is no obvious earth rod pit, so the cost involved in investigating and correcting what is there may well not be something the customer is happy to consider.

The RCD in this case is no providing supplementary protection against direct contact - there are no sockets on this circuit, although there could easily have been. (external lighting and a waterfall pump)

I have always taken it that any 30mA should be tested at 5I, but having done some browsing others seem to be suggesting that the 5I test is only needed when the RCD is providing protection against direct contact - is that what you were getting at Spark?

If so, I would still think testing it would be preferable, in case the circuit is changed in future to include a socket.

It seems that I am not going mad though, and that there is no obvious way to test at 5I when the earth is poor?

Gavin

 

[plugwash]

if you just want to test the RCD put your testers earth probe on the supply side neutral to the RCD.

 

[Jim2287]

50v and 25v alert is feature of Megger and fluke (and maybe other testers) ... to alert you the installation raises to above 'safe touch voltage(50v in house, 25v construction/argicultural) ... so i dont think it can be turned off

 

[RF Lighting]

Did you try weeing on the earth rod?

 

[securespark]

Did you ramp test?

 

[pcboffinuk]

sorry but I'm not a registered wee-er

Yes, it passed the ramp test, just the 5I that it wouldn't do due to the earth.

I guess its something I will have to bear in mind for future reference, since there are quite a lot of TT around here, even though we are supposedly urban.

I will use the neutral to get a reading when I go back I think, and move on.

Cheers for the replies

Gavin

 

[Spark123]

If the RCD is not used for supplementary protection against direct contact it doesn't need checked at 5I anyway.
Seems strange that the voltage should get up to 50v with 150mA, is indicating an external resistance 330> ohms which will probably be the root of the problem.