Exactly. Case closed.

If I may be permitted to have my 10 pence gents.

IMO on a new install all max values should be met and simply bunging an RCBO on a circuit which fails through what seems to be poor design or possibly a high resistance joint seems a spurs and straw to me, readings should be within limits RCD or not for new installations.

If it were me I would certainly not be happy with this and further investigation is definately needed.

On the other hand as more than one circuit is affected then possibly an issue with the test equipment???

As for the opinion of the chap with the RN measurement being suitable as an alternative.....

I agree but some cable and device combinations are easy to stray out of the max Zs permitted

Take a 4mm t/e with its 1.5mm cpc say thats feeding a fixed load of 26A so a 32A type C rcbo would be used. The cable is well able to carry the load but the cpc although usually able to clear the fault thermally will cause a high r2 value.

As for the guy suggesting using the RN value I think he needs a refresher on the purpose of the testing and form filling.

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James

Could the OP's colleague be suggesting that the RCBO will take care of an earth fault due to the max Zs of 1667 ohms, but also suggesting that a L-N loop test be carried out to ensure that disconnection times would be met by the overload part of the device in the event of a short circuit?


Steven

The easy option for testing all the circuits for IR in one go, is gone!

Regards

this is what i was trying to get at the zs to high? but dont worry its protected by rcd it just seems wrong

so an rcd will trip in 0.4 secs the max that a human can take apparently so i suppose we are like the breaking capacity of a fuse lol
but this is what i feel. wouldnt the high resistance in cables slow fault currents down ie making the rcd trip slower

Remember a RCD tester tests at the rated current eg 30mA which lets face it is not much of a fault to produce this level of current.

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James

i think its finally sank in thanks the rcd is so sensitive that it will never get to the point of being dangerous theory anyway.

as soon as it detects inbalance of 30ma it will trip

still agree with getting zs down thou but now i get many thanks

I agree. The circuit still need to disconnect within a reasonable time (say <5s) on a L-N fault. L-N loop impedance within the tabulated values for Zs would allow the magnetic element to do just that.
- Andy.

Maybe I'm easily confused, but 411.4.4(ii) seems to say selecting an RCD is acceptable even on TN systems and I don't recall any requirements for R2 alone (not since the "alternative method" and 5s disconnection for sockets disappeared with the 16th Ed).

- Andy.

Another reason that causes concern on this high R1+R2, High R1+R2 may suggest a slack connection on a live terminal which can cause heat to be generated raising a fire risk that won't be detected or protected by the RCBO

how have you determined what the maximium permissable measured earth fault loop impedance is for the device you are using?

I trust the answer will not be" from the table in the onsite guide" or "from the table inside the cover of the NICEIC certificates pad" seeing as it seems so critical.

Andy

ps. how many times did you take a reading and was it constant?

"There is an earth - and it has been proved. The problem is that the resistance of the earth causes the R1+R2 to be too high."

why is it not R1 causing the problem?

1.0 and 1.5 T+E both have the same R2, but different R1+R2.

"There are various circuits, all with the same problem. One of the worst ones is external lighting, it's all been concreted now so there is no chance of running in a new cpc." if it is three core SWA then R2 is same size plus the armour, if it is two core SWA there should be plenty of armour, any other cable I would expect to be in a duct and be able to be pulled out for replacement

the same is true of 4.0 and 2.5 mm T+E both have the same R2, but different R1+R2.

Andy

You might like to tell us a bit more about the circuit. What is it serving, what size conductors, what current rating and type of overcurrent protection, what length of cable run, what type of cable, what values are you getting for R1 and for R2, what is your measured Ze, what is the method of earthing? The phase to neutral test will enable you to measure R1 (well 2 x R1 actually, then divide by 2) but have no idea of R2.
How long have you been "in the game"?

The phase to neutral test will enable you to measure R1 (well 2 x R1 actually, then divide by 2) but have no idea of R2.
How long have you been "in the game"?

Not on a conventional lighting circuit it wont.

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Regards

Dave(not Cockburn)

there are 3 options available for a circuit with a high Zs

1 . increase size of cpc
2 . Decrease size of cpd
3 . install additional rcd protection.
al options are acceptable .

Now you say that increasing cpc size is not possible .so can a smaller cpd be used?(not knowing the circuits in question apart from outside lights ) for example are there any ring mains involved? can a 20 or 16 amp cpd be used ? usually ample for rings except in kitchens .a little more information will help.
Main concern really like stated before ,is if the circuit's have been designed properly,(which in a normal size house isnt rocket science)
why is the Zs to high .main things to check as listed by others posted above.
1. is meter correct and on correct settings , batteries and leads all ok?try another meter ,same results?
2.have connections been inpected for damage or corrosion and that they are tight?any decorators or others removed outlets?
not trying to state the obvious but we have all been there at a problem and overlooked something obvious at one time or another.

Either;

a. The designer got it wrong.
b. The meter is on the blink.
c. There are loose connections.
d. The cable came via the old silk road.

Any chance you might have an earth somewhere not properly tight? e.g. a loose terminal somewhere

Re-reading the OP, maybe the taking a PSCC reading is related to checking ADS for the circuit compliance. It sounds to me like its only just out of Zs for BS7671 and if readings are OK for PSCC to satisfy ADS, and the Zs is sufficiently low and protected by an RCBO anyway so can be tolerated then I wouldnt bother about it and just make a note on the cert. I'd be more concerned about poor terminations if it was too much higher than expected than the R1+R2 for Zs..

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