RCBO on TNCS system. - Earthing and testing

[RF Lighting]

You can take a reading of the external earth loop impedence value (Ze) by connecting your tester earth lead to the earth bar in the RCD enclosure, the live (or phase) lead to the supply side live of the RCD (top right terminal) and the neutral lead to the supply side neutral terminal of the RCD (top left terminal). You can only get an ELFI value from your sockets by either linking out the RCD (which is not something you should do with out being able to isolate it) or if you tester has a low current earth loop setting (sometimes refered to as trip lock) Post us the make and model number of your tester (or a picture) and we can see if it has this setting.

HTH

Rob

 

[plugwash]

your tester should have come with a set of probes that you can use to probe at the input terminals of the RCD.

note: as its a TT installation, earth fault loop impedance WILL be too high to trip a MCB or blow a fuse, thats why on a TT installation you have to have everything covered by some form of RCD (best practice is a 100ma time delay for everything then a normal 30ma for sockets etc but a 30ma covering everything is quite common).

 

[philrosenberg]

Cheers guys.
Unfortunately, although I have various leads and clips that fit my multimeter all I have for my loop tester is a flex with a plug on the end and a single probe that plugs into a 3 or 4 mm jack on.

It's an anologue Robin 4111. It has 100 Ohm and 2 Ohm modes but both trip the rcd.

I'll have to see if any of the labs at work have one I could borrow that may do the job.

Just out of curiosity, how do you guys isolate everything when you put a new CU in?

 

[plugwash]

your supposed to call out the DNO and get them to isolate but many sparkies just pull out the service fuse themselves.

 

[Steve]

Out of interest, i'm guessing that the electricity company claimed that "all their installations are TNCS and TNS" because earthing TT installations isn't strictly their problem.

What they should have said was "All of the installations that we have responsibility for earthing are TNCS or TNS, however there are some installations that are TT".

 

[Adam_151]

your supposed to call out the DNO and get them to isolate but many sparkies just pull out the service fuse themselves.

I know of at least one that does it live... The DNO notice if they are gone and charge the customer for resealing, and for elderly people on a pension, etc.... part p doesn't seem to get taken note of either...

Not recommened, but I'm sure given suitable care, confidence, and practice (hopefully on dead tails) one could learn to do it in a safe manner, afterall its how the REC work. Usual disclaimer applies though DONT TRY THIS AT HOME!

 

[JohnD]

All the best installations these days have a 100A DP isolator as a Main Switch in the tails. In your case you have your RCD which will safely isolate the CU (provided you remember to turn it off ) Putting cables in and out of that RCD would have been much less fiddly than putting them into the metalclad CU.

I suppose if you kept an isolator and 25mm tails handy until you had a card saying the meter was due for replacement, you might charm the DNO fitter into connecting the new rails for you.

 

[philrosenberg]

Fair enough then, so if the rcd starts leaving us in the dark on a regular basis then its not a simple job to replace. Well so far its only nuisance tripped once since we bought the house last summer so I'll keep my fingers crossed for the future.

Thanks again for all the help
Phil

 

[plugwash]

Cheers guys.
Unfortunately, although I have various leads and clips that fit my multimeter all I have for my loop tester is a flex with a plug on the end and a single probe that plugs into a 3 or 4 mm jack on.

can we see this lead?

 

[JohnD]

Replacing the RCD is simple, but doing it safely requires great care and/or pulling the suppliers fuse.

If you ever do replace it I would go for a 100mA 100mSecond (time delayed) RCD for the entire house which will be less prone to nuisance tripping; but in this cse you would need a 30mA 30mS RCD for the downstairs sockets (the one you have would be suitable, unless the entire CU is being changed for a modern one).

The trouble with a sensitive RCD for the whole house is that all the lights will go off when the fault is caused by e.g. a kettle or lawnmower. This makes it difficult to see what you are doing, which can be dangerous if you are carrying something heavy, or running down the stairs, or up a ladder changing a lightbulb.