Testing electrode/EFLI
- Thread starter Sean30
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I didn't know the DNO fitted RCD's on installs? I'm guessing from the title it's a TT installation?
I suppose you would have to take it between the incoming line terminal of the RCD and the earth rod?
Sorry I think edited my post while you replied. I'm not sure how else you could do it then. But i'm not a spark and i'm sure one will be along shortly to clarify.
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Can you post a picture of that "RCD". It may be one of the obsolete voltage types. In which case it may need to be removed..
I dont have a pic and I am not there to take one, but it doesnt look perticularly old if that helps
Am I right in thinking some testers have a no-trip setting using three test leads, so you include the incoming neutral?
Am I right in thinking some testers have a no-trip setting using three test leads, so you include the incoming neutral?
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I have often worked when different companies control different parts of the supply to a site. It is common to have moulded breakers with tripping coils connected to an RCD unit which has two thumb wheels one for current and one for time and these can be set quite high for example 1 amp at 5 seconds and there is often an arrangement between the companies as to how these are set.
Even meters without no-trip technology can normally test the systems when the values are so high. Of course the panels are always locked as required by BS7671 and each company can't normally re-set the others RCD's.
However the RCD tester will highlight any non-conformance by other company so the practice of turning down the settings is soon found out.
But this has never been the case with a TT supply. The supplier must have a earth of 21 ohms or better so if you measure the earth rod then add 21 ohms to reading this will suffice. You are allowed to use both measurement and enquiry and if you enquire the DNO will normally quote 21 ohms for TT. 0.8 ohms for TN-S and 0.35 ohms for TN-C-S.
I have debated these readings many times. The DNO does not need to inform you if they change the transformer supplying your installation. So if you measure for example 0.25 ohms on a TN-C-S system at the meter and then on a ring main (before 2008) on 32A B type 1.4 ohms should one fail it? If the DNO changed supply it could then measure 1.5 ohms. It seems reasonable to fail until one considers a factory with a 300A supply which would likely have a reading of around 0.1 ohms at head and at 0.35 most of the supplies to large machines would fail. Sorry I don't have the answer. But if the reading on a socket passes on the day I would personally pass it without any correction for actual against guaranteed ELI.
As to three leads I have found the connections in the meter very confusing. I used top of range robin and the PFC was auto measured line to neutral as it should be and ELI measured Line to Earth as it should be. But cheaper meters have given the PFC measured line to earth when using plug in system. So where I can I use the wandering leads so there is no chance of error as to what pair the meter is measuring with.
Even meters without no-trip technology can normally test the systems when the values are so high. Of course the panels are always locked as required by BS7671 and each company can't normally re-set the others RCD's.
However the RCD tester will highlight any non-conformance by other company so the practice of turning down the settings is soon found out.
But this has never been the case with a TT supply. The supplier must have a earth of 21 ohms or better so if you measure the earth rod then add 21 ohms to reading this will suffice. You are allowed to use both measurement and enquiry and if you enquire the DNO will normally quote 21 ohms for TT. 0.8 ohms for TN-S and 0.35 ohms for TN-C-S.
I have debated these readings many times. The DNO does not need to inform you if they change the transformer supplying your installation. So if you measure for example 0.25 ohms on a TN-C-S system at the meter and then on a ring main (before 2008) on 32A B type 1.4 ohms should one fail it? If the DNO changed supply it could then measure 1.5 ohms. It seems reasonable to fail until one considers a factory with a 300A supply which would likely have a reading of around 0.1 ohms at head and at 0.35 most of the supplies to large machines would fail. Sorry I don't have the answer. But if the reading on a socket passes on the day I would personally pass it without any correction for actual against guaranteed ELI.
As to three leads I have found the connections in the meter very confusing. I used top of range robin and the PFC was auto measured line to neutral as it should be and ELI measured Line to Earth as it should be. But cheaper meters have given the PFC measured line to earth when using plug in system. So where I can I use the wandering leads so there is no chance of error as to what pair the meter is measuring with.
T
tonylec
We generally would take our measurement from between the incoming side of the cutout and the electrode tail, now that of course entails removing the cutout fuse to stick your probe in.
Depending on where you are in the country and what arrangement your scheme has with the DNO you can go on a short (1/2 day) course on removing the cut out and a sealing kit of "blue" seals to re-seal it (thats the arrangement we have here).
I should point out that if you are not trained or permitted to remove a cutout don't do it, it has the potential to be very dangerous if the casing is old and falls to bits!! and if you don't have spares you are in an embarrasing situation at the least!!
You could get the reading from the input side of the RCD, but in many ways taking that cover off live could be more dangerous. The RCD may well be yours (they usually belong to the customer) if it was fitted by SEC engineers who re-wired the house at some point they would have tagged it but that doesn't always mean they own it
Depending on where you are in the country and what arrangement your scheme has with the DNO you can go on a short (1/2 day) course on removing the cut out and a sealing kit of "blue" seals to re-seal it (thats the arrangement we have here).
I should point out that if you are not trained or permitted to remove a cutout don't do it, it has the potential to be very dangerous if the casing is old and falls to bits!! and if you don't have spares you are in an embarrasing situation at the least!!
You could get the reading from the input side of the RCD, but in many ways taking that cover off live could be more dangerous. The RCD may well be yours (they usually belong to the customer) if it was fitted by SEC engineers who re-wired the house at some point they would have tagged it but that doesn't always mean they own it
T
tonylec
PS check whether PME (TN-C-S) is available to you, you're in the SEC grid area and the SEC guy who converted a cutout for us this week reckons most of their area has it available (they are alowing PME even when there is not an earth at every distribution pole!!), the conversion is free and negates the need for your stake and TT arrangements
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