Zs = Ze + (R1 + R2) its the Ze part that suffers from a poor earth (electrode)?
Nuisance tripping - Earth Leakage Device on a TT system
- Thread starter mikgle
- Start date
Zs = Ze + (R1 + R2) its the Ze part that suffers from a poor earth (electrode)?
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I don't think that there is any doubt that the voltage controlled breaker has got to go. The simplest thing for you to do (if you intend to keep nthe CU as it is) would be to replace it with a 100mA RCD, preferably time-delayed. As I said before, without the time delay, you may not get discrimination from the 30mA RCD on the sockets, so a fault on the socket circuit could take out the whole installation. That's not really compliant with current regs (separation of circuits to avoid inconvenience) and, since the 100mA RCD would be one you had fitted, I imagine (but am open to correction) that (unlike the CU etc.) the consequences of your installing that RCD probably does have to comply with current regs.
That was very standard accepted (and compliant with 16th ed.) practice in 2005 - which, of course, was prior to the 17th edition.
Indeed, I believe that's the situation. However, as above, if you installed a 100mA RCD I imagine that it probably would have to be time-delayed in order for 'what you had done' (and the consequences of 'what you had done') to be compliant with 314 of the regs. The practising electricians here will be able to advise you whether that's the case.
Kind Regards,John.
Does the earthing conductor connect through the device?
As you say it is 100mA then it is more likely an RCD as opposed to a Voltage Operated ELCB.
Not to get confused that RCDs were also called ELCBs in their early days!
My guess is that some sort of mains suppressor in a device is taking current to earth when there is a surge on the system.
When it was written back in 2005 the 16th edn regs were the current version which permitted this setup.
There is no requirement to bring everything up to todays standard, the regs basically say that before you carry out any addition or alteration that the earthing and bonding should be brought up to scratch and the existing system should be adequate for your additions/alterations. Have a read of regulation 131.8
As for rewireable fuses they are still a current standard (BS3036) and there are a lot about, although it is not the norm to install them in domestic premises nowadays.
I think I can just about understand what you folks are trying to say, but I think that exchange above will totally confuse most readers.I think you mean a satisfactory Zs
Having a 100mA RCD obviously won't alter Ze or Zs, so won't 'ensure that one achieves a satisfactory' impedance.
As far as I am aware, the reason for the traditional 100mA overall RCD in a TT installation which does not have (30mA) RCD protection for all final circuits is that the (beacuse of the high Ze) Zs with a TT system is generally not low enough to provide acceptable disconnection times with an overload device (MCB or fuse). Those circuits which are not protected by a 30mA RCD in the CU therefore need some other RCD protection in order to achieve the required disconnection times. To achieve that, it doesn't have to be a 100mA RCD - 30mA would be even better. The 100mA tradition/practice presumably resulted from an attempt at discrimination - but, as I've said in recent poste, if it's not time-delayed, then I don't think that discrimination from a 30mA RCD can by any means be guaranteed.
That's my understanding, anyway.
Kind Regards, John.
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All very valid comments, duly taken onboard... I can't remember about whether it had an earth lead going to the device, I only had time for a quick look as guests unexpectedly arrived! Definitely labelled "Earth Leakage Circuit breaker". also didnt know until later about these voltage devices; next time I will take a camera and photograph the board....
many thanks.
I must confess that I've been assuming all along (from your initial comments) that the present device was a voltage-operated ELCB and if it's labelled 'Earth Leakage Circuit Breaker' I imagine that more-or-less clinches it. However, as Spark123 has pointed out, if it actually has 100mA written on it, that makes no sense (since voltahe-operated devices are voltage-operated!). Are you sure that it doesn't say 100A, which was a typical rating for such devices? As Spark123 also says, the 'decider' is really whether the path to earth from the installation's CPCs goes through it. If so, then it's a voltage-operated ELCB. If it has no 'earth' connections, it can't be - and then, I guess would probably be an RCD.
However, one thing which I think we've all been forgetting a bit is that you said this was between the DNO's cutout and meter. As someone pointed out to you early on, if that's the case, it's theirs, 'their problem' and certainly not something you are allowed to touch.
Kind Regards, John.
John, from my perspective I am not convinced mikgle is qualified. From the questions he raises and the answers he is giving he certainly hasn't done any time with a knowledgeable electrician. Everything seems to have come from books - even they are out of date!
I did ask him If he is qualified to what extent? Time served apprentice? C&G qualifications? Competent Persons scheme? I know non of these prove competence but it should give an indication to us of what level to pitch our responses.
Remember what he is proposing does require Local Authority Building Control Notice unless he is up North and proberbly even there.
He doesn't seem to know the difference between an RCD and an ELCB and my concern is that he has now decided to fit a consumer unit without full knowledge of what that entails. There is no mention of a PIR to determine the quality of the circuits in situ, no mention of the state of the bonding if any. Best we have had is that it is a TT system with a Zs of 125ohms.
I totally agree. To be fair, though, you were part of (or, at least, perpetuated) that totally confusing exchange
Again, I agree. As I hope you will have seen, I (who have, and claim, no formal qualifications in this field) pitched my responses at what I regarded as an appropriate level - explaining things as clearly as I could 'from first principles', almost lecture-style. Whether the op is meaningfully qualified or not, that seemed to be the appropriate level. You may, or may not, agree with my approach.
One thing that does rather concern me is that I do come across genuinely 'well qualified' (on paper) electricians whose level of understanding, certainly of the underlying principles, is no better than we've seen here.
Kind Regards, John.
Maybe when he wrote "only just" (as in qualified) he meant "barely", not "newly".
Your comment reminds me of something which we all tend to forget, and which is potentially a cause for concern ...
...I don't know what the situation is with electricians but in many walks of life, including major professions such as medicine, law, engineering etc., some (sometimes all) of the qualifying exams have a pass mark of 40% or 45% (e.g. degree-based qualifications). That begs certain questions about the 55% or 60% which some legitimately 'qualified' people didn't know, or got wrong, in their exams - and certainly makes one stop and think about 'how qualified' some people really are to practise their trade or profession.
Kind REgards, John.
Some qualified people are open to more learning, education and gaining experience before they feel confident to provide a service on their own without supervision by an experienced person. These almost always become good service providers but even then they continue to be open to learn more.
Others assume their piece of paper means they are able to work on their own and are infallible. Sadly for the their customers they are not infallible and will continue to make mistakes if they remain arrogant that their qualification alone means they have all necessary knowledge.
The present "education" and "qualification" schemes for some trades seem to promote the second type. It does after all increase profits for the training schemes at the expense of the customers who use these "qualified" ex-students.
Others assume their piece of paper means they are able to work on their own and are infallible. Sadly for the their customers they are not infallible and will continue to make mistakes if they remain arrogant that their qualification alone means they have all necessary knowledge.
The present "education" and "qualification" schemes for some trades seem to promote the second type. It does after all increase profits for the training schemes at the expense of the customers who use these "qualified" ex-students.
Indeed so, and that's how it should be in any trade or profession; learning certainly doesn't stop with a 'qualifying exam'. That's how I felt about this thread. The op recognises some deficiencies in his knowledge, asked questions and is showing his willingness and keeness to learn more - which is something I applaud.
Quite, and we've all met some of them. I suppose the worst of all are those who not only feel infallible in relation to their own work, but are so convinced of their omniscience that they assert incorrect advice to others; one sometimes sees that in forums such as this.
Kind Regards, John.
No, I meant "newly", hence my request for advice.....
Getting back to the subject under discussion, why does a 100mA device not give discrimination over a 30mA one? And why would a 100mA device trip 9 times out of 10 before the 30mA one? Does'nt seem logical, unless there is a problem with the 100mA one....
[Don't you think it would be a good idea to conduct a PIR on the circuits first? You say you are qualified, just, does this mean qualified to complete your own building control notice through a competent persons scheme or are dealing direct with the LA building control.[/quote]
Yes, will be doing a Period Inspection, unfortunately I had to cut the initial visit short, so need to return once the clients are back.
Will be going thru the LA; and I'm not intending to fit a new CU, just replace this suspect ELCB / RCD. What IS the difference between the two?
Yes, will be doing a Period Inspection, unfortunately I had to cut the initial visit short, so need to return once the clients are back.
Will be going thru the LA; and I'm not intending to fit a new CU, just replace this suspect ELCB / RCD. What IS the difference between the two?
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