Nuisance tripping - Earth Leakage Device on a TT system

[JohnW2]

Getting back to the subject under discussion, why does a 100mA device not give discrimination over a 30mA one?

As I explained before, if you have a 100mA RCD and a 30mA one in series and then gradually increase the (L-E) fault current (detected by RCDs as a L-N imbalance), the 30mA one will obvioulsy trip first. However, if a large L-E fault current arises suddenly (which is what happens with most real faults), the theoretical disconnection times of both devices will both be extremely short - milliseconds or less (but obviously subject to a bit of variabilitty between different devices), so there is really no guarantee that the 30mA will have broken the circuit before the 100mA one operates.

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....

Indeed, that's not logical at all. It could be due to a problem with one of the RCDs OR (as above) it may simply be due to individual variation, such that the (very short) disconnection time of the 100mA one happens to be slightly shorter than the (also very short, but not quite so short) disconnection time of the 30mA one in the face of a large L-N imbalance (large L-E fault current).

Does that make sense?

Have you yet clarified what the device between DNO' cutout and meter actually is - an RCD or a voltage-operated ELCB?

Kind Regards, John.

 

[JohnW2]

.... this suspect ELCB / RCD. What IS the difference between the two?

They are utterly different.

You hopefully understand the RCD, which is the only one of the two which is now installed. An RCD senses small differences (e.g. 30mA) between the current flowing in the L and N conductors. If such a difference is (usually with the higher current being in the L conductor), the assumption is that some current is finding a route from L to earth other than through the N conductor - i.e. because of a L-E fault in the installation or, say, current being conducted through a human being from L to E. The device therefore trips if the difference in currents exceeds the threshold of the device (e.g. 30mA). In the case of a human L-E path, a 30mA RCD will usually (at least, with a fairly 'healthy' victim) prevent the electric shock being fatal.

With a voltage-operated ELCB, the CPCs of the installation are not connected directly to earth but, instead, go to earth via the ELCB. If the voltage of the CPC exceeds a certain value (usually 50V) above the potential of the earth (due to some L-CPC fault) the device then disconnects the supply.

Is that clear enough, or do you need to know more, or have further explanation?

Kind Regards,

 

[mikgle]

Does that make sense?

Have you yet clarified what the device between DNO' cutout and meter actually is - an RCD or a voltage-operated ELCB?

Kind Regards, John.[/quote]

Yes I understand what you're getting at John.....
And no, I need to do a return visit and Unfortunately clients are away presently. I dont think there were any earth wires going to the device, so probably correct are the suggestions it is actually an RCD - it def. said 100A, 100mA. ELCB. And of course accept the advice that a type S (time delay) RCD would be better..... thnx

 

[JohnW2]

And no, I need to do a return visit and Unfortunately clients are away presently. I dont think there were any earth wires going to the device, so probably correct are the suggestions it is actually an RCD - it def. said 100A, 100mA. ELCB. And of course accept the advice that a type S (time delay) RCD would be better..... thnx

I look forward to hearing what you discover when you go back. As we've explained, "100mA" really makes no great sense in terms of an ELCB; although it is obviously current through a (voltage-operated) ELCB's coil that causes it to trip, the threshold for that tripping was/is usually stated in terms of the voltage (usually 50V) required to result in a current which causes the device to operate - since tripping at a certain CPC-earth voltage is what it was designed to do.

I suppose it's just possible that what you have is an RCD labelled 'ELCB', but that would be extremely odd! A photo of it when you go back would be very helpful.

KInd Regards, John.

 

[mikgle]

.... this suspect ELCB / RCD. What IS the difference between the two?

They are utterly different.

You hopefully understand the RCD, which is the only one of the two which is now installed. An RCD senses small differences (e.g. 30mA) between the current flowing in the L and N conductors. If such a difference is (usually with the higher current being in the L conductor), the assumption is that some current is finding a route from L to earth other than through the N conductor - i.e. because of a L-E fault in the installation or, say, current being conducted through a human being from L to E. The device therefore trips if the difference in currents exceeds the threshold of the device (e.g. 30mA). In the case of a human L-E path, a 30mA RCD will usually (at least, with a fairly 'healthy' victim) prevent the electric shock being fatal.

With a voltage-operated ELCB, the CPCs of the installation are not connected directly to earth but, instead, go to earth via the ELCB. If the voltage of the CPC exceeds a certain value (usually 50V) above the potential of the earth (due to some L-CPC fault) the device then disconnects the supply.

Is that clear enough, or do you need to know more, or have further explanation?

Kind Regards,

Very clear. However, I'm not old/experienced enough to know whether the old type RCD's were called previously ELCB's or whether an old device labelled "ELCB" as in this case, is in fact a voltage-controlled device - this is what was confusing me as to what the device actually is. As I say, I dont think there were any earth wires going to it just the meter tails which would suggest it is an rcd; so the comment made earlier about me not knowing the difference is a bit unfair I think, as it seems in this case there is no difference...or have I completely lost the plot?

 

[riveralt]

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.

Well thats good to hear.

Will be going thru the LA;

Well you need to speak to them before you start any work - do you know what will be involved and the costs associated with this process?

and I'm not intending to fit a new CU, just replace this suspect ELCB / RCD.

This appears contrary to what you have already written.

I have bought a new CU and intend to fit it this week! Many thanks, mike

I am still intrigued by what you mean by qualified - What course have you taken?

 

[ban-all-sheds]

Clearly not one which requires him to know, or even know how to find his way around, the Wiring Regulations.

 

[Spark123]

I suppose it's just possible that what you have is an RCD labelled 'ELCB', but that would be extremely odd!

It isn't that odd, we have all probably come across them from time to time, it just depends on the age of the device.
For example try looking for an MK LN5760 on ebay: clicky

 

[Spark123]

When it was written back in 2005 the 16th edn regs were the current version which permitted this setup.

Are you saying then, that the current regs do NOT allow this kind of setup? I have the current regs and cannot find any reason why not, unless you can enlighten me. Regards,

The current edition of the regs requires all unprotected concealed cables such as T&E in a partition or wall <50mm to be protected by a 30mA RCD.
(Reg 522.6) which is nigh on every domestic installation I know of!
The 16th edn didn't have this requirement.

 

[JohnW2]

I suppose it's just possible that what you have is an RCD labelled 'ELCB', but that would be extremely odd!

It isn't that odd, we have all probably come across them from time to time, it just depends on the age of the device.
For example try looking for an MK LN5760 on ebay: clicky

Thanks - one learns something every day! In that case, I suppose the simplest explanation is that what the op has is a 100mA RCD labelled 'ELCB'.

Kind Regards, John.

 

[mikgle]

When it was written back in 2005 the 16th edn regs were the current version which permitted this setup.

Are you saying then, that the current regs do NOT allow this kind of setup? I have the current regs and cannot find any reason why not, unless you can enlighten me. Regards,

The current edition of the regs requires all unprotected concealed cables such as T&E in a partition or wall <50mm to be protected by a 30mA RCD.
(Reg 522.6) which is nigh on every domestic installation I know of!
The 16th edn didn't have this requirement.

I see, well the skts circuits are protected by the 30mA rcd, not sure about the lights - will have to check the other ccts when I do the Periodical but does it then become mandatory that I have to upgrade the protection on existing circuits, or can I just advise? Remember, what I am contemplating here to resolve the nuisance is a virtual like for like replacement of an existing rcd device - I am not adding any new circuits. This is the area I am unsure of. Thanks.

 

[riveralt]

When it was written back in 2005 the 16th edn regs were the current version which permitted this setup.

Are you saying then, that the current regs do NOT allow this kind of setup? I have the current regs and cannot find any reason why not, unless you can enlighten me. Regards,

The current edition of the regs requires all unprotected concealed cables such as T&E in a partition or wall <50mm to be protected by a 30mA RCD.
(Reg 522.6) which is nigh on every domestic installation I know of!
The 16th edn didn't have this requirement.

I see, well the skts circuits are protected by the 30mA rcd, not sure about the lights - will have to check the other ccts when I do the Periodical but does it then become mandatory that I have to upgrade the protection on existing circuits, or can I just advise? Remember, what I am contemplating here to resolve the nuisance is a virtual like for like replacement of an existing rcd device - I am not adding any new circuits. This is the area I am unsure of. Thanks.

I really do fear for this client when this guy returns.

 

[JohnW2]

Very clear. However, I'm not old/experienced enough to know whether the old type RCD's were called previously ELCB's or whether an old device labelled "ELCB" as in this case, is in fact a voltage-controlled device - this is what was confusing me as to what the device actually is.

Well, I'm plenty old enough but, like you, not experienced enough to know the history of terminology. In recent times, it's certainly the case that 'ELCB' is normally only used refer to an (obsolete) 'voltage-operated' device - but it seems as if when ('current-operated') RCDs started to appear, some were intially also called 'ELCB'.

As I say, I dont think there were any earth wires going to it just the meter tails which would suggest it is an rcd; so the comment made earlier about me not knowing the difference is a bit unfair I think, as it seems in this case there is no difference...or have I completely lost the plot?

I think some of the comments may have been a bit harsh, but there obviously is a difference between voltage- and current- operated devices, and it was you who, at the very start, suggested that it might be a voltage-operated one. I can also sympathise with the view that you ought to understand the difference - since, although voltage-operated ELCBs are no longer installed (at least, not in normal domestic situations), there are still a good few out there which you will encounter, and need to understand. However, that's really a criticism of those who have taught you, not yourself.

More generally, I do share some of the concerns that some people have expressed about your apparent level of knowledge and hence 'competence' to do the sort of work we are talking about, although I recognise that you are keen to learn and therefore will eventually become very knowledgable. Are you just working alone, or are you working with or for someone? When one starts out, it's obvioulsy always good to have an experienced person nearby, whose experience and advice one can take advantage of, as one acquires one's own experience.

Kind Regards, John.

 

[ban-all-sheds]

When it was written back in 2005 the 16th edn regs were the current version which permitted this setup.

Are you saying then, that the current regs do NOT allow this kind of setup? I have the current regs and cannot find any reason why not, unless you can enlighten me. Regards,

The current edition of the regs requires all unprotected concealed cables such as T&E in a partition or wall <50mm to be protected by a 30mA RCD.
(Reg 522.6) which is nigh on every domestic installation I know of!
The 16th edn didn't have this requirement.

"The following user says thank you to Spark123 for this useful post:
mikgle (today) "

mikgle - you're a qualified electrician. Didn't you know about the regulations for concealed cables?

 

[mikgle]

Very clear. However, I'm not old/experienced enough to know whether the old type RCD's were called previously ELCB's or whether an old device labelled "ELCB" as in this case, is in fact a voltage-controlled device - this is what was confusing me as to what the device actually is.

Well, I'm plenty old enough but, like you, not experienced enough to know the history of terminology. In recent times, it's certainly the case that 'ELCB' is normally only used refer to an (obsolete) 'voltage-operated' device - but it seems as if when ('current-operated') RCDs started to appear, some were intially also called 'ELCB'.

As I say, I dont think there were any earth wires going to it just the meter tails which would suggest it is an rcd; so the comment made earlier about me not knowing the difference is a bit unfair I think, as it seems in this case there is no difference...or have I completely lost the plot?

I think some of the comments may have been a bit harsh, but there obviously is a difference between voltage- and current- operated devices, and it was you who, at the very start, suggested that it might be a voltage-operated one. I can also sympathise with the view that you ought to understand the difference - since, although voltage-operated ELCBs are no longer installed (at least, not in normal domestic situations), there are still a good few out there which you will encounter, and need to understand. However, that's really a criticism of those who have taught you, not yourself.

More generally, I do share some of the concerns that some people have expressed about your apparent level of knowledge and hence 'competence' to do the sort of work we are talking about, although I recognise that you are keen to learn and therefore will eventually become very knowledgable. Are you just working alone, or are you working with or for someone? When one starts out, it's obvioulsy always good to have an experienced person nearby, whose experience and advice one can take advantage of, as one acquires one's own experience.

Kind Regards, John.

As newly qualified, my knowledge is obviously near the start of the learning curve, hence this forum. I would like to work alonside more experienced sparks but unfortunately noone is taking youngsters on around here at the moment.
Unfortunately other replies seem to think that once qualified one will know everything there is to know, certainly some replies have been no help whatsoever, unlike yours, and seem intent on picking holes in everything I write. Obviously there are some out there with no day job (does make you wonder why, doesnt it?) with a lot of time on their hands... If one wants to question the efficacy of the education system - start a forum on it.... I would be happy to contribute but this has little to do with this specific problem.
I think this forum has now run its useful life so am signing off; thanks to all who provided useful, positive inputs. Kind regards.