**CU Changes And YOU**

[nickblake]

Basically means that if you carry out a cu upgrade with say a lighting circuit with no cpc then you'd mark it on the electrical installation cert one thing you would have to do is the bonding to the services gas water etc , in the case of no cpc's you must mark the Cu to prevent people installing metal clad class 1 face plates and fittings

 

[domestic spark]

Basically means that if you carry out a cu upgrade with say a lighting circuit with no cpc then you'd mark it on the electrical installation cert one thing you would have to do is the bonding to the services gas water etc , in the case of no cpc's you must mark the Cu to prevent people installing metal clad class 1 face plates and fittings

so you cant condem an installation (lighting circuit) for having no earth)??? or any other circuit?


would this be because, wiring is old and work would have not been done to 16, 17th ed etc!! (dependant on age est)

or because there are no metal fittings?

if you were doing a PIR what code would this be??

with/ and without metal fittings.?

 

[lloyd1980]

A great thread BTW

I have glanced at this as about to price for a CU change
Just a quick one, domestic 30ma RCBO . Will trip when there is a 30ma imbalance between the line and neutral conductors

If I was to do an IR test at 500V dc between neutral and earth prior to install;

0.00 would indicate a short and would then indicate definate fault once installed

my question is this, if your Ir readings are low but above 1M ohm ( min ac ceptable reading )

at what point would the RBCO start to register a fault???? eg if the IR reading are low 2.6Mohm or in region of this fig, would an acceptable IR reading still trip the RCBO?

obviously its not a short but its showing signs of deterioration - at what point would the RCBO trip, when there is a short only????
I have known RCBO's to trip without there being a short, just preasure faults where conductors are kinked as been stuffed behind accesories.

just wondering does anyone have a definative answer to this
Thanks

 

[tedy]

A great thread BTW

I have glanced at this as about to price for a CU change
Just a quick one, domestic 30ma RCBO . Will trip when there is a 30ma imbalance between the line and neutral conductors

If I was to do an IR test at 500V dc between neutral and earth prior to install;

0.00 would indicate a short and would then indicate definate fault once installed

my question is this, if your Ir readings are low but above 1M ohm ( min ac ceptable reading )

at what point would the RBCO start to register a fault???? eg if the IR reading are low 2.6Mohm or in region of this fig, would an acceptable IR reading still trip the RCBO?

obviously its not a short but its showing signs of deterioration - at what point would the RCBO trip, when there is a short only????
I have known RCBO's to trip without there being a short, just preasure faults where conductors are kinked as been stuffed behind accesories.

just wondering does anyone have a definative answer to this
Thanks

I'm new here, but I think the tripping point when "the RBCO start to register a fault" is not so much related with IR (isolation resistance) but very much in relation with the csa of the cable, it's length, and it's impedance Zs.
Basically the RCBO is "watching" fluctuations of the current in cable, and in between cables

I don''t think you will find an definitive answer to your question, like a precise value ... because this varies with the length of the and CSA in principal.

And lastly, we have 30 mAmps, 100 mAmps and other, to clearly show us that is the (MAXIMUM) variation of the current in between the wires at tripping moment. (this is my own supposition)

 

[rryles]

A great thread BTW

I have glanced at this as about to price for a CU change
Just a quick one, domestic 30ma RCBO . Will trip when there is a 30ma imbalance between the line and neutral conductors

If I was to do an IR test at 500V dc between neutral and earth prior to install;

0.00 would indicate a short and would then indicate definate fault once installed

my question is this, if your Ir readings are low but above 1M ohm ( min ac ceptable reading )

at what point would the RBCO start to register a fault???? eg if the IR reading are low 2.6Mohm or in region of this fig, would an acceptable IR reading still trip the RCBO?

obviously its not a short but its showing signs of deterioration - at what point would the RCBO trip, when there is a short only????
I have known RCBO's to trip without there being a short, just preasure faults where conductors are kinked as been stuffed behind accesories.

just wondering does anyone have a definative answer to this
Thanks

To trip the RCD part of a 30mA RCBO will require at least 15mA of fault current. This corresponds to an IR reading of less than 15.3K. If the fault is Neutral-Earth then it will take an even lower resistance before the RCD trips (As Neutral-Earth voltage should be a small fraction of 230V).

So if you get an IR > 1M, then this is a long way off tripping a 30mA RCD (including RCBO).

 

[i=p/u]

good thread , if doing any work on any circuit, i thought was ok to IR test at 250v between l-n, n-e, l-e as this would save you removing all elements and sensitive equipment.and getting more than 2Mohms is this ok????

 

[nickblake]

you will find that even testing at 250 volts you will get low IR reading due to the resistance of the load , the easiest way to carry out thi test is to short out the L and N then test to the cpc and mark that that test has been carried out this way you are unlikly to damage any sensitive equipment etc , especially will all the low voltage transformers,pir fans etc , if i can unplug and disconnect every thing i will and test that way but doing that you may have neons in the circuit which may well cause false readings

 

[saabuldin]

you will find that even testing at 250 volts you will get low IR reading due to the resistance of the load , the easiest way to carry out thi test is to short out the L and N then test to the cpc and mark that that test has been carried out this way you are unlikly to damage any sensitive equipment etc , especially will all the low voltage transformers,pir fans etc , if i can unplug and disconnect every thing i will and test that way but doing that you may have neons in the circuit which may well cause false readings

When shorting L and N and doing IR test between L+N and earth. Can this test be done using 500v or do you have to stick with 240v.

 

[Lioned]

Hi ,have tried to read back and ,unless i've missed it,cant see it yet.When changing CU then its a must to upgrade main earth connection to 16mm,but what about main bonding to water and gas ?
Quite often you will find some kind of earth to the water but maybe only 4 or 6 mm or smaller ! If you cannot reasonably get a new 10mm to the water can you just leave as is but advise should be done and code 4 ?

 

[clark]

Please could you clear up the earth and neutral IR query and test to be done at just 250v.

 

[chiplard]

Please could you clear up the earth and neutral IR query and test to be done at just 250v.

It should be carried out at 500v. Table 61, P158 of the BRB unless it is reasonably unpractical to disconnect equipment.

 

[rryles]

Hi ,have tried to read back and ,unless i've missed it,cant see it yet.When changing CU then its a must to upgrade main earth connection to 16mm,but what about main bonding to water and gas ?
Quite often you will find some kind of earth to the water but maybe only 4 or 6 mm or smaller ! If you cannot reasonably get a new 10mm to the water can you just leave as is but advise should be done and code 4 ?

Codes are for PIRs not EICs. There is however the section on the EIC for "departures from BS7671". However, as I understand it, earthing and bonding is the one thing that must be brought up to current regs for any work that you do - no exceptions.

 

[malcolmsanford]

Regulation 131.8 is the guide for earthing and bonding on alterations or installations

Then capter 54 for the set of regulations.

 

[mike2790]

Please could you clear up the earth and neutral IR query and test to be done at just 250v.

insulation tests at 250v is not a legitimate test, this is only done to give yourself peace of mind that you arent going to run into all sorts of faults which a residual current device will detect, ie faults which were already present in the installation which a fuse or a 60898 mcb etc would not have previously detected.
also....
a 30mA rcd has a maximum allowance leakage of 30mA before it will trip. in reality a ramp test will tell you at exactly what leakage that specific rcd/rcbo will trip at. i usually find between 23-27mA is the norm. this is not a required test and is done in addition to the required rcd tests and is done again to give peace of mind that the device not only complies but is also not too sensitive.

 

[malcolmsanford]

insulation tests at 250v is not a legitimate test, this is only done to give yourself peace of mind that you arent going to run into all sorts of faults which a residual current device will detect, ie faults which were already present in the installation which a fuse or a 60898 mcb etc would not have previously detected.
also....
a 30mA rcd has a maximum allowance leakage of 30mA before it will trip. in reality a ramp test will tell you at exactly what leakage that specific rcd/rcbo will trip at. i usually find between 23-27mA is the norm. this is not a required test and is done in addition to the required rcd tests and is done again to give peace of mind that the device not only complies but is also not too sensitive.

I do realise that the BS 7671-2008 is not a statutory document and so therefore technically every test in it is not legally binding.

As the tests and testing procedures, as stated in this and the GN 3 is industry recognised as being "legitimate" why do you consider IR to regulation 612.3.2 not therefore legitimate. Can we assume that the whole of section 612 to be the same?

 

[ukpablo]

i think mike2790 is referring to the pre test at the CU mentioned by the OP (or soft test as it has been called between neutral and Earth bar at 250V) to determine if any faults are present before doing any work on CU changes.

 

[malcolmsanford]

i think mike2790 is referring to the pre test at the CU mentioned by the OP (or soft test as it has been called between neutral and Earth bar at 250V) to determine if any faults are present before doing any work on CU changes.

So why is it not legitimate?

 

[ukpablo]

im not saying an IR test is not legitimate, just does not state the test between Neutral and Earth bar at CU as a pre test prior to a CU change in GN3 or BS 7671. This is something that was pointed out as a thing u may want to do by the OP. Thats the way i read it anyhow?!
i see what u are saying malcomsford and im not stating that u are wrong either, im just reading between the lines on this thread????!

 

[mike2790]

So why is it not legitimate?

as chiplard has stated above, IR tests should be performed at 500v unless it is not practical to disconnect all loads. obviously in a domestic dwelling 99% of the time it IS practical to disconnect all loads to enable a 500v IR test, so in this instance a 250v IR test would not be legitimate. the section of the regs you pointed out does highlight that a 250v test IS ok if there is an item of equipment which is not practical to disconnect, but ONLY that circuit, so all others have to be tested at 500v. you missed the point a little bit with what i was getting at anyway but i do realise the good book is open to a lot of interpretation. just tell me whether you would pass your 2391 or any other exam which involves testing at some point if you IR test at 250v? the 250v test described in this thread is just a quick test on neutral and earth which will give you an idea whether or not the fancy new rcd board will trip like a hippie on acid, turning your few hour board change into an all nighter lifting floor boards.

 

[malcolmsanford]

Mike I don't take the view that what your saying is not in good practice correct, what I'm saying is that when you use words such as legitimate then it conveys that the 250v Test is not right.

I fully agree in an initial verication there can be no excuse for not doing a full 500v DC test across LIVE/LIVE and then LIVES/EARTH. But the post was for a board change and therefore literally either a full or semi PIR on the existing installation.

I don't know about you but I would indeed, unless I was truely concerned with the installation, just do a 250v LIVES/EARTH only test and see what that got me. If I had a poor result then I would indeed do each circuit individually again at 250v LIVES/EARTH until I found which was giving me that poor reading, Then look at the individual circuit more closely. If I got good results then I would indeed do the 500V test but again only LIVES/EARTH

I always found that the regs as a guide was invaluable, but it gives scope for thought and interpretation. I know as a competant person if I was changing a CU on an installation that was 50 yrs old perhpas even wired in VIR that I would be doing a lot more testing before I fitted the board than a 10 yr old installation that had no obvious signs of tampering, Each installation would though both be tested to ensure that they were safe and still in a condition to be used, it would just mean different levels and perhpas even different ways of testing.

To just say that someting is not legitimate conveys that it must never be done, and I'm sorry that is wrong. I will not comment on the way individulas are trained, but fortunately I was trained to think and to assess, which to me is what a competant person is.

Please don't think I'm carping at you, as I said at the start your not incorrect for an initial verication every time, but on a PIR and so a board change, you are giving scope by the BS 7671-2008 to assess the installation.