r1 r2

[daza25]

how do i test the r1 r2 on a lighting circuirt. and what shudd the reading be

 

[securespark]

Same way as any other circuit.

Depends on cable length & csa, mainly.

 

[GaryMo]

I would suggest you purchase a copy of Guidance Notes 3 (GN3).
It's an invaluable aid to anybody carrying out inspection and testing.

 

[ericmark]

I use a very old BA22d plug and a wandering lead with crocodile clip connected to a 13A socket so I can easy connect either the low impedance ohm meter or the earth loop impedance tester to a lamp socket. However the BA22d plugs are very rare and likely you will need to just use probes on meter.

Many years ago lighting power had different meter to socket power and there were two different tariffs. So many use to break the law and use the cheaper lighting power for items like irons. After the war although one tariff was used many houses only had two sockets so the BA22d plugs were still used.

However there was often no earth on lighting supplies so rather dangerous not sure if any law was passed or if Woolworths stopped making them but the plugs disappeared.

Mine is very carefully guarded both so I don't lose it and so no one uses it to power other items.

Do remember R1 + R2 needs a special low ohm ohmmeter not a multi-meter the proper ones are normally combined with a high voltage ohmmeter for insulation testing.

 

[ricicle]

how do i test the r1 r2 on a lighting circuirt. and what shudd the reading be

You mean R1+R2......

 

[ericmark]

Sorry missed the readings.

If you have a RCD on the lighting circuit then the R1 + R2 reading could be very high and the only real limit is the 3% volt drop. With a lighting circuit having the units even placed we would normally consider it like the ring main with less than the whole load. Not seen any figures but I would consider a 6A MCB circuit would be considered as 4A for calculating volt drop so maximum R1 + Rn would be around the 1.725 ohm mark. If working on 6A then 1.15 ohms and since most ceiling roses are only rated 6A in real terms 6A is max size for a MCB.

Where a RCD is not used then R1+R2 with a TN-C-S system will be the max earth loop impedance minus the earth loop impedance of the supply. So for a B6 MCB it would be (230/(5x6)) - 0.35 = 7.32 ohms but since this is higher than the R1 + Rn permitted for volt drop so in real terms still 1.725 ohms.
Using a C6 MCB = 3.48 ohms again volt drop will be limiting factor.
Using a D6 MCB = 1.57 ohms so here the R1 + R2 is limiting factor.
With a 6A fuse permitted Zs is higher than with MCB's so volt drop is limiting factor.

So to conclude with 5/6 amp protective device if the R1 + R2 is lower than 1.15 ohms it will pass with TT or TN-C-S system. If above 1.15 ohms then you need to consider all factors and one could argue that up to 7.32 ohms could pass.

There is also a TN-S system where 0.8 is subtracted instead of 0.35 but unlikely you have TN-S system.

What I hope I have showed it is not like the insulation test where 1Mohm is considered as pass in nearly all cases but it requires some calculations and who ever signs the installation certificate or Minor works has to decide what are the reasonable limits for that installation. Had I just said better than 1.15 ohms without giving reasons for coming to that conclusion I am sure many on here would question my figures. They may anyway! But at least you have some idea what will pass.

You will also note we are in the main looking at very low readings and 612.2.1 states:- A continuity test shall be made. It is recommended that the test be carried out with a supply having a no-load voltage between 4 V and 24 V, d.c. or a.c., and a short-circuit current of not less than 200 mA.

i.e. not a standard multi-meter.

 

[ban-all-sheds]

If you have a RCD on the lighting circuit then the R1 + R2 reading could be very high and the only real limit is the 3% volt drop.

I've never liked the idea of bunging an RCD in to work round the problem of too high a fault-loop value. On a TN system you ought to be able to get under the limit - if not then something is wrong, and saying "Oh well I'll use an RCD then" is not good.

 

[ericmark]

I would agree that the R1, R2, Rn figures should be within expected limits and high readings are cause for investigation.

However what I was trying to say is that there is not a point where one would say it has failed when using a RCD.

I have my software to give cable length which would correspond to readings taken and we know 40 meters of cable should give us a reading of around the 1.5 ohm mark.

I do think the volt drop limits should reflect type of lighting used. We all know with a ballast start fluorescent fitting that just a few volts below ratting can cause them to fail but with a HF fitting with an inverter the volt drop has to be quite dramatic before it makes a scrap of difference. So as the designer why should I not be able to use this in my design?

I feel too much of the BS7671 regulations are aimed at Domestic installations and there should be more latitude to allow one to design an installation according to what is installed.

Sockets I will agree have to supply what ever is plugged in but where fixed lighting is used why can't I stipulate a switch mode supply and use the huge voltage latitude they will work with?

I had to set up temporary lights in a cable tunnel and I used 20 x 60W fluorescent lamps first 15 set to 127 volt and last 5 at 110 volt it worked well and I can see no reason why we should be stopped using a set-up of this type?

Too many rules and not enough common sense now.

 

[Spark123]

There is also a TN-S system where 0.8 is subtracted instead of 0.35 but unlikely you have TN-S system.

Why is it unlikely?

 

[GaryMo]

If you have a RCD on the lighting circuit then the R1 + R2 reading could be very high and the only real limit is the 3% volt drop.

I've never liked the idea of bunging an RCD in to work round the problem of too high a fault-loop value. On a TN system you ought to be able to get under the limit - if not then something is wrong, and saying "Oh well I'll use an RCD then" is not good.

I have to agree with BAS here

 

[RF Lighting]

However the BA22d plugs are very rare and likely you will need to just use probes on meter.........Mine is very carefully guarded both so I don't lose it and so no one uses it to power other items.

http://www.google.co.uk/#hl=en&sour...=f&aqi=&aql=&oq=&gs_rfai=&fp=73ccac14cbe0e30c

 

[ericmark]

If you have a RCD on the lighting circuit then the R1 + R2 reading could be very high and the only real limit is the 3% volt drop.

I've never liked the idea of bunging an RCD in to work round the problem of too high a fault-loop value. On a TN system you ought to be able to get under the limit - if not then something is wrong, and saying "Oh well I'll use an RCD then" is not good.

I have to agree with BAS here

I have not said it is a work around, but that the volt drop limits are tighter so no real point in even giving figures. However if you want to suggest a ball park figure at which you would consider the ELI to be acceptable when a RCD is used I am not stopping you. No one else seems to have answered the guys question and given him any figures to go by. I realised it's easy to make a mistake when answering a question like this and I would admit any mistake if I have made any. So give some readings you would expect.

One thing I am aware of is people ask questions to check on other tradesmen's work and if one doesn't give the exceptions it is likely some guy will be getting ear ache over some valid readings.

There is also a TN-S system where 0.8 is subtracted instead of 0.35 but unlikely you have TN-S system.

Why is it unlikely?

Maybe you know of loads, but I can only think of one in last few years and that's a boat with on board isolation transformer, which is rather a special case.
Yes some of the firms I worked had step down transformers on premises supplying only them, but not really a standard house.
So I can't recall a single house where it was confirmed it was a TN-S supply. A few we thought it was but on application to DNO it transpired it was TN-C-S.
I am sure somewhere there are houses with TN-S supplies, but I would expect to find something special like powered from their own generator.
It was because there was a slight chance, that somewhere in the country they do use TN-S supplies I made the comment.
I have a few times wanted a TN-S supply, for example caravan sites, but each time I have asked DNO they have labelled it as TN-C-S, and since if anywhere on route the neutrals and earths are combined it becomes a TN-C-S it is impossible for any electrician to tell by looking at supply head. Sometimes we can see it's a TN-C-S but we can not confirm it as being a TN-S by a visual inspection.
So if the ELI exceeds 0.35 ohms I have always phoned for conformation as to supply type, and each time the DNO have sent someone out to check, and each time a fault has been found and corrected, and it was declared as TN-C-S.

So I am open to be told how common TN-S supplies are in standard domestic dwellings, confirmed by the DNO, but not going to hold my breath waiting for replies. Really only people who can tell us are those who work for DNO's and would be interesting to hear from them.

 

[ericmark]

However the BA22d plugs are very rare and likely you will need to just use probes on meter.........Mine is very carefully guarded both so I don't lose it and so no one uses it to power other items.

http://www.google.co.uk/#hl=en&sour...aql=&oq=&gs_rfai=&fp=73ccac14cbe0e30c[/QUOTE]

Thanks if I ever lose mine then I know where to go.

 

[Electrifying]

I am sure somewhere there are houses with TN-S supplies, but I would expect to find something special like powered from their own generator.
It was because there was a slight chance, that somewhere in the country they do use TN-S supplies I made the comment.

Ha Ha, you're having a laugh.

We've got whole housing estates around here that are TN-S.

And we're only 12 miles from you

I think you're getting mixed up somewhere - best get your hard hat on for the replies

 

[RF Lighting]

I would reckon that 50% of all supplies round here are TN-S, 40% TN-C-S and 10% TT, but as the cast iron cutouts are getting replaced, YEDL (or CE) tend to convert them to TN-C-S with a bonded sheath.

There are loads and loads of TN-S round here both old ones on PILC and new ones on split-con.