Which way do you do it?H

[uksparks]

Hi,

When changing a board for someone, what method do you use to obtain R1 + R2?

When I do it, I cross connect at the CU and measure at each point, and on a radial link between and measure at each point.

Does anyone just calculate it on a ring by doing this:

(r1 + r2) /4 = R1 + R2

Just seems to take me a ages doing a board change and wondered if I'm missing a trick here?

 

[Stuuk1]

Get a copy of GN3 and do exactly as it says and you will know you are doing it correctly..

 

[Meggerman]

yes, (r1+r2)/4 = an expected value of R1 + R2 at sockets. I do that calc, then at the end when doing my Zs readings do Zs-Ze to get a calculated R1+R2 and it should be thereabouts (unless spurs). A lot of DB change EIC's I see have nothing in the R1+R2 box!

 

[uksparks]

Like I said I do the three step process and cross connect etc to check for polarity etc, just wondered who calculates it on a board change opposed to going round each and every point measuring it?

 

[uksparks]

yes, (r1+r2)/4 = an expected value of R1 + R2 at sockets. I do that calc, then at the end when doing my Zs readings do Zs-Ze to get a calculated R1+R2 and it should be thereabouts (unless spurs). A lot of DB change EIC's I see have nothing in the R1+R2 box!

Thats bad leaving boxes empty! Zs-ze shouldn't be relied upon due to parralel paths etc, but I guess it will give you a rough idea. Presumably then you don't go round each socket or light fitting then?

 

[Meggerman]

I don't cross connect on a db swap. Like I stated I take Zs reading, then Zs-Ze that is my R1+R2.

- - - Updated - - -

I take Zs at every point.

 

[uksparks]

But surely you are not relying on that to be correct due to parrallel paths?

 

[uksparks]

Surely though you would better off just adding the calculated R1 + R2 to Ze to get your Zs?

Calculated being as above.

 

[Meggerman]

Im not doing initial verification of all the circuits, Im changing a consumer unit. I just interested in correct OC protection, satisfactory insulation resistance and satisfactory disconnection times.

 

[Meggerman]

Surely though you would better off just adding the calculated R1 + R2 to Ze to get your Zs?

Calculated being as above.

I thought you were trying to save yourself some time. Why would I want to calculate it then measure it again as Zs?

 

[uksparks]

True, I think I just have a nasty habit of going over the top, I suppose it doesn't hurt, just a waste of time!

 

[Meggerman]

True, I think I just have a nasty habit of going over the top, I suppose it doesn't hurt, just a waste of time!

Well I cant knock someone for being thorough

 

[uksparks]

Very true, I suppose it's just something I have always done to satisfy what I'm doing being relatively new to the trade I prefere to be thorough, maybe time for a change, I'll get things done quicker then.

I picked up another rewire today which was good, 4 bed house, 1930's, nice job, the bloke works for WPD so I'll get him to stick an isolator in and a new shiny service head etc, I'm sure he can wangle that for free.

 

[Meggerman]

Very true, I suppose it's just something I have always done to satisfy what I'm doing being relatively new to the trade I prefere to be thorough, maybe time for a change, I'll get things done quicker then.

I picked up another rewire today which was good, 4 bed house, 1930's, nice job, the bloke works for WPD so I'll get him to stick an isolator in and a new shiny service head etc, I'm sure he can wangle that for free.

Nice one, Obviously go the whole hog on testing that install being a new job an all.

 

[uksparks]

Yes indeed, I'm not going to be starting it until start of April anyway, but it appears to e all agreed, just need to go and measure up etc.

 

[Delboybully]

You cant calculate R1+R2 by taking Ze from Zs due to parallel paths. You should measure R1+R2 at every point on a radial or a ring circuit. This proves polarity and that each point has CPC continuity (you shouldnt be livening up an unsafe circuit). You should then measure Zs and put in the highest value on your certificate.

The testing on a CU change is the most important bit and can take some time depending on how many circuit and how many points

 

[Meggerman]

You cant calculate R1+R2 by taking Ze from Zs due to parallel paths. You should measure R1+R2 at every point on a radial or a ring circuit. This proves polarity and that each point has CPC continuity (you shouldnt be livening up an unsafe circuit). You should then measure Zs and put in the highest value on your certificate.

The testing on a CU change is the most important bit and can take some time depending on how many circuit and how many points

I think your confused with initial verification

 

[Delboybully]

You shouldnt make live an unsafe circuit. To prove it is safe you need to do R1+R2 and IR. But hey, thats how I see it. If I am wrong then I am willing to be corrected (it would save me some time on board changes)
You cant calculate R1+R2 by substracting Ze from Zs due to parallel paths. What about spurs?

 

[guest119]

Thats bad leaving boxes empty! Zs-ze shouldn't be relied upon due to parralel paths etc, but I guess it will give you a rough idea. Presumably then you don't go round each socket or light fitting then?

You need full testing after a cu change.
Including zs and polarity at every socket (which you will prove when doing r1, r2).
Also you must prove the switches at both sides of a twin socket at one of the visits.

Boydy

 

[D Skelton]

Technically there is no requirement to test after a CU change, those who do, and the extent to which they do it are complying with best practice guidelines only.

Zs-Ze isn't a recognised method although it can give a rough idea of what your R1+R2 values are, remember we aren't talking initial verification here. Bear in mind that during periodic inspection and testing even an R1+R2 is highly unlikely to be free of parallel paths either.

Me, I use test method 2 (no need to take off accessories as you can test at the screw [earth pin at sockets]) to every accessory and Zs at each point. A Buzz it 'n See to check polarity at sockets and a global IR will have been taken prior to old board coming off.

Mind you, last time I did a CU change was over a year ago! I'd be rusty now

 

[ipf]

Technically there is no requirement to test after a CU change, those who do, and the extent to which they do it are complying with best practice guidelines only.

Zs-Ze isn't a recognised method although it can give a rough idea of what your R1+R2 values are, remember we aren't talking initial verification here. Bear in mind that during periodic inspection and testing even an R1+R2 is highly unlikely to be free of parallel paths either.

Me, I use test method 2 (no need to take off accessories as you can test at the screw [earth pin at sockets]) to every accessory and Zs at each point. A Buzz it 'n See to check polarity at sockets and a global IR will have been taken prior to old board coming off.

Mind you, last time I did a CU change was over a year ago! I'd be rusty now

A very rough idea with the RCD involved, I'd say.

 

[NDG Elecs]

I generally r1r2 at every socket as how can you otherwise determine the OPD is correct. Take an instance last week, was getting decent r1r2/4 values at all sockets bar two. These were higher and then higher still. Spur off a spur, so changed OPD to 20A, and gave option for return to sort it out if they want.

I am not mad on the 'buzz it' socket testers but I do have one and do use it (mainly for determining what's on each breaker though).