continuity of cpc

[flukey]

hi all,
got some testing to do where there are some large expensive looking light fittings that i would rather not remove for testing

what methods do you use to get round this problem as i dont want to get caught up in an only fools and horses type moment

cheers in advance

 

[Spudnik]

If you are just checking cpc continuity then easiest way would be a long wander lead connected to MET or cpc bar in cu and to your then use the other to probe the fitting, assuming they are metal of course.

Or temp link in cu from mcb to cpc bar. then on to light fittings with switches on and probe fitting and try and get other probe into lamp holder for R1.

 

[flukey]

hi jason.
need to get decent r1 r2 reading without dismantling but although fittings look expensive i dont think they will be metal construction

 

[Spudnik]

Are the circuits looped at the switch or behind the fitting?

Sounds like a difficult situation you have.

If its for a PIR then you may have to mark it done on the test sheet as a limitation explaining why.

Other than that, im afraid your only other option is to take the fittings down, or maybe get the client to do it?

If you explain the reasons why, you may find them fairly accommodating.

 

[flukey]

looped at the fittings, just my luck

better check my insurance is up to date first then.

cheers for the reply

 

[WarrenG]

Like Jason said ask them to take down the fitting only so that you can test (they don't have to touch the electrical side) or alternatively explain the work that has to be carried out and get them to sign something to cover you for any damage that may result in the removal of the fittings?

 

[mrloy99]

hi jason.
need to get decent r1 r2 reading without dismantling but although fittings look expensive i dont think they will be metal construction

can you not use one of them lightmates that everyone was talking about?
Its unlikely that you will have to dismantle the fitting.Most have a sort of cowl covering the ceiling rose,just drop this down to access connector block.Or maybe its on the landing and you can access ceiling rose/jb from roof void.Or maybe it DOES have an only fools and horses type winch.Are you sure that this fitting is at the end of circuit?

 

[flukey]

hi mrloy. its ground floor lighting and all the lights have same sort of fitting. cover not removable. the 2 screws going through cover are holding the light to the ceiling.

 

[mrloy99]

hi mrloy. its ground floor lighting and all the lights have same sort of fitting. cover not removable. the 2 screws going through cover are holding the light to the ceiling.

Recording as LIM sounds like the way foreward then if its a double insulated fitting,and you can't get to the earth.Maybe you could take your reading at the switch,if you don't want to risk the fitting.

 

[flukey]

funny you should say that mrloy, just flicking through gn3 and noticed on page 35 that the r1 r2 reading is in fact being taken at the switch.

never noticed that diagram before because ive always taken reading at the fitting as that was the way i was taught, but thinking logically the switch would be the extremities of r1 r2 would it not.

would certainly make my life easier

 

[Spudnik]

At the switch is fine.

You could also do the loop test using 2 wire method if the MCB will hold.

Could always swap it out for something bigger if not whilst you do the test.

I thought you needed to get behind the fitting.

Wander lead will only be any good if they are metal fittings and not class II

 

[flukey]

hi jason.
never said i needed to get behind the fitting, just asked for alternative methods like the one thats now been highlighted.

we were advised against testing at the switch even on my 2391 but thinking about it logically the switch is the furthest point and if you get continuity reading there then all is well in the fitting.

obviously would wander lead the fitting as well if it was needed, but now i am questioning why we are taught to test at the fitting at all

can open, worms everywhere

 

[Spudnik]

hi jason.
never said i needed to get behind the fitting, just asked for alternative methods like the one thats now been highlighted.

I realise that now ive re read your OP

What you say about testing at the switch and it being fine at the fitting is not always the case, however if you test EVERY switch and you are sure its the last switch on the circuit then you can be sure all is ok.

Exactly the same as testing at the last fitting.

 

[whensbait?]

ive had a similar problem before, the switch isnt usually the highest r1r2 value as the switched live at the fitting adds those extra few metres from the switch, but wouldnt have thought it would be by a very noticable amount? well not enough to go through the hassle of removing fittings. is this end of line?

 

[flukey]

thats exactly how ive always seen it whensbait. i reckon thats another cock up in gn3 but it would certainly be the way to go in my predicament.

 

[mrloy99]

thats exactly how ive always seen it whensbait. i reckon thats another cock up in gn3 but it would certainly be the way to go in my predicament.

OK consider this example I have just made up.
I have a circuit 50 metres long.
resistance of copper at 20 degrees(all approximate to illustrate example)
1.5mmsq =12 milliohms/metre
1.0mmsq =18 milliohms/metre
so 1.5/1.0 t+e =30 milliohms/metre
Now if I measure at ceiling rose (line side,forgetting about switch drops for a moment) R1+R2=50x30=1500 mohm or 1.5ohms
So,say we have a distance to switch of 10m
we would now have to add the switch wire lengths to get reading at ceiling rose.:12x20=240mohm +1500=1740 or 1.74ohms
Now if I measure at the switch( only one leg of twin)
we would have to add( 12x10=120 )+(18x10=180)=300mohm+1500=1800
or 1.8ohms. So it looks like the switch in this example wins.

Of course if we are using loop tester we should bear in mind Jasons advice that we may need a circuit neutral when testing thru rcds.

 

[flukey]

i am completely with you mrloy

not sure if i have missed something but i checked in my on site guide today and the testing method in there has been changed to measuring at the switch instead of the fitting too.

brown book shows fitting, red book shows switch

perhaps i wasnt paying attention when i did my 2391 in july last year but that was to the 17th and i tested at the fittings on my practical when surely i should have tested at the switches according to gn3 and osg

 

[Spudnik]

I thought the brown book showed it at the switch as well?

Still, now you know testing at the switch is ok!

 

[flukey]

Brown book definitely at fitting jason

ive attached a copy below for you

 

[Spudnik]

Brown book definitely at fitting jason

ive attached a copy below for you

Nothing there?!

 

[flukey]

keeps saying cant display the image

its page 68 of the brown one

 

[Spudnik]

Ok no prob,

Ill have a look later.

 

[flukey]

Managed to attach it now

 

[flukey]

heres the latest update as well for those that havent got it

 

[Spudnik]

Interesting that they have now moved to the switch.

I agree with testing there as i would have thought, that would be the furthest point on the circuit at the last ceiling rose, but then i guess it depends on how the installation was wired.

 

[flukey]

hi jason
they kept that one quiet. i agree its best at switch too and will certainly make my life easier

 

[WarrenG]

I was always taught to take readings at all and record the highest reading which should be at the furthest end of the circuit i.e. switch

If a higher reading was obtained which wasn't at the furthest point then further investigation is required.

 

[Spudnik]

I agree, however, most electricians never go round all of the fittings and switches to see which has the highest value on a PIR, even though they should.

 

[bahco]

i was allways told to test at the pendant never the switch, like people have said it all depends on which way its wired.

 

[flukey]

hi bahco
thats how i was taught too. the question is now whats the reasoning for the method to be changed in osg and gn3. can only summise that as its continuity of cpc the furthest point will be the switch not the pendant irrespective of how its wired.

 

[bahco]

well if that is the case then why have they only realised that now?

they need to make there mind up! haha probably just doing it to **** us off

 

[Spudnik]

Just reading through the bits in the OSG it says:

'test between line and earth at each point in the circuit'.

So that includes fittings and switches.

So they havent really changed their minds like we thought.

 

[flukey]

thats what it said in the old book too.

why would they change the diagram then

as per previous post, thinking logically the switch is the end of the line for the cpc so perhaps like bahco says they must have just realised lol

 

[mrloy99]

hi bahco
thats how i was taught too. the question is now whats the reasoning for the method to be changed in osg and gn3. can only summise that as its continuity of cpc the furthest point will be the switch not the pendant irrespective of how its wired.

If there is any doubt,both readings should be compared.There could be 2(or more)lights off the same switch,and in this case the higher reading is unlikely to be at the switch.

 

[iceman]

hello iv just rewired my house for on of me jobs for nic, i been testing continuity of cpc today on a lighting circuit suppying downstairs after iv linked out at the c/u thn connect my metre leads to cpc and switch line the reading im gettin for R1 + R2 is somethin like 0.67 witch is fine but when i comfirm polarity by flickin the switch im gettin a terrific value somethin like 88.9 ohms lol i always thought there was ment to be no reading at all as the switch is closed, or am i just pickin up the resistence of the cable??? im gettin the same sort of thing when comfirmin polarity for landing light swiched upstaires and downstairs with strappers.

also theres a garage suppy feeding 4 socket outlets and 3 lights when messuring Zs do i take the reading frm the fuse spur suppyin the equiptment?
any help would be great cheers

 

[mrloy99]

hello iv just rewired my house for on of me jobs for nic, i been testing continuity of cpc today on a lighting circuit suppying downstairs after iv linked out at the c/u thn connect my metre leads to cpc and switch line the reading im gettin for R1 + R2 is somethin like 0.67 witch is fine but when i comfirm polarity by flickin the switch im gettin a terrific value somethin like 88.9 ohms lol i always thought there was ment to be no reading at all as the switch is closed, or am i just pickin up the resistence of the cable??? im gettin the same sort of thing when comfirmin polarity for landing light swiched upstaires and downstairs with strappers.

also theres a garage suppy feeding 4 socket outlets and 3 lights when messuring Zs do i take the reading frm the fuse spur suppyin the equiptment?
any help would be great cheers

I think the furthest light,as it would be the extremity of the circuit.
As for your r1,r2 readings,when my meter leads are short circuited it shows 00.0 and open circuit 1(scale to 200) What readings do you get on megohm scale.(I think you meant to say switch is open)

 

[beaver74]

funny you should say that mrloy, just flicking through gn3 and noticed on page 35 that the r1 r2 reading is in fact being taken at the switch.

never noticed that diagram before because ive always taken reading at the fitting as that was the way i was taught, but thinking logically the switch would be the extremities of r1 r2 would it not.

would certainly make my life easier

i would say that switch line to earth at fitting would be the extremities as the feed go from fitting to switch then back co fitting

 

[iceman]

hi i testin the insullation resistence on megohm scale set at 500v and tht was all fine the only probley i had wos a dead short on the garage supply between neutrel and earth it was doin me head thn when i went to do Zs i forgot the luminare wos still fitted, i best not do tht on me assessment or the N I C guy will send me straight to hell lol

 

[Guest123]

also theres a garage suppy feeding 4 socket outlets and 3 lights when messuring Zs do i take the reading frm the fuse spur suppyin the equiptment?
any help would be great cheers

Hey mate.

You would need to measure Zs at - your consumer unit, the socket outlet circuit and the lighting circuit within the garage, not quite sure what you mean with the fuse spur supplying the equipment bit.

Can you clarify?

Cheers.

 

[moorhouj]

Back to switch vs fitting - for a loop in wiring - would have expected R2 to be greatest at switch, as extra CPC from fitting to switch, but R1 greatest at fitting as have to add phase path to and back from switch. So for this arrangement, in 1mm, the fitting should be greater by the resistance of the length of a single cable from fitting to switch, but in 1.5/1.0 this drops to 0.5 of this. But if it's JB wired the relative distances come into play, and any other arrangement can give either as max, so you really do need to measure both unless you are sure how its wired.
Jack

 

[iceman]

hi theres a 20 amp circuit breaker suppying the the garage witch feeds socket outlets and a light circuit the supply comin in goin to a j/b then to a 13amp fuse spur i wanted to knw when messuring Zs do i take it at spur i wouldnt of thought i take it frm light fitting radial sockets cos there are seperate circuits

thanks

 

[Spudnik]

Zs is measured at the furthest point on the circuit.

So, just guessing, at the last light fitting on that 20A radial, or literally, the furthest point from the CU.

 

[iceman]

so i gotta eather take a reading for Zs at the futhest light fitting or the socket outlet witch ever is the greater???

 

[Guest123]

Hi.

Sorry I thought you had a DB in there.

If it's just the one circuit then yes you just find the firthest point on it, and measure your Zs, from there.

Although if coming out of that joint box that you have, a feed to socket outlets and then another feed to a fused spur in turn feeding your lights then I would take 2 readings and record the highest.

Cheers.