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

 

[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.