2330 lvl 2 Practical test. R1+R2 for swa?

domino · Feb 13, 2011

Help!

Currently doing the practical test for Level 2 2330. Have to inspect and test this week coming. I think I have a good grasp of all the dead testing we need to do BUT have just been swotting up in the Onsite Guide and I've found something that has not been mentioned so far at college.

One of the circuits is a length of 2.5mm² 3 core SWA from a consumer unit to a surface mounted metal clad Fused Connection Unit. The CPC is one of the 3 conductors but of course the armouring is also earthed at both ends.

We need to enter an R1+R2 value on the test sheet for this circuit but according to the OSG, Method 1 can't be used for swa or micc cables. We definitely haven't been taught this yet, so I was wondering how you guys obtain this value in the real world?

I do appreciate there are 2 parallel earth paths down the cable. Would you remove the armouring earth connections and measure across phase and earth? Or maybe connect both earths and phase and record that reading?

Help!!

malcolmsanford · Feb 13, 2011

Are you sure your not getting a little confused with using the sheath of the MICC and the armour of the SWa as your CPC. If your using a designated core of the SWA as your CPC then you do R1+R2 as normal.

High Tower · Feb 13, 2011

use method 2, end to end of cpc

domino · Feb 13, 2011

Armour is not used as CPC in this case.

@malcolmsanford.

I couldn't put a link between the live and the earth in the consumer unit could I, or there'd be 2 parallel earth paths, one down the conductor used as CPC and one down the armouring . If I pulled them out of the board and linked them in a chocblock then measured across load side of FCU, would this then give me R1+R2 as you say, and exclude the path down the armour.

@High Tower

I have to find a value for R1+R2 on the inspection sheet, once I'd measured R2 end to end how would I go on from there?

To confuse me even further, an electrician down our street has since told me to take both the CPC and the armour earth from the same terminal on the CCU earth bar, link them both to R1 and take that as my R1+R2 reading.

AAAArrGGhh!!

Sorry if I seem long winded. Just trying to make everything clear to all (especially me).

Many thanks for all help so far.

High Tower · Feb 13, 2011

R1+R2 is method 1, that is where you link both of them out and take a reading at one end.

R2 is method 2, measure continuity of just the cpc end to end and record that on you sheet as R2 value.

I would earth cpc armor ouny at the supply end

malcolmsanford · Feb 13, 2011

Dom that is what you do once you have terminated your armour before you connect up your banjo fly lead or CPC into the Earth bar you croc clip out the LINE and CPC at the CU and then you take your R1+R2 at the furthest point. Obviously if it's a socket or a light circuit you have to take at each point.

What your spark as told you is if you use the conductor and the armour combined as your CPC. In some case espicially on larger SWA's and because it is a large cable, you may want to use a 2 core with armour as your CPC, you may find that the armour itself is not sufficient. That is when you normally calcualte as per reg 543.1.3 using your adiabatic equation, to see if is can comply.

Even after doing the calculation you may find that it is still too small and therefore would run a 3 core SWA and use a combination of the armour and the conductor that is when you do the test as he said.

High Tower · Feb 13, 2011

It's normally 1.5mm 3 core that's used for this assessment hence use the grey as a cpc with the appropriate sleeve and only connect armour at supply end, then use method 2

domino · Feb 13, 2011

Thanks Guys, I think understanding is almost dawning.

Here is a quote from the OSG:

Test method 1 can only be used to measure (R1+R2) for an "all insulated" installation, such as an installation wired in "twin and earth". Installations incorporating steel conduit, steel trunking, micc and pvc/swa cables will produce parallel paths to protective conductors. Such installations should be inspected for soundness of construction and test method 1 or 2 used to prove continuity.

This above only applies then to swa cable where the armour is being used as a CPC.

If I understand you both correctly then, in my case I should only earth the armour from the gland in the consumer unit, not from the gland in the fused connection unit to the earth therein (I did use the grey wire as CPC).
I believe we are required to give a value for (R1+R2) and not R2, so having done the above I can simply pull phase and earth from the MCB and Earth Bar, connect them together and take the (R1+R2) reading at the FCU (furthest point), just as with any other non ring final circuit. I'm sure this is what the assessors are looking for (basic stuff) and it was simply me trying to do some background reading that was confusing the matter.

Here's hoping that's it.