R1 + R2 test on SWA

[Ponty Massive]

Evening all,

May seem like an obvious question to some, but how would you test the R1+R2 on a 2-core SWA cable supplying a submain? TT supply, rod at house and additional rod at garage?

 

[LlandrilloSpark]

If your using two core to a submain then I usually run a separate 10mm cpc then get the R1+R2 by linking the seperate cpc with the line in the armour, or if your using the armour for earth then bond both ends with an earth ring and get it from one of them, IMO I don't think it's good practice to use the armour for a submain

 

[sparky82]

As all Tera Tera circuits should be RCD protected I would carry out R2 test (Core to core + Core to Armour) providing gland is tied into earth system via frying pan/banjo

 

[johnboy6083]

I don't like usin SWA for CPC in an environment, where the sheath is likely to be damaged.

 

[sparky82]

agree about SWA as Submain earth, Would also run seperate earth

 

[Ponty Massive]

There is a 2-core SWA from the house board down to the garage. The board in the garage has it's own rod, SWA seems to be banjo'ed the garage end.

 

[LlandrilloSpark]

Its banjo'd one end to just to bond it because its a exposed conductive part which could become live under fault conditions, If the armour is not the cpc and the garage has a rod and theres no separate cpc then there is no loop between line and earth from house db to garage db, hence no R1+R2, IMO

 

[Ponty Massive]

Its banjo'd one end to just to bond it because its a exposed conductive part which could become live under fault conditions, If the armour is not the cpc and the garage has a rod and theres no separate cpc then there is no loop between line and earth from house db to garage db, hence no R1+R2, IMO

I agree, but for the case of testing at the house DB....R1+R2 or R2 (for that circuit) how is it possible? The R2 reading would have to be via the TT rods?

 

[LlandrilloSpark]

Now I may need correcting here but the final circuits from the garage db like the lighting and sockets are what need the cpc's, if their Zs is up to scratch and they have the rcd protection than good, same applies to the main final circuits in the house, the cable supplying the garage db is a distribution circuit not a final circuit, pretty much the same as what supplys the main board from the road, and there is no R1+R2 on that cable.
What im saying is its not possible, there is no R1+R2 to get, as for R2 if you clipped on the MET and then onto the other end of the supply cable, the R2 you would get would travel through the garage rod, then mass of earth and maybe eventually pick up the suppliers earth provision

 

[Ponty Massive]

I've never really come across this one before !!!!!! What do you put on the cert (R1+R2 or R2, at least one column must be completed), just N/A?

 

[LlandrilloSpark]

Well the armour is connected to earth so I reckon you could measure an R2 reading and put that on the form, I cant see where you can get R1+R2 so thats the only thing you can do ?

 

[Ponty Massive]

But that would mean testing and getting an R2 circuit reading for / from the other board. From what I can see there is neither R1+R2 or R2 for this circuit from the house DB??

 

[johnboy6083]

Simples mate. You link live to armour at the each point, and thats your reading. As armadillo sparks mentioned, the only other option is a wander lead, and measure R2.

 

[malcolmsanford]

I still find it hard to grasp that unless there is a serious environmental reason that would persuade you not to, why you would not want to use the armour of say a 10mm SWA as the CPC, as that armour needs to be a minimum of 22.6mm to be the copper equivalent of 10mm and is in fact 41mm for a PVC 70c SWA, so nearly twice as much.

As it is a TT system the chances are that your going to have to protect that sub main distribution circuit with an RCD as you will most likely not get a low enough disconnection of 1 second if it over 32 amps or 0.2 for under with just a normal MCB/FUSE. So for the Zs of the circuit you would put 500ohms if your using a 100mA RCD, and then test for times.

I normally would calculate R1 + R2 for a SWA cable that is utilising the armour as a CPC as such. using a wander lead, measure the R1 and then the Armour R2 and then combine it with the Ze to find the Zs.

 

[truckster]

May be wrong but I would link live to armour at garage end and class that as R1/R2. The armour should be bonded at the supply end i.e. the house as that is where the associated protection for that cable is.

I would also test to see if there were any difference in potential between the armour of the sub and the rod at the garage and if so isolate the armour in the garage from the garage rod. (seperate earthing systems) Probably unlikely if both are TT