Issue with r2 reading on ring final circuit due to parallel paths

[Geo9073]

Good evening,



Been on and off on this project, they’re all brand new flats and now I’ve come back to carry out EIC tests.



It’s all first fixed in steel conduit and have stud frames with caddy clips on, so naturally been told to earth back boxes. Before I left I mentioned to the QS running job it’s best to leave any ring final circuits off so fly leads can be disconnected to get a true reading of the circuit.



I’ve now come back to everything screwed back, painted and basically signed off, so they don’t want any sockets taking off. I’m getting far to low r2 reading (r1, 0.49, rN 0.49, r2 0.17). And basically been told to write down the results that I get and that’s it.



So now I’m posting on here to get some advice, is it acceptable to write down these results I got? In my opinion I’ve not really proven the integrity of the circuit, could be disconnected and touching a back box for all I know, and can you still carry out the formula to see what sort of R1+R2 readings i should get with these results (r1+r2/4)?

 

[Pretty Mouth]

Is it wired in steel conduit throughout the circuit?

 

[Geo9073]

Is it wired in steel conduit throughout the circuit?

Not continuous, just straight bit with a spout within the walls at each point

 

[Pretty Mouth]

My copy of GN3 is outdated, but in the section on continuity of protective conductors it has a section on parallel paths. At the end it says "It is often impractical, and sometimes impossible to carry out testing with some or all of the parallel paths disconnected, and the inspector must be aware of this" - it could be argued that this is the case here.

 

[westward10]

If they are just isolated conduit drops to the accessories they shouldn't affect r2.

 

[Geo9073]

My copy of GN3 is outdated, but in the section on continuity of protective conductors it has a section on parallel paths. At the end it says "It is often impractical, and sometimes impossible to carry out testing with some or all of the parallel paths disconnected, and the inspector must be aware of this" - it could be argued that this is the case here.

So will results showing r1-0.49, rn-0.49, r2-0.17 and R1+R2-0.16 be acceptable?

I was always under the impression my r2 should be x1.67 and within 0.05?

 

[westward10]

I can't see how isolated sections of conduit affect r2.

 

[mainline]

So will results showing r1-0.49, rn-0.49, r2-0.17 and R1+R2-0.16 be acceptable?

I was always under the impression my r2 should be x1.67 and within 0.05?

If the conduit is earthed even if it's not continuous, but the back boxes are earthed, it will effect the r2 resistance.

 

[brianmoooore]

If the caddy clips secure the conduit sections to the steel frames, there is a parallel earth path between sockets on that frame. If a socket near one end of the RFC is on the same frame as one near the other end of the RFC, it would drastically reduce r2.

 

[HappyHippyDad]

Good evening,



Been on and off on this project, they’re all brand new flats and now I’ve come back to carry out EIC tests.



It’s all first fixed in steel conduit and have stud frames with caddy clips on, so naturally been told to earth back boxes. Before I left I mentioned to the QS running job it’s best to leave any ring final circuits off so fly leads can be disconnected to get a true reading of the circuit.



I’ve now come back to everything screwed back, painted and basically signed off, so they don’t want any sockets taking off. I’m getting far to low r2 reading (r1, 0.49, rN 0.49, r2 0.17). And basically been told to write down the results that I get and that’s it.



So now I’m posting on here to get some advice, is it acceptable to write down these results I got? In my opinion I’ve not really proven the integrity of the circuit, could be disconnected and touching a back box for all I know, and can you still carry out the formula to see what sort of R1+R2 readings i should get with these results (r1+r2/4)?

You've mentioned that you will be 'writing down' the results. Is this for someone else to fill out the EIC, or are you filling out the EIC?

I would be making a note on the EIC about the comments that @Pretty Mouth mentioned about GN3. I.e. that it is impractical to remove all parallel paths. I'd also be emailing the chap who told you to 'just write them down', to email you that, so you have a record.

 

[westward10]

If you are going to quote parallel paths you need to state what they are.

 

[Pretty Mouth]

I was always under the impression my r2 should be x1.67 and within 0.05?

That would be the expected value for a ring wired in 2.5mm2 T+E with a 1.5mm2 CPC (with no parallel paths), but that is not the only way of wiring a ring. In the case of a ring wired in singles, using conduit or trunking as the CPC, the containment probably isn't going to be wired in a ring in the same way that the live conductors are. Even if it was, it would be a ball ache trying to r2 it!

So an r2 isn't always going to be possible for all rings, and calculating an expected r2 isn't always possible.

If it was me, I would carry out the tests, making sure that each and every point had a good R1+R2 to it, and note on the cert that the circuit had multiple parallel paths due to conduit, structure etc.

 

[Pretty Mouth]

If you are going to quote parallel paths you need to state what they are.

Is that required?

 

[Geo9073]

That would be the expected value for a ring wired in 2.5mm2 T+E with a 1.5mm2 CPC (with no parallel paths), but that is not the only way of wiring a ring. In the case of a ring wired in singles, using conduit or trunking as the CPC, the containment probably isn't going to be wired in a ring in the same way that the live conductors are. Even if it was, it would be a ball ache trying to r2 it!

So an r2 isn't always going to be possible for all rings, and calculating an expected r2 isn't always possible.

If it was me, I would carry out the tests, making sure that each and every point had a good R1+R2 to it, and note on the cert that the circuit had multiple parallel paths due to conduit, structure etc.

Ok thanks, sorry should mentioned it was all done in twin and earth that’s where I was getting the x1.67. But I will put down on the notes that unable to remove parallel paths due to finished/ signed off rooms, and get it it in writing from QS as mentioned on here.

 

[westward10]

What are the parallel paths?