Can’t measure R1+R2, can I calculate it instead?

[Botanical]

Hi forum,

A question about R1+R2 testing. I’m testing a lighting circuit. It’s an old circuit with old colours. In the control panel feeding the circuit there is no earth cable feeding the lighting circuit, only an earth cable bonding the control panel to the strands of the armoured cable which goes onto the feed the lighting circuit. With no earth cable to link to the live, I can’t get an R1+R2, am I allowed to rearrange Zs=Ze + R1+R2 to find R1+R2 or is this not acceptable, any help would be appreciated, many thanks

 

[westward10]

How will you obtain a Zs if you have no cpc (cpc not an earth cable).

 

[timhoward]

Is the control panel using metal trunking or conduit as a CPC?
What happens at the other end of the armoured?

 

[MFS Electrical]

Hi forum,

A question about R1+R2 testing. I’m testing a lighting circuit. It’s an old circuit with old colours. In the control panel feeding the circuit there is no earth cable feeding the lighting circuit, only an earth cable bonding the control panel to the strands of the armoured cable which goes onto the feed the lighting circuit. With no earth cable to link to the live, I can’t get an R1+R2, am I allowed to rearrange Zs=Ze + R1+R2 to find R1+R2 or is this not acceptable, any help would be appreciated, many thanks

Short answer is no, you can’t calculate R1+R2 from the Zs.

(This will be a good example for learners or people looking to further their knowledge on Inspection and testing. It will require a little imagination though.)

The reason we can’t calculate is that the R1+R2 will give the absolute worst case scenario for calculated max Zs and therefore disconnection providing its correctly carried out.
Why it gives worst case scenario is because there will be no parallel paths in circuit to lower the Zs.

How this could affect safety;
Hypothetically, Take a boiler circuit earthed Via a CPC but also via the manifold bonded to the boiler.
Now imagine all of the pipework was replaced with plastic and now all you have is the CPC earthing the metal boiler case. This cable happens to take ludicrously long and arduous route to the boiler and the design was done by someone visiting Screwfix of a weekend to pass some time. So let’s say that the cable is on the small side and therefore the maximum Zs is borderline, couple that to a few lesser than acceptable terminations and you have a perfect storm.
The Zs has been tested previously on an EICR, before the pipework was replaced and came back well within limits, the spark calculated Zs because it was a bit of a ballache to measure it.

Well now that pipework has been removed. As has the parallel path it brought with it. The boiler develops a fault causing a short between live and earth internally. The actual Zs of the cable is too high to allow disconnection (a stretch I know but it happens) and the casing remains live. The homeowner feels a little chilly and goes to turn the boiler on. Boom the poor sod is dead.



So why did it happen? Also TL;DR

The parallel path provided by the pipework originally lowered the resistance (impedance) of the earth fault loop path significantly, so much so that the MCB would trip within 0.4 seconds.

Unfortunately the pipework having been replaced with plastic no longer brings an earth to the boiler, the cable itself didn’t either thanks to some really terrible connections. Now because of this the Impedance is so high that disconnection didn’t happen.

The spark calculated R1+R2 because the Zs seemed ok, now that meant that the terrible connections were missed. If he’d measured R1+ R2 he’d have found this issue.

All of these things added up to the homeowner receiving a fatal electric shock.


So to round off, that’s why we can’t calc R1+R2.

 

[MFS Electrical]

Hi forum,

A question about R1+R2 testing. I’m testing a lighting circuit. It’s an old circuit with old colours. In the control panel feeding the circuit there is no earth cable feeding the lighting circuit, only an earth cable bonding the control panel to the strands of the armoured cable which goes onto the feed the lighting circuit. With no earth cable to link to the live, I can’t get an R1+R2, am I allowed to rearrange Zs=Ze + R1+R2 to find R1+R2 or is this not acceptable, any help would be appreciated, many thanks

So can you not measure the R1+R2 using the SWA strands? I’m assuming here that the strands are the CPC.

 

[cliffed]

Short answer is no, you can’t calculate R1+R2 from the Zs.

(This will be a good example for learners or people looking to further their knowledge on Inspection and testing. It will require a little imagination though.)

The reason we can’t calculate is that the R1+R2 will give the absolute worst case scenario for calculated max Zs and therefore disconnection providing its correctly carried out.
Why it gives worst case scenario is because there will be no parallel paths in circuit to lower the Zs.

How this could affect safety;
Hypothetically, Take a boiler circuit earthed Via a CPC but also via the manifold bonded to the boiler.
Now imagine all of the pipework was replaced with plastic and now all you have is the CPC earthing the metal boiler case. This cable happens to take ludicrously long and arduous route to the boiler and the design was done by someone visiting Screwfix of a weekend to pass some time. So let’s say that the cable is on the small side and therefore the maximum Zs is borderline, couple that to a few lesser than acceptable terminations and you have a perfect storm.
The Zs has been tested previously on an EICR, before the pipework was replaced and came back well within limits, the spark calculated Zs because it was a bit of a ballache to measure it.

Well now that pipework has been removed. As has the parallel path it brought with it. The boiler develops a fault causing a short between live and earth internally. The actual Zs of the cable is too high to allow disconnection (a stretch I know but it happens) and the casing remains live. The homeowner feels a little chilly and goes to turn the boiler on. Boom the poor sod is dead.



So why did it happen? Also TL;DR

The parallel path provided by the pipework originally lowered the resistance (impedance) of the earth fault loop path significantly, so much so that the MCB would trip within 0.4 seconds.

Unfortunately the pipework having been replaced with plastic no longer brings an earth to the boiler, the cable itself didn’t either thanks to some really terrible connections. Now because of this the Impedance is so high that disconnection didn’t happen.

The spark calculated R1+R2 because the Zs seemed ok, now that meant that the terrible connections were missed. If he’d measured R1+ R2 he’d have found this issue.

All of these things added up to the homeowner receiving a fatal electric shock.


So to round off, that’s why we can’t calc R1+R2.

Agree on calc R1+R2 No Way…. this property is existing, performing a Zs test would prove the integrity of the earth fault loop & hopefully this will be low enough to operate the Mcb/Rcbo.
R2 would be the better way to test cpc connection… R1+ R2 only necessary on initial verification of the property

 

[MFS Electrical]

Agree on calc R1+R2 No Way…. this property is existing, performing a Zs test would prove the integrity of the earth fault loop & hopefully this will be low enough to operate the Mcb/Rcbo.
R2 would be the better way to test cpc connection… R1+ R2 only necessary on initial verification of the property

Yeah I agree R2 can be better, although it depends where you are and if the building is in use or not. Some people won’t tolerate a long wander lead being trailed about in say an office or similar.
It can also be good to indicate of other potential issues like loose or poor connections whereas an R2 test won’t. The other consideration is a ring circuit, R1+R2 gives you a clear cut answer to whether the ring is wired properly or where someone may have wired a ring within a ring.

 

[pc1966]

Maybe the OP can clear up something. Is there a CPC in the form of the cable armour?

If so then it might not be practical to isolate it at the panel, but you can still link the L to it and measure R1+R2 at the accessories (lights, in this case).

What you don't have is any simple means to know if that armour CPC is continuous or if any accessories are being earthed via unrelated metalwork. One thing you can do is to measure R1+R2 and do R1+Rn and Rn+R2 and then see what you can solve for each of them. If things are generally good in terms of cable joints, etc, then you should find R1+R2 and Rn+R2 and very much the same, though if the L is looped a long way to switches, etc, then the R1 term might be a touch higher than Rn.

Assuming the are close, then R1 will be approximately (R1+Rn)/2 and so you can estimate R2 from your R1+R2.

At this point if you know roughly the cable spec you can look up the conductor resistance per metre and the same for the armour and see if your measurement ratio is roughly in line with that.

• If in agreement to reasonable degree (10-20% ish, say) then you have almost certainly measured your R1+R2 and can then do your calcs for Zs, etc.
• If you get R2 significantly higher than expected it is a bad sign, suggesting poor connections at glands, rusted accessories boxes, etc.
• If you get R2 significantly lower than expected then you have parallel paths somewhere. Not as worrying, but as @MFS Electrical points out they could well be ones you cannot or should not depend upon for safety.