Very low CPC Readings

[chloebear]

Hi All,

Just been testing out some existing ring circuits in an existing building and I am when checking out Rn & R1 on the ring circuits all seems Ok with readings between 0.89 & 1.12 for 2.5mm singles, but when checking the cpc I am getting very low readings of between 0.01 & 0.03 for the same circuits as above????????? The majority of the installation is in steel conduit & box trunking, but as far as I can see the conduit system has not been used as the cpc.

Any thoughts??????????????????????????

 

[rocker]

If the steel conduit us earthed and in contact with CPCs from both legs it will lower resistance of CPC when measuring from both ends. What are your R1R2 measurments?

 

[Sparky Ninja]

Are you removing the connections of the cpc's to the conduit at every accessory?

 

[chloebear]

R1R2 mesurements come out at some low reading like 0.01 or 0.03

 

[rocker]

Is the conduit system earthed?

 

[chloebear]

No, I have not disconnected the link cpc to the back box of all the accessories - I have elected to take the readings at a socket within the ring and taken the mesurements from there. There are 2,5mm cpc singles in and out of the sockets through the conduits, so I would logicly assume that these have been wired directly the same as the Line & Neutral.

 

[chloebear]

the way the system is cabled it doesnt look as if the conduits have been used as the cpc, that is what is so confusing. It looks as if the system has been wired in the standard way with the Line, Neutral & CPC all in 2.5 singles. There are cpc tails to each of the back boxes and all the box trunking is earthed.

 

[rocker]

That's the problem then, if you think you are measuring 2.5 sq mm resistance against around 70sqmm resistance of the conduit system. The backbox fly leads and any other links to the conduit should be removed to get an accurate reading of the CPC's resistance.

 

[rocker]

Yea that's the thing, you have a cpc, BUT it's also attached to the conduit system (to actually earth the system, rather than to use the system as the CPC, thus the CPC's resistance appears very low when measured (when the cpc is attached to the conduit system) because of the lower resistance down the conduit's path, if you get what I mean.

 

[Sparky Ninja]

You should have a very good understanding of parallel paths if you are doing this kinda testing.

Otherwise it is likely that you have never obtained an accurate value before.

 

[chloebear]

So in this case it leaves me with 2 options as I see it - (1) accept that the system has been installed using the conduit etc as the cpc and note this down in the appropriate way on the report or (2) disconnect every accessory and cpc tail to the backbox and take the reading of the cpc that way (boy, if only there were 72 hours in every day)

 

[Sparky Ninja]

If time is a factor then I would:

a) Obtain a r2 value, to verify continuity of the ring final,
b) Measure R1&R2 and R1&RN at every outlet, to verify polarity,
c) Take an R2 value (wander lead/method 2) from the overcurrent devices position to the final conduit position to verify integrity of the earthing through the conduits.
d) Carry out a visual check on the integrity of the conduit connections, considering location and existing corrosion etc.
e) MEASURE Zs (as clearly calculating is inaccurate here) at every outlet to assure disconnection times are achieved.

 

[rocker]

If time is a factor then I would:

a) Obtain a r2 value, to verify continuity of the ring final,
b) Measure R1&R2 and R1&RN at every outlet, to verify polarity,
c) Take an R2 value (wander lead/method 2) from the overcurrent devices position to the final conduit position to verify integrity of the earthing through the conduits.
d) Carry out a visual check on the integrity of the conduit connections, considering location and existing corrosion etc.
e) MEASURE Zs (as clearly calculating is inaccurate hear) at every outlet to assure disconnection times are achieved.

All of which (apart from maybe the cont of the cond system) you should do anyway. I would add to this make a note on the cert saying 'socket ring wired with 1.5mm CPC as standard, however steel conduit system connected to CPC giving exceptionally low readings. Care should be taken to ensure continuity of cPC at all points on ring when any maintenance carried out on circuit' or something like that.

I can't see any real problem with that, but to be 100% you should d/c all flyleads and measure the resistance of the CPC itself. As you say though, there are only 120 hours in a day and, in real terms, being connected to the conduit system could be classed as actually beneficial.

 

[Knobhead]

Here we go back to the old argument of installing a CPC in conduit. If the conduit installation is good there’s no need for a back up.

 

[rocker]

Here we go back to the old argument of installing a CPC in conduit. If the conduit installation is good there’s no need for a back up.

Spot on there, Tone. I would call a CPC in a decent conduit system a 'back up' against morons and cheapskates installing bits of PVC.

Cond has much better resistance values than tiny cpc cores, and is virtually indestructable in likely conditions.