R!+R2 Question

[bren19]

when testing R1+R2 should the cpc be taken out of the earth bar and linked to the phase condutor? The reason I am asking is because my supervisor is complaining that my R1+R2 readings are higher than Zs readings, this is due to parrallel paths (conduit wiring system). So should I leave the cpc in earth bar so that these paths are included in R1+R2?
Thanks

 

[telectrix]

all depends on whether or not the conduit is your cpc

 

[bren19]

No conduit is not usde as cpc

 

[telectrix]

so , disconnect cables and measure the R1+R2 on the L and cpc alone.

 

[JUD]

R1+R2 is a measurement of the resistance of the line and cpc conductors only and shouldn't include any parallel paths.

 

[Farmelectrics]

Be aware to make sure though that you disconnect the earth from the back box to the socket plate as this will give you paralel earths especially if it's in conduit

 

[Farmelectrics]

Sometimes your r1 r2 readings can be higher than your zs the meter dosnt lie you have to record what ever the meter says

 

[JUD]

Be aware to make sure though that you disconnect the earth from the back box to the socket plate as this will give you paralel earths especially if it's in conduit

I think this is the basis of his original question.

If he disconnects from the earth bar then there can't be any parallel paths. The meter will only be reading the resistance of the line and cpc.

 

[bren19]

Thanks for everyones input, I just wanted to clarify that I was right to carry on testing and recording as I have been before being questioned about my readings. Due to the installation (large commercial install) there are so many paths to earth so every circuit on this DB has a higher R1+R2 than Zs

 

[Farmelectrics]

jud what i meant to say is if the conduit goes back to db and the flying earth lead from back boxes is still connected even if you take earths out of earth bar at board and link testing at sockets for r1 r2 you will get a false reading as the conduit is acting as earth as well

 

[JUD]

jud what i meant to say is if the conduit goes back to db and the flying earth lead from back boxes is still connected even if you take earths out of earth bar at board and link testing at sockets for r1 r2 you will get a false reading as the conduit is acting as earth as well

No it won't.

The conduit will no longer be part of the circuit under test as it doesn't form part of the loop i.e the resistance down the line conductor to the link in the DB and back up the cpc to the other test lead is all that the meter will read.

 

[Farmelectrics]

Jud if the containment is conduit and it's tubed back to the board this is an earth path if you disconnect live and eart at db and link you then test at sockets or light switch etc if there is a flying lead from socket or light switch etc to the back box it need to be disconnected as this will give you a parralel earth path ie a false reading

 

[JUD]

Yes it is a parallel path when the cpc is connected to the earth bar because the earth bar is linking the cpc and conduit together and making it one loop.

Think of it this way....

Take a length of wire. Strip both ends and also strip a section of the insulation from the middle (this simulates the cpc connected to the back box at the socket). Take a reading from the middle to one end and record the result. Now link both ends of the wire together (this simulates the cpc and conduit connected together in the earth bar) and take a reading from the middle to the link and you'll get a lower reading i.e parallel paths.

If it helps try drawing a picture.

 

[Farmelectrics]

Done the above test many times jud and tried the tests with and without flying lead to back boxes with the flying lead connected you get a much lower reading now what is that telling me it's picking up other paths or have I been doing these tests all wrong

 

[brman]

Thanks for everyones input, I just wanted to clarify that I was right to carry on testing and recording as I have been before being questioned about my readings. Due to the installation (large commercial install) there are so many paths to earth so every circuit on this DB has a higher R1+R2 than Zs

I reckon you are doing it right and it is quite worrying that your supervisor doesn't understand how parallel paths affect Zs measurements. Perhaps you need to suggest he does his 2391 again.........