Ze+(R1+R2) is lower than measured Zs

rob121 · Aug 2, 2011

Hi All,
Currently in the process of my NIC application. Completed one job and had it signed off. Just installing a socket at home on its own circuit directly under CU. Going through my test results cant understand the difference in readings.
Measured Ze is 0.12
Measured R1+R2 is 0.01
Was expecting a result around 0.13
Instead results keep coming back between 0.19 and 0.22 which I think is quite a bit more than the expected. Bearing in mind its run in 2.5 T+E and the socket it about 6 inches under unit. Anyone any ideas about the difference in readings?
All leads were nulled, each test was done twice as I thought it could of been a dodgy reading. I know its not a huge amount and nothing to worry about but couldnt understand why its so different. Its a megger m/f im using

Got building control coming out tomorrow then it shall be time to book site visit!

Thanks for all the support!

Rob:icon4:

Johnny-G · Aug 2, 2011

The extra impedance is probably due to the RCD, assuming there is one.

Edit: Or that you're using a non trip test for the direct Zs measurement...

Guest123 · Aug 2, 2011

RCD impedance

Socket contacts

Meter accuracy

No trip loop testing will always throw up differing results, just the nature of the beast.

chopper · Aug 2, 2011

i never find them to be the same when comparing measured and calculated results , always a sligh diffrence, n my opinion .04,and .05 is not much or am i missing something here

rob121 · Aug 2, 2011

cheers for the replies! Now I know for next time.
No .04 isnt a lot but when the R1+R2 is only 0.01 it made me wonder if I was doing something wrong.

Thanks again,

Rob

telectrix · Aug 2, 2011

the fact that the measured value is different from the calculated value shows that you have measured the values not tried to fudge them. don't worry about it. as said previously, this can be expected.