zs on socket

[soulman]

hi while taking a zs at the same socket 3 times the readings were slightly out 0.50, 0,48, 0.51 is this fine as my lecturer says the same socket should give exactly the same reading. Also he states that on a tncs system the pfc & pefc should be identical when measured the difference was 100 amps.

cheers

 

[Sidewinder]

soulman,

Look at the %age error in those readings, calculate it and then tell me that they are not the same!

You are talking about an spread of 30 milli ohms!

What did you do inbetween the readings, had you nulled your meter leads.

There are a hundred things that could change a reading by 30 milli ohms!

 

[Doc Hudson]

There will always be slight variations due to the resistances of the connection between your test leads and the accessory being tested. Is this with a plug in lead/adapter or meter probes/croc clips. Also if testing sockets through a plug in lead/adapter you also can have resistive effects due to the on off switch in the socket outlet itself. The main thing is that you reading is within the tabulated max values. The pfc & pefc are proportional to the voltage and the earth loop impedance path, as the supply voltage can fluctuate these reading can also fluctuate, the essential point is does the pfc/pefc exceed the breaking capacity of the protective device. I would only worry if you readings are anywhere near the max permitted values.

Doc H

 

[soulman]

thank u both much appreciated

 

[alchemist]

PSSC and PEFC will differ except when taken at the CCU with main earth disconnected, because of parallel earth paths. Testing L-N the current can use those conductors to get back to the supply the supply transformer, L-E can go via the CPC to the CCU but from the CCU can also go via anything you have main bonding to. So PEFC should be higher from somewhere on the circuit.

At least this is what I think is correct, happy to be corrected if it's not.

Agree with the other comments on the variation in readings. How hard you press with the probes can change the reading that much. If you use crocodile clips or something that doesn't require you to hold it in place it's more likely to be exactly the same, but for 0.01ohms, there's not much point.

 

[soulman]

thanks mike