High Zs readings

[murf20]

I have read a bit on here about getting high Zs readings compared to (R1 + R2) + Ze tests. I have just replaced an old fuseboard with a Contactum dual RCD CU.

Ze is 0.2 on a TNCS system

• On the ring circuit I have an (R1 + R2) of .54 but a measured Zs of 1.39 -- PSCC of 384, PFC of 158
• On the cooker circuit an R1 + R2 of .27 but a measured Zs of 1.18 -- PSCC of 918, PFC of 207

There is a similar situation on the immersion heater circuit but this stays within the Zs limits.

An added problem that I have is that there is only one final ring, bigger than recommended, but going by Ze + (R1 + R2) I would have thought that I would have been OK recommending dividing it in 2. The fact that the Zs problem exists on other circuits as well suggests that there is other reasons as well.

I have read some really good answers on this from people about the non trip test being unreliable, I'm using a Di-log 9083p by the way, and the effect of the RCDs themselves. I have checked all the connections on the CU.

Why the big difference in PSC and PFC on a TNCS?

They have loads of electronic kit plugged in which wasn't all disconnected when i did the Zs tests. I was wondering if some of this could be producing a bit of current on the protective conductor and distorting results?

Any thoughts appreciated



I

 

[telectrix]

both the MCBs and the RCDs can affect your readings. if you want the readings to be within limits on your cert, use the calculated values Zs= Ze+(R1+R2)

 

[i=p/u]

or you could do the tests from the start disconnecting everything... but adding it up is quicker. why is pefc differnt from his pscc if tn-c-s is roughly same length

 

[Des 56]

He could have a high resistance on a neutral connection between the consumer unit and the cut out

 

[Richard Burns]

The large differences between PSCC and PEFC (assumedly); are these reflected at the origin? If not then it looks like you have some high resistance joints for your CPC. Should be easy to check at the CCU for the cooker.
If it is at different at the origin then a problem with your main earth connection at the earth bar.
Have you tried checking the fault currents at the CU but after the RCD, this would eliminate a high resistance through the RCD. (oh yes and do non trip test at the origin to eliminate different method variability)

 

[Richard Burns]

Just still thinking out loud is the earth bar dirty, varnished etc and not giving good connections? But more likely Tels suggestion.

 

[spark 68]

Hi Murf,

There was quite a long discussion about this very thing on this thread here:

http://www.electriciansforums.net/i...ical-installations/41772-zs-reading-help.html

 

[Cj24]

When testing impedance what exactly are you testing, I know it's something to do with your earth path to earth if a fault occurs

 

[Richard Burns]

For a zs measurement you are measuring the impedance of the red line shown in the EFLI diagram (for a TNCS) below.
This is to ensure that for the appropriate circuit protective device, in case of a fault (of negligible impedance) to earth there will be enough current flowing to cause it to trip.
If a higher current flows the CPD will trip faster, so you want the current as high as possible (so long as it is not above the breaking capacity of the device). Therefore for a high current you must have a low resistance (impedance) and this is what you are measuring.

Ze is measuring this value at the origin so that you can add this to R1+R2 rather than measuring Zs directly.

 

[murf20]

Thanks i=p/u. I have gone by what I have added up for now but I will be back there on Tuesday, when I will disconnect everything again and check. The Dilog tester uses a 15ma current for the non-trip test, which I think may be smaller than some of the others. This may make it more susceptible to noise on the circuit according to the di-log FAQs.

 

[murf20]

Thanks Richard. Stupidly I forgot to check PSCC at Origin when doing Ze. I am relying on Ze + (R1 + R2) until Tuesday when I go over it again. The cooker switch has a socket outlet so I took R1 + R2 from there. Connections all OK at MET and CU - new MET and 16mm cables between PME terminal at the head, MET and CU. Will do non trip test, and by pass RCD on Tuesday.

 

[murf20]

Thanks telectrix. I have used the calculated values for now but would like to be able to explain exactly why Zs tests as it does. If it is resistance across the RCD and MCB then this would stop the MCB operating within the specified time limits?

 

[murf20]

Thanks Des but L-N is fine so isn't it just the Live to Earth path to check?