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[davino]

hello,
I done a bit of testing for the first time today, but am not sure of some of the terminology...

i tested r1*r2 on a domestic lighting circuit and got 0.80
then i tested the zs and got 0.50
my boss then told me that the r1r2 was to high to have a zs of 0.50
can some once tell me in plain english the relationship between zs ze and r1r2 as he blinded me with science , and am only a apprentice....
thanks
________
YAMAHA YBR 125

[ericmark]

OK what you have is common as R1 + R2 measures the cable only where as an earth impedance meter measuring Zs is also including any other earth paths so water pipes and gas pipes may also be earth paths.
The theory is the incoming supply will be around 0.35 on a TN-C-S supply. And with a ring main with say 79 meters of 2.5mm T&E cable the line wire will be 79 x 0.036Ω = 0.711Ω and the earth wire will be 79 x 0.058Ω = 1.1455Ω so R1 + R2 will be 1.8565Ω but taking a midway point on an earth loop impedance tester you would measure 0.464125Ω as the cable is doubled up and only half way around circuit.
Sorry Grand Children have arrived so must go.
Eric

[ericmark]

The figures I picked represent the maximum length of twin and earth 2.5mm² which can be used in a ring main with a 32 amp MCB giving a volt drop of 11.25 volts where permitted is 11.5 volt being 5% of 230 volt. The figures are all in the 17th Edition but sometimes it is a little hard to work it out.
Table 4D5 for example gives Voltage drop per amp per meter. This however is the line and neutral so for line earth you need to take the 18 mV/A/m and 29 mV/A/m into account so 9 mV/A/m and 14.5 mV/A/m = 23.5 mV/A/m and of course ohms is volts divided by amps so 18 mV/A/m also is 0.018 ohms per meter armed with this we can calculate what the results should be.
With your example of lights it is likely to feed a bathroom and hence have extra bonding conductors so we look at the results. Assuming 1.5mm T&E with 1mm earth then 0.035 ohms per meter so at 0.8 R1 + R2 = 22 meters seems OK and at Zs = 0.5 then minus 0.35 Ze gives 0.15 so total length = 4.1 meters which seems far too short so seems error is in the earth loop impedance measurement. This can be meter error or extra bonding. Once you know it is the earth loop impedance measurement maybe you can workout where the error came from.
Eric

[davino]

Hi again, thatks for the reply..

ok , might have this wrong here but please bare with me...
As the consumer unit was 16th ed' ( no duel rcd etc) we upgraded the mcb's to rcbo's , now as this carries 30ma protection we no longer need the bonding in the bathroom from the light to the bath radiator and sink pipes,, and under the sink in the kitchen...so could this be a factor of the low zs as you stated extra bonding could be a reason
thanks
________
Vapir oxygen

[ericmark]

If the low Zs reading is due to any parallel paths then the reading is correct. But looking at reading given seems unlikely.
Many people when checking the Ze reading do so with main switch on and everything running. Really, the reading should be made with everything off and all earths disconnected from main incoming earth.
Although we assume an Earth Loop Impedance of 0.35 ohms on a TN-C-S supply it could once the water pipes and gas pipes etc are connected be well below that figure.
So to start with would you say that 22 meters of cable seems around the right figure for what you know. Remember if will allow for a +/- error of 0.1 then between 19 and 24.5 meters how does that match up?
Looking at 0.5 reading and comparing with 0.8 the Ze reading would be need to be very low and a really good parallel path also. As a result one starts to look at the meter.
Normally one selects a socket at work and one writes on it the measured Earth Loop Impedance each time one takes the meter out you first retest that socket. Should something go wrong with the meter it then gives one a good indication that it has gone out. Once inspection and testing becomes a regular job you soon get to know what to expect. This is exactly what happened with your boss he looked and realised something was wrong.
You post is unclear to me. If your saying because it is not RCD protected you have removed the bathroom bonding then this will not explain the reading.
If you are saying the bathroom is bonded and should you remove it. Then yes for test but no on permanent basis and in real terms I would not remove but just note that it was in place on the paperwork.
Also all circuits in bathroom would need to be RCD protected to be able to remove earth cross bonding not just the circuit you are working on.
Do remember this is only an excise as you may use inquiry, measurement, and calculation to get results and you do not need to duplicate so once you have measured R1 + R2 there is no need to then use the earth loop impedance tester as well. It is a good idea but not required.
I will guess your boss just wants you to realise why you got the reading and once you tell him you think because of parallel earths you got the reading then he will accept it he has done what he wanted and made you think.
Eric