Srange results

[roukel01]

This has probably been talked about time and time again, but today following a board change I carried out some testing.

On every circuit, the R1+R2 result was higher than the Zs result.

How come???

 

[telectrix]

it normally is. Zs=Ze+(R1+R2)

 

[roukel01]

sorry, wasn't watching what I was typing.... what I meant to say was R1+R2 was higher than the Zs result........ on every circuit.

(I've corrected the starting thread now)

 

[telectrix]

that often happens. the R1+R2 are purely the circuit resistances, whereas Zs is that + Ze. now you measure Ze with the earth dissed from the installation. now when you reconnect the earth, you introduce parallel paths through the main bonding, water, gas, etc. this lowers the effective value of Ze and hence Zs, often to below the R1+R2 value.

 

[roukel01]

I can understand connecting bonds can reduce Ze due to parallel paths, but I can't see how it can reduce the Zs value lower than the recorded R1+R2

 

[telectrix]

hmmm, see what you mean. did you null your leads for R1+R2?

 

[nickblake]

think about ohms law and parrallel earths for example if you have 2 wires each measuring 20 ohms wired in parallel the resistance across those wires will be half of the rating so will be 10 ohms , this is the reason why parallel earth paths can reduce the Zs reading ,but the R1R2 reading will be as measured as you have taken these reading with out any parallel earth in the circuit ,as you would have disconnected the R1 and R2 from the board joined them together and tested at the furthest point

see what i mean

 

[roukel01]

To me, it still doesn't explain why the results were less than R1+R2. I think it must be my meter.


If I measure Ze with bonds disconnected, I'd get say 0.11 ohms. I then connect the water and Gas bonds, measure Ze again, and get perhaps 0.09 ohms. If I then measure the R1+R2 of a length of cable and get say 0.04 ohm and then connect this to the installation and measure the earth loop, surely I shouldn't be getting a result of say 0.03 ohms. But this is what was happening...

 

[telectrix]

it's not only parallel paths. it's the way measurements are taken. also, you have the internal bits of RCDsand MCBs affecting the Zs reading.

 

[Amp David]

I've had this and thought my meter must be faulty, so had it calibrated. Even now still get similar readings.

As tel just said, it must be the workings of the meters that does it also.

Used to bother me a bit, but as long as my calculated values are within the maximums then i don't worry about it

 

[telectrix]

if you think that's strange, i had one . a 16th ed install. RCD as main switch. Zs on all circuits around 350ohms. Ze was 0.2ohms. took a"Zs" on the CU live busbar, 347ohms. internal workings od RCD affecting the value to a ridiculous extent.