Zs Conundrum

[shazbutz]

No question is a stupid question! Apologies if this one is.......

With a Ze of say 0.38 Ohm and (R1+R2) = 0.44 Ohm based on the formula Zs should be 0.82 Ohm.
When measuring the live circuit for Ze would the parallel earth paths of gas and water feeds due to their lengths and materials increase the measured Zs? If so what order of magnitude should be expected?

Or, should the resistance values be lower because of the additional paths?

:rolleyes4:

 

[jaydee81]

hi measured zs would usually be lower due to parallel paths

 

[telectrix]

the Zs would be reduced by the parallel paths, but using no-trip test setting ane effects of MCBs/RCDs could make it higher.

 

[shazbutz]

the Zs would be reduced by the parallel paths, but using no-trip test setting ane effects of MCBs/RCDs could make it higher.

Why is that then?

 

[ackbarthestar]

Why is that then?

The No-trip setting is usually a 15mA window for 40ms max. some use less time or a DC current with a pulsed AC over the top . Both methods rely on the electronics of the instrument to determine the volt drop and hence the resistance.
The more series switch contacts placed in the circuit the higher the likely overall resistance caused through the contact resistance of the switches.

 

[shazbutz]

So, if I use the other setting the trip would be activated but I'd get a more realistic reading albeit a reduced value in relation to the calculated Zs.
On that basis is there any benefit in a measured Zs as opposed to a calculated one for the EIC?

 

[ackbarthestar]

So, if I use the other setting the trip would be activated but I'd get a more realistic reading albeit a reduced value in relation to the calculated Zs.
On that basis is there any benefit in a measured Zs as opposed to a calculated one for the EIC?

I would say that its 50-50. sometimes a low current level window gives a reasonable result and if its below the maximum Zs for the MCB .
I don't now usually bother, but make a note just in cae.

On other occasions, possibly such as yours, I might use a different method. That is: By calculation...

However, I have a set of jump leads, made back in 1989, when RCD testers were in the process of being developed as part of the electrician's stable test kit, which I used to link out those troublesome RCCBs and so that I could get a reasonable Zs value on each circuit.

Now, this is not recommended so I cannot suggest this as an option as its antiquated and they now know that C21 electrical current travels differently ( as if) and it means you might have to work live, which is definitely not recommended, unless you pull the service fuse first, again not recommended, by me.

So you have two methods available at the moment.
1/ Use a good quality loop tester such as a Megger single
2/ Zs = Ze + ((R1 + R2) * Correction factors)