Summat for sparkies...

If Ze is 0.15 and Zs is 0.58 what's the point of measuring R1+R2? Surely you could just calculate it?

The earth-loop of the circuit is Zs-Ze so R1+R2 must be 0.43 surely?

Or did I miss summat really basic lol

Ze & R1R2 do not have parallel paths. Zs does. so is often lower.


i wouldnt try and make it meet the requirements of the formula

if it is below the required zs values for the circuit config dont worry about it

Not mine found on talk electrician...sounds good though aye..

AJT said:

Ze & R1R2 do not have parallel paths. Zs does. so is often lower.


i wouldnt try and make it meet the requirements of the formula

if it is below the required zs values for the circuit config dont worry about it

Not mine found on talk electrician...sounds good though aye..

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Huh? Ze is the earth-loop at the origin while Zs is the earth-loop at the furthest point on the circuit? Could you explain?

If Zs is lower than Ze my head would explode lol

The use of the transposition of the Zs = Ze + (R1+R2) formula to back calculate R1+R2 is not a valid calculation because the value of Zs obtained by measurement is a live test on a final circuit (or at least, not the origin) and so all protective measures including bonding are in place and the presence of the bonding conductors will reduce the effective value of the external loop impedance compared to the accurate measured value of Ze that is taken with the main earth disconnected from the installation.

The Zs as calculated from Ze + (R1+R2) is the worst case scenario for the measured circuit.

The measured Zs is the best (and real) case scenario and could be significantly lower than the calculated value.

Using the measured value of Zs will give an optimistically low value of R1+R2 if used in the transposed formula.

Check here
None of this is mine just found on net mate...Far to clever for me.

AJT said:

The use of the transposition of the Zs = Ze + (R1+R2) formula to back calculate R1+R2 is not a valid calculation because the value of Zs obtained by measurement is a live test on a final circuit (or at least, not the origin) and so all protective measures including bonding are in place and the presence of the bonding conductors will reduce the effective value of the external loop impedance compared to the accurate measured value of Ze that is taken with the main earth disconnected from the installation.

The Zs as calculated from Ze + (R1+R2) is the worst case scenario for the measured circuit.

The measured Zs is the best (and real) case scenario and could be significantly lower than the calculated value.

Using the measured value of Zs will give an optimistically low value of R1+R2 if used in the transposed formula.

Check here

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So if Ze was measured with MET disconnected then Zs measured with ALL circuits (including MET) connected could provide false values? Got it

Phew I'm glad about that mate..way over my head.

AJT said:

Phew I'm glad about that mate..way over my head.

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Lol. Argument with Building Inspector. If PFC (Prospective Fault Current) on the final circuit is massively more than required (looking for 5 times rated trip value or better) who gives a shit what R1+R2 is? Final circuit Zs is 0.58 so at 230 volts we're looking at pfc of 398 amps which is more than 10 times trip value of 32 amps. Like I care about R1+R2? Not! Lol.

Him Her said:

Lol. Argument with Building Inspector. If PFC (Prospective Fault Current) on the final circuit is massively more than required (looking for 5 times rated trip value or better) who gives a shit what R1+R2 is? Final circuit Zs is 0.58 so at 230 volts we're looking at pfc of 398 amps which is more than 10 times trip value of 32 amps. Like I care about R1+R2? Not! Lol.

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wheres the trip hazard sign when you need one!!!

but agree with ATJ over my head... would need to research this to comment further.. good luck with it all

Lol, if the results do not make sense 90% of the time it will be down to parallel paths

Electrics is bullshit lol

Him Her said:

Lol. Argument with Building Inspector. If PFC (Prospective Fault Current) on the final circuit is massively more than required (looking for 5 times rated trip value or better) who gives a shit what R1+R2 is? Final circuit Zs is 0.58 so at 230 volts we're looking at pfc of 398 amps which is more than 10 times trip value of 32 amps. Like I care about R1+R2? Not! Lol.

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The r1 + r2 is to show there's a earth path for That circuit rather than finding a way back through bonding etc. This is why the earths have to be disconnected when testing

slacky1230 said:

The r1 + r2 is to show there's a earth path for That circuit rather than finding a way back through bonding etc. This is why the earths have to be disconnected when testing

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Sorry M8, you'll have to explain that. We get Ze with the earths disconnected but do Zs with all earths connected?

Lifelong learning said:

On the topic of PFC: a 3-phase system has a PFC of 7kA. This PFC of 7kA has been calculated from the measured single-phase PFC of 3.5kA. If a single-phase final circuit is taken from a 3-phase distribution board, does the CPD serving the single-phase circuit require a breaking of at least 3.5kA or 7kA? Logic tells me that the CPD needs single-phase PFC breaking capacity because that is all that can be pulled through the breaker in the event of a short circuit between L-N on that circuit. This is a hypothetical scenario.

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Are you a Bot?

Mmm, I think you're a Bot.

Off Topic but seeing this old thread dragged up, I was out of the loop for a while and so have missed quite a few things so I hope I'm not out of place but can anyone tell me if they know what happened to Him Her who was a regular contributor but no longer appears.

Good question.Last posted Dec 2020. Last login Dec 2021. Didn't always see eye to eye with him, but he was always interesting.

Anyone?

nara said:

Good question.Last posted Dec 2020. Last login Dec 2021. Didn't always see eye to eye with him, but he was always interesting.

Anyone?

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I always thought he was a reincarnation of speccy bins. Very similar

IANB said:

I always thought he was a reincarnation of speccy bins. Very similar

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Wow! That’s going back a bit!