How to work out no value of Zs to Ze being 0.35 ohm

[Anthony Deakin]

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Hi all, ive been asked this question
A direct measurement of Zs cannot be carried out. The supplier gives a Ze value of 0.35 ohm and the line and cpc resistance as 36.24 milliohm/m. If the circuit is 31metres in length the earth fault loop impedance will be?

What is the formula please?

Thank you

 

[timhoward]

Are those units intended to be the same or do you mean Ze of 0.35 ohms and 36.3 milliohms/metre (maybe for 1.5mm T+E).

If that is 36.3 milliohms/metre:
The circuit's estimated R1+R2 will be 31 X 0.0363 = 1.13 ohms
Then best you can do is just add them, so 1.48 ohms.

It sounds like the supplier has just given you the max allowed TNCS value anyway.
You probably know that ignoring any parallel earth paths makes this an estimated worst possible value, assuming all connections are good.

 

[timhoward]

What is the formula please?

Zs = Ze+(R1+R2)

 

[timhoward]

If @Anthony Deakin returns it would be interesting to know whether this is a real situation or an exam question.

The other curious thing is that a moderator has changed the original to read 36.3 ohms per meter which is surely completely implausible. I doubt the human eye could see a wire with that resistance!

I still think it's most likely 1.5mm csa T+E , as my book has that as 36.24 milliohms / m

 

[westward10]

If @Anthony Deakin returns it would be interesting to know whether this is a real situation or an exam question.

The other curious thing is that a moderator has changed the original to read 36.3 ohms per meter which is surely completely implausible. I doubt the human eye could see a wire with that resistance!

I still think it's most likely 1.5mm csa T+E , as my book has that as 36.24 milliohms / m

My bad I mis edited think I have it correct now.

 

[telectrix]

My bad I mis edited think I have it correct now.

30 minutes in the naughty corner.

 

[Anthony Deakin]

Are those units intended to be the same or do you mean Ze of 0.35 ohms and 36.3 milliohms/metre (maybe for 1.5mm T+E).

If that is 36.3 milliohms/metre:
The circuit's estimated R1+R2 will be 31 X 0.0363 = 1.13 ohms
Then best you can do is just add them, so 1.48 ohms.

It sounds like the supplier has just given you the max allowed TNCS value anyway.
You probably know that ignoring any parallel earth paths makes this an estimated worst possible value, assuming all connections are good.

Thanks guys, to be clear it was 0.35 ohm for the ze and 36.3 milliohm/m.

I’ve been told by a nerdy electrician that the correct answer is 1.475 ohm but no idea how they got to this.

cheers

 

[timhoward]

I’ve been told by a nerdy electrician that the correct answer is 1.475 ohm but no idea how they got to this.

Ah well I just rounded it a bit here and there, after all what's 5 thousandths of an ohm between nerds ;-)
Was this an exam question or a real installation?

 

[Anthony Deakin]

Ah well I just rounded it a bit here and there, after all what's 5 thousandths of an ohm between nerds ;-)
Was this an exam question or a real installation?

Thank you very much, at least I know the formula.