Earth loop

[sjl468]

Just been testing a house lighting circuit.
Ze was 146.6ohms. Zs was 141.7 ohms.
R1+R2 for this circuit is 0.69 ohms.
Why is Zs so much lower than Ze?
By the way i disconnected all the other earths in the house to carry out this test just to stop parallel paths.

 

[oharaf]

Ze was 146.6ohms.

What is the earthing arrangement for this property??

Zs was 141.7 ohms.

What MCB / RCBO is protecting the circuit?

R1+R2 for this circuit is 0.69 ohms.

Why is Zs so much lower than Ze?

This is the least of your worries!!!!!!

By the way i disconnected all the other earths in the house to carry out this test just to stop parallel paths.[/quote]

That is a start!!!

Max Ze for TNS is 0.8 ohms, TNCS 0.35 ohms and TT 21 Ohms

Are you having a laugh or is this real???

 

[Adam_151]

I hope thats a TT install, or you better be ringing the DNO...

 

[oharaf]

Even if it is TT someone should be getting out an earth rod at least on a temp. basis and if the earth is supplied by the DNO then calling and declaring an emergency

 

[Adam_151]

Max Ze for TNS is 0.8 ohms, TNCS 0.35 ohms and TT 21 Ohms

I think you've confused yourself, 21 ohms refers to the DNOs electrode

You are allowed upto 200 ohms, depending on what RCDs you have... anything over that is unstable (or anything over 100 depending on who you ask)

 

[oharaf]

Sorry Adam yes you're right

 

[Spark123]

Just been testing a house lighting circuit.
Ze was 146.6ohms. Zs was 141.7 ohms.
R1+R2 for this circuit is 0.69 ohms.
Why is Zs so much lower than Ze?
By the way i disconnected all the other earths in the house to carry out this test just to stop parallel paths.

I doubt you can disconnect everything to stop parallel paths. You should never disconnect main protective bonding and liven up the system anyway.

 

[sjl468]

Sorry. It is a TT.
So lower than 200 is ok but I'll still have a look at the rod.
I didn't disconnect the main earth just the protective bonding to gas/water and all other circuits. I turned everything else off too. So there was only the one lighting circuit connected and energised. And I was the only person in the house at the time.
This is why it confused me a little.

 

[davelx]

Sorry. It is a TT.
So lower than 200 is ok but I'll still have a look at the rod.
I didn't disconnect the main earth just the protective bonding to gas/water and all other circuits. I turned everything else off too. So there was only the one lighting circuit connected and energised. And I was the only person in the house at the time.
This is why it confused me a little.

Even with the MPB disconnected, if there is supplementary bonding in the bathroom there is probably a parallel path to earth via the water and/or waste pipes, unless these are plastic of course.

 

[Spark123]

Even the metal back boxes of accessories can aid in providing a parallel path to earth - but as I said before you should never energize a system with the MPBs disconnected, even if you are the only person in the house. It is a point blank no-no.

 

[ban-all-sheds]

Wasn't there a heated debate about that on the IET forum a few years ago?

Even got to the stage of the author of one of the well known books on I&T (Anthony Hinsley) joining in, I think.

Search there for Hinsley & you'll probably find it.