Strange readings on ze and zs

[Paul Glover]

Studio flat recently re-wired.
Ryfields board with main earth coming in and connects to side of ryfields (sealed unit).
Flat’s own head outside has main isolator, from there 25mm tails to inside and then in main switch on DB. With 16mm earth feeding board.

Ze at main switch on DB with main earth disconnected - 0.12 ohms.
Ze At main isolator below head 0.41 ohms with main switch off.

Zs on circuits 0.71 ohms with R1+R2 0.21 ohms.

First question ... how can the ze At the board be greatly lower than at the head isolator with no parallel earths.

Second question... is the ze 0.41, as the Zs of all circuits is going by that result rather than the 0.12?

Help please!

 

[westward10]

Where have you disconnected the main earth, in the dwelling board. Is this main earth definitely connected from the local head or does it derive from the source.

 

[Strima]

Poor/loose connections somewhere.

Also are you using a no trip setting on your MFT?

 

[westward10]

You say the Zs on the circuits is 0.71, surely they are not all the same.

 

[Paul Glover]

Where have you disconnected the main earth, in the dwelling board. Is this main earth definitely connected from the local head or does it derive from the source.

Zs is an example from one circuit, they are all around the 0.41 higher than the R1+R2 readings from each circuit

 

[Nigel]

To be honest I cannot make head not tail of your set up.

 

[Paul Glover]

Poor/loose connections somewhere.

Also are you using a no trip setting on your MFT?

All connections tight and checked. Tried all settings, varies slightly but not much

 

[Paul Glover]

Where have you disconnected the main earth, in the dwelling board. Is this main earth definitely connected from the local head or does it derive from the source.

Definitely from the flats own head (local) it’s off the sheath, TNS from the flats supply

 

[Strima]

So your main earth goes to a marshalling point before the DB?

 

[Wilko]

Hi - my take on what you've said is ...
1. Ze - Zdb = 0.29 Ohms and this is a circuit that's made from a length of 25mm, 16mm, 60A fuse (?) and some connections. How long are these conductors?
2. You've said the external impedance of the Flat's CU is 0.41 Ohms, so this is used in the EIC as Zdb, it's not the Ze as that's at the Ry before the distribution circuit to the Flat.
But I may be completely off track .

 

[Toneyz]

What is the supply? if TN-C-S (PME) where is the N-E link Sorry I'm a bit confused upon the setup any pictures?

 

[westward10]

Hi - my take on what you've said is ...
1. Ze - Zdb = 0.29 Ohms and this is a circuit that's made from a length of 25mm, 16mm, 60A fuse (?) and some connections. How long are these conductors?
2. You've said the external impedance of the Flat's CU is 0.41 Ohms, so this is used in the EIC as Zdb, it's not the Ze as that's at the Ry before the distribution circuit to the Flat.
But I may be completely off track .

You trying to confuse us

 

[Wilko]

I did warn at the bottom of the post .
Perhaps @Paul Glover could pen a wiring diagram?

 

[Octopus]

Up front RCD ?

They can produce unusual results

 

[Paul Glover]

Up front RCD ?

They can produce unusual results

Nope. No RCD except inside the board. Just main switches.

 

[Paul Glover]

What is the supply? if TN-C-S (PME) where is the N-E link Sorry I'm a bit confused upon the setup any pictures?

It’s tns at the main isolator, I can’t tell in regards to the ryfields board as it has a seal. All I can see is a main earth bolted on to the side of the unit. It has baffled me. It is as if in between the isolator and the DB main switch the neutral has found another earth. Which is impossible because I put it in...

 

[westward10]

The Ryefield is likely to be PME internally you will have a common neutral/earth bar. Not always but very often.

 

[Toneyz]

The Ryefield is likely to be PME internally you will have a common neutral/earth bar. Not always but very often.

Your right on that one westward all the Ryfield boards that I have installed have had a N-E link.

 

[Paul Glover]

What is the supply? if TN-C-S (PME) where is the N-E link Sorry I'm a bit confused upon the setup any pictures?

It’s tns at the main isolator, I can’t tell in regards to the ryfields board as it has a seal. All I can see is a main earth bolted on to the side of the unit. It has baffled me. It is as if in between the isolator and the DB main switch the neutral has found another earth. Which is impossible because I put it in...

 

[Paul Glover]

Your right on that one westward all the Ryfield boards that I have installed have had a N-E link.

So the main isolator at the head (flat) is tns the ze will be 0.41 ohms. Which still leaves the question why in the earth is the Zdb lower than the ze with no parallel earths and it being isolated?