Strange Ze

[Steve D]

Hi All,

I was doing my first periodic on saturday since passing my 2391 and I was getting some strange readings.

It was a terraced house in Ipswich on a TNCS supply. I tested Ze and got about 1.2 ohms, which on a TNCS I thought to extremely high. I then went up to the service head and pulled the fuse to do a Ze there, just to eliminate any potential issues with testing through the meter (I know there shouldn't be a problem, but I was clutching at straws)
Anyway, Ze on the service head was 1.17 ohms.

All my actual Zs readings were ok.
For example on a socket very close to the origin I tested 0.36 ohms. I thought this might have something to do with the main bonding lowering the reading, so I disconnected it and retested and got 0.60 ohms.
I know at the end of the day it's the Zs readings that count, but I was just a bit uncomfortable with this weird Ze reading

I was a bit flummoxed to be honest. Why would a Ze be so high and the Zs readings be fine ?

And more importantly, do I need to investigate this with the DNO ? Is it feasible for me to be writing this high Ze reading on my report ?

Cheers in advance for your comments.

Cheers
Steve

 

[Andyfm]

Inform the DNO, if its tncs they should arrive pretty quickly to sort it out (though i`ve read on here that they`re not as quick as they used to be - taking days rather than hours)

 

[Sparky Ninja]

Hi All,

I was doing my first periodic on saturday since passing my 2391 and I was getting some strange readings.

Well done

It was a terraced house in Ipswich on a TNCS supply. I tested Ze and got about 1.2 ohms,

OK

which on a TNCS I thought to extremely high.

Correct

I then went up to the service head and pulled the fuse to do a Ze there,

HUH???

just to eliminate any potential issues with testing through the meter (I know there shouldn't be a problem, but I was clutching at straws)

Correct

Anyway, Ze on the service head was 1.17 ohms.

Are you sure?

All my actual Zs readings were ok.
For example on a socket very close to the origin I tested 0.36 ohms. I thought this might have something to do with the main bonding lowering the reading, so Idisconnected it and retested and got 0.60 ohms.

Why?

I know at the end of the day it's the Zs readings that count, but I was just a bit uncomfortable with this weird Ze reading

Huh?

I was a bit flummoxed to be honest. Why would a Ze be so high and the Zs readings be fine ?

Because there could be a problem with the supply earth, and the existing earthing could be using the existing main bonding to gain its better potential.

And more importantly, do I need to investigate this with the DNO ?

Yes

Is it feasible for me to be writing this high Ze reading on my report ?

You write the readings obtained.

 

[sparkylad]

Best not tell the DNO you tampered with the service head though eh !

 

[chiplard]

The high Ze will be caused by either a poor main earthing connection at the supply board or a problem with the Supply Live or Neutral. This is why the DNO for a TN-C-S system will come out ( The earth for a TN-C-S is supplied via the neutral ). Whereas with a TT / TNS system they wont as they dont have to supply the earth.

 

[Risteard]

Tampering with the fuse is bad enough but why would you energise an installation with main protective bonding conductors disconnected from the MET? That's extremely dangerous.

 

[sparkylad]

Tampering with the fuse is bad enough but why would you energise an installation with main protective bonding conductors disconnected from the MET? That's extremely dangerous.

I was also very worried by that statement from the OP. mate !

 

[hasel5]

Probably a poor connection with the guys leads on to the met had that loads of times