different Zs readings on a ring final?

[scott p]

should'nt the Zs readings on a ring final be roughly the same at each outlet? the reason i ask is that i'm getting different readings (.08ohms-.45ohms) is this normal or could it be the tester. cheers

 

[telectrix]

the further you go from the DB, the higher your readings will be. you should measure the continuity first. r1, r2, r n. then R1+R2. shgould only be a small difference socket to socket.

 

[Lou7]

Could it be a spur off the ring ?

 

[gpz900jim]

if you cross connect the lives and cpcs into a figure of 8 at the cu, when you do your r1/r2, readings at all sockets should be broadly similar, except for when you take the reading from a spur, which should be higher

 

[telectrix]

re-reading the op. 0.08 is a ridiculously low reading as Ze even on a tncs is rarely as low as that.

 

[mechelec]

re-reading the op. 0.08 is a ridiculously low reading as Ze even on a tncs is rarely as low as that.

must be a distribution transformer in the front room

 

[telectrix]

or an earth rod to australia

 

[Sparky Ninja]

The Zs test is the earth fault return path, so the current under test will find the best/fastest path back to the transformer. This wont split evenly down the parallel resistance in the ring, most of it will flow down the lower impedance route.
So when testing a Zs on a ring final, the further you test from the origin (resulting in both legs return to the CU being of equal length), the higher the Zs will be.

if you cross connect the lives and cpcs into a figure of 8 at the cu, when you do your r1/r2, readings at all sockets should be broadly similar, except for when you take the reading from a spur, which should be higher

(R1&R2)

 

[redface]

i have had differnt readings on socket but found if i took the socket off and done the test at back the readings were ok it was just a socket with high resistance or a bad switch .
anyone els found this?

 

[Sparky Ninja]

i have had differnt readings on socket but found if i took the socket off and done the test at back the readings were ok it was just a socket with high resistance or a bad switch .
anyone els found this?

Quite regularly mate, yes. Sometimes some jiggering around helps, otherwise it could be the socket itself. Do check your lead out with another point to verify it's resistance as well.

 

[redface]

oh yes and switch socket switch on off on off