very high zs

[mickles]

hi everyone, but stuck at the mo with a zs of 6.49ohms. simple job of fitting spur to existing ring. all other sockets on the ring are also 6.49 ohms. the normal ring health checks are good with ratios maintained throughout. initial thought was poor distributor earth. ze checked out at 150 ohms, badly corroded and tarnished MET and distributor earth cable. after cleaning, ze (all earths disconnected) 0.12. reconnected earths and checked ze from the consumer unit earthing block and the MET. both reading 6.49 tried disconnecting main bonding conductors, no effect. tried disconnecting circuit earths one at a time, no effect.
supply is TNCS, and the c/u and meter are in a very damp cellar. the ring i have worked on is protected by rcd so is quite safe using the 1667 reg but there are other non rcd protected circuits, so i am not happy leaving it like this.
in short, the ze is ok untill the consumer unit earth is connected to it then it increases to 6.49
any ideas

 

[trev]

Loose terminations somewhere?

 

[Octopus]

Ze checked out at 150 ohms and then you mention TNCS??

 

[mickles]

ze was 150 until met and main distributor bonding conductor cleaned then fell to 0.12

 

[Richard Burns]

Not quite sure i have fully followed what the changes and readings were, however if your Ze of 0.12 ohms was from the main earthing conductor directly (standard test method) and then you were later measuring Ze from the MET and then from the CU earth bar and both Zes were 6.49ohms even after cleaning then I would say that you have a problem with the MET being dirty inside the terminals and not providing a low resistance connection to the CU.
If you change the MET block and, if required, the earth cable from MET to CU you may find the Ze is now OK.


For the socket circuit:
If you were measuring Zs also at 6.49 ohms for the socket circuit then this is odd as the socket circuit has zero resistance, so by calculation what would be your R1+R2 +Ze?

If this is within limits then you have reduced the high resistance joint to being MCB, RCD, Main switch, earthing conductor, MET.
since this includes the MET and earthing conductor likely to be the culprit.

 

[tgtworsley]

try bypassing the rcd and test. rcd may give very high reading

 

[mickles]

thanks gents, i will try replacing the met and consumer unit earth and let you know what happens.everything points to the main earthing connections.

 

[mickles]

apolgies for delay, new met fitted, new main earthing conductor fitted, distributor earth cable cleaned on every strand, everything now fine. all down to corrosion.

 

[somersetsparks]

Maybe you should consider putting a light smear silicone grease on the ends of the cable at the MET to prevent it happening again.

 

[Richard Burns]

Thanks for letting us know the outcome, glad it is all sorted.

 

[Octopus]

Thanks for letting us know the outcome, glad it is all sorted.

Good feedback - out of interest what is the Ze now?