Strange Readings

[SLIPSHOD & SLAPDASH]

Powered up a sub main today and got some odd readings, it was getting late and I think I  may have done something wrong but gave up for today and will be going back tomorrow.

TNS supply. We fitted a switchfuse (45A) & from this ran 35m of 3c 10mm SWA. Fitted a duel rcd board on the end with 7 circuits. The Ze was 0.35. The R1+R2 was 0.08 but the Zdb was 2.00. Checked all connections & seemed ok. I put N & E probes both on N (other on L) & got 0.45. Double checked with another meter. What have I missed  ?

To reward you all I will post a few pics soon of the existing installation.

 

[Manator]

You have missed nothing, I am guessing that you used a Fluke? I sometimes get some weird readings and shake my head, change of batteries normally sorts it out.

Last time it was down to the fuse in the probe not connecting properly.

 

[Nicky Tesla]

have you put the tail into the main switch incorrectly and ended up with some resistance

 

[NozSpark]

I had a similar thing with one of SBS Dave's main switches.... Had a dodgy reading and found that the cage on the outgoing Neutral didn't tighten down even though the screw was tight as you like...

Could it be possible you have a similar thing with the L???

 

[SLIPSHOD & SLAPDASH]

We couldnt get access to recheck the supply so are going back tommorow. I was thinking  a bad earth at the switchfuse as i had the lead clipped to the MET.

 

[binky]

Sounds like you were tired more than a real issue.

 

[SLIPSHOD & SLAPDASH]

Part 2.

I didn't tell the whole story yesterday. My Fluke gave a reading of 120 ohms Ze with the bonds connected as we could not turn off the supply. I then got 2.0 ohms Zdb. My mate puh-puhed the readings and got his Ideal Suretest out which gave .35 Ze and 2.00 Zdb. We both had a long day etc so called it quits.

Today we returned and I watched him test again. I have never used the Suretest. It starts testing as soon as the probes are touching, no need to press buttons. He got the .35 ohm reading before the earth lead was even connected & then argued with me he was instructed that you could do a 2 wire test. When I pointed out  that maybe so, but not the arrangement he had used as how could it possibly be a Z reading without a earth connection he went quite & red in the face.

Anyhow, turned off main switch, disconnected main earth and tested & double tested with both meters, 120 ohm. Did I mention its a commercial premises 3 phase & N  100A supply, all underground via a lead sheathed cable with a 951 earth clamp (very old, it has the Earth connection do not remove legend) and no soldered or crimped earth tail. No rcd's  anywhere and looks like never been TT.

Called UK power network explained situation and waited.  I went to another job. Engineer called at 17.45 to say he would be there in an hour, I replied don't bother they have gone home it will have to be tomorrow. He asked more and  I chatted about the clamp but its never been TT and he agreed as its a lead sheath. Now, what's the bet they will just get on with a repair or try to wriggle out of the situation. Unfortunately we cant be there tomorrow.