Zs Reading On Streetlights

[Russ1109]

Hi all.

After a bit of advice on this please. I work on streetlighting as my main job. We have a feeder pillar which basically feeds 2 streetlights

Ze at the Feeder Pillar is a PME system and was 0.30 ohmns. R1+R2 was measured as 0.86 so my total Zs will be 0.30+0.86. 1.16 ohmns.

As this job was being used for a NICEIC inspection I thought I'd measure the Zs as well just to check the reading. Reading taken at the 2nd lighting column was 17.08 ohmns. I then checked the 1st column and was 16.88 ohmns

Cable feeding 2 x columns is 6mm 2 core using swa as the earth

After checking the earth connection several times I was unable to get the reading lower.As the earthing termination and cut outs were quite old and dated we've even been back and reglanded the cables into the column and fitted modern cut outs. and reterminated all earth bonding and the Zs readings are the same. The NICEIC inspector who came on the day was baffled by this and was unable to explain the problem.

If you measure the Ze and add the R1+R2 the installation will pass but if you measure the Zs at the last column it will fail so which is it?

Any ideas please

 

[Andy™]

sounds like you have a connection missing somewhere

is the armour connected to the earth terminal, and not just the column?

Any pics?

 

[Russ1109]

The swa is terminated into a gland which is attached to a brass glanding plate on the cut out.From that plate we run a 10mm to the earth terminal within the cut out out and also take a 10mm to a earth block,from there we run a 10mm to the col and then the door. Everything is bonded and terminated correctly and all connections are tight.Surely if we did have a loose or poor connection we wouldn't have have acheived a good R1+R2 reading

 

[Steve3948]

Have you tried a different tester ?

 

[Andy™]

but is there a connection betwwen earth terminal of DNO head and your cutout?

 

[Russ1109]

We've even used 2 different testers so rules that out.

From the DNO cut out we've run a set of DI tails and earth into a main cut out which then feeds out to col 1 and then loops out to col 2

 

[Sidecutter]

Well from what i was told a few years ago when the digger drivers slice through a streetlight cable they twist the cables together and rap the joint in the bread bag that there sandwhiches were in. so that could be your problem dodgy digger drivers have been about

 

[Steve3948]

Have you tried R2 on its own ?

 

[Andy™]

Have you tried R2 on its own ?

probably too far apart

 

[Evans Electric]

An interesting post.     I can't offer anything at the moment  but I would have imagined a good Zs reading purely because the pole is bonded to your earth and its buried in the ground , like a giant earth rod.       Are they fed through RCDs ?      Has the armouring deteriated through water ingress / capilliary action .

 

[binky]

not getting resistance trough screwheads?

 

[davetheglitz]

Just a thought - how did you do R1 + R2? Short at pillar end or at streetlamp end? If shorted at the streetlamp end then you would have to get good contact to different terminals to the Zs measurement - so Binky's point of terminal connection comes into play.

 

[Russ1109]

The R1 R2 test was carried out by shorting out at the pillar and taking the reading at the streetlighting column which there are only 2 on the circuit.Fisrt one is 30m from the pillar and the other is another 30m away.

In reply to a previous post the circuit is back up by a BS88 fuse in the main cut out in the feeder pillar 16A. The DNO cut out has a BS88 25A fuse.No RCD on the circuit.

The armour looks ok at the column as they were both re glanded into new CW20 glands