Zs complies?

[DT1991]

Okay, that's fine then....

Well i suggest you re-gland up the wire armouring on both ends of this SWA cable to ensure grip and tightness and then install earthing nut's or banjo's to each gland with fly leads to earth bars. By the way, what size is this SWA cable we are talking about??

It's only a 2 core 4mm2. And I have done what you have said and still received the same readings, no change whatsoever. So this process identifies a break in the armourings I would say?

The company I work for received a contract of about 30 of these installations, I can see this problem arising on every site.

 

[Flanders]

Never come across a break in SWA before, did you install the cable ? if not it may have a joint in it

 

[Engineer54]

It's only a 2 core 4mm2. And I have done what you have said and still received the same readings, no change whatsoever. So this process identifies a break in the armourings I would say?

The company I work for received a contract of about 30 of these installations, I can see this problem arising on every site.

I've never seen that situation in 40+ odd years!! That would be almost impossible without substantial damage being done to the cable... Maybe a bad joint in the cable, ....is the length of cable run visible throughout??

 

[tony mc]

I would get a fresh set of eyes to have a look IMO it will be something simple that you are missing it happens to all of us at some time !

 

[telectrix]

couple of years back , i came across a situation where the WSA was well glanded but has a high Zs. turned out to be that while the SWA was earthing to the metal box, the earth bar had no continuity to the box, even though it was bolted to it. this was on a distribution switch fuse.

 

[DT1991]

Thanks for your input everybody.

 

[Guitarist]

If the swa is buried and the outer sheathing has been damaged, it's quite possible for the wire to corrode away and leave you with what you describe. In this case, it would need to be replaced, as you cannot guarantee a good earthing of the existing wiring which isn't corroded.

 

[timbobelfast]

Are you sure your meter is 100% working??

 

[telectrix]

first you need to dis. fly links at both ends ., then wander lead and measure the R2 of the armour. that's your starter for 10.

 

[Silly Sausage]

The company I work for received a contract of about 30 of these installations, I can see this problem arising on every site.

Eh?!?!? Why?
I think I want shares in this lad's company.

 

[onions1066]

Have you fitted flying leads from the banjos or just bolted them? Sounds like you've got bad connection between the bar and the casing? I had the same as what tel described with a bad earth to the casing the other day. But I'm with Archie on this there is no way you could expect this on every site. It is unbelievably rare for the armouring to be damaged and the phases in tact.

 

[D Skelton]

Now lets go back to the main switch, it's a 100mA RCD so a touch potential of 50v can be applied to ohms law calculated. 50 divided by 0.1 is 500. So I would be allowed 500ohms at any point of the installation? Is this correct

I haven't anything to add regarding the installation as others have all made good points. The only thing worth noting is that as the installation is TN-S, the max Zs allowable for a 100mA RCD would be 2.3kOhms, not 500 Ohms. You are only required to apply the touch potential of 50V to TT systems.

 

[telectrix]

I haven't anything to add regarding the installation as others have all made good points. The only thing worth noting is that as the installation is TN-S, the max Zs allowable for a 100mA RCD would be 2.3kOhms, not 500 Ohms. You are only required to apply the touch potential of 50V to TT systems.

a very good point. but then again, 411.4.9 does suggest usint the values in table 41.5 for TN systems. ( it does say "may be applied" ).

 

[D Skelton]

Indeed Tel, although I chose my words very carefully at the end when I said "You are only 'required' to apply..."

 

[Mike66]

About 10 years ago I ran a temporary surface laid 6.00mm SWA cable to a site office & connected it both ends. When it was tested I could not obtain any readings at the site office.

On a closer visual inspection on the SWA cable it was very floppy about halfway along it's run.

We cut the SWA cable & found that all that was keeping the cable together was the external PVC coating. All of the conductors etc had not been jointed.

The cable was sent back to the manufacturer & it was decided that the cable was made like this in it's manufacturing process.