Ze & Zs-- unacceptable readings

[telectrix]

hi, just been to a Call-out, mobile home site. one homeowner has had a faulty socket replaced and the electrician who did the job, quite correctly filled out a MWC, problem is his Zs reading was 51.2 ohms. now, the supply comes approx. 100yards from the cut-out ( which is boxed on the supply pole) in 2 core 6mm SWA, using the armour as earth ( circa 1960's). I checked his readings with MFT and got 49.5ohms, then measured Ze at the cut-out, 0.22ohms PME system. think it's pretty clear, it's the resistance of 100yds of armour that is the problem. the home has a 30mA RCD , front end 16th ED. so my query is: do i leave well alone, or dis. PME and convcert home to TT. which will hopefully make the installation comply. replacing the cable is not a option, considering that half the park will have to be excavated.

 

[Martin Dawes]

From my understanding, the 30mA RCD means that the 51.2 ohms reading isn't a problem.

 

[pushrod]

50Ω is a big reading - is it worth looking for a corroded high resistance joint at one end or the other that can be re-terminated?

 

[Ponty Massive]

Due to the RCD it can be 1667 ohms and still activate within the required time.

 

[Ponty Massive]

2-core swa using armouring as cpc should give a resistance of about 1 ohm over 100m, probibly should be worth a look for corrosion somewhere on the joints!

 

[nick611]

also it should,nt be on pme dont think csa of swa will be 10mm a minimum requirement of pme.

 

[telectrix]

points taken. will check both end connections 2morrow. told home owner there's nothing to woory about as she has RCD. with regard to your post, nick. DNO converted to PME with that cable already in place. so, as per my orig. post, do you think i should convert the home to TT and dis the home end of the armour?

 

[J@mes]

I'd make the glands off again if there is enough movement to do so and see what if that makes any difference to the readings.

 

[telectrix]

i forgot to mention. the cable is not glanded. the armour is earthed at both ends with jubilee clips. suspect oxidation, but it started to p*** down so that,s first thing to check tomorrow

 

[J@mes]

Ah ok in that case I would get the clips off and give it a dam good wire-brushing or going over with emery paper to clean up the faces and see if that helps.

 

[Deleted member 9648]

You should carry out an R1R2 test to confirm a low continuity ....which there obviously wont be.As the others have said almost certainly down to poor terminations of the SWA and corrosion.

 

[telectrix]

that's the plan. however, if i can't get the reading down to an acceptable value, do i TT it and get an even higher reading. also, from the MWC done by the spark who did the socket replacement, the RCD is listed as 100mA, so i will check this and replace with 30mA if so.just a bit confused as to why he's done a MWC for a like for like socket replacement.

 

[J@mes]

Not sure on the TT front, I guess if you TT and RCD it then the earthing will be correct, rather than it being a poor condition PME installation that is covered by an RCD...if that makes any sense?

Probably not, but it sounds right in my head!

 

[telectrix]

mine too

 

[WayneL]

Not getting what you mean by a 'mobile home' site.

Would this not be classed the same a caravan site, and therefore not be allowed on a PME(CNE) system?

Are you sure the SWA is not just earthed at the supply end - and the 'mobile home' installation is TT'd as it's supposed to be......this would make more sense of your reading

Nothing wrong with PME coming in as main supply to park, but shouldn't be run to caravans/mobile homes!

100mA up-front RCD also points to it being a TT installation.


Just my thoughts.

 

[telectrix]

it's not exacly mobile homes as in caravans. it's those prefab homes like a large static. but set on bricks. called park homes. and the swa armour is providing the earth

 

[WayneL]

it's not exacly mobile homes as in caravans. it's those prefab homes like a large static. but set on bricks. called park homes. and the swa armour is providing the earth

OK, we'll go with this for now.

The thing I would be more concerned about, rather than the Zs of the 'prefab home' installation(as this is RCD protected), would be this:

How are the services entering the 'home' (extraneous), bonded to the MET.
Bearing in mind that the MET, technically, is at the cut-out.

Unless I'm mistaken, where PME conditions apply, the Main Bonding Conductor has to be sellected in relation to the supply neutral conductor (up-stream of the cutout), and I don't think the armour of 6mm 2-core will suffice - I might be wrong.
It certainly isn't good enough with a resistance of 49.28 ohms.

 

[telectrix]

yuo're right there. i assume that originally it was tns, from MET is 16mm to jubilee clip on SWA, then approx 80m to home where earth taken off SWA. Manweb, or whoever it was called then, converted this mess to PME some years ago. at the moment all i am concerned about is making it safe. will discuss options for improvement once i,ve done the best i can with what's there

 

[WayneL]

It certainly sounds like the jubilee clips might be your main problem - you've got high resistance joints somewhere.......is it not possible for you to gland both ends into an appropriate enclosure.

The 16mm should be alright at the cutout end - you could use same size at 'home' end.

Just need someone to confirm the 'copper equivalent CSA' of 6mm 2-core SWA.

then approx 80m to home where earth taken off SWA.

Does 6mm work out okay for 'volt-drop' over 80 meters?

 

[nick611]

there is a guide that gives swa csa,s.i dont think 6mm will comply.if you tt the supply at meter position you will comply as long as swa csa is 6mm