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.