swa submain query

[james beckford]

I have been asked to install a db in a wooden outbuilding, the building has been erected on the base of an old garage.

There is an old 2 core SWA cable running from the main DB in the house underground coming up in the building

I have tested out all the cables and continuity and insulation all fine, the house is tncs.

My question is this, can i use the armour as the earth back to the main DB ? i fitted the board on and connected in a socket and Zs and trip times all good, well within limits or do i need to replace the old cable with a 3 core ?

ps i have disconnected everything pending solving this problem.

client wasnt too happy when i said that i would prefer to dig out cable, said another sparks had told him it would be fine,

Thanks in advance for the usual helpful answers

 

[JD6400]

I can see into the future...

Glenn... The Teacher!

He reminds me of a couple I have had !

 

[davethsparky]

I always use 3 core when i'm running small single phase SWA

And my justification for this that my wholesaler doesn't keep 2 core as the price difference is negligable and they haven't the space to keep another load of drums of armoured

 

[GLENNSPARK]

im lookin bear with me

dont go looking too far...
where on about containment being used as an earth path here....

 

[james beckford]

nope brain now too wine soaked, what am i supposed to be looking at glenn ?

 

[trev]

James, you have a 3 phase motor and a three core swa, can the motor spin?

 

[JD6400]

I always made sure I got the answer right to all the questions , mind !

 

[GLENNSPARK]

nope brain now too wine soaked, what am i supposed to be looking at glenn ?

a 3 phase motor is what we call a ballanced load....
we dont need a neutral for it...
3 cores=3 phases...the armourings provide the earth-fault path...
this is really what i wanted you to internalise james...
you have a 2 core SWA existing....
so would it not be a reasonable (and true) statement to use the armourings as the earth-fault path?

 

[D Skelton]

James, you have a 3 phase motor and a three core swa, can the motor spin?

As long as you can turn the three core cable fast enough then yes, of course the motor can spin!

 

[james beckford]

i was fairly confident that i could, but like dave said, i always use 3 core if im installing, purely cos i dont trust people not to fiddle with nuts and bolts, also lack of tt system in old garage showed that sheath was being used for earth, still not sure about 3 phase motor as not worked on any, covered testing out 3 phase circuits on C&G but never used in anger

 

[micknew]

Your not an electrician are you, are you the garage owner or a builder/demolition contractor?

 

[davethsparky]

Nah I said I always use three core because of the lack of availability of two core at the wholesalers who do the best free coffee around here. Nothing to do with any concerns about its suitablity for the job.

Don't trust people not to fiddle around with nuts and bolts? That's utter nonsense, who's going to open up an enclosure and start fiddling about undoing a nut and bolt?

 

[GLENNSPARK]

i was fairly confident that i could, but like dave said, i always use 3 core if im installing, purely cos i dont trust people not to fiddle with nuts and bolts, also lack of tt system in old garage showed that sheath was being used for earth, still not sure about 3 phase motor as not worked on any, covered testing out 3 phase circuits on C&G but never used in anger

so..
going off on a slight tangent here:
if you get 230V (nominal) to either neutral or earth from any 1 of 3 phases....
why do we get 400V (nominal) between lines (phases)?

 

[JD6400]

Come on fella's , it is like the Spanish inquisition ! Admittedly I would like to see the answer to Glenn's second question in post 7 , just to be sure . But I think he is a bit more than a Electrical Trainee , all be it he may not have a full knowledge of the job yet !

 

[D Skelton]

so..
going off on a slight tangent here:
if you get 230V (nominal) to either neutral or earth from any 1 of 3 phases....
why do we get 400V (nominal) between lines (phases)?

Coz if you add 230 to 230 you get 460! But the clever people who make the electricity take a bit away to make it a little safer for us to touch. Shame really coz sometimes I could really do with my microwave being a bit more juicy and I could really do with them extra 60 volties!

 

[james beckford]

dave,

the same bloke who swapped his loft light for a 13A socket and then ran his hydroponics off it