SWA terminated into CU - relying on metal case as sole earth connection

[Cigaro]

Hi,

I have come across a two-core SWA cable connected into a metal cased CU. The "banjo" is bolted to the case of the consumer unit and the consumer unit's earth bar is then electrically connected to the case by one of its supports, which are metal.

Is this acceptable? There is good continuity between the cable armour and the earth bar but surely it would be usual to run an extra earth wire between the bolt and the earth bar - too many variables otherwise - what if someone forgets to sand the paint off the case where the bolt goes through, for instance, and it doesn't make a good connection?

It might be totally fine and I'm overthinking it, I was just slightly shocked as I wouldn't have thought it was acceptable to rely on the case to provide the sole earth connection for the entire consumer unit!

 

[EFLImpudence]

I have come across a two-core SWA cable connected into a metal cased CU. The "banjo" is bolted to the case of the consumer unit and the consumer unit's earth bar is then electrically connected to the case by one of its supports, which are metal.

Ok.

Is this acceptable?

Well, if the objective is achieved then it is fine.

There is good continuity between the cable armour and the earth bar

That is the objective.

but surely it would be usual to run an extra earth wire between the bolt and the earth bar

You could do - then you wouldn't need the other one.

too many variables otherwise - what if someone forgets to sand the paint off the case where the bolt goes through, for instance, and it doesn't make a good connection?

What if someone does it your way and forgets that?

It might be totally fine and I'm overthinking it, I was just slightly shocked as I wouldn't have thought it was acceptable to rely on the case to provide the sole earth connection for the entire consumer unit!

It's not. That will be achieved by the earthing conductor.

The case and the armour is then connected to that.

 

[ban-all-sheds]

I have come across a two-core SWA cable connected into a metal cased CU.

In what context have you "come across" it, and have concerns?

 

[333rocky333]

Anyone know why swa glands even come with a banjo.
For years metal conduit and pyro was connected in a similar fashion just with sockets, bushes, lockrings etc

 

[ban-all-sheds]

What would you do with a plastic enclosure with no banjo or a piranha nut?

 

[Risteard]

For years metal conduit was connected in a similar fashion just with sockets, bushes, lockrings etc

Everyone here uses flanged couplers.

 

[Cigaro]

I have been inside this consumer unit to disconnect some existing circuits which will not be required any more. The CU will need to be moved and new circuits run, I'm aware of the rules and I am going to get a qualified electrician to do this.

I'm starting a course in electrical installation at college later this year (late career change) and asked the question out of curiosity. For instance, if you have a metal socket backbox, even though it should be electrically connected to earth through the screws, it's common to add an extra lead from the earth terminal to the backbox, right?

Thanks for putting my mind at rest, it just seemed 'wrong' so I thought I'd ask

 

[Spark123]

If the metal backbox is sunk then no need for the link as it is earthed via the screws.

 

[333rocky333]

The earth via a socket screw would be nowhere near as good continuity as a tight gland and locknut, hence the fly lead
That fly leads not compulsery if the sockets earthed direct.
However if the swa is glanded in a backbox then that fly lead becomes compulsary.

In petrol stations its compulsory to have a flylead to the swa gland and thats in addition to one of the cores used as earth, if its in an area where its not screwed into an EX rated box

However your right to think that some casings have a finish that reduces earth continuity, and was quite common with black enamelled boxes, however its a known problem and training teaches you to clean up the area to acheive better connection

 

[333rocky333]

For years metal conduit was connected in a similar fashion just with sockets, bushes, lockrings etc

Everyone here uses flanged couplers.

I like them, but there rarely used round here now, they seem to have the idea there only for fire protection, I think that was the original idea and they were supplied with lead washers back in the day, we used to use them for conduit drops to fire alarm call points.
Round here we have a big shopping area called Lakeside, all the backareas and stairwells are done in black enamel conduit, which seemed a strange choice to me as its now going rusty

 

[JohnD]

stories are occasionally told of an incident where the screws, in a bathroom, had become badly corroded due to damp, and lost contact, and the wall became live (IIRC due to the damp or leak)

 

[333rocky333]

What would you do with a plastic enclosure with no banjo or a piranha nut?

Yes, good point, I did see some swa glands packaged with earthing nuts rather than banjos, so maybe that will catch on

 

[Iggifer]

A piranha nut is considerably more expensive than a banjo though, and unfortunately, a lot of people are too (IMO) price driven.

 

[333rocky333]

True, but ive always used a 6mm brass bolt, brass nut, and 2 brass washers, so cost of fitting a banjo can creep up, gutter bolts IMO are rough, some wholesalers sell a 2 pack incuding an allen key of the earthing nuts.
I did note there quite dear on ebay, luckily for me the firm pays