SWA earthing arrnagements

[s6stu]

Morning All!

I have recently started work on a commercial site with a company installing air-con, the system that I have questions on is thus;


35mm2 4 core SWA as main supply to the board, where the armour is used as a CPC, which then feeds 10 x 3 phase circuits, supplied via 1.5mm2 4 core again using the armour as a CPC. All of these cables enter on the same plate of a metallic water proof box and then off to the board inside.


I have been asked to replicate the above system with the other identical 'pods', that have already been installed, which house the air-con coils etc. The way in which the other pod/system has been earthed, is with a 35mm2 main earth from the main supply SWA to the MET, and the outgoing circuit SWA's have not been individually earthed, but have 2 earth cables, with a nut and bolt, near each bunch of SWA's linking the armour up to the MET, but not via a banjo but with a nut and bolt in the metal box. The metallic box has
been bonded to the MET.

So, is this arrangement ok, as the armoured is being used as a CPC in all cables and the cables are all located on the same metal plate, in close proximity, this guarantees earth continuity?!? Or, as I suspect, should each outgoing SWA have its own earth connected to the MET?

Thanks for your time, I hope this makes sense!

Stuart

 

[madmac]

Stuart, this is quite common unfortunately and as long as there is continuity between the outgoing armour's and the met it should be fine, however, I would not be happy with this personally!
The outgoing armour's would be better fitted with brass banjo's on the inside of the D.B. and I normally bend the banjo upwards from the gland plate and link them all together with 6mm2 earth wire, with crimps and brass nuts/bolts/washers/spring washers. and then take a 6mm2 flying lead up to the MET thus giving a guarantee earth connection to all the outgoing armour's.
I also use this method for IP rated units as the last thing we want to do to water proof enclosures, is to drill the banjo onto the gland plate!!!

 

[Knobhead]

What’s the metal plate made of? In some circumstances the plate alone will suffice without banjos.

 

[s6stu]

Thanks for the replies and advice,

The box I would say is painted steel, schneider IP rated metal box, if that helps?

...... If the SWA is individually earthed for the out going circuits, am I right in thinkng that 1.5mm2 will be fine, as the live conductors are this size?

 

[bomjac]

Metal Box - Use serrated washers every time. Gives a good earth connection/continuity as well as extra protection of the glands working loose over time - especially with a clearance hole.

 

[stitch]

I dont use the banjo earthing tabs any more, but use these instead.

Earthing Nuts - Speed up metal cable gland installation and EMI / RFI screening

They have teeth on one side that cut and grip into the metal work of the enclosure and then a grub screw that locks the nut to the gland.

 

[s6stu]

Thanks again for replies, will install separate cpc's for what it's worth. Will 1.5 mm2 suffice?

 

[benji]

personally i would never use 1.5mm for power ,always used 2.5mm as minimum ,1.5 for control wiring if there short runs

 

[s6stu]

1.5 mm2 is already run as power, only about 3A per circuit so will be fine, I assume 1.5 mm2 is fine as cpc?

 

[Andy 7671]

Gonna jump straight in and say 1.5 is ok upto 19 amps.

We were surprised as well.

Limitations, ...?

 

[lamb]

IMO the banjo's must always be used on the gland plate, except where glanding into threaded holes (eg besa boxes).
The correct method is to remove the paint from the gland plate before glanding, and to reinstate the protective coating afterwards (eg galv spray)