armour cable 1

[cn100]

should the armour of swa be earthed at both ends when this is not the main earth cpc,or is earthing at one end acceptable.also how to earth the armour in plastic boxes. some good practice or regulation would be appreciated. this for u.k. industrial sites.
thanks
cn 100

 

[ScottyUK]

Assuming this is a multicore cable and not a single, I would say bond both ends to earth. If it is a single core carrying AC, you can get image currents flowing in the armour which will reduce the thermal rating of the cable. In this case you should be looking at aluminium armour, not SWA, and considering bonding only one end to earth.

Assuming you're using a standard brass gland to make off the cable at your plastic box, we tend to use a normal banjo and take a bond of 6mm[sup]2[/sup] or larger as required from the bolt hole in the banjo to the earth terminal within the enclosure. Use a large washer to spread the stress from the nut on the opposite side of the box from the banjo or you will fracture the plastic. I'd avoid counting on the armour as CPC in this situation, so include a core as the CPC.


I've tried to summarise the practices typically employed at our site. They aren't necessarily written down in tablets of stone, but we consider them to be safe practice. I might learn something from this too.



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If we learn from our mistakes,
I'm getting a great education!

 

[Sparkyman]

I agree with ScottyUK.
cn100, If you are unfamiliar with armoured cable glands, banjos etc have a look at cmp-products.co.uk

 

[cn100]

thank you scottyuk and sparkyman for your replies as some areas are grey and left to individuals and good practice.always good to know how others do it.
here's another one for you.how do you generally rate distribution board supplies and i mean main feed cables in when a db board is feeding other db boards and can easily get overloaded. i'm talking mainly 3 phase industrial supplies. also any experience in connecting up several high bay lights at 400w each,circuit breaker size, how to all be switched from one position ect.. any ideas are good ideas.
thanks again
cn100

 

[Sparkyman]

The best way to size dis boards and their feeder sizes is to have an accurate load schedule to start with for the application. Then you can work out a load for each board, sub-board etc and you can then rate the feed and feed cable accordingly. Then add a bit for future requirements and away you go.
Then you need to determine another driver, i.e. initial installation cost against lifetime cost. What I mean here is that most cables are rated full load current at 70 deg C, therefore if a 25mm sq cable will suffice it will be cheaper to buy and install than a 50mm sq cable. However the 25mm will be at 70 deg C, the energy that heats it has to be found somewhere and it's in the IsqR losses, so it costs you more in running costs than putting in a 50 mm sq.
High bay lights, install a contactor to switch them on.
Sorry I'm brief, good lady just said teas ready.