SWA - Prefered earthing method

[the_jinj]

I have always used the armour as the cpc when using SWA cable but have noticed people sometimes use 3 core with one of the cores acting as cpc (as well as armour sometimes)

As long as the glands are made off correctly the armour is more than sufficient so do people use 3 core to safeguard against bad termination at the gland? Or maybe if insulated gear is used? Don't trust the steel as a cpc?

BTW - not saying I think using 3 core is wrong and can see that it provides extra cpc protection but raises the cost and size of cable need when 2 core would have sufficed.

 

[Adam_151]

I think its a mixture of the risk of it being a bad connection* / pulling out**

and...

Also, it lowers Z's, obviously the Z's using only the armour on its own have got to be ok to cause disconnection (incase someone puts a spade through it), but when using fuses, you might need 0.4 disconnection at the end of the cable (say its feeding a masterseal S/O at the end of a garden), but if someone spades through the cable, then 5s would be ok for that (or is everything that happens outside the E-zone 0.4?)

Or if you were feeding a submain, using the core provides a lower "Ze" at the submain board, ... allowing r1+r2 of final circuits to be higher


*I belive there is a reg somewhere that says something agaisnt using the armour as CPC on a dockyard, as all the salt in the air.... rust, and all that

** I seem to remember TT saying that he had seen in a factory, several SWAs caught by a fork lift truck, and if the armour provides the CPC, then the CPC is the first connection to be pulled out...

 

[Pensdown]

More often than not in a domestic situation it's probably belt a braces. It does make terminating easier in plastic boxes although you still have to use the banjo and a flying tail.

If your doing any high integrity circuits (section 607) then the armour and the extra core gives you your 2 sperate earth paths. Just out of interest I ran a 6mm SWA through Amtech and the armour comes out equivalent to 15.97mm

 

[plugwash]

I always reccomend 3 core to diyers because you don't know how good a job they will do of the glanding.

TTs point about the armor getting ripped out of the gland is a good one too. Especially if the cable is connected to a large enclosure with long tails.

 

[the_jinj]

Say in a domestic situation where the cable is properlly terminated and clipped, there is (or should be) no chance of the cable being pulled out. The cross section of the armour is >= it's copper equivalent so should provide more than adequate readings.

I can see good arguments for using the extra core for added integrity in some commercial properties or special situations but I get the feeling the way 90% of folk will do this is to run the extra core - mainly to add the extra piece of mind to the installation - but surely this could lead to people badly terminating the steel (or not at all - or not in glands) as they have the cpc covered for their needed readings etc? so not adequatelly covering the spade through cable scenario?

 

[plugwash]

on the other hand if an earth core is used only one gland has to hold up to deal with the spade situation.

still a good idea to test the armour and core seperately though.

also with the larger sizes you have to be carefull about the armour CSA, its can be a bit on the low side with the larger sizes and remember a particular area of steel is NOT equivilent to the same area of copper.

 

[the_jinj]

General concensus so far is 3 core is the way ahead (3 core you know the scoreTM?)
For the sake of a few pence extra on the smaller sized cables it may well be worth adding the extra piece of mind.

 

[Pensdown]

I always reccomend 3 core to diyers because you don't know how good a job they will do of the glanding.

TTs point about the armor getting ripped out of the gland is a good one too. Especially if the cable is connected to a large enclosure with long tails.

I would agree with both comments. If the cable is not glanded off properly and some of the SWA strands end up outside the gland it will not only reduce the CPC but the gland will never tighten down properly on the rest. Under these circumstances It could be pulled out with very little effort

 

[Spark123]

I think it is to do with wether or not the armour will comply with table 54G, which if you use a core for earth it complies. If the armour of a 2 core cable is not up to 54G * then the adiabatic equation can be used.
(* the ratio of the K values for steel and copper also needs to be applied.)



edit: spelling

 

[the_jinj]

I always reccomend 3 core to diyers because you don't know how good a job they will do of the glanding.

TTs point about the armor getting ripped out of the gland is a good one too. Especially if the cable is connected to a large enclosure with long tails.

I would agree with both comments. If the cable is not glanded off properly and some of the SWA strands end up outside the gland it will not only reduce the CPC but the gland will never tighten down properly on the rest. Under these circumstances It could be pulled out with very little effort

I suppose it's not only DIY'ers that can terminate badly though......

 

[Pensdown]

Are you trying to say that sparks make mistakes........never

 

[the_jinj]

Are you trying to say that sparks make mistakes........never

Nooooo....especially no NICEIC registered ones