Question about earthing the armour of SWA

[mak]

If cpc core is present i.e. it's either 5 core three phase or 3 core single phase — is a banjo connected with a brass screw into steel trunking adequate to earth the armour(provided trunking is earthed to the earth bar)? Or would you put fly leads in between then a fly lead to an earth bar?
Thanks.

 

[telectrix]

only answer is to measure resistance between points. I once has a TNC-S with SWA armour acting a SPC. Although Earth Bar was screwd into Metal box, The Earth Bar - Box measured 100 ohms.

 

[Dustydazzler]

As Tel says, the only way to be 100% of continuity is to measure it ...

Like Tel I have found some pretty shonky connections with high resistance in the past

 

[pc1966]

I too have seen problems with mechanical joints being far from good electrical joints!

If in doubt put in flying leads, that might be down to having low faith in the mechanics staying good due to corrosive environment, or vibration (or idiots) risking loosening of trunking fixing bolts.

Also I would add that if you are looking at big cables / high PFC then you really should check if the trunking arrangement could cope with the fault currents from a cable-armour fault, in those cases you might need a decent girth of flying lead to survive a fault without further damage.

 

[littlespark]

The OP mentioned a core being the main cpc, so earthing the armour is really bonding.

If glanding onto metal trunking or enclosure, and it’s not needing IP requirements, then the normal glands and locknut should be fine.
For a little extra grip… serated washers.

 

[Julie.]

The problem isn't necessarily the loop impedance for the remote end, but what happens if the SWA is punctured shorting line to armour?

Will that cause sufficient current to operate in a suitable time?

I have seen situations where the remote supply (sub board, large load etc) has a suitable Zs due to a core being used as CPC; but the SWA's armour is so poorly connected that a fault to the armour would just pass current forever, or even burn out the connection leaving the armour live.

So the question becomes, would the connection proposed result in a suitable Zs on the armour?

Personally I like to see well over-the-top effort to ensure a solid low resistance connection to all protective conductors be it the cores, or/and the armour.

 

[Dustydazzler]

Its an interesting point that if the armour becomes live due to the live core touching the outer armour if the armour is very badly earthed would it actually trip the ocpd , the same applies to any metal container I guess.
if we are being honest how many of us actually test the armour ?

 

[pc1966]

if we are being honest how many of us actually test the armour ?

If using the armour as CPC then every time you do an R1+R2 measurement.

Assuming there is only one sub-main, no bonded service pipes, etc, creating parallel paths!

If you are using a core as CPC (or parallel CPC, etc) then you can disconnect it at one end and measure the total of Rcpc + Rarmour and if you have the design figures or a good idea of the installed length then you can see if they add up to a sane value.

Again, assuming no parallel paths on the metal at armour ends.