Earthing SWA

[Knobhead]

Right I’m going to throw a rock in the water.

There’s been a lot of debate about earth clamps and SWA. To my mind, conventional clamps shouldn’t be used. But I kept on thinking back to cable jointing and constant pressure springs. I had a dig around and found 3M can supply the springs and copper braid separately from joint kits.

This could save a lot of time and trouble for you guys.

• SB1 12mm
• SB2 19mm
• SB3 25mm
• SB4 38mm

Don’t even think of putting you’re standard bonding wire in to the spring, it has to be braid.

 

[pencilpusher]

What is it you are confirming with your Zs test of the distribution circuit?

I suppose I am confirming that the protective device will operate in time if the armoured cable is damaged for example?

I need to show a ZS on the CU STR for "garage distribution circuit" I think I am with it now. The SWA will be earthed at the CU end (TNCS) and so use that to measure ZS at the garage CU?

Have I got this right yet!?:banghead:

 

[Knobhead]

Hi Tony,

this is very interesting, do you have a link I can look at ?

Cheers

Spartykus

Have a look at the OP The clue is in there!

 

[JD6400]

Another querie. If the garage has a TT arrangement and I am filling in the Shedule of Test Results for the house CU, how do I measure ZS for the garage distribution circuit. It has its own earthing system and I am not sure how I can show a ZS for this circuit. Advice gratefully received.

PP

I would measure it on the t+e at the adaptable box as the swa is isolated from the house end , that is what i think you are saying anyway ?
Why the swa has not been earthed from the house end then isolated from the garage cu is beyond me , may as well not be there for what use it is , apart from the mechanical of course !

 

[Deleted member 26818]

I suppose I am confirming that the protective device will operate in time if the armoured cable is damaged for example?

I need to show a ZS on the CU STR for "garage distribution circuit" I think I am with it now. The SWA will be earthed at the CU end (TNCS) and so use that to measure ZS at the garage CU?

Have I got this right yet!?:banghead:

Yes that's right, you are confirming that the CPD will operate in the event of an earth fault on the SWA cable.
To measure the Ze (Zdb) at the TT CU, you would use the live conductors of the SWA and the TT earth conductor.

 

[SPARTYKUS]

EARTH BONDING, 3M SB1 SB2 SB3 SB4 EARTH BONDING KITS, COPPER BRAID, FORCE SPRINGS

 

[JD6400]

Hi Spin

I think i have been missing the point , so if you have a good Zs even if it is not back to the supply board the circuit is fed from you will still get operation won't you ?
Have always just assumed it needs to be back to the circuit origin at the cu and not further on !
Thanks for that , that is if i have not just spouted tosh !

 

[pencilpusher]

Thanks guys. I am replacing the T&E to the adaptable box and running SWA instead to avoid the need for an RCD at the house end. I cannot understand why the original contractor earthed the armoured at the garage CU end either but thats what I have) My querie was the ZS value for the house CU in relation to the STR. I understand the Ze (Zdb)for the garage but I just need to confirm that you are saying I should use the live conductors of the SWA and the earthed armour for the ZS. Thats the only bit I need confirming please.

Thanks for all your help.

PP

 

[Deleted member 26818]

I think that pencilpusher is refering to an installation, where the house is TN-C-S, and in order not to export the PME earth, the shed/garage has been TT'd.
I'm assuming the SWA is earthed at the house end, and not at the shed/garage end.
As such to measure Zs for that cable, you would have to use the TT end of the live conductors and the TT end of the armour.
However to measure Ze/Ra at the shed/garage CU, you would again use the live conductors of the SWA, but instead of the armour, the earth conductor that connects to the rod.
If you measure Zs, and obtain an acceptable value, then yes the CPD will operate, irrespective of the actuall path the earth path takes.
However, it would be better if the earth path is back to the circuit origin, preferably via the CPC, as you would not be able to guarantee any other route.

 

[JD6400]

I think what he has is T+E up to a adaptable box then swa to the garage but the T+E and the swa are not joined ?
But you answered my bit at the end of your response, thanks again Spin .

 

[Deleted member 26818]

I think what he has is T+E up to a adaptable box then swa to the garage but the T+E and the swa are not joined ?
But you answered my bit at the end of your response, thanks again Spin .

Not good, in fact quite dangerous.
If the SWA is not earthed at the supply end and the Zs are not low enough for the CPD to operate, the earth fault will be exported past the RCD protecting any sockets or circuits in the shed/garage.

 

[JD6400]

Not good, in fact quite dangerous.
If the SWA is not earthed at the supply end and the Zs are not low enough for the CPD to operate, the earth fault will be exported past the RCD protecting any sockets or circuits in the shed/garage.

So if reading is good enough it is OK but not a good place to be as in the inability to guarantee the reading in the future .
I had not given it a thought until reading this thread just always taken it that the CPD would not operate in a fault , but it would if the right conditions exist at the time of fault !
Obviously would not use this method in million years but was interesting to think about it !

 

[pencilpusher]

Thanks. Some confusion here. At the moment I have T&E joining the SWA (adaptable box) but the SWA is only earthed at the garage CU not the supply origin. The cpc from the T&E is isolated via a connector block in the adaptable box) As I am changing the CU at the house and want to avoid having an RCD in the new house CU (discrimination issues then as garage is TT with RCD too) , I am changing to SWA all the way. I will earth the SWA at the supply end rather than the TT CU end as it is now.

I now understand the measurement of ZS (for the house CU STR) and the Ze (Zdb) for the garage CU.

Thank you both.

Regards PP