On reading installation guide, im told to earth the screen to earth.
Do I actually have to twist the foil screen and terminate or will the copper earth be in contact with the foil screen inside the cable?

Catch my drift?

You can cut the foil screen away from the point at which you have stripped the outer sheath and simply connect the copper earth wire - which will be in contact with the foil screen in the cable.

Also, I think I recall reading somewhere that it is best to only earth the cable screen at one end of the cable?

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M.I.E.T - Forfeited this due to The I.E.T's ridiculous membership rules!

Edited: 04 January 2013 at 02:12 PM by M.Joshi

You do need to be careful what you earth the screens to and where. Getting it wrong can affect the usability of the final cable.

Earthing one or both ends is a good debate . . . in reality, there are reasons for both approaches, and you need to chose the right one.

Just out of interest, SK, what is the particular application you are looking at ?

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Eur Ing Graham Kenyon CEng MIET

The application is the LV thermostat range by Heatmiser. (it does indeed instruct to earth at one end only).

I would earth the control panel end and insulate the field device end

spooky just done one of these.... didnt earth the screen and all works fine..

suppose it depends on where your cables run..

That would seem to be correct for this application. If that's what the manufacturer instructs then that's what should be done. Some field devices powered by batteries or by the control cable may well have a screen connection on the board that should be connected - but of course not additionally connected to earth at that end.

Many CAT 5 applications connect two devices, each having their own power. Deciding which end is more difficult in those circumstances, and the general advice from BS IEC61000-5-2 is to connect screen at both ends, but of course on long runs, buildings with multiple supplies, etc., deciding what to do can be tricky.

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Eur Ing Graham Kenyon CEng MIET

Whilst this works, I'm sure the manufacturer wouldn't want to guarantee the EMC performance, and possibly therefore could be considered "not conformant" with EMC Regulations 2006 and maybe a departure from BS7671: 2008(2011)?

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Eur Ing Graham Kenyon CEng MIET

And of course in the real world we all follow manufactures instructions too the letter. As to Heatmiser stats you would only need to worry about the screen if you where using the network part rather than just the power and switch.

This is the reason for mentioning it, i.e.open up the discussion, not to criticise.

Problem with manufacturers' instructions is that they may not be practicable (or even work) in every case you come across.

If that's the case, no problem. However, had problems with links out to "switch-only" devices acting like antennae in the past: depends on the environment & too many other factors.

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Eur Ing Graham Kenyon CEng MIET