Screening control cables

[edexlab]

I've seen different methods of screening on control signal cables , some people twist the braid and connect directly this is usually badly done unless there is a proper clamp available , or connect a single core conductor to the braid in various ways, ie solder or via the tube cut from a ferrule crimped over the braid and single conductors
Another thing is whether or not to screen at both ends.

Where I work the method is to cut the braid short , twist, ferrule and crimp a white single tail, with heat shrink applied over.
Some do both ends, some one end only , and I'm wondering who's right and which is the more acceptable method of screening the cable.

Generally the cables are CY, FYI ,Tronics and pre manufactured screened cat 5 for networking, often in the same tray/basket as motor cables etc wired in SY
(Personally I don't think this is a good idea when everyone does it differently )

Any opinions welcome also if anyone can point me to some further reading I'd be grateful!

 

[edexlab]

Forgot to say control cables generally 24V dc or ac although occasionally depending on customers this can be a different voltage

 

[Darkwood]

I've seen different methods of screening on control signal cables , some people twist the braid and connect directly this is usually badly done unless there is a proper clamp available , or connect a single core conductor to the braid in various ways, ie solder or via the tube cut from a ferrule crimped over the braid and single conductors
Another thing is whether or not to screen at both ends.

Where I work the method is to cut the braid short , twist, ferrule and crimp a white single tail, with heat shrink applied over.
Some do both ends, some one end only , and I'm wondering who's right and which is the more acceptable method of screening the cable.

Generally the cables are CY, FYI ,Tronics and pre manufactured screened cat 5 for networking, often in the same tray/basket as motor cables etc wired in SY
(Personally I don't think this is a good idea when everyone does it differently )

Any opinions welcome also if anyone can point me to some further reading I'd be grateful!

Professionally good practice is to use the correct glands and follow through the screening as needed for the circuit, the one end or both end debate relates to what the actual control cable is doing and is crucial in some sensitive measuring gear to avoid circulating currents that produce their own fields and can corrupt control data..... Having said that some are essential to screen down both end like a VSD controlled motor cable.

There is no one answer fits all, its about knowing the job your doing and usually like VSD's the manufacturers instructions will explain the earthing and screening required, as for other controls its down to research and experience.

 

[shanky887614]

why dont you use castle glands? they will be a lot less hastle than soldering.

tbh we sleeve the braid and put either a ferrule or lug on the end

 

[UKMeterman]

No,
Never pigtail, the inductance of the pigtail reduces the effectiveness of the shield. You need a clamp that makes metal contact between the case and the shield, preferably all the way round the cable.

 

[Deleted member 26818]

Just tease out the screening, twist the strands together, slip some G/Y sleeving over and terminate.
Don't earth out in the field, as it can cause feedback or something.
Also most of the equipment is plastic and there is often nothing to earth to.

 

[tigerpaul]

We either use the proper brass glands for SY or CY cables, or pull a few braids out with a terminal driver and sleeve up with Green or green/yellow, then put the cable through a stuffing gland.
The screen is generally connected down everywhere except the field device.
For example if we have a cable that leaves panel A and goes to junction box B then to isolator C and finally ends at field device D, the cable screen would be connected everywhere but at the end of line, an there it would be made off and connected into a floating connector block.
All this is done in accordance with the specification, and then if it is wrong it can be altered with ease, the designers get it wrong quite often!

 

[netblindpaul]

If it's EMC compliance you are looking at then you need to be looking at the standards here:
Electromagnetic compatibility EMC - Electrical & Electronic - BSI Shop

 

[edexlab]

why dont you use castle glands? they will be a lot less hastle than soldering.

tbh we sleeve the braid and put either a ferrule or lug on the end

I never said I do it this way ,just that I have seen it done by others

 

[rmc86uk]

This is quite interesting as I was arguing with someone at work about it the other day. At my last company it was all BMS work and if it was an AHU fan motor fed from an inverter we would pull out the braiding, sleeve g/y and connect to earth. Things like space temps and valves the screen was connected at panel end and in the field just wrapped back round the cable as there was no connection for it in the field device. At this new company which is different work all together he said it should only be done at one end. I said both but not sure what is the correct procedure.

 

[UKMeterman]

Have a read of EMC Information Centre it explains how to terminate in great detail.

 

[shanky887614]

This is quite interesting as I was arguing with someone at work about it the other day. At my last company it was all BMS work and if it was an AHU fan motor fed from an inverter we would pull out the braiding, sleeve g/y and connect to earth. Things like space temps and valves the screen was connected at panel end and in the field just wrapped back round the cable as there was no connection for it in the field device. At this new company which is different work all together he said it should only be done at one end. I said both but not sure what is the correct procedure.

we only do it supply end from panel to iverter then inverter to motor (not connected motor end)

i would get a hellerman sleeve tool and put a sleeve over it instead of bending the screen back

 

[Darkwood]

we only do it supply end from panel to iverter then inverter to motor (not connected motor end)

i would get a hellerman sleeve tool and put a sleeve over it instead of bending the screen back

The screening for a motor cable run off a vsd should be grounded at both ends... I yet to find an vsd that doesnt stipulate this to in their installation instructions.. the screening if vsd output cables serves a different purpose to say that of a cable for a measuring device.

 

[SheffSimon]

The screening for a motor cable run off a vsd should be grounded at both ends... I yet to find an vsd that doesnt stipulate this to in their installation instructions.. the screening if vsd output cables serves a different purpose to say that of a cable for a measuring device.

We use SWA generally, which provides both a adequate screen and ease of grounding at both ends.

 

[edexlab]

Don't try telling the Germans that!

I'm back at a job we installed a few months ago, ripping out Swa to control panels and replacing it with NYY because they don't recognise this cable type as a product which complies with Vde

It's really a case of sour grapes, the German contractors that normally do work for this company lost out on price ( to us ) to supply power to the equipment we're installing, and as he is actually testing the job and signing it off he says that it has to be done with a cable he is familiar with !
His colleague said he was impressed with the standard of work we'd done, but it was out of his hands!