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Topic Title: SWA Cable- Does Armour prevent Electomagnetic Interference?
Topic Summary: Electromagnetic Interference between LV & ELV systems
Created On: 21 November 2012 10:30 AM
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 21 November 2012 10:30 AM
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nad

Posts: 410
Joined: 14 January 2005

Hi, I'm Nad and I design electrical installations.

My question regards some external CCTV cameras. Could we use the same duct to run the 230V power and the coaxial cables, if using SWA cable for the power?

Basically does anyone know if the steel wire armour of a SWA cable screens the LV circuit from electromagnetically interfering with adjacent ELV circuits?

Any opinion or experience on the matter will be gratefully received.

Regards,

Nad

-------------------------
Nad

*Regularly edited due to spell cheque misdiagnosis
 21 November 2012 10:37 AM
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OMS

Posts: 22864
Joined: 23 March 2004

Generally, no - armour has almost no screening effect (particularly at higher frequencies). Think of it as the equivalent of a sieve.

That said, the emc emission from a small camera supply will be low and not likely to influence a coax.

Why coax to the camera btw.

In principle, I'd suggest you use seperate ducts in a pit and duct system - if only because the external power supplies may also need to be supplying other equipment than cameras (and thier relevant illuminators perhaps)

Failing that - perhaps have a look at how you are transmitting the camera signals - coax may not be the best idea for future proofing

Regards

OMS

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Let the wind blow you, across a big floor.
 21 November 2012 10:42 AM
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Legh

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I have run cat6 cable in the same underground ducting as SWA. There has been no EMI effects over the past two years. Although that doesn't mean that there may not be problems in the future, dependent upon loading.

The regs suggest as a minimum a distance of 100mm between LV and ELV. (see section 528.1 and -2 for more info)

Legh

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http://www.leghrichardson.co.uk

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 21 November 2012 11:23 AM
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OMS

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Cat 6 is a bit different in construction from a sheilding perspective than coax though, Legh ?

Regards

OMS

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Let the wind blow you, across a big floor.
 21 November 2012 11:37 AM
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Legh

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I agree, but there maybe problems with Insulation and withstand voltages under fault conditions. and possibly with 3-phase cabling

Legh

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http://www.leghrichardson.co.uk

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 21 November 2012 11:40 AM
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OMS

Posts: 22864
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The earthed armouring of the SWA would protect from that I would have thought ?

I was thinking more along the lines of the cat6 being STP whereas coax has almost no shielding from EMC

regards

OMS

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Let the wind blow you, across a big floor.
 21 November 2012 11:51 AM
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Legh

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Ok, I'll go along with that: 1500V IR and 300V operational, comms screening

Legh

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http://www.leghrichardson.co.uk

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 21 November 2012 11:52 AM
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AJJewsbury

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What sort of duct is this? Is it underground, or just part of a building?

Might specifically shielded (e.g. BS 8436 or FP) power cables be suitable?

Alternatively, do the cameras need mains (a.c.) power? Most I've seen just use 12V d.c. with 'shotgun' cable (d.c. pair + co-ax together) - seems sufficient for most, including PTZ ones with IR illuminators.

- Andy.
 21 November 2012 12:36 PM
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spinlondon

Posts: 5494
Joined: 10 December 2004

SWA does appear to provide some screenining.
My fluke magic wand won't tell me if any of the conductors are live, without me breaking into the cable and exposing the cores.
Bloody anoying if you're attempting to trace an underground circuit and there's a number of cables to choose between.
 22 November 2012 10:28 PM
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cmatheson

Posts: 223
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I beleive the armour will provide some screening but at low frequencies and it should be connected to (logic) earth at one end only. A ferrite doughnut around the power cable is usually all that is needed to do the job.

-------------------------
Chris Matheson
 22 November 2012 11:12 PM
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nad

Posts: 410
Joined: 14 January 2005

Thanks guys, been away from the desk for a few days so sorry for not posting sooner.

In answer-

The duct is buried in the ground. We only have the one duct on the shortest route, the other route is much much longer.

Was opting for coax due to the length of the runs (the furthest is about 250m).

I'm thinking the runs are too long for using DC over shotgun cable?

I had thought about an aluminium screened multicore (swear I remember FP200 being screened but their website doesn't mention it?) for the power but am more comfortable with an armoured cable. Does anyone know if there is a SWA cable out there with an EM screen built in?

Also, does anyone have any experience of screened coaxial cable? Such a cable with an aluminium screen outer layer over (and insulated from) the copper braid?


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Nad

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 23 November 2012 01:36 PM
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GB

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FP400 is an armoured fire resistant cable which had an internal foil (if memory ok) bt you wold need to confirm EM properties.
Although I would not forsee many issues with your original proposal, fibre optic signal cable??
 26 November 2012 02:31 PM
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AJJewsbury

Posts: 17795
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Also, does anyone have any experience of screened coaxial cable? Such a cable with an aluminium screen outer layer over (and insulated from) the copper braid?

'sat' style co-ax (PF100 etc) usually has a foil as well as a braid, but in contact with each other.

I'm thinking the runs are too long for using DC over shotgun cable?

Depends on the current and choice of cable... failing that using DC at a higher voltage and then voltage regulation at the load end should work.

Another possibility might be to run the co-ax inside flexible metallic conduit (e.g. http://uk.rs-online.com/web/p/cable-conduits/2275256/ ) although your length of run might make that impractical.

- Andy.
 26 November 2012 10:18 PM
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cmatheson

Posts: 223
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CAT6 is usually more resistant than coax to induced interference because of common mode rejection with twisted wire pairs. What happens is that any induced currents occur in both cores which are connected to the inputs of a differential amplifier with the result that the induced voltages cancel each other out.

As for preventing high frequency emissions to acheive EMC compliance, normally all that is required is to clamp a ferrite core like RS part 116-1487 to the cable.

-------------------------
Chris Matheson
 27 November 2012 09:42 AM
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AJJewsbury

Posts: 17795
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Would 250m be pushing UTP a bit?

Another option might be to use fibre for the video - that way you can do what you like with the mains side without fear of interference.

Google found this: http://compare.ebay.co.uk/like...emTypes&var=sbar&cbt=y but you can probably find a branded one somewhere, if you want to use analogue video.

Fibre should be upgradable to ethernet at a later date too if you need to keep your options open for IP cameras.

- Andy.
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