Screening control cables

[edexlab]

SWA does not provide adequate EMC screening.
End of.

Obviously I'm here asking about this subject, so I'm not an expert, still reading up on it
I wouldn't have used SWA with EMC screening in mind.....
but as a cable for power ie to motors etc , wouldn't it be better than something like NYY when run in close proximity?
I know ideally there should be some separation, but often cables of all types are in the same containment , which is often going to be a potential problem.

 

[netblindpaul]

If you are working to EMC reg's then they will take priority over BS7671, even if BS7671 applies.
SWA is a "BS7671" cable.
It is about as good as a collander at keeping water in as regard to EM emissions.

 

[edexlab]

I've been reading an ebook/guide to EMC and methods of minimising effects using components or screen techniques , interesting stuff!

Yet it seems to me on jobs I've been on or seen (exception being data centres) that apart from larger installations it's pretty much ignored .....

Separation within the same containment ie using one side of basket/tray for signal/control a small gap and the remaining side for power and screens connected at one end is the main method used although the actual "Separation" gap used I think is questionable sometimes

 

[netblindpaul]

I've been reading an ebook/guide to EMC and methods of minimising effects using components or screen techniques , interesting stuff!

Yet it seems to me on jobs I've been on or seen (exception being data centres) that apart from larger installations it's pretty much ignored .....

Separation within the same containment ie using one side of basket/tray for signal/control a small gap and the remaining side for power and screens connected at one end is the main method used although the actual "Separation" gap used I think is questionable sometimes

Yep that explains why I end up picking up the pieces when these things go wrong because the muppets who design and install them don't understand the requirements and why these requirements are in place.

 

[edexlab]

Shouldn't be too bitter Paul!
I bet you make a half decent living out of these mistakes,
but I'm surprised that nowadays with technology moving on that there's very little taught about this at colleges etc , I remember touching on it vaguely and being told " Don't worry too much about it"
My last apprentice had the same advice!

 

[shanky887614]

Shouldn't be too bitter Paul!
I bet you make a half decent living out of these mistakes,
but I'm surprised that nowadays with technology moving on that there's very little taught about this at colleges etc , I remember touching on it vaguely and being told " Don't worry too much about it"
My last apprentice had the same advice!

vsd were mentioned less than micc when i did my 2330.

 

[SheffSimon]

SWA does not provide adequate EMC screening.
End of.

I have read a number of sources that SWA is perfectly adequate, and have been told the same by both ABB, and on a Control Teqhniques training course many moons ago, so I would be interested to know why you think otherwise?

 

[netblindpaul]

Because I was told exactly the opposite, during my training, and it was in our documentation when I worked for the drive and control company.

 

[davesparks]

I have read a number of sources that SWA is perfectly adequate, and have been told the same by both ABB, and on a Control Teqhniques training course many moons ago, so I would be interested to know why you think otherwise?

At a basic and simplified level it is due to the armour strands all being laid up in the same direction and occasionally having gaps between the strands.
SY or DWA offer better screening as they have aver lapping strands in the screenso that gaps are much smaller.
The best screening is afforded by a complete metal covering such as a foil screen, steel conduit or micc

 

[Darkwood]

In reference to VSD motor cables then I have to agree with the fact that they are poor choice for screening, the armour will provide some screening protection at low frequencies hence your volt-stick won't see if the cable is live but due to the nature of harmonics and the high frequencies that are seen in relation to using VSD's then SWA serves little if any EMI protection.

I only ever use SY, Lapp or Igus which comes in many formats but as I'm usually handling these cables alot I know where to get them, although SY has snuck into the standard wholesaler been sold (mis-sold) as a armoured flex which it is then put to so many incorrect uses.

If you search hard enough you can get screened SWA although most of what I have seen is for signal and data use and HV/MV power cables.

So as a solution for screening IMO then standard SWA doesn't qualify and not only can this see illegal EMI but can cause drives and motors to prematurely fail... also another very big issue with harmonic noise is rcd masking where it can stop rcd within the same installation from actually seeing earth leakage faults.

 

[SheffSimon]

From some training notes...

Some drive manufacturers have recommended specialised cables using one or more braided copper screens. Measurements at Control Techniques have consistently shown that low-cost steel-wire armoured cable, as widely used in the UK, gives equally good results. The reason for this appears to be that although the steel wire is a less effective screen (Z[SUB]T[/SUB] is higher than for the copper braid), the losses in this frequency range are higher so that the actual current flowing in the screen is less.

Also,

http://www.brookcrompton.com/pdf-files/GAMBICA_Installation_Guide_Power_Drive_Systems.pdf also suggests that galvanised steel wire armour is an effective screen, and cables with open woven braids are not suitable.

I would class SY as an open woven braid......discuss

 

[Darkwood]

A good little link... points I noted .. SWA would satisfy only if tightly wound, very few SWA's meet this criteria and only the more expensive brands - I have been in debate not long ago about poorly manufactured swa where all the armour had clumped together instead of the usual small gap between steel strands and left a 10mm open unprotected area. So if you are going to use swa I can see it been acceptable but only on a tight wound construction.... as you rarely stipulate this or know what armour you will get (cheapest per meter wins) then I would say a lot of the SWA on the marker for LV applications would be unsuitable due to the competitive market and corner cutting to reduce manufacturing costs.

The braiding about open woven been not acceptable then yes... cannot argue that - some SY has very poor coverage , I do use the tight woven brands where the underlying insulation is completely covered.

 

[shanky887614]

dark isnt it only dw swa that would comply?

i havent seen any swa that has the armour all the way around except on old installs.


you can get sy with a black covering to offer uv protection but this would have to be a special order from the manufacturers

 

[netblindpaul]

OK, fair cop, I was always taught that SWA was no good, and as a manufacturer, we would not accept it for connection between our drives and motors, our documents forbid it.
However, I doubt that adequate EMC screening would be achieved with a "normal" SWA purchased from your average wholesaler at the moment in the UK, and installed by a "normal" electrical installer in the UK.
There is too much time pressure on them to throw it in rather than terminate it in accordance with the requirements such as that in the Gambica documents.

I would add that if you are going to connect a BS5467, (or a BS6346 which is no longer) cable to a vibrating device then you must ensure that the manufacturer agrees that the cable is suitable for connection to a vibrating device.

 

[UKMeterman]

The problem with steel wire armour is that it just forms a very long thin slot for the radio interference to leak through, any decent emc coverage is in the form of a mesh.

 

[Darkwood]

dark isnt it only dw swa that would comply?

i havent seen any swa that has the armour all the way around except on old installs.


you can get sy with a black covering to offer uv protection but this would have to be a special order from the manufacturers

As I don't use it as a choice myself so knowledge of it for this purpose is limited but do know some SWA is compatable although the cheap standard SWA you get everywhere IMHO wouldn't be good enough, you can also get SWA with integral screening but the cost is rising beyond simply finding an alternative method. A lot of my cables of choice are suitable for energy track and screened to the teeth and so many ranges and styles that all installs can be catered for.

 

[shanky887614]

As I don't use it as a choice myself so knowledge of it for this purpose is limited but do know some SWA is compatable although the cheap standard SWA you get everywhere IMHO wouldn't be good enough, you can also get SWA with integral screening but the cost is rising beyond simply finding an alternative method. A lot of my cables of choice are suitable for energy track and screened to the teeth and so many ranges and styles that all installs can be catered for.

nice, by dw i meant double wound. you get swa with a lot more steel in it than normal, personally i would like a screen but then my cable of choice is sy or twisted pair (belden 8760 etc)

 

[SheffSimon]

SY only goes up to a certain size range though I think, and the voltage rating is 300/500, a lot of customers won't accept that.

If you have got big motors on VSD's then what would you use other than 3 core SWA?

 

[shanky887614]

SY only goes up to a certain size range though I think, and the voltage rating is 300/500, a lot of customers won't accept that.

If you have got big motors on VSD's then what would you use other than 3 core SWA?

thats a big motor that needs bigger than 150mm 4c sy

 

[SheffSimon]

thats a big motor that needs bigger than 150mm 4c sy

True, but not uncommon on long conveyors on power stations, quarries etc. We have installed conveyors with 250kW and 355kW LV motors, currents up to 750A, on VSD's. Some long cable runs too, SWA's in parallel, and I've never seen any connected in SY. It would just not be accepted by the customer engineer, nowhere near robust enough in that environment.

We always run the protective conductor separately, so 3 core SWA's.