Data cables in electrical control panels

In practice the 50mm rule is a bit arbitrary and only really  makes sense in terms of avoiding EMC problems over long distances (metres of parallel runs). You can come a lot closer if the region is only short, but by how much and what else needs doing is highly specific to the situation. If in doubt earthed screens allow things to touch opposite sides of the screen and not see each other under most circumstances . Basically the design authority needs to be able to work out what is going to happen EMC wise. Inside the box the external wiring standards are not really applicable, but the geometry is understood, by someone at least. Voltage related spacing  is the really easy part of the question as insulation to hold off mains is less than 1mm of PVC. Understanding the nuances of EM takes half a lifetime ;-)
Mike.

Hi Mike,

Makes sense. Thanks for your answer.

As for perfect planning I’ve seen data cables whacked into control panels, twisted around mains cables like spaghetti, working flawlessly for years. Whenever I design systems like this, I try and ensure data and power are kept apart as much as possible. 

When you have a LAN port 10mm away from a mains connection on a product, this is somewhat difficult ;-) 

Thanks. 

mapj1 said:

In practice the 50mm rule is a bit arbitrary and only really  makes sense in terms of avoiding EMC problems over long distances (metres of parallel runs).

This is NOTHING to do with EMC.With EMC in BS 50174-2, there are conditions for zero separation.,

This 50m min, however, is listed as a fundamental safety requirement in BS 6701.

mapj1 said:

In practice the 50mm rule is a bit arbitrary and only really  makes sense in terms of avoiding EMC problems over long distances (metres of parallel runs).

This is NOTHING to do with EMC.With EMC in BS 50174-2, there are conditions for zero separation.,

This 50m min, however, is listed as a fundamental safety requirement in BS 6701.

I do not agree.

 Then that standard is either incomplete or technically unjustifiable, I do not have it to hand to see their safety explanation for 50mm.  It should not matter which standards we look at, if they are not based on the real physics that applies to the situation, they are unhelpful.

We know that wires from the various 'bands' have to meet somewhere, not least where mains is converted to ELV of course, and the real amount of insulation to hold off mains may be little more than a thin -but well specified to be CE marked- coat of varnish by the time you get deeper into the guts of the power supply. (so called triple insulated magnet wire allows you to ditch the inter-winding papers )

If you open an ethenet mag jack, designed to hold off 500V or more, the wires are a lot closer together but 'globbed' to prevent arc strike.

I suggest the actual problems in the field that come with network cables in power boxes and vice versa are usually related to EMC, not voltage breakdown of the insulation of the cables.  To jump an air gap of 50mm needs in excess of 30kV !!  Once an arc is struck of course a lot less. This is why we are happy to work with bare hands on open systems isolated from the mains by a 3mm or greater air gap at the mains switch or MCB.

The exception needing real separation is probably the few remaining bare copper phone lines sharing poles or crossing under bare mains overheads, and there I think we want a lot more than 50mm!

I'm happy to say the 50mm rule is a bit arbitrary.