Band I and Band II Segregation

steve100 · Apr 11, 2013

I am aware that Band I (ELV) and Band II (LV) circuits should be kept segregated. My understanding of the reason behind this is firstly to reduce noise interference on data cables as well as control, alarm and fire circuits. Secondly, to ensure that no emf is induced into the Band I (ELV) cables or circuit causing a higher voltage in the ELV circuits. Thirdly, if the LV cable become damaged, because it would be segregated from the ELV circuits it would reduce the likelihood of a higher voltage appearing on the ELV circuit and reducing the likelihood of someone receiving a shock. Can someone please tell me if my assumption/understanding is correct?

However I do have another query about my second point, ordinary twin and earth cable carries the same current but in opposite directions so very little magnetic field is produced as they cancel one another out. So is the concern about induced emf’s still a great concern when using T&E/Multicore cable?

Marvo · Apr 13, 2013

This question isn't as straight forward as you might think it is. I'm not UK based so I'm unfamiliar with the ins and outs of the UK regs but what I would say is that there's an important difference between segregation and separation. You could possibly have ELV and LV cables in the same trunking and segregation might be achieved by just using certain types of cable. Separation would require separate containment or maybe trunking with a divider in it.

Hopefully one of the UK guys will point you in the right direction regarding the requirements in your local regs. The problem with areas like this is that there will be multiple regs covering it from different angles ie the electrical regs cover the electrical safety aspect whereas the fire alarm regs might cover the operational aspects of the data side of things.

With your second question, it's the constantly changing AC electrical fields withing a cable that cause induced and capacitively-coupled voltages to be carried from one cable or wire to another. T&e cable has conductors that run in a straight fashion inside the cable, data cable has twisted pairs which helps cancel out any induced noise.

D Skelton · Apr 13, 2013

There are recommended distances for segregation to reduce interferece on Band I circuits however the separation part as you say comes down to limiting a fault occuring that could result in a higher voltage appearing on a Band I circuit. You are allowed to run Band I and Band II circuits together so long as all cable's insulation is rated at the higher voltage.

Your second point about PVC/PVC multicore cable is spot on. On AC systems there is very little problem or concern with induction when using T+E to supply one circuit.