SELV and 230V circuit separation

knxhome · Jul 31, 2011

It is planned to install a central 24V SELV supply in a domestic residence, e.g. to power LEDs.

According to my reading, BS 7671:2008 chapter 414.4.1 (ii) requirements for protective separation between a SELV and 230V circuits offer the following options

a) double insulation -or-
b) reinforced insulation -or-
c) basic insulation and protective screening

for the highest voltage present.

Which value should be assumed here for "highest voltage present"? 230V or 230V plus potential voltage spikes?

Or to put it more simple: which cables (in the 1.5 to 10mm2 range) would be suitable for the SELV circuit according to option a), i.e. where SELV and 230V cables may run next to each other without further spatial separation?

Thanks in advance.

Deleted member 26818 · Jul 31, 2011

The highest voltage present means 230V.
If you intend running both the LV and SELV cables in the same trunking or conduit, then the SELV cable has to be rated for 230V.

knxhome · Jul 31, 2011

spinlondon said:
The highest voltage present means 230V.
If you intend running both the LV and SELV cables in the same trunking or conduit, then the SELV cable has to be rated for 230V.

I have been inclined reading 414.4.1 (ii) the same way as you.
However, this appears to be a relative low requirement for the insulation.

According to the above interpretation the wiring of SELV circuit could be T&E, at least from the insulation requirement.
I certainly would not use T&E for a SELV circuit due to
a) potential confusion with the 230V circuits -AND-
b) earth should not be connected on the device side

Therefore, any suggestion for suitable SELV cables (1.5 - 10mm2) are still welcomed.

jeremy · Jul 31, 2011

knxhome said:
I have been inclined reading 414.4.1 (ii) the same way as you.
However, this appears to be a relative low requirement for the insulation.

According to the above interpretation the wiring of SELV circuit could be T&E, at least from the insulation requirement.
I certainly would not use T&E for a SELV circuit due to
a) potential confusion with the 230V circuits -AND-
b) earth should not be connected on the device side

Therefore, any suggestion for suitable SELV cables (1.5 - 10mm2) are still welcomed.

There shouldn't be confusion as selv runs should be kept to a minimum because of the current rating/ voltage drop; the transformer would normally be in close proximity to the load. To negate the worry you could label the T&E, "SELV".
Snip the cpc back to the outer sheath, it doesn't have to be used

knxhome · Jul 31, 2011

jeremy said:
There shouldn't be confusion as selv runs should be kept to a minimum because of the current rating/ voltage drop; the transformer would normally be in close proximity to the load. To negate the worry you could label the T&E, "SELV".
Snip the cpc back to the outer sheath, it doesn't have to be used

Planned is a CENTRAL 24V supply, as stated in the first post. Therefore the SELV circuits and LV circuits may run in parallel in certain sections of the house.

The problem of voltage drop has been taken into account.

jeremy · Jul 31, 2011

Please accept my sincerest apologies, I missed the "central" part. The selv should be earth sheathed if running in parallel to disperse any induced voltage. Hope this helps

knxhome · Jul 31, 2011

jeremy said:
Please accept my sincerest apologies, I missed the "central" part. The selv should be earth sheathed if running in parallel to disperse any induced voltage. Hope this helps

No problem, central SELV supply is not so common..... yet for LED illumination potentially attractive, due to their relative low wattage. Further it will be 24V and not the more widespread 12V.

Re earth sheathed:
The 24V supply will not carry any data signals. It is 24V DC, supplied from industrial-strength SELV-certified power supplies.
Therefore I do not see a need for earth shielding concerning EMI.

jeremy · Jul 31, 2011

Re earth sheathed:
The 24V supply will not carry any data signals. It is 24V DC, supplied from industrial-strength SELV-certified power supplies.
Therefore I do not see a need for earth shielding concerning EMI.[/QUOTE]
What will stop the higher voltage being induced into the elv cable