SELV Fusing

I think I can argue that fusing of only one pole of 2-wire SELV is required.

Note before I start the argument: This is NOT necessarily the case for multi-pole SELV (where at least two poles are required to be protected against overcurrent) or three-phase SELV, where in delta, the lines should be protected against overcurrent, and in star, the lines other than star point lines should have OCPDs. Nor is it the case for FELV or LV where electrical separation is used as there may be risk of electric shock.

First, does SELV require fusing for ADS ? No ... SELV requires protective separation from Earth, as well as limitation of voltage (and hence touch-voltage after one or more faults). Even after first fault to Earth, provided an appropriate voltage is applied for the conditions (e.g. in BS 7671 dry condition 50 V AC/120 V DC, wet condition 12 V AC/30 V DC, although note these voltages vary between standards), there should be no shock risk.

Which leaves us with overcurrent protection (faults and if necessary overload current). Assuming no other faults, overload current is provided by a single OCPD (in 2-wire SELV). There are two scenarios which chiefly address fault current:

• Transformer-type source, where, either after secondary line-to-line fault, or one line to Earth then second line to Earth, a single OCPD in the primary (and/or a OCPD in the secondary) could provide overcurrent protection
• Electronic (switching-type) supply, either after secondary line-to-line fault, or one line to Earth then second line to Earth, the current-limit in the supply device, or a single OCPD in the secondary of the supply device, will provide protection.

It's a little more tricky to see the fault scenarios if you are distributing a single source for a number of circuits, and require especially if choosing one SELV line to be common, and providing OCPDs in the other, but I'm sure if you draw out the two fault scenarios (e.g. common first, then line in a final circuit, or line in a final circuit then common), fusing just the non-common lines should still work.

In PELV, where necessary (i.e. protection against overcurrents not provided in primary, or converter, or the supply feeds a number of final circuits) it's normal to fuse the lines which are not neutral or mid-point conductors, and the neutral/mid-points ought not to be fused.

I think I can argue that fusing of only one pole of 2-wire SELV is required.

Note before I start the argument: This is NOT necessarily the case for multi-pole SELV (where at least two poles are required to be protected against overcurrent) or three-phase SELV, where in delta, the lines should be protected against overcurrent, and in star, the lines other than star point lines should have OCPDs. Nor is it the case for FELV or LV where electrical separation is used as there may be risk of electric shock.

First, does SELV require fusing for ADS ? No ... SELV requires protective separation from Earth, as well as limitation of voltage (and hence touch-voltage after one or more faults). Even after first fault to Earth, provided an appropriate voltage is applied for the conditions (e.g. in BS 7671 dry condition 50 V AC/120 V DC, wet condition 12 V AC/30 V DC, although note these voltages vary between standards), there should be no shock risk.

Which leaves us with overcurrent protection (faults and if necessary overload current). Assuming no other faults, overload current is provided by a single OCPD (in 2-wire SELV). There are two scenarios which chiefly address fault current:

• Transformer-type source, where, either after secondary line-to-line fault, or one line to Earth then second line to Earth, a single OCPD in the primary (and/or a OCPD in the secondary) could provide overcurrent protection
• Electronic (switching-type) supply, either after secondary line-to-line fault, or one line to Earth then second line to Earth, the current-limit in the supply device, or a single OCPD in the secondary of the supply device, will provide protection.

It's a little more tricky to see the fault scenarios if you are distributing a single source for a number of circuits, and require especially if choosing one SELV line to be common, and providing OCPDs in the other, but I'm sure if you draw out the two fault scenarios (e.g. common first, then line in a final circuit, or line in a final circuit then common), fusing just the non-common lines should still work.

In PELV, where necessary (i.e. protection against overcurrents not provided in primary, or converter, or the supply feeds a number of final circuits) it's normal to fuse the lines which are not neutral or mid-point conductors, and the neutral/mid-points ought not to be fused.