Isolation Transformer Control Circuit

davezawadi (David Stone):

This is quite normal. For a circuit with no access, one cannot receive a shock, and the control circuit probably controls contactors. Even if one side is Earthed, does this make the circuit safer? The answer is no, and 2 faults are required to make any risk, even if contact were possible.

Correct regarding protection against electric shock. But it's not just about protection against electric shock, and not always normal for machinery controls.


In general, BS EN 60204-1 control panels would use PELV for controls, to help faults in control wiring to show up ... for safety and business continuity reasons of course. This is due to the fact that for safety, digital control output signals are "sourcing" (switched from the "+" conductor) in negative-earth DC controls, or non-neutral AC conductor. Where, in odd circumstances, positive earthing is used for DC controls, the digital control output signals would be "sinking" (Switched from the "-" conductor) for the same reasons, but this is quite rare.


Unless there's a good reason, PELV should be used in preference to SELV.


Clause 6.1 of BS EN 60204-1 clearly says this (my highlight):

6.1 General

The electrical equipment shall provide protection of persons against electric shock by:

– basic protection (see 6.2 and 6.4), and;

– fault protection (see 6.3 and 6.4).

The measures for protection given in 6.2, 6.3, and, for PELV, in 6.4, are a selection from

IEC 60364-4-41. Where those measures are not practicable, for example due to the physical

or operational conditions, other measures from IEC 60364-4-41 may be used (e.g. SELV).

So, I'm not saying SELV is definitely wrong in this case (SELV is necessarily used for some physical communications circuits, such as Ethernet, for example, which are used for control). However, it depends on whether PELV was practicable for the particular circuit or application, and more importantly whether there would be a controls safety issue if a fault in the control wiring were not detected ... the understanding the OP has, that PELV is normally used for controls in machinery, is 100 % correct.

davezawadi (David Stone):

This is quite normal. For a circuit with no access, one cannot receive a shock, and the control circuit probably controls contactors. Even if one side is Earthed, does this make the circuit safer? The answer is no, and 2 faults are required to make any risk, even if contact were possible.

Correct regarding protection against electric shock. But it's not just about protection against electric shock, and not always normal for machinery controls.


In general, BS EN 60204-1 control panels would use PELV for controls, to help faults in control wiring to show up ... for safety and business continuity reasons of course. This is due to the fact that for safety, digital control output signals are "sourcing" (switched from the "+" conductor) in negative-earth DC controls, or non-neutral AC conductor. Where, in odd circumstances, positive earthing is used for DC controls, the digital control output signals would be "sinking" (Switched from the "-" conductor) for the same reasons, but this is quite rare.


Unless there's a good reason, PELV should be used in preference to SELV.


Clause 6.1 of BS EN 60204-1 clearly says this (my highlight):

6.1 General

The electrical equipment shall provide protection of persons against electric shock by:

– basic protection (see 6.2 and 6.4), and;

– fault protection (see 6.3 and 6.4).

The measures for protection given in 6.2, 6.3, and, for PELV, in 6.4, are a selection from

IEC 60364-4-41. Where those measures are not practicable, for example due to the physical

or operational conditions, other measures from IEC 60364-4-41 may be used (e.g. SELV).

So, I'm not saying SELV is definitely wrong in this case (SELV is necessarily used for some physical communications circuits, such as Ethernet, for example, which are used for control). However, it depends on whether PELV was practicable for the particular circuit or application, and more importantly whether there would be a controls safety issue if a fault in the control wiring were not detected ... the understanding the OP has, that PELV is normally used for controls in machinery, is 100 % correct.