Isolation Transformer Control Circuit

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.

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.

Thank you for your detailed response, this clarifies my initial assumptions based on what I had found which is good to know I was on the correct track as I was not 100% certain. 


I also had a read through BS EN 60204 this morning and found further guidence on control supplies in particular Section 9.4.3 Protection against maloperation due to earth faults, voltage interruptions and

loss of circuit continuity which goes further into detail.  


Just in my opinion, but I always feel slightly uneasy seeing anything above extra-low voltage for control circuits especially where there are several touch points i.e. controls, actuators etc  and the environment requires addtional measures to prevent electric shock. With todays technology, extra-low voltages i.e. 24VDC can easily be utilised even for larger applications with power contactors etc. 


This installation is >40 years and the Iso Transformer is only a recent modification, in an ideal world I would rather see they utilise extra low voltage but that will require a substantial financial investment to upgrade.


Thank you all for your help.

Apologies, I mis-interpreted isolating transformer being 1:1.


Again for the same reasons the system is usually earthed, as you will have noted. Controls and interlocks may be mains voltage, but usually PELV is used for safety in testing, fault finding and maintenance.

Just in my opinion, but I always feel slightly uneasy seeing anything above extra-low voltage for control circuits especially where there are several touch points i.e. controls, actuators etc and the environment requires addtional measures to prevent electric shock.

From purely a shock perspective an unearthed secondary isn't that bad at all - as the first fault rather than making some exposed-conductive-part live merely degrades the system to a kind of TN arrangement (just as if you'd installed a N-PE link in the first place) - it then takes a second fault, on the other pole, to create a voltage difference between two different exposed-conductive-parts and as long as those parts are both earthed (or just bonded together) you then get a short and the overcurrent protective devices will usually provide ADS. Such separated or IT arrangements are actually preferred for some safety critical systems where not failing on first fault is an advantage - although an insulation monitor to warn of the presence of a first fault is preferable.


  - Andy.

The sort of 'insulation monitor' for a 40 year old system may be as simple as two lamps in series with the centre grounded. A single fault extinguishes one lamp, and the other comes on bright, but you can continue to keep the power on long enough to bring the machines to a safe state.

The 2 lamps technique used to be common on London underground, small ships with DC mains, and specialist things like cranes with DC motors.

Actually the modern all electronic monitor is not much different - replace the lamps with opto-couplers and ballast resistors, and drive some alarm electronics.

Yes, as Mike says there are cases where to simply remove the power on first fault could also be dangerous - for example, equipment keeping a patient alive, hence medical IT systems.