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Standards for a PSU to be considered as "SELV"

Tim Watts
Hi - quick standards question, for hobby purposes only:


Is a declaration of being "Class II" and within the appropriate voltage band, sufficient for a PSU to be considered as SELV for usage scenarios in BS7671?


I'm doing a Raspberry Pi project with my son to PWM drive some 30V LED outdoor fairly lights. So I'm choosing a PSU that's nice and safe.


The candidate PSU I have is this one: 30V PSU (RS) or possibly this one 30V PSU (-10C) for it's lower operating temperature (it's going to be in the shed, occasional drops below 0C possible)


First one is defined as Class II, it's 30V. It also claims medical approval. A whole bunch of quoted standards on the datasheet above. Apart from "Class II", not sure if I should be looking at any particular standard that defines it as meeting SELV PSU requirements. Isolation voltage is given as 4kV?


Second one claims very little but implies Class II by lack of protective conductor pin.


Pretty sure either is a good choice (noting operating temperature limits of the first), but academically I have been curious what defines a SELV PSU if it doesn't actually have the word SELV printed on it?


Many thanks as always for letting me tap the font of knowledge that this forum is :)


Tim


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    Just to add that the standards that BS 7671 specifically lists are primarily for conventional wound transformers - BS EN 61558-2-6 (safety isolating transformers and power supply units incorporating safety isolating transformers) and BS EN 61558-2-8 (transformers and power supply units for bells and chimes) - but there are other product standards which might give equivalent protecton - the first device you referred to mentioned EN60950-1 which will be known in the UK as BS EN 60950-1 (Information technology equipment. Safety. General requirements) (although that standard has now apparently been superceded by BS EN 62368-1 (Audio/video, information and communication technology equipment. Safety requirements)). You'd have to read the standard proper to confirm the details (which aren't freely available on the web) - but perhaps your local library could help with that.


    I wouldn't be too worried about the 'electronic' designs per se. The traditional switch-mode PSU normally had a transformer between the "mains" and ELV sides (if a much higher frequency, smaller and more efficienet one that would traditionally been the case) - which together with opto-coupling in the feedback loop which can provide isolation from the mains every bit as good as a traditional safety isolating transformer. Not all designs will provide safety isolation of course, but it is possible.


      - Andy.
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  • 0
    Just to add that the standards that BS 7671 specifically lists are primarily for conventional wound transformers - BS EN 61558-2-6 (safety isolating transformers and power supply units incorporating safety isolating transformers) and BS EN 61558-2-8 (transformers and power supply units for bells and chimes) - but there are other product standards which might give equivalent protecton - the first device you referred to mentioned EN60950-1 which will be known in the UK as BS EN 60950-1 (Information technology equipment. Safety. General requirements) (although that standard has now apparently been superceded by BS EN 62368-1 (Audio/video, information and communication technology equipment. Safety requirements)). You'd have to read the standard proper to confirm the details (which aren't freely available on the web) - but perhaps your local library could help with that.


    I wouldn't be too worried about the 'electronic' designs per se. The traditional switch-mode PSU normally had a transformer between the "mains" and ELV sides (if a much higher frequency, smaller and more efficienet one that would traditionally been the case) - which together with opto-coupling in the feedback loop which can provide isolation from the mains every bit as good as a traditional safety isolating transformer. Not all designs will provide safety isolation of course, but it is possible.


      - Andy.
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