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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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  • 0
    Tim


    BS 7671 recognises various sources for SELV. It has to be "separated" and Extra Low Voltage i.e 50V or less AC.


    Safety isolation transformers to BS EN 61558-2-6 and BS EN 61558-2-8 are in my view the safest as the electronic type SELV sources being electronic will probably fail short circuit. Below is the actual wording from BS 7671.


    The following sources may be used for SELV and PELV systems:

    (i) A safety isolating transformer in accordance with BS EN 61558-2-6 or BS EN 61558-2-8

    (ii) A source of current providing a degree of safety equivalent to that of the safety isolating transformer specified

    in (i) (e.g. motor-generator with windings providing equivalent isolation)

    (iii) An electrochemical source (e.g. a battery) or another source independent of a higher voltage circuit (e.g. a

    diesel-driven generator)

    (iv) Certain electronic devices complying with appropriate standards, where provisions have been taken such

    that, even in the case of an internal fault, the voltage at the outgoing terminals cannot exceed the values

    specified in Regulation 414.1.1. Higher voltages at the outgoing terminals are, however, permitted where, in

    case of contact with a live part or in the event of a fault between a live part and an exposed-conductive-part,

    the voltage at the output terminals is immediately reduced to the value specified in Regulation 414.1.1 or

    less.

    NOTE 1: Examples of such devices include insulation testing equipment and monitoring devices.

    NOTE 2: Where higher voltages exist at the outgoing terminals, compliance with this regulation may be assumed if the

    voltage at the outgoing terminals is within the limits specified in Regulation 414.1.1 when measured with a

    voltmeter having an internal resistance of at least 3 000 ohms.
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  • 0
    Tim


    BS 7671 recognises various sources for SELV. It has to be "separated" and Extra Low Voltage i.e 50V or less AC.


    Safety isolation transformers to BS EN 61558-2-6 and BS EN 61558-2-8 are in my view the safest as the electronic type SELV sources being electronic will probably fail short circuit. Below is the actual wording from BS 7671.


    The following sources may be used for SELV and PELV systems:

    (i) A safety isolating transformer in accordance with BS EN 61558-2-6 or BS EN 61558-2-8

    (ii) A source of current providing a degree of safety equivalent to that of the safety isolating transformer specified

    in (i) (e.g. motor-generator with windings providing equivalent isolation)

    (iii) An electrochemical source (e.g. a battery) or another source independent of a higher voltage circuit (e.g. a

    diesel-driven generator)

    (iv) Certain electronic devices complying with appropriate standards, where provisions have been taken such

    that, even in the case of an internal fault, the voltage at the outgoing terminals cannot exceed the values

    specified in Regulation 414.1.1. Higher voltages at the outgoing terminals are, however, permitted where, in

    case of contact with a live part or in the event of a fault between a live part and an exposed-conductive-part,

    the voltage at the output terminals is immediately reduced to the value specified in Regulation 414.1.1 or

    less.

    NOTE 1: Examples of such devices include insulation testing equipment and monitoring devices.

    NOTE 2: Where higher voltages exist at the outgoing terminals, compliance with this regulation may be assumed if the

    voltage at the outgoing terminals is within the limits specified in Regulation 414.1.1 when measured with a

    voltmeter having an internal resistance of at least 3 000 ohms.
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