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SELV Fusing

Martyn

Hi All,

I recently had a discussion with regard to fusing SELV lines, traditionally I always fused the +ve line feeding equipment.

A colleague recently mentioned that he had read an article from Sage? Arguing that both lines should be fused, from memory this ties up with 60204:2019.

A bit of thought suggests this would be wise as obviously with SELV there is no ref to earth for ADS. 

I cannot find the article, can anyone point me in the direction of some relevant info.   

Thanks

Martyn

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    As soon as one side is chassis, or ground, it is not SELV, but PELV.  Generally vehicle and trailer electrics do use a chassis return, and there is very clear sense of the other one being the side you fuse or switch.

    Also in some systems it is safer to not fuse and to allow it to run to failure - both high and low power systems.

    As an example of low power, most ELV bathroom fans are not fused for example on the ELV side, rather there is a one time thermal cut-out in the transformer primary.

    As a common  example of unfused higher power consider the car or lorry starter motor - there is no sensible fuse size that will give much  better protection than the motor windings melting, and that is safely contained,  so that may as well be the limiting factor.

    Mike.

Reply
  • 0

    As soon as one side is chassis, or ground, it is not SELV, but PELV.  Generally vehicle and trailer electrics do use a chassis return, and there is very clear sense of the other one being the side you fuse or switch.

    Also in some systems it is safer to not fuse and to allow it to run to failure - both high and low power systems.

    As an example of low power, most ELV bathroom fans are not fused for example on the ELV side, rather there is a one time thermal cut-out in the transformer primary.

    As a common  example of unfused higher power consider the car or lorry starter motor - there is no sensible fuse size that will give much  better protection than the motor windings melting, and that is safely contained,  so that may as well be the limiting factor.

    Mike.

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