Was looking into an issue of a shock* reported from a SELV system, the supply is of switchmode design. Approx 90v-ac was registered on digital multimeter between metalwork associated with the SELV side and the earth from a socket. Insulation test across the power supply comes up as >1000 and the problem definatly stops when the power supply is turned off at the spur, so looking at capacitive leakage across it rather than insulation breakdown, Now I know switchmode power supplies can leak a bit due to filter capacitors etc, but never come across one though could result in a perceptable shock* before. The problem can be made to disappear by earthing one side of the ELV system but not really a proper solution. Using the meter on current as the link to earth I measured a current of approximatly 273 MICRO amps which of course is very low current, but I think too high for this, even though I was always taught that one milli-amp was the threashold of being able to perceive it. Came across a document that suggested 250 micro-amps was the limit for this leakage (dropping to 100 for medical devices). But unsure where in the world the document came from. Tested a few other SELV power supplies and the highest I could get was 63 Micro-amps. The same brand supply as the one giving the issue is installed at serveral sites and they all give the same results, so it seems to be a poor design, rather than a failure.
So it seems that the supply is leaking quite a bit more across it than is nice, and a little higher than the top end of what is probably considered acceptable. Hopefully a linear transformer based supply is being sourced at the moment to deal with the issue, the supply is the standard boxed 12V 1A security psu with backup battery, type and up until recently the majority of them were linear, never gave any problems and rarely failed. Are we now expected to accept poorly designed switching power supplies that cause issues like these and probbaly dont last as long, in order to make the whole thing £10 cheaper?
*Shock is a bit strong, it might be dependant on person, the report was a shock, I can perceieve there is something there, and a colleague cant feel anything........ to me Its a similar level to if you have an open cpc on a circuit with a brass lightswitch, with the open circuit bit of cpc capacitivly coupled to the cirucit conductors, the surface of the switch takes on an almost rough feeling, its not painful, or even uncomfortable, but can be felt and will light a volt stick.
So it seems that the supply is leaking quite a bit more across it than is nice, and a little higher than the top end of what is probably considered acceptable. Hopefully a linear transformer based supply is being sourced at the moment to deal with the issue, the supply is the standard boxed 12V 1A security psu with backup battery, type and up until recently the majority of them were linear, never gave any problems and rarely failed. Are we now expected to accept poorly designed switching power supplies that cause issues like these and probbaly dont last as long, in order to make the whole thing £10 cheaper?
*Shock is a bit strong, it might be dependant on person, the report was a shock, I can perceieve there is something there, and a colleague cant feel anything........ to me Its a similar level to if you have an open cpc on a circuit with a brass lightswitch, with the open circuit bit of cpc capacitivly coupled to the cirucit conductors, the surface of the switch takes on an almost rough feeling, its not painful, or even uncomfortable, but can be felt and will light a volt stick.
