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Detecting a faulty diode in a bridge rectifier?

Discussion in 'Electrics UK' started by ericmark, 18 Jan 2021.

  1. ericmark

    ericmark

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    So with a switch mode power supply rated 100 - 250 volt if a diode fails with a 230 volt supply it seems likely it would still work, but would be passing DC through any RCD, so when PAT testing today how do you ensure there is no DC which will stop an RCD working?

    Do modern PAT testers detect this?
     
  2. JohnW2

    JohnW2

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    1...You're lucky that BAS is nor around since, if he were, you would probably be "sent to the headmaster" for starting a question with "So" :)

    2...I Suppose the 'bureacratic' answer is probably that (I presume) current PATesting 'rules' do not require such a test (so "modern PAT testers" would presumably not implement such a test), which probably makes the question currently moot.

    3...I presume that you are thinking of one of the diodes failing 'open circuit'. If it failed 'closed circuit', the full input voltage would appear across one of the other diodes, in which case there would be a bang, after which the PSU would not work at all.

    4...I'm not so sure that the PSU would necessarily still 'work' (in a usable fashion) with one of the diodes O/C. In that situation, the PSU (including it's oscillator) would only be powered during every other half-cycle of the 50Hz input. The output of the PSU would therefore have a massive 50Hz component, and (if the output were DC) I presume that there would not be any smoothing on the output that had much effect at 50Hz. I suppose that whether or not the result would be perceived as 'working' would depend upon what the PSU was supplying.

    5... Unless you have managed become better informed than I have so far managed, we don't seem to know how much of a DC component is necessary to "stop an RCD working" (I presume you mean a 'Type AC' RCD) - and I would imagine that the total current drawn by most small SMPSUs would very probably be such that any degree of DC component thereof might not be enough to significantly impair the functioning of an RCD.

    I suppose the bottom line is (1) above - that you probably should not worry about a hypothetical aspect of PATesting which (I presume) is not currently 'required'.

    Kind Regards, John
     
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  4. davelx

    davelx

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    Would such a failure be dangerous? If not, then it is of no relevance to PA Testing. As I'm sure we all know, PAT is to do with safety, not usability/functioning
     
  5. JohnW2

    JohnW2

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    Eric's concern is that if a fault such as he described resulted in a significant DC current flowing in the mains supply, that might impair the functioning of a ('Type AC') RCD, such that the RCD might not function as it should in response to a fault in something else (or someone receiving an electric shock from something else) - which potentially could be dangerous.

    As I've written, whether it is a significant 'real cause for concern', I don't know.

    Kind Regards, John
     
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