Supply failure warning device

Hi All,

The RCBO that supplies the compressor in my treatment plant tripped randomly the other day. As I have been doing some maintenance on it and subsequently monitoring it I noticed quite quickly. This was the first time it had tripped since installation 20 months ago and hasn't tripped again.

The RCBO is in a CU in the detached garage which itself is supplied via sub main direct off the incoming supply to the house.

So I thought some sort of monitoring device might be prudent. Originally I thought I could utilise a non maintained emergency light that I have kicking about but realised it wouldn't have enough life if the supply failed at night. I go into the garage daily but not necessarily first thing. Has anyone any (low cost) suggestions?

I'm thinking maybe batten lampholder, red lamp, supply from garage lighting circuit and, what, an NC relay with a supply from the outgoing terminals of the plant's RCBO?

We're not talking life critical, hospital standard here, but if the supply's off too long my poor little microbes suffer!

Thanks for any suggestions!

  • Do you want something that raises an alarm back at the house ? I have used a garo no/NC contactor for this sort of thing - slightly wasteful in that the contactor is on all the time - well most of  the time, but easy in that it easily wires into the Wylex CU off the end of the bus-bar (cue booing and hissing from the purists who will claim the CU is now a death trap as it is not a Wylex contactor - but they do not make one with that contact configuration) and then a pair of wires can be brought out and the  contacts can be used for almost anything - in my case shorting two spare wires in the armoured CAT5  cable that accompanies the SWA down the garden and the rest of the alert is at my end.

    I have also in another life connected a small siren and  car battery to a modified  EM light fitting to warn of generator failure, but that was very much a case of what we had to hand at the time, which was pretty much that.


  • If you want to avoid tapping into another circuit, you could get your NC relay to trigger an LED lantern running of some decent sized alkaline batteries.  Modern LEDs only need a few milliamps to produce a very bright light.

  • Hi Mike, thanks for that. The garage is only 30m from the house and one of the windows is clearly visible from the kitchen so a simple red light in the garage window would suffice. I'm probably being a bick thick but wouldn't that Garo unit monitor the whole busbar whereas I only want to monitor the output of a specific rcbo.

  • Thanks Simon. Have you seen the price of batteries!! Tapping into the lighting circuit in this instance is quite simple though.

  • Depending on the RCBO, and space in the CU, could you use an auxiliary contact side-mounted on the RCBO itelf (example)? Doesn't tell you if the supply's lost due to an upstream device of course.

    What you do with it then is another matter. Perhaps binary input to an RPi for a rudimentary logger, and perhaps email notification if you can get wifi? Or just use it to switch a lamp in the window on your lighting circuit.

  • Nice one Jam, never heard of those before. Just checked and Lewden do them so I think that's the way to and no coils permanently energised!

    Thanks all.

  • No worries (c: I use them quite a lot for indication to SCADA systems on remote plant, which is pretty much what you're doing.

    Do check when you order which type you're getting: Some indicate Open/Closed, some Open/Tripped and some are selectable / both. Plus obviously it needs to be compatible with the RCBO it's fitting to.

  • mine is on wires - the bus bar is cut short ;-)

    Its my own property....


  • The sister unit to that sort of  aux contact    is the one that links up to allow you to force  trip with a small solenoid inside - the 'shunt trip coil'  Useful to kill supplies when flooding is detected or the fire alarm goes off or similar.
    (Wylex do not make those either....)


  • I'm thinking maybe batten lampholder, red lamp, supply from garage lighting circuit and, what, an NC relay with a supply from the outgoing terminals of the plant's RCBO?

    Hm, whatever would the neighbours think of a red lamp in the garage? ;-)

    The simplest way would be to have a little green light connected to the circuit: when it is on, all is good.

    An alternative would be to mount a small contactor/relay in a suitable box so that if the power goes off, a lamp (or siren, etc.) comes on. It does not have to be in the CU, but if there is space in there, one mounted on a bit of DIN rail would be acceptable.