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Hardware Thread, Power in suite keeps tripping! in Technical; I think 30mA RCD is too low for that many machines. The page linked to by @plexer shows that you ...
  1. #16

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    I think 30mA RCD is too low for that many machines.

    The page linked to by @plexer shows that you can have up to 3.5mA leakage per device and it's considered OK; with 34 machines you've got 68 devices so even if they each only have leakage of 0.5mA you're going to trip.

    Murphy's law says that you get more leakage as the machines are switched on; it then drops to a lower value - this is why you can switch them on one at a time without problem.

    You either need to increase the trip value for the RCD or split the circuits so that you have multiple 30mA RCDs (eg 1 per 10 machines)

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    We get it in one suite after holidays. I put it down to the lack of heating and the computers getting cold and damp. If I stagger the switch on it's ok.

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    Andrew_C's Avatar
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    Not quite clear here; is it the RCD (far left) or the MCBs that are tripping?

    I would concur with those above that a single RCD is wrong for that many PCs, but find it hard to explain why it trips overnight. Unless the cleaners' vacuum is the final straw?

    Some bios have a facility to wake on power restoration, which whilst not curing the problem, could make your life a bit easier.

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    Quote Originally Posted by srochford View Post
    I think 30mA RCD is too low for that many machines.

    The page linked to by @plexer shows that you can have up to 3.5mA leakage per device and it's considered OK; with 34 machines you've got 68 devices so even if they each only have leakage of 0.5mA you're going to trip.

    Murphy's law says that you get more leakage as the machines are switched on; it then drops to a lower value - this is why you can switch them on one at a time without problem.

    You either need to increase the trip value for the RCD or split the circuits so that you have multiple 30mA RCDs (eg 1 per 10 machines)
    Ours have been changed to 100ma DCC not happy, but everything is earthed or double insulated and nothing is plugged or unplugged on a regular basis (read never).

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    If you've got a spare ups it may be worth setting that up on 1 pc and leaving that pc on 24 / 7. - It will then tell you the exact time the power cuts off, and will also tell you if you get any major fluctuations in voltage. (asuming its a reasonable ups!)

    Steve

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    Have had this kind of thing a few times now.
    First time was with a re-wired server room. Every night it would trip, in the end the then NM stayed here all night and waited for it to trip, seemed the draw that came from the backups starting and then copying to USB disks caused it. Got trip changed for slightly less sensitive one and things were fine (server room has been re-wired again since then and this was taken into account this time!)

    Second time was a dodgy PSU in a computer, it only tripped once though as the PSU gave up and it was during the day.

    I agree with the suggestions that each ring should have an RCD on it. That way you wont lose all your computers every time and it will then narrow down whats causing it (only one RCD should then go)

    Just my 2 cents

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    Just to muddy the waters further...
    when this became an issue in our suite, it turned out that the circuits weren't as 'dedicated' as I thought.

    One ring had been expanded to put a socket in a room next door... and the problem was in there *harrumph*

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    Cheers everyone for the helpful comments so good points to take home. Couple of bits I want to clarify.

    Quote Originally Posted by robk View Post
    Looking at that board, all of the leakage current for all of the machines is being measured by that one RCD. This lead to nothing but problems with us.

    Leaking CRT monitors, power supplies and simple filtered sockets all add up to a large background level of earth leakage. Anything slightly off will push it over the edge.

    We ended up fitting RCDs per bank of sockets, but its not a cheap fix.

    Problem is, turning off banks of machines will drop the "background" level, and hide the issue!

    Try unplugging a system (at the wall) one at a time, see if it stays on over night?

    RobK
    So when trying to find the fault I can only really do 1 at a time as turning off a whole bank makes the test a bit biased. I thought this in my head but wasnt sure if I was just making it up as when trying the method below never had a trip!

    Quote Originally Posted by Gerry View Post
    Turn everything off at the plug except 1/2 the monitors. If it trips you know which 1/2 is causing the problem. Repeat until you're left with the faulty monitor(s).
    Im not sure if this method will properly work as stated above. If anyone could give a definate answer it would be helpful just to clear things up in my head.

    Quote Originally Posted by srochford View Post
    I think 30mA RCD is too low for that many machines.

    The page linked to by @plexer shows that you can have up to 3.5mA leakage per device and it's considered OK; with 34 machines you've got 68 devices so even if they each only have leakage of 0.5mA you're going to trip.

    Murphy's law says that you get more leakage as the machines are switched on; it then drops to a lower value - this is why you can switch them on one at a time without problem.

    You either need to increase the trip value for the RCD or split the circuits so that you have multiple 30mA RCDs (eg 1 per 10 machines)
    I think this is the route we will take when I get some more money to play with. Add an RCB for each row rather than the lot. Just out of interest is this a quick job for a spark it would it take some time, I know the rcbs themseleves are a few quid each.

    Quote Originally Posted by Andrew_C View Post
    Not quite clear here; is it the RCD (far left) or the MCBs that are tripping?

    I would concur with those above that a single RCD is wrong for that many PCs, but find it hard to explain why it trips overnight. Unless the cleaners' vacuum is the final straw?

    Some bios have a facility to wake on power restoration, which whilst not curing the problem, could make your life a bit easier.
    Yep sorry its the rcb not any of the mcbs they stay where they are. I have now turned the function on all the bios's to save me some time.....theres something about flicking a switch which turns on multiple appliances which I just dont like though.....

    Quote Originally Posted by steveg View Post
    If you've got a spare ups it may be worth setting that up on 1 pc and leaving that pc on 24 / 7. - It will then tell you the exact time the power cuts off, and will also tell you if you get any major fluctuations in voltage. (asuming its a reasonable ups!)


    Steve

    Thats a spot on idea will fish out the one old ups we have and give it a bash to see what happens.

    Quote Originally Posted by elsiegee40 View Post
    Just to muddy the waters further...
    when this became an issue in our suite, it turned out that the circuits weren't as 'dedicated' as I thought.

    One ring had been expanded to put a socket in a room next door... and the problem was in there *harrumph*
    This is definately not the case...although of topic but upstairs in 1 of our classroms the people that installed 1 of the projectors/audio managed to install the switch for it in the classroom next door

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    Quote Originally Posted by titch View Post
    I think this is the route we will take when I get some more money to play with. Add an RCB for each row rather than the lot. Just out of interest is this a quick job for a spark it would it take some time, I know the rcbs themselves are a few quid each.
    What you would need are RCBOs, which are combined RCDs and MCBs. Not cheap, but they are simple to fit and you won't need a new board.
    something about flicking a switch which turns on multiple appliances which I just don't like though.....
    Nothing wrong with that, unless you have a problem with the inrush current. This could cause fast acting (Type B or C) MCBs to trip. We suffered this, and had to put time delay contactors on 2 of the 3 circuits we have centrally switched.

  10. #25
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    Would pat testing all my equipment help or would this be a waste of time, as Im not exactly sure what the a PAT tester tests for?

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    You don't LEGALLY need to PAT (Portable Appliance Test) but you MUST have a way to prove your equipment is safe. PAT is one easy method to do this.

    Using an appropriate tester (not a cheap pass/fail model) you could measure the leakage from each PC/monitor. This would enable you to calculate the total leakage for the suite. If this is over, or even near, 30mA then that would explain your problem.

    The 30mA/30mS is a level where you shouldn't die if you are connected to the mains. Following a risk assessment, you may be able to de-rate the device. Depends largely on the wiring and kids in your school. Our electricians were of the opinion that we don't need RCD protection at all in the suite.

  12. #27
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    This problem means you must have some sort of earth leakage. We had this same problem and we had similar issues when staff were getting static shocks when touching a light switch. We had to get electricians in to resolve it.

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    ALL PCs leak to earth. This is in the very nature of the power filtering used. The only question is how much, and how many you have on the same RCD.

    Static shocks are entirely different and will be related to the type of carpet fitted, and the humidity.

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    I think 17th Edition of the regs calls for rcd on all sockets?

    As said above all pc psu's leak to earth some more than others and some more so when they get older as well.

    Ben

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    Quote Originally Posted by plexer View Post
    I think 17th Edition of the regs calls for rcd on all sockets?
    As I understand it, only those over 32A or likely to be used outside the zone of equipotential bonding, but I will have a look later to confirm.



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