RCD trips randomly !!!!!

Discussion in 'Electricians' Talk' started by moz, Nov 19, 2012.

  1. moz

    moz Member

    I have a 12 way MK Sentry Consumer Unit (Split Load) which has an 80A 30mA RCD supplying 3 MCBs (40A, 2 x 32A). It was installed by a friend who was a qualified electrician (now sadly dead) about 2006/7.

    The RCD has just started randomly tripping. Approximately once a day, at no particular time, or if anything has just been turned on/off. For example the last time it happened was during the night. (no boilers, or clock operated devices kicking in).
    When reset, everything is fine. But it reoccurs, maybe a day or so later.

    If there was an appliance or circuit fault, surely the RCD would just trip repeatedly. Which it doesn't. Are these RCD devices (MK Model No.5780S) prone to failing?

    On a previous occasion when it tripped, I was sitting at my computer, which is supplied by a multiblock that has surge protection. A fraction of a second before the power failed I heard a very short, quiet buzz, which I think could have been coming from the multiblock. I stress that I am only guessing where the sound came from, it lasted for a fraction of a section, and of course I can't replicate it. But was this a power spike, being handled by the surge protection unit. I don't know how they work. Would an incoming spike trip an RCD? If so, and I am getting spikes, who do I contact? My energy supplier (EDF) or what?

    I could get a spark to fit a replacement RCD, but they cost £35 quid (plus fitting), which is a lot of money for a pensioner and we're not even sure that's the problem.

    Thanks in advance for any suggestions/advice.
     
  2. bluetobits

    bluetobits New Member

    Finding the cause of an intermittently tripping RCD often requires professional test gear and experience. The tripping RCD indicates there is a fault and yes, the buzzing multiblock could be the culprit, especially if the spike detector has operated. They are usually only a 'one-time' device so it would be prudent to replace it anyway. Unplug it from the wall and see if it still trips. The RCD is unlikely to be faulty and is very likely doing the job it was designed to do. If this does not solve the problem you may have to call in a pro.
     
  3. wally

    wally Screwfix Select

    could be a problem to find & as blue says it may need further investigation by a decent sparky.   The random faults are the worst to find.    If different items seem to trip the RCD then you could be looking at a neutral to earth fault on any circuit controlled by the RCD as with this type of fault any current flowing from any item on any circuit could trip it, it does not mean that these items are faulty
     
  4. If it happens as regularly as every day or two, then it might be a good candidate for trying to track it down by a process of elimination? Starting with the computer and all its associated paraphernalia (yes, I had to look up the spelling - so what...). Unplug everything to do with the pooter from the wall. If you can, perhaps try and eliminate that whole room - speed things up?!

    Moz, can you do without your computer for a day or so? Or are you like the saddos on this forum...
     
  5. moz

    moz Member

    Thanks guys.
    Surge Protecting multiblock now removed. Watch this space.
    The more I think about it, the more I think that the short buzz must have come from it. My hearing isn't 100%, so it was unlikely to come from another room. I'm in a quiet room off a hallway. The hallway has no elecrical items, and just one twin power outlet. So no noise sources there. It had to be in the room I was in, which has no electrical appliances, other than the computer paraphernalia, via the 10 way multiblock.
    Anybody know how these spike arrestors work? Do they buzz briefly when activated. This one has a "Surge Protected When Lit" LED, but you're not likely to see that, unless permanently staring at it, or there are spikes coming thick and fast!!!!
    MORE IMPORTANLY - would "activation" cause an RCD to trip. And possibly equally as important, if it did activate, why am I getting spikes? And could they cause damage to other sensitive electrical/electronic items, e.g. tv, radio, CH Controller, or anything else that isn't powered via a transformer?
    Also, are they "one time" as suggested by bluetobits, or do they reset? All I know is that mine came from somewhere like Maplin, and it has SMU UK20810 on the back.
    If after removing, the problem is cured, do you think I should refit a new one? They're freely available on Amazon and the like (Belkin, etc.). Or are they one of those things that are just "nice to have", but retailers try to kid you are essential?
    Thanks again.
     
  6. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Anything on the same circuit could give you a momentary buzz on your SP. The most random thing that switches on and off, would be the fridge/freezer.
    You have to remember, that the surge protection is not monitoring what you have plugged into it, but monitoring the supplying circuit. Abnormality on the circuit may be picked up(even if it's not enough to activate it) and it may be the same abnormality that the RCD is seeing.
    I'd play with the fridge thermostat dial a few times(might have to wait about sometimes), see if I could get thr RCD to trip.

    Just saying.

    Mr. HandyAndy - Really
     
  7. moz

    moz Member

    Thanks HA. I've been keeping an eye on both fridge and freezer. They're both cycling on and off frequently.
    I've also changed temperatures - they're both Bosch Exxcel with push button selection. Also gas boiler cycles on and off. Unless one of these suddenly causes an unusual surge, I don't think they're the culprit. What would cause a fridge or freezer to do this?
     
  8. Dunno. Dodging smoothing caps on the motors?

    I wonder if, since the RCD supposedly trips at 30mA, there could be multiple circuits all contributing tiny leakages so's the overall residual is hovering chust below this 30mA level, and then the extra li'l spike caused by summat is enough to tip it over t'edge?

    I don't know if I'm talking wubbish?
     
  9. moz

    moz Member

    Not Surge Protector!!!! Occurred again in middle of the night. Nothing on, other than fridge and freezer, see above.
    Being a (retired) scientist, I am quite analytical and logical in my problem solving. So I thought about this strategy to attempt to identify which circuit (choice of 3) is possibly causing the tripping.
    I ask my friendly spark, to temporarily remove the RCD and run a full busbar from the DP isolator across all MCBS. Would the MCB on the errant circuit trip, if/when these 30mA hiccups occur? Or aren't MCBs sensitive enough? They certainly seem sensitive, when a bulb blows!!!
     
  10. Bazza-spark

    Bazza-spark Screwfix Select

    Hi Moz

    DO NOT remove the RCD.  It is there for your protection and if you remove it you run the risk of electrocution.

    The RCD will trip with an out of balance between the live and neutral conductors of 30 milliamps.  This limits any voltage rise on conductive parts such as water pipes, sinks etc to a safe level.  If you remove or bypass the RCD you will be relying on 6 amp or 32 amp MCB's to operate.  These will only operate on overload, not on out of balance so under fault conditions the conductive parts can become live at mains voltage and the results can be fatal.  (6 or 32 amps fault current instead of 0.03 amps).

    Contact your local reputable electrician.  He will be able to carry out some investigative work to try and locate the fault safely, and ramp test the RCD to ensure it is not faulty.  Insulation resistance testing the circuits may reveal some problem, and he will then be able to start tracking it down.  If all circuits are clear he may then be able to temporarily replace your MCB's with RCBO's before bypassing the RCD to track down which of the circuits is causing the problem. (An RCBO operates the same as an MCB and an RCD in 1 package.)  However there are many other things that can be carried out prior to this.

    Kind regards

    BS
     
  11. moz

    moz Member

    I'd only suggested he bypassed the rcd on a temporary basis, to maybe help identify which (if any) circuit is at fault. Didn't realise mcbs were that insensitive - why do they go on bulb failure?
    I fully understand the purpose of an rcd. Still waiting for my electrician to get back to me, for testing to commence. Tripping just once a day, it's going to be difficult. I like your idea that the rcd could be checked. I half suspected that immediately, but someone said they were reliable. The symptoms just started happening. No changes to wiring, etc. no new ( or ancient) appliances.
     
  12. Bazza-spark

    Bazza-spark Screwfix Select

    Hi Moz

    They go on bulb failure because when the element fails it can put a short between live and neutral temporarily.  This allows a very high current to flow for a short time exceeding the tripping current of the MCB.  Tended not to happen very often with rewireable fuses as they were not as sensitive.

    RCD's are reliable, but like anything mechanical can weaken or fail over time.  How often have you used the button marked "Press to test"?  It is recommended that this is done frequently however in the modern life with computers, electronic clocks, TV recorders etc people tend not to bother because they have to go round resetting clocks etc.

    There are 2 possible reasons for it tripping:-

    <ol>[*]The RCD is faulty - unlikely but possible.[*]There is an intermittent fault and it's doing what it is designed to do.</ol>
    With all due respect, let a qualified electrician sort it for you.  As a retired scientist you are obviously a very intelligent person, however this requires special skills so please leave it to someone with the correct training and test gear to carry out the work safely.

    Where abouts in the country are you?

    Kind regards

    BS
     
  13. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    Another possibility is MICE. Eaten the sheath and leaving wires perilously close to each other, many factors possibly bringing them together(in particular shorting to the earth) including the mice running past them.

    Mr. HandyAndy - Really
     
  14. moz

    moz Member

    My spark (son of a pal), came around. Did a couple of checks, not sure what; and suspects RCD (thanks MK). Which has now been replaced.
    Sorted - I hope.

    I think I have a moused cable that supplies hob and hood only (used also to supply oven). When the guy installed the consumer unit, this circuit kept tripping.  He therefore connected it to non RCD side of board, because it would have entailed ripping up most of the ground floor to access an integral single cable that runs across the concrete foundations from one corner of the house to the other.
     
  15. Mr. Handyandy

    Mr. Handyandy Screwfix Select

    "He therefore connected it to non RCD side of board, because it would have entailed ripping up most of the ground floor to access an integral single cable that runs across the concrete foundations from one corner of the house to the other."

    Very naughty/dangerous/possible 'present' fault(neutral to earth)

    Mr. HandyAndy - Really
     
  16. decreenisi

    decreenisi New Member

    Old thread reawakened. Have just started having occasional RCD trip problem. Happened 3 times in last 48 hours. Not obviously connected with turning anything on or plugging anything in. Twice this happened early morning when we're all in bed and the only things that could possible be to blame, I would think, would be fridge/freezer or boiler. Cant think of anything else that might come on of its own accord.

    I've read this thread and others and know I need to go through what could be a very tedious process of elimination assuming this is a faulty device I have plugged in somewhere. At present the lighting remains on but everything else goes. And that is very, very annoying.

    Saw a mention elsewhere of using plugin RCDs to try to identify a faulty device, benefit being that they would trip without taking the entire mains out. Is this a valid idea or am I better off just getting an electrician in straight off.

    Funny thing is I have had this problem a couple of years ago. And it just 'went away' before it got irritating enough to force me to get on and try to fix it.
     
  17. unphased

    unphased Screwfix Select

    As a slight digression, this just proves the point I make about dual RCD CUs not complying with 314.2. In the event of the RCD tripping several circuits are being lost and a fault on any one, or more than one, can be the cause. RCBOs limit the fault to just one circuit and makes fault finding a lot easier.
     
  18. FatHands

    FatHands Well-Known Member

    I'm inclined to agree with you UP. I know some houses are cabled up side - side (opposed to up and down) but this still doesn't get away from the fact that you could wake up in the night and not have any lighting because of a fault in the kitchen.

    Just boils down to cost. That said, big house, 10 minute walk to the board.... got to be RCBO's surely.

    Cheers
    Fats
     
  19. unphased

    unphased Screwfix Select

    Well, if people could get through the cost barrier and invest in an RCBO board it would certainly make life a lot easier all round. Price isn't everything. People will happily pay for RCBOs if the electrician takes the time and trouble to explain their benefits. I don't think I have installed a dual RCD board for the last two years. Denmans Curve 14 way complete with 8RCBOs £99 plus VAT. Not exactly breaking the bank. Probably £50 more than a cheap dual RCD. I have had no problems at all. Must admit, though, the quality of the casings has improved somewhat. They used to suffer from broken clips holding the earth and neutral bars in place but they have been strengthened and even include a spare in case.
     
  20. FatHands

    FatHands Well-Known Member

    I agree. Lots of people (understandably) moan about the inconvenience when an RCD board goes in and its then discovered that their hoover or freezer is on the verge of taking the RCD out in the middle of sunday dinner :"this never happened before"
    Residual protection from RCBO has to be the way to offer protection, minimal disruption and more direct troubleshooting for the electrician (and therefore cost saving in diagnostic time).
     

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