MCB

Discussion in 'Electrics UK' started by Grebe, 4 Jul 2004.

This topic originated from the How to page called Miniature circuit breakers (MCBs).

  1. Grebe

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    I have a consumer unit protected circuit in the garage which is fed from a 16amp MCB from the house consumer unit. When using a power saw in the garage the house MCB trips but the garage 16amp one dosn't. Anyone any ideas? the electics are about 18 months old.
     
  2. breezer

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    dont use a power saw.

    i am wondering is it an rcb in the house or an rcbo

    is there an rcd in the garage?
     
  3. brown-nought

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    What type of power saw are you using - is it a workshop type with an induction motor??

    It would be helpful it you could describe it and how it is connected a but further.

    Does it trip immediately of after a few seconds/minutes?

    Induction motors can have large in rush currents and need special starters circuits. Are your MCB's both type B's or is the garage on a type "C"?

    Also its worth noting that having wo 16a breakers doesn't provide any discrimination - i.e. ensuring the smallest MCB/fuse trips first. (Obviously don't up grade the house MCB if the circuit hasn't been designed for a higher MCB).
     
  4. Grebe

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    Thanks for the replys, The saw is a comercially available diy type connected via a 13 amp plug. The saw will run, then when I restart it it trips out.
    It is portable and I have used it from an extension lead from the house circuit without any problem. i am begining to think that the MCB is over sensative. Both house and garage CU are fitted with type B MCB's
     
  5. brown-nought

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    Ok, if it’s a DIY type it’s unlikely to be an in rush current problem. Type B MCBs are normal in a house.

    Few more questions:-

    1) What’s the power of the saw?
    2) What fuse is in its plug?
    3) Does it only trip on a restart – always?
    4) Do you run it for long or short periods?
    5) Has it stopped spinning before you restart?
    6) Does the 16A house MCB trip with other similar loads in the garage – other power tools for example?
    7) (Sorry for this one but) you are sure the house MCB is 16A not 6A?
    8) What make is the house MCB?

    It could be a faulty MCB – the best way to test this is to swap it for another 16A MCB – however without knowing your (or who else is reading this) electrical skill/competency level I would advise being very careful if you undertake a swap. If you are at all unsure get a professional in.

    Ideally, swap the house MCB for another 16A MCB – if you have a spare / alternative in the consumer unit swap them. If the garage unit is the same brand you may be able to swap the two MCB's over. (Or you could buy a new one of course) However only swap the MCB for another 16A B type.

    If the problem stops (or follows the MCB swapped to the garage) suspect its faulty. (You can only be sure if the suspect MCB is not in the circuit – if its put in the garage it could still indicate another problem if it trips). If it’s faulty I’d bin the suspect MCB.

    However, If the problem continues I’m not so sure what the action to take beyond getting the saw tested.

    I hope this is helpful – however as I said, please be careful if you start swapping MCB’s. Consumer units have permanently live parts even after the isolator/rcd are turned off. It’s a bit like changing a blade with the saw still plugged in.

    BN.
     
  6. Grebe

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    Brown-nought
    In reply to your questioons
    1. 1600W
    2. 13 amp
    3. Sometimes but never on initial start up
    4. It's a chop saw so it only runs for a few seconds each time
    5. Yes, the blade has stopped on each occasion
    6. No, nothing else has caused the problem
    7. House is a Crabtree. with split load RCD. MCB is 61/B16 16A
    Garage is a Legrand with B16 MCB for the sockets and B6 for the lights plus 63A RCD.
     
  7. brown-nought

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    Hi Grebe,

    Thanks for the answers - nothing apart from the 2 16A MCB's we discussed before stands out.

    MCBs trip for two reasons

    1) Fault current – short circuit etc – quick bang.
    2) Overload – circuit demanding too much power for the protection device can take time to trip fior small overloads.

    Apart form checking the House MCB, as we discussed before, the only other thought I have had is to check the overload scenario. What else if anything is drawing power through the house 16A MCB? Is it supplying anything else ... immersion heater, other garage appliances etc?

    Two ways to test (1) switch it off and see what else goes off ... (2) if you are electrically competent and own a clamp meter check the ambient current flow.

    I’ve done a quick overload calculation based on general MCB characteristics.

    16A MCB (ln = 16) assume already overloaded to 113% (trip > 1 hour) gives 18A. (i.e. a near worst case scenario 1.45 trip in under 1 hour is 23A by the way.)

    Continuous Saw rated at 1600W =~ 7A.

    To trip in less than 10 seconds you would need 2.5-4 x ln the rated load (24-64A)

    Therefore at 24 < 18 + 7 (just)

    At 48 (3xln) 48 – 18 needs a surge of 30A to trip.

    I cannot find a time current curve for a plug fuse to BS1362 so I’ve no idea if a 13A would hold under a 30 amp surge?

    Assume small load 6A lights

    6 (lights) + 7 (saw) = 13A

    24 > 13.

    Surge needed is 48 – 6 which is 42 A – seems too big for a 13A fuse (~3.23x ln)


    Overload theory may hold up if the circuit is already loaded. If you only have the Saw and say 100w of lights then I cannot see the house MCB would trip due to an overload. Therefore it could only be tripping due to a circuit fault current or faulty MCB.

    Bottom line is:-

    Check the house 16A circuit for overload.
    Swap/test the MCB.
    Have the Saw tested

    May be worth calling in a professional if you get stuck.

    Good luck.

    BN
     
  8. guymark

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    I realise the topic is old but others might still be finding it (as did I) through Google and still have an interest.

    I had a similar sort of system with a 20A MCB in the workshop then feeding via SWA a remote "woodshed" which has its own little RCD and MCB setup.

    In fact the whole chain is like this: HOUSE 50A type C > Workshop 20A MCB > Woodshed 16A MCB

    When I plug in a 3KVA site isolation transformer I used to find it would sometimes work fine, sometimes trip the 20A MCB (but not the 16A) and sometimes (rarely) just trip the 16A. (Different makes, Wylex and Square D)

    I swapped in other breakers but ended up with type C breakers for the same current rating and the problem vanished.

    I accept that a 3KVA site transfomer isn't exactly "domestic" but it is also hardly a "trade only" device as many decent DIY tools now come in 110V flavours.

    I know the inrush currents while the transformer mags-up can be quite dramatic - and it seems that properly rated type-B breakers really are at their limit in handling it. Type C seems to take it in its stride.

    The only other device I have had problems with tripping, is "Fluffy" - a class IV argon ion laser which ticks over at 30-35A or therabouts. the problem is that the inrush current is massive - two HUGE toroidal transformers on the power supply. This was why I ended up having a 50A type C in the house to power the workshop - as otherwise it would trip 4 times out of 5 when the laser was initially switched on.

    In the short term (waiting for breaker to arrive), I ended up with a very crude but effective "starter" consisting of nothing more elaborate than a 1KW floodlight connected over the laser's power switch via a push button. To switch the laser on without tripping a breaker you start by pressing and holding the button - light briefly tries to flash bright and then instantly dims, then you throw the main power switch on the laser and start to power the rest of it up. It just means that the floodlamp limits the inrush current to magnetise the transformers which have no load on them yet - just magnetising current - then you can proceed as normal.
     
  9. securespark

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    Yikes! What's the cable length, csa and Zdb like?
     
  10. guymark

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    Through the house, the cable is 10mm, then it is 6mm SWA for about 8 metres to the workshop where it goes into a split load CCU.

    All workshops stuff is on RCD but woodshed feed is not - the CCU feeds the woodshed on a 4mm SWA cable, via a 20A MCB.

    This cable is about 10 Metres long. In the woodworking shed, there is a 16A breaker for sockets and a 6A breaker for lights with an RCD overlooking both.

    Naturally it was all done before the requirements of TimeWastePart-P but JUST in time to make use of the new colours :)

    Nothing seems to get even slightly warm even when Fluffy is on, so seems safe enough :)
     
  11. slup

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    You shouldn't have the lights and power on the same RCD in your woodworking shed.

    If you cut through a cable, you will be left in complete darkness with a spinning machine in your hand...
     
  12. guymark

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    Thank you.

    That IS a fair point.

    All machines are fixed except 110V ones but nonetheless if it trips for any another reason your point is right.

    Will give it some thought.

    Kind Regards
    Mark
     
  13. ban-all-sheds

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    Disconnection times?
     
  14. guymark

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    Well the laser pulls between 30 and 35A - and as it is normally only run at about 1 watt output, tends to be around the 30-31A range - It connects Via a 32A Ceeform coupling and a 32A type C breaker - Type B simply trips out most of the time it is used - unless I do the "soft-start bodge".

    Sure, I would prefer a type B breaker for a faster disconnect - but 32A C seems about the best that I can use. The RCD is a 30mA unit which in fact trips at a little under 21mA if memory serves me.

    Always happy to learn from those who know better though. If you can think of a better solution than a 30mA RCD / 32A type C MCB to protect a unit pulling 30A or so with very high inrush I would be very happy to hear your thoughts.

    For now, the system seems safe, nothing has a breaker larger than it needs, nor a type C unless a type B trips when otherwise within rating.

    All underground cable is SWA and despite TNCS, there are also several copper grounding rods at the workshop (for other purposes but link to the CCU anyway) .

    It seems a safe installation to me but constructive criticism always appreciated - eg: the fellow who made the suggestion about taking the lighting off the RCD.

    Regards
    Mark
     
  15. ban-all-sheds

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    Is the fault loop resistance under the limit for a C32?

    If it is, then it's fine, if it's not then it's not. An RCD should not be used as a band-aid to cover up an underlying unsatisfactory circuit.
     

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