Type B v Type C MCBs

Discussion in 'Electricians' Talk' started by Mr Spark, Jan 26, 2004.

  1. Mr Spark

    Mr Spark New Member

    Can anyone tell me the difference between a Type B and a Type C MCB ? Will a Type C MCB work in a consumer unit that has Type B MCB's installed in it, or are they totally two different sizes/spec ?
     
  2. hillheader

    hillheader New Member

    Type C MCBs take longer to trip than type B. They are genrally used for inductive loads such as motors, and discharge lighting where there are high start-up currents. For a domestic consumer unit, most of the MCBs will be (and should be) type B. Domestic exceptions are rare.
     
  3. Mr Spark

    Mr Spark New Member

    Top man !

    Many thanks.
     
  4. ban-all-sheds

    ban-all-sheds New Member

    Domestic exceptions are rare.

    Until, that is, you get totally hacked off with your MCBs tripping every time a bulb blows and you decide to fit Type Cs.

    And before anybody starts bleating on about disconnection times, remember it's 5s for a lighting circuit....
     
  5. Milton Bradley

    Milton Bradley New Member

    However you cannot get Type C MCB's for Newlec or Volex Consumer Units, (as I hear), so if thats the case, your'e stuck with the Type B MCB's.
     
  6. ban-all-sheds

    ban-all-sheds New Member

    Wow - another reason for not using Volex.
     
  7. Damocles

    Damocles New Member

    Still trying to figure the practical difference between a 6A mcb type B and type C. Given an existing circuit using 1.0mm cable, just which sort of faults are better protected by "B" than the "C" breaker. C has a higher 'must break' current, but does it make much difference in practice? the short circuit current would be enough to trip it in either case. Using type C should give better protection from nuisance tripping when a bulb goes.

    I am beginning to wonder if the thinner cable might actually be safer than a circuit wired in 1.5mm, since its higher resistance will better limit the short circuit current.
     
  8. The Trician

    The Trician New Member

    So long as it complies with the regs and meets the 5s disconnection time:)
     
  9. Brightspark

    Brightspark New Member

    With an MCB (BSEN60898) the loop impedance value's are the same for both 0.4 and 5 secs disconnection. However when using a 'c' type breaker the loop impedance is half the value of the 'B' type i.e. 32amp 'B' type limit is 1.5ohms, 'C' type limit is 0.75ohms.
    So in other words to still comply with the 5sec or 0.4 sec disconnection time you need to be in these limits.
    On the other hand for a 6amp MCB a 'B' type is 8ohms and a 'C' type 4ohms so for a domestic installation you would need a very long lighting circuit to be over the 4ohm limit and in most cases you would still comply.
     
  10. Damocles

    Damocles New Member

    I reckoned anything less than 50m of 1.0mm cable would be fine,which covers most houses. So why does anyone use type B at all for lighting if C will reduce nuisance tripping?

    The current/trip time curves I was looking at suggested that these devices will trip within 5 seconds at lower currents than needed for instantaneous trip. I wouldnt do it, but you might use one in a circuit where the impedance was too great to trigger fast trip, but would still satisfy.
     
  11. Earthman

    Earthman New Member

    So the $64K question - has anyone on this forum actually fitted Type Cs to their own lighting circuits and seen this as a cure to nuisance tripping or are we all talking theory. ;)
     
  12. Damocles

    Damocles New Member

    I notice someone said always fit 10A/1.5mm lighting circuits in new installations. How is nuisance tripping on these too? type b 10A has characteristics more like type C 6A as far as resistance to surges is concerned.
     
  13. supersparky

    supersparky New Member

    To be honest unless you have your circit near the max, ive never herd of a 10amp tripping on a light blowing.....but then ive only fitted 10amp as std for a short while...

    ss
     
  14. ban-all-sheds

    ban-all-sheds New Member

    My 6A Type 1s trip if you so much as look at them sideways.....
     
  15. Damocles

    Damocles New Member

    type 1? that should be even worse than type B for low current tripping. I think we are getting to the secret here finally. The typical current surge when a bulb blows must be around the trip value for 6A type B... Higher nominal current breakers will have a highter surge resistance even when type B. A 6A B plainly is unsuitable for an incandescent bulbed lighting circuit. So why doesn't someone acknowledge this formally in the regulations?

    That leaves everyone who has already got 1mm wiring and 5A semi enclosed fuses, which are changed to 6A B's. Still seems to me type C's are preferable and (in general,with all the normal provisos) perfectly acceptable. Any suggestions?
     
  16. The Trician

    The Trician New Member

    Just pondering a point - the role of the mcb is to protect the cable here isn't it? So its a case of checking much current it takes to trip a 6A type B mcb and then compare this with the rating of 1.00mm for example. Can the cable withstand the potential fault current without its insulation being damaged? I haven't checked the time/current graphs in the regs but thats where the answer should lie.:)
     
  17. raystock

    raystock New Member

    The on site guide table 7.2B p 51 gives guidence to the application of BS EN 60898 etc protective devices
    Hope this clarifies the situation
     
  18. Damocles

    Damocles New Member

    Rather depends how long the cable is. larger the cable, the lower its resistance so the bigger the current. Reckon that means that for the same duration of current and length of cable, it will get to exactly the same temperature. But then a thick cable will have more stored heat. Take longer to lose its heat giving more time for the insulation to flow and fall off.

    The shorter the cable, the bigger the current will be. 10cm of 1.0mm might even start to glow? I assume 10m should be just fine, but I also do not know....lets see... 10m x1.0mm might be about 500Amps for the duration of the fault,so 10cm would be 50,000 Amps? Guess not. would be limited by supply impedance. if it trips in 0.01 seconds then thats equivalent heating to a current of 5A for 1 second for the 10m cable. Guess it could withstand that. Reckon it would survive 50A for 1 second too. Not sure whether a 1.0mm cable can withstand 500A for 1 second, would have to do some maths on heat capacity. So i would not advise having a circuit shorter than 1m.
     
  19. limestone cowboy

    limestone cowboy New Member

    I've just put my fork truck charger on a Type D because it tripped Type B and C. Is this OK?
     
  20. The Trician

    The Trician New Member

    I've just put my fork truck charger on a Type D
    because it tripped Type B and C. Is this OK?

    Depends upon the size - cross sectional area of the supply cable to the charger and also the protective device you have used to supply is with - see above post(s)

    What size and type of cable have you used to supply the charger and what current rating is the type D mcb?
     

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