Type B v Type C MCBs

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

  1. limestone cowboy

    limestone cowboy New Member

    Sorry about lack of information. The MCB in the distribution board is 3 pole 20A Type D. Then there is one length of 4 core 2.5 sq mm SWA cable, 32 metres long. This goes into a 4 pole isolator, comes out as 4 core flex which goes into the charger. The front panel of the charger has a 3 pole 20A breaker which is used to start/stop the charging cycle. Everything is 3 phase. The black core in the SWA isn't used. While I don't want nuisance tripping, it would be good to know the cable wont melt. Thanks.
     
  2. fusefinder

    fusefinder New Member

    single phase questions only please :)
     
  3. supersparky

    supersparky New Member

    well, yea what you have done is fine, but you will probrably now have to upgrade the size of the cpc to meet d/c times, as type d brings in whole new problems

    ss
     
  4. Milton Bradley

    Milton Bradley New Member

    You should really have a 30MA trip RCD protected Incomer on a distribution board containing 1 or more type D mcb's.
     
  5. fusefinder

    fusefinder New Member

    I calculated your maximum allowable Earth Fault Loop Impedance (based on reg 413-02-26): as 1.732 ohms max.

    Using the adiabatic equation the minimum CSA for the CPC to be 3.88mm2

    which would probably mean that the SWA is OK but the 4 core flex you mention between the Isolator and the charger may need looking at?

    I'm only a humble adult trainee so I would appreciate any and all correction if I'm off the mark here. TheTrician?
     
  6. fusefinder

    fusefinder New Member

    sorry, I also meant to say I was answering limestones query not original post. Calculations based on type D MCB.
     
  7. limestone cowboy

    limestone cowboy New Member

    Thanks Humbug. The flex came on the charger and is 2.5 sq mm so I'll change it for SWA, which would be more durable as well.
     
  8. The Trician

    The Trician New Member

    SWA to the charger sounds like a good idea, though not completely necessary unless you need to provide more robust mechanical protection. A quick fix would be to buy a roll of 10.00mm single earth and run this from the earth terminal on the charger back to the dist board just to be on the safe side. There are tables in the regs which give the resistance of steel armouring per metre but you only need a bad connection/corroded swa gland to screw your earth loop impedence figures. 50 metres of 10.00mm earth from screwfix is only about £16 but how much is a new length of swa?
     
  9. Damocles

    Damocles New Member

    dont know if this discussion is missing the point here. Does anyone know if it reasonable for such a charger to need type D or does it have some fault which is causing surges?
     
  10. The Trician

    The Trician New Member

    Good point Damocles, but I'm guessing that if the charger has several charging modes - e.g. equalizing, etc then it will 'float' the charging current once the battery is up to full charge. If this is how it works, there will be current surges when the charger goes on an off, if the battery is extremely flat then there will be an initial heavy current draw, hence the nuisence tripping.

    One thing I missed and which might help.
    Limestone cowboy, does your charger actually USE the Neutral - does it have to have the Neutral core connected for it to work? Or is it just 3 phase?
    If the Neutral core of the SWA is a spare one, you could mark it up at both ends with some earth sleeving and use it as an earth wire. If you do this you MUST identify as an earth. Everything helps when it comes to EFLI when you are using swa. Just a thought:)
     
  11. limestone cowboy

    limestone cowboy New Member

    You are right. The charger does have an equalising mode. The cold weather makes the battery go very flat and so the tripping with the type B MCB got worse.

    The charger only uses 3 phase, no neutral, and the black core is taped up both ends. I'll do what you suggest with the earth sleeving and use it as an earth wire.

    This leads to a related but different question, just out of interest; is it not sufficient to rely on the steel armour for the earth? I was on a job recently and the electrician ran a separate earth wire along side the SWA. Was this because it was underground cable?
     
  12. The Trician

    The Trician New Member

    I calculated your maximum allowable Earth Fault Loop
    Impedance (based on reg 413-02-26): as 1.732 ohms
    max.

    Using the adiabatic equation the minimum CSA for the
    CPC to be 3.88mm2

    which would probably mean that the SWA is OK but the
    4 core flex you mention between the Isolator and the
    charger may need looking at?

    I'm only a humble adult trainee so I would appreciate
    any and all correction if I'm off the mark here.
    TheTrician?

    Humbug,
    Haven't checked your numbers but I think you are on the right track. Would need to do a bit of swatting up on adiabatic equations etc to be sure:)
    Sounds like you are being thorough here though, which is always a good thing. More speed to adult learners I say - I'm one myself and I don't have the answers to everything!

    I've never met anyone who does have the answers to everything but I've met a few who think they have:)

    Keep up the good work and if you are training to be a trician I'm sure you'll make a good one.
     
  13. The Trician

    The Trician New Member

    It's not strictly necessary to run a seperate earth with swa but it's a good idea. I have heard stories from different contract electricians who used to do a lot of installation work at the factory where I was based, and many of them said that the regs demanded that you should not rely soley on the armouring to provide a satisfactory earth path but there is nothing in the regs as far as I know which actually states this. What should happen is that you should fit 'banjo's to each gland and fit a brass nut and bolt to secure the 'banjo' to the enclosure/control cabinet/dist board, then run a single earth cable from this to the earth terminal in the cabinet/dist board. From this you should run another earth cable to the outgoing swa to the machine/charger/whatever, thus providing a 'thru' earth connection from swa termination to swa termination.

    Again, its all about achieving a low enough earth fault loop path to meet tyhe disconnection times given in the regulations.

    Hope this helps
     
  14. ban-all-sheds

    ban-all-sheds New Member

    type 1? that should be even worse than type B for low current tripping...

    Yo. And they're 5A ones....
     
  15. ban-all-sheds

    ban-all-sheds New Member

    It's not strictly necessary to run a seperate earth with swa but it's a good idea.

    Doesn't GN6 say that the use of separate protective conductors running parallel with the armouring of cables is to be avoided?
     
  16. The Trician

    The Trician New Member

    It's not strictly necessary to run a seperate
    earth with swa but it's a good idea.


    Doesn't GN6 say that the use of separate protective
    conductors running parallel with the armouring of
    cables is to be avoided?

    Not sure here B-A-S - will have to check and get back to you, but which section of GN6 are you specifically referring to here?
     
  17. ban-all-sheds

    ban-all-sheds New Member

    I too will have to check and get back
     
  18. The Trician

    The Trician New Member

    Have had a quick look at GN6 but I can only find a fleeting reference to the subject in question. There is mention of the effectiveness of a parallel conductor being nulified by the magnetic effect of the armouring when running supplementry earth conductor in addition but I can find mothing else. I'm not quite sure how this works, or the reasoning behind the statement, anyone else know?
     
  19. ban-all-sheds

    ban-all-sheds New Member

    I was going by what Paul Cook says in his "Commentary on IEE wiring regulations" rather than the GN itself.

    In the book he says the advice is in the "Protection against overcurrent" section, and it says it's to be avoided "as the inductive impedance of such a parallel conductor is likely to be such as to make little contribution to the current-carrying capacity of the armouring. It is also usually not necessary.
     
  20. The Trician

    The Trician New Member

    Yes, I think this is correct. Haven't studied the phenomena in depth though. Might be a good one to post on the IEE site?

    I suppose there's an argument where an extra 'Earth' is handy in case of mechanical damage to the armouring of the swa, or where one end of the cable has pulled out of the swa gland. We used to see this in industry where a fork lift truck had inadvertantly caught an swa when he was stacking pans/crates against the wall. The swa armours would be yanked out of the gland at one end of the run, thus earth continuity would no longer exist. Sometimes the whole cable would be 'stretched' by the experience!!
     

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