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Discuss Air Circuit breakers in the Commercial Electrical Talk area at ElectrciansForums.co.uk.

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  1. hadleigh sparks
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    hadleigh sparks Guest

    Hi Everyone , i havnt fitted one of these before and i need to do so as a retrofit into and existing panel board. the reson for this is a down grade in their load and therefore i wish to fit a protective device to protect the current wiring against the new lower load. I also intend to fit a Voltage optimisation unit in line and by lowering their main protective device i can fit the correct size unit for their load.
    1 can i simply cut a hole in a spare panel install and fix the breaker and then cut and divert the busbars via Flexis through the breaker and the unit then of to their load ?
    2 is their a good place to go and get some training ( is it needed or as they as simple as they look )

    Thanks for taking the time to answer
    kev
     
  2. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    Why are you installing an Air Ciruit Breaker?
    Why not use an MCB?
    If you do install it, just use an enclosure to house the ACB, break into the tails between the meter and the CU, connect the ACB, then from the ACB to the optimisation unit, then to the CU.
    To be honest, it would probably be simpler to det the DNO in to down rate their fuse.
     
  3. hadleigh sparks
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    hadleigh sparks Guest

    Hi Spin , like the bike !
    They are currently pulling 625 amps peak per phase , i wasn't aware they made MCB's that large. I have contacted the DNO however their is no fuse between the sub and the incoming supply , they are an integral part of a loop main ( i think ) their is 8 large tails feeding into the bottom of their switch gear going directly to the customers main switch and then Bars. So thought the best way would be to install an ACB.
    However im open to suggestions
    thanks again
     
  4. Knobhead
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    Knobhead Guest

    What current are we looking at? I don’t like the idea of flexibles in switchgear due to very high possible fault currents. I’ve seen bus bars bent due to fault currents, flexibles will jump about like zebedee.
    Some ACB’s can be down rated without the need to change the unit.
    Can you give more details?
    Make, model, current, etc.
     
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  5. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    Sorry, thought you were doing domestic and instaling one of those Vphase optimisation units.
    No ACBs are more appropriate.
    As Tony says, we would need a lot more information, probablly some photos of the existing switch gear and panels.
     
    Last edited: Feb 16, 2012
  6. hadleigh sparks
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    hadleigh sparks Guest

    Hi We are looking at a maximum current drawn of 625 amps per phase , so I need to allow for approximately 750 amps per phase. I would be retrofitting the ACB into the existing panel board. At the moment all they have is a main switch which is rated at 1200 amps fed direct from the sub station next door this goes direct on the bus bars and these feed directly of to switch fuses and then on to sub mains to different parts of the factory.
    The existing panel board is approximately 20 - 30 years old
    thanks
     
  7. hadleigh sparks
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    hadleigh sparks Guest

    thanks very much , however the existing gear is simply a large main switch with vertical bus bars at the rear of the chamber . above the main switch their is a spare panel with a spare void behind where i could mount an ACB within ( there is loads of room). behind this is the bus bars ( their must be at least 3 feet behind the cover plate to the bus bars ).
    I dont have any photos unfortunately.
    May i ask are you suggesting using copper bar to bolt on to rear of the ACB from the existing bars and then diverting back to the bars in bar again? Or do you have a better suggestion of cable to use ? or even indeed a recomended manufacterer i could contact for help on this point
    thanks again for your help
     
  8. Knobhead
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    Knobhead Guest

    Most ACB’s will be adjustable down to 75% FLC (some even lower) which would give 900A
    I really can’t understand the need to down rate the protection to the board. It won’t affect the loading on it so billing won’t be affected. The ACB is there just to protect the boards bus bars.
     
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  9. spinlondon
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    spinlondon Trusted Advisor

    Location:
    Harlow Essex
    Is the optimisation unit current sensitive?
     
  10. hadleigh sparks
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    hadleigh sparks Guest

    hi guys
    At the moment their is no overcurrent protection between the bus bars and the sub station.
    I am looking to fit a voltage optimisation unit in between the sub station ( via the main switch) and the bus bars . therefore in order to protect this unit and the wiring to and from it i will need to install some sort of overcurrent protection device. This is why i was thinking of retrofitting and ACB into the existing panel board system. Obviously i dont want to fit a unit that is to large as they are very costly so if i can down grade the wiring with an ACB to suit their current load plus 20% for expansion the whole thing works.
    hope fully this explains it a little clearer
    thanks again
     
  11. Engineer54
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    Engineer54 Guest

    No disrespect, but i don't think your competent to be mucking around with a switchboard of this size going by your above proposals!! You need to maintain the switchboards fault current ratings bracing, so you would know flexible and bracing is a contradiction. You also don't cut into a switchboard and start altering or cutting into bus bars unless you know exactly what your doing of..

    The 1200A ACB may also have connections to protective relays, have you checked?? That ACB IS the over current protection between the transformer and the bus bars !! As Tony has already told you ACBs of this size have a wide setting arrangement in many cases down to 60% or less. I'm also guessing that you also have little idea how to set or reset the parameters of the protection curves on an ACB

    There is no need to replace this breaker anyway, as it's there to protect the bus bars and possibly the transformer, not the outgoing cable/loads, they will be protected by the distribution MCCBs or switch fuses...

    Let me put it another way, an electrician shouldn't be making these sorts of dicissions on a sub-station supplied main switchboard, that is the work of a qualified Engineer...

    Mark my words here, get something like this wrong, and you'll know all about it, ...but fast!!
     
  12. johnboy6083
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    johnboy6083 Band Member Trusted Advisor

    Location:
    South east
    i have to agree witbh the others. Setting an ACB to cascade with the DNO's gear will require extesnive knowledge and experienceof protection devices. Also, as theyve mentioned, you have overcurrent protection to your Busbars in the panel, and so you can then feed final circuits or submains from other protective devices that are tapped from the busbar.
     
  13. Knobhead
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    Knobhead Guest

    I’ve seen the aftermath of over stressed bus bars. Lumps bitten out of 8” X 3/8” copper isn’t a pretty site. The bars have to be supported and braced to withstand the tremendous magnetic forces between them under fault conditions.

    You still haven’t said what make the ACB is or the switchboard.
     
  14. netblindpaul
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    netblindpaul Trusted Advisor

    Location:
    South Wales
    E54, Tony, can you now understand some of my comments!

    BTW I agree with you both on this.
     
  15. Engineer54
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    Engineer54 Guest

    Apart from anything else Tony, when people don't understand the ''From build'' of a switchboard, that Form build level can be destroyed when you start mucking about with internal dividing sections etc within the panels. A high Form 4 panel can easily be reduced to a Form 1 around that ACB.

    It's also not just a question of fabricating a Bus Bar arrangement, that's the easy part, the bracing of those bus bars to the switchboards KA fault level, can be neigh on impossible. You can't just bolt them to the dividing panels, they have to be bolted to the main panel structures. Or as you say, in a fault the snaking bars will take those thin panels with them ...lol!! Switchboards are designed AROUND the required KA fault level, to which it can be potentially subjected...

    We don't know what size the supplying transformer is, but a 1MVA TX can quite easily supply 20+ KA into a fault, and for a considerable time too, if the protection hasn't been set correctly!! This is Not something an electrician should be contemplating off his own back, this needs an Engineers involvement, as well as the manufacturers of this Switchboard.
     
    Last edited by a moderator: Feb 16, 2012
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