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Circumstances for NC contactor?

Discussion in 'Electrics UK' started by Mike2007, 14 Oct 2021.

  1. Mike2007

    Mike2007

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    When is a normally closed contactor used (not normally open)?
     
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  3. EFLImpudence

    EFLImpudence

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    When you want to switch off something when the contactor is energised -

    it saves having to power the contacter continuously for items that are usually on.
     
  4. JohnW2

    JohnW2

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    Well, such functionality might be wanted for all sorts of reasons, albeit I would imagine extremely rarely in relation to a domestic electric installation.

    One reason for wanting it is so that the 'default state' (e.g. if there is a failure in the supply or circuitry controlling the contactor) leaves the contactor contacts closed, hence any supplied equipment 'on'. It may sometimes me more appropriate/desirable to have a contactor (in its activated state) 'switch something off', rather than 'switch it on'.

    Kind Regards, John
     
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  5. bernardgreen

    bernardgreen

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    Battery powered emergency lighting can be switched via the normally closed contact of a contactor whose coil is connected to the mains.

    Sometimes where a contactor is normally ( quiescently ) energised the contacts are labelled in the circuit diagram as QC ( closed when coil energised ) and QO ( open when coil energised )
     
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  6. 333rocky333

    333rocky333

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    We use them for store and car park lights, by using them, not done the maths, but apparently the coils are energised for a shorter time during day turning lights off, than overnight with the coil turning them on.
    The contactors seem to last longer.
    Also as John says if the contactor fails the lights default to on till replaced
     
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  8. ericmark

    ericmark

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    I used them with compressor de-load valves, if valve is opened at start and the valve leaks there is too much pressure in cylinder for it to open so it ruptured the fuse, but if de-energised on run then since held open the pressure can't build up in the cylinder.
     
  9. oldbutnotdead

    oldbutnotdead

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    Very handy (in large buildings with no windows) having the NC connected to indicator lamps on light switches....NO connected to the actual lights
     
  10. bsr

    bsr

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    Electrically controlled door locks eg mag locks?
     
  11. SUNRAY

    SUNRAY

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    Most of the controlled lighting circuits in supermarkets is with NC contactors.

    Typically the lights are on for around 18 hours a day and these days big supermarkets are 24 hours.

    The lights are often controlled by a BMS (Building Management System) so an added bonus is the lights will come on if the BMS fails.
     
  12. crystal ball

    crystal ball

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    In ye olde relay logic systems NC is used to switch other relays or devices as required
     
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