Maglock wiring - Don't get it!

Discussion in 'Homework Help' started by EddieG, Jan 19, 2014.

  1. EddieG

    Thread Starter New Member

    Jan 18, 2014
    3
    0
    I'm trying to figure out this wiring (attachment) for an electromagnetic door lock which must release under several conditions (i.e. fire alarm, Manual reset key, etc.) The fire alarm relay (8) is written NC, but the note next to it says NOHC. Can anyone help me understand the logic steps behind how the fire alarm and manual reset processes work?
     
  2. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,547
    2,371
    NOHC is normally open, held closed.
    This is a switch that in its normal state is open, but the application hold it closed.
    It looks fairly straight forward, I assume when the lock is under power it is ON?
    Max.
     
  3. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    When fire alarm contact is closed, which it normally will be, pressing reset key momentarily, will pull in CR-1.

    CR-1 will be "latched" in by it's own contact CR-1A, until such time as fire alarm or power failure.

    It will then have to be re-latched.
     
  4. EddieG

    Thread Starter New Member

    Jan 18, 2014
    3
    0
    Inwo, Are you saying that the Fire Alarm contact is incorrectly labelled as NOHC, and instead it should be NCHO?
    The manual reset is largely why I'm confused. CR-1B has to be energized / closed for the door lock to be on and makes this failsafe, meaning any failure will open the doors. If the manual reset is never pressed, CR-1 will never energize and the circuit will not start working as wanted.
     
  5. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,547
    2,371
    CR1 has to be on to arm the circuit. If the NOHC was NOHO it would never be possible to arm it in the 'normal' sense, CR-1B and TR-1 are shown open, but these would be closed in the 'armed' state.
    Max.
     
  6. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    I guess I chose the wrong term.:p

    It is a normally open contact in that it is open when the fire alarm is not powered.

    But "normally" the fire alarm will be powered, and contact will be closed.

    It's confusing to me too!

    I drew a diagram in another thread with a limit switch that stops the operation in a home position.

    It shows a NC contact, yet it will be open when the machine is not in use and waiting to be started.:confused:
     
    EddieG likes this.
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,547
    2,371
    This is normal designation for limit switches etc, there are specific symbols to describe each condition, NO, NOHC or NC, NCHO, each has there own designated symbol.
    If this is a relay contact, then it is described by the label as per the schematic shown.
    Max.
     
  8. alfacliff

    Well-Known Member

    Dec 13, 2013
    2,449
    428
    fire alarm circuits are normally closed loops. when alarmed, the open. that way, if a wire breaks they go into alarm, thats for safety, if they were normally open, and a wire broke, you would never know it, untill you had a fire and it didnt go off.
     
  9. EddieG

    Thread Starter New Member

    Jan 18, 2014
    3
    0
    I understand all the posts (and thank you all for those) but there are still two things I don't get .. (maybe I'm a little thick!)
    1) Why is the Fire Alarm relay labelled NOHC. If the definition of NOHC is "A type of switch that is connected normally open but the contacts are held closed when the circuit is de-energized."
    then this circuit would hold the Fire Alarm relay closed when the fire alarm goes off and CR-1 would remain energiized and therefore the door would not release like it should. I understand how the circuit works if this relay is a NC (which matches the symbol), and is opened when there is a fire..

    2) The Manual Reset Key. Am I correct that pressing the Manual Reset Key (closed) will only allow the CR1 relay to become energized when initially powered up, but it will have no effect after that?
    If that's correct, then what's the point of this key of a manual reset key that doesn't reset anything? Isn't it the Key On/Off Selector that will reset the circuit to it's initial state?
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,547
    2,371
    We don't know at this point what the actual Fire alarm relay consists of or what holds it in, but I suspect that in the normal safe condition, the circuit picks up the relay and it will drop out under detection of fire, loss of power etc, so in this condition it is a N.O. contact that is held closed by the virtue of its relevant 'safe' circuit relay.
    The N.O.H.C is not normally a relay contact designation , it is usually reserved for other mechanically operated limit switches, so I believe they have taken a little license with the drawing, but I think the intention is plain.
    All the circuit contacts are AND'ed so the Lock is kept energized under power until any of the AND (or NOT AND) are open, releasing the lock.
    At least this is the way I interpret it?
    Max..
     
    EddieG likes this.
  11. inwo

    Well-Known Member

    Nov 7, 2013
    2,433
    315
    It might have been best if the circuit was drawn showing this as an external NC contact from FA.

    Nc refers to relay contact position when coil is not energized. Not the position it is normally in.
     
    EddieG likes this.
Loading...