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Relay dry contact

Discussion in 'General Electronics Discussion' started by ivan.stanic, Feb 19, 2017.

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  1. ivan.stanic


    Feb 19, 2017
    Can someone please explain me what is the difference between dry contact (voltage free contact) and wet relay contact.
    For example, in fire alarm systems, on fire alarm control unit board, some sirens and speakers are connected over relay dry contact. Why did we connect these elements in that way insted of regular relay contact? What is benefit of dry contact?
    Last edited: Feb 19, 2017
  2. Bluejets


    Oct 5, 2014
    There are 2 instances but since you mention one I will explain that first.
    Wet contact in the context of voltage free means there is no voltage applied by the controlling circuit.
    Ideal for complete isolation from the controlling section.
    An example would be between 12v control and 240v mains circuit, the latter using the voltage free contact.
    Of course also the reverse is true.

    The other instance is from when tilt switches and the like were actually a sealed glass tube with internal contacts which were closed by the use of mercury, hence wetted contact.
  3. Minder


    Apr 24, 2015
    Another description of a dry contact it is not responsible for switching the ultimate load of the circuit.
    Also any current that the contact carries is switched by an alternate means.
    IOW it does not switch any current when operated.
    Bluejets likes this.
  4. ivan.stanic


    Feb 19, 2017
    Thank you so much for the answer.
    Unfortunatelly english isn't my native language so I have decided to upload the regular NC relay scheme to be more clear.

    Can you please tell me how would dry contact appear on that example?

    Many thanks.

    Attached Files:

  5. Minder


    Apr 24, 2015
    It wouldn't, that is an example of a whetted contact.
    It is responsible for braking the primary load.
    The dry contact is not responsible/used for switching when current is flowing.
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