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themal relays explained

Discussion in 'General Electronics Discussion' started by gjoo, May 22, 2013.

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  1. gjoo


    May 22, 2013
    I am confused with this statement I read in an old electricity book. " in many circuits you will see overload relay contacts in series with the line. These contacts are normally closed, but if the temperature of the device becomes excessive , the thermal relays will close, causing the overload relay to become energized and opening the line."

    I don't understand , is the thermal relay normally open and when it gets hot it closes pot an electromagnetic on line which energizes to open it's contacts. So the thermal relay acts as a switch. Isn't this unnecessary?
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    Nov 17, 2011
    That's exactly how I'd intepret this statement.
    It would be unnecessary if the thermal relay has a rating that is sufficient to directly carry line current and to open under load. However, that would require thermal relays of different ratings - unless you would use a high power relay for low power loads - overkill. Therefore, you use a low power thermal relay to control the power relay of appropriate rating.
  3. GreenGiant


    Feb 9, 2012
    there are 2 different relays, there are overload relays, and thermal relays.

    I would hazard a guess that the thermal relays are much lower rated than the overload relays hence the need for both. When the thermal relay closes it energizes the coil of the overload relay which causes it to open.
  4. gjoo


    May 22, 2013
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