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Millivolt contacts

I am not stating this as fact . I seem to remember that at some Honeywell school I attended

probably 40 or 45 years ago the instructor said all M-H switch contacts were designed with

a small raised portion in the center which made them compatible with MV. If the control was used even once on line voltage the nubs would be gone and the switch was no longer compatible with MV.


  • Tim McElwain
    Tim McElwain Member Posts: 4,480
    The rasied portion

    was used to insure a sure contact on 120, 24 or millivolts. Almost all switches are rated for use on all three. I have never had an occasion to find out if after applying 120 volts to a switch if it would not be usable on millivolts. I believe it would. I worked as an instructor for Honeywell for 5 years and have many years experience with switches on millivolt systems. Maximum MV drop across any switch is 10 millivolts. The only exception is some thermostats which have a fixed anticipator will drop around 100 to 115 mv's depending on the thermostat
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