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amperage to voltage relationship of mechanical switch ratings

Discussion in 'General Electronics Discussion' started by GeoNOregon, Oct 31, 2016.

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  1. GeoNOregon

    GeoNOregon

    14
    1
    Jan 30, 2014
    The stated amperage rating of all types of electrical switches seems to vary based on the voltage level.

    It also seems the amperage goes up when the voltage goes down.

    What I do NOT see is consistency in how much the amperage goes up when voltage goes down.

    I also have not been able to find an explanation for this on the Internet.

    I frequently run across switches which only have one stated 'amperage at voltage' rating.

    How do you determine what the amperage rating is, if you want to use the switch at a lower voltage, when the relationship appears to be inconsistent?

    Thanks, in advance,

    GeoD
     
  2. Harald Kapp

    Harald Kapp Moderator Moderator

    10,993
    2,498
    Nov 17, 2011
    When a mechanical contact (pair) opens (a switch, a relay...), the air between the opening contact surfaces is ever so slightly inonized, i.e. it becomes electrically conducting. Thus the current tends to flow in spite of the opening contacts through the ionized air between the contact surfaces.
    The higher the open circuit voltage, the more this voltage will keep open an ionized channel between the surfaces. The current flowing through this channel needs to be below a certain limit to allow the air to de-ionize, become insulating again and interrupt the circuit - as it is intended by the operation of the switch.
    The lower the voltage, the more current is required to keep the air ionized (or in other words the higher the limit for the current to allow de-ionization of the channel).
    This is why high voltage goes with low current and vice versa.

    I don't know of any safe way other than getting this data from the manufacturer of the switch. There are some factors involved like
    • material of the contatcs
    • width of the gap between open contact surfaces
    • speed at which the contact pair opens
    • allowed (or possible) degree of pollution
    • ...
    The manufacturer knows these parameters and also is able to test the switch design for different applications. Unless you have a rating for your application, the only safe assumption is that the current may in no case exceed the given limits.
     
  3. Colin Mitchell

    Colin Mitchell

    1,417
    313
    Aug 31, 2014
    All these values have been generated through tests of the individual switch and it is a value based on the manufacturers consideration as to how he guarantees the performance of his switch.
    It to do with the surface contact of the switch, how fast it opens and closes and how often the manufacturer thinks the switch will be operated.
     
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