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#1 KernowCooper



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  • Name: Dave
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Posted 27 February 2016 - 08:02 PM

As we get a lot of enquiries about switches here is a brief write up.


As we know most switches are stamped a amps rating lets say 10amps, so if you subject the switch to 10amps through service its life span would be very short before failure.


Why ?


Whats not generally known all switches when designed have a "Wetting Current"


Wetting current is the minimum amount of electric current necessary for a switch contact to carry in order for it to be self-cleaning. Normally this value is far below the switch’s maximum current rating. So a 10amp maximum switch would have a wetting current of around 7amps to maximise its MTF (Mean Time To Failure), remember my posts quoting amps load of switches and wiring of load (A) + 30% which is the wetting current + 30%.


Manufactures of Automotive switches don't list the wetting current but heavy duty industrial switches most likely have a spec sheet showing the max and wetting current.


So rule of thumb on switches work out the amps on the circuit and select a switch with a reserve minimum of 30% + and your switches will last a lot longer.



Rule of thumb using AC Switches on a DC Circuit


You should know that AC gives switch contacts a easier time because On AC, the voltage drops through zero 120 times per second, which helps to extinguish arcing as the contacts open. On a DC Circuit this dropping to zero does not happen and the switch contacts are under much more constant load than AC


Therefore if you use a 120/10a or a 240v 10a switch on a 12v 10amp circuit the switches life will be reduced, and under switching the lighter construction internals will eventually arc shut and you will loose control of on/off control of the circuit.


The trend now is to see on most switches from a cheap source (ebay - far east) is just to list the AC voltage and amps, If you must use these types of switches on a DC circuit then its best to use them to power a relay or as a rule of thumb (mine)  I divide the amps on the switch  by 3.


If you wish to see a good example of a DC switch then work the original toggle switch made by Lucas for the side/headlights and wipers, now repeat the test on some of the rubbish I've removed over the years sold by accessory shops in the blister packs.


The internals of AC Switches are very cheaply constructed compared to true DC Switches as are the switching times internally.

Edited by KernowCooper, 27 February 2016 - 10:39 PM.

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