Paralleling Circuits to Extend Switch Life


#1

I have seen many Rocker and toggle switches wear out and stop functioning prior to their expected life. Though the only way to really fix a problem of premature failure is to go with a more robust switch (higher current rating and/or longer life rating) this is not always possible on an existing project due to limited size constraints. One suggestion that I have passed on is that if you can find a switch that has twice the poles and run your circuits in parallel it can extend the life of your switch.
One example was when using SPDT rocker switch SW340-ND to run two separate motors. Even though the voltage and current draw of the motors was with-in the specifications of the switch, after using for a short period of times I started to see failures. Here is the schematic of the original circuit.

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With a set cut-out in my panel I was limited in options when looking for a replacement switch. Using a DPDT rocker switch CWI419-ND I was able to run the circuit in parallel. The initial thought being with less current flowing through the individual contacts in the switch I would be able to run the switch without having the failure issues we were seeing in the original switch and no modification was needed to the panel.
In reality, the wear and eventual failure of switches usually occurs during opening and closing, and double pole switches almost never open and close both poles at exactly the same time. This means that during the most critical time, all of the current and arcing will be focused on only one of the two poles. Thus the extended life I was experiencing is most likely a result of redundancy, because once the first pole fails, the second pole can take over, but at the very best, it would only double the life. If one were having premature failures with a single pole (less than half the expected life), then a double pole would, at best, make it last up to the expected life. This however would not be a guarantee.
Here is the schematic using the DPDT switch.

Note: Using a DPDT can’t hurt, but it does not solve the problem of inadequate contact ratings for the application. You must also understand that a DPDT switch cannot handle twice the current just because you paralleled the poles. With parallel connections, the current should still never exceed the rating of a single pole, for the reasons described above.