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What is some information on switch contact resistances varying from phase to phase?

In any given 3 phase AC system, the load from each phase will not be same, so some phases will have a different lifespan then the others depending on the load cycles they experience. Contacts as they make or break will erode overtime, and will have different resistance values, so it is expected to see some variation in resistance values measured, assuming we are measuring them with an 100 Amp meter. The contact erosion will be faster under load (especially with fluctuating loads) but you also have a mechanical wear of the contacts, and the curve shown in manual 6045-1 on page 48 refers to ideal conditions.

In that Figure 37, if we are interrupting 100 Amps only, we should be able to obtain 500 to 600 load operations, but that life assumes that you have constant interruptions with no peaks in the system. If you had any faults, or higher rated interruptions, the contact and switch life will be shortened. We can not go back and continue to expect the 100 Amp interruption life.... This is where it becomes important to check your contact resistance and determine your end of life, as the curve is not very valid after a severe or abnormal application/interruption.

Regarding the resistance variation, the switches are checked prior to shipping for contact resistance, so we do not typically see much of a variation in contact resistance,maybe 5 to 9 micro-ohms. Once it hits the field, the change in resistance will vary from phase to phase, depending upon your application/load. The end of life is determined at 240 Micro-ohms, and if your contact resistances are within that value you are OK per our tests.


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