Differences Between Magnetic Contactors

Somewhere in that company will be a standards manual for switchboard design and manufacture. What does it say on the topic?

These are the duty cycles, there was a previous thread where an explanation was attached. Lazy to search out, try the search feature for it- or look here Page 5.

GA.

Before the advent of 'pre-switching' capacitor-specific contactors, NEMA ICS2 recommended that one can wind a few turns of the contactor-to-capacitor wire on a 6" mandrel, maybe 6 turns to form an air-cored inductor. This inductor would offer a high impedance to the 15kHz spike that would appear when switching on multiple capacitors. A good common-sense solution. However, now that specifically designed AC6 contactors are available, one must use them.

A capacitor's switching inrush transient can be as high as 20 times the rated current and it can last for about 4 to 5 cycles. A magnetic contactor's making capacity in AC-3 is only 10 Ie (Ie - Rated Opertaional Current). So, if you use a AC 3 rating contactor for Capacitor Switching, it will not be able to "make" the switching current and might damage.

A contactor rated at AC-6b - is designed for taking up the effects of the high magnitude of the capacitor inrush.

Alternately, manufacturers sell AC-3 Contactors for Cap. Switching with a set of current limiting resistances in series, so that the inrush current is limited by the resistance and now the AC-3 rated contactor can handle the switching inrush.

I differ with other replies in this post. A contactor with a set or resistors and pre-closing contacts doesn't make it AC 6b. The contactor is still AC 3, but only the current is brought down within it s making capacity. But, if a contactor is designed for AC 6b, it will be able to "make" the capacitor switching inrush without the need for any current limiting resistor.

Any contactor can have an assigned rating as per the utilisation category AC1,2,3 or 4 etc. Its rating will come down as the severity of the application increases. AC 6b happens to be the utilisation category for switching capacitors. Contactors with early closing contacts with current limiting resistors and / or reactors were developed just for this purpose. Needless to say any conventional contactor will also have an AC6b rating, albeit low as one goes into multiple capacitors switching. It may be interesting to note that switching of multiple capacitors can give rise to inrush currents of the order of 180 times the rated current. It is really in such cases that these special contactors make themselves an obvious choice.