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Power Defense MCCB Zone Selective Interlocking


ZSI stands for Zone Selective Interlocking. This is an optional feature that can be ordered with PXR20, PXR20D, and PXR25 trip units. Section 3.8 in the PXR manual has some additional information.

The breakers in a system should be coordinated so that the breaker closest to the fault will trip first to clear the fault. This is done using pickup settings and programmed time delays for the short time and ground fault trip functions. The furthest downstream breakers have the shortest delays, and as you go upstream, the delays get longer. This is to minimize outages.

ZSI is used to eliminate the delay in the case of a bus fault.

With ZSI wiring, the ZSI output of the downstream (feeder) breakers is wired to the input on the upstream (main/tie) breakers (and the other feeder breakers). This way, if a feeder sees a short time or ground fault, it sends out a signal to all the other breakers saying, "I see a fault" and all the breakers know to wait per the programmed time delays. If there is a short time or ground fault between the main and feeder breakers, the feeder breaker doesn't see it. Now the main breaker sees a fault, but it doesn't get an input saying someone else sees it. So the main breaker says, "Oh no, I'm the only one seeing this, I should ignore the time delay and trip right now!"

How to Make a Breaker Delay or Not Delay

If a trip unit is ordered with ZSI, this comes enabled/on by default. This means that the breaker will NOT delay per the programmed settings unless it sees an input on Zin.

If you want a breaker to always delay per the programmed settings, you can install a self-restraining jumper from Zout to Zin. You can also use PXPM to disable ZSI. If the trip unit has a display (PXR20D or PXR25), you can navigate to Protection -> ZSI and set this to off.

The wiring diagrams have a ZSI diagram showing the connections between breakers, and the self-restraining jumper.