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PD-NF and PD-RF Zone Selective Interlocking


ZSI stands for Zone Selective Interlocking. The PXR trip units in PD-NF and PD-RF breakers come with ZSI off/disabled in the trip unit as a factory default. If you want to use ZSI, you will need to enable ZSI in the trip unit.

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 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 for the customer.

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

The ZSI setting can be toggled between on/enabled and off/disabled by navigating in the menus (Setting -> ZSI) or using PXPM (Setpoint Configuration -> Current Protection Configuration). As long as this setting is off/disabled, the trip unit will always delay per the programmed settings. You can tell what the setting is by looking at the trip unit display. If the bottom row says "ZSI" next to the battery icon, then this setting is on/enabled. If it does not say "ZSI" next to the battery icon, then this setting is off/disabled.

A self-restraining jumper from 24 to 21 (Zout to Zin) can also be installed.

  • If ZSI is set to off/disabled, this jumper is redundant, and removing the jumper does NOT enable ZSI (that needs to be done in the trip unit).
  • If ZSI is set to on/enabled, this will make the trip unit delay per the programmed settings.

The unrestrained tripping time is shown on the LS time/current curves, and is 75ms on a 60 Hz system (80ms for 50 Hz). This is how fast the breaker will trip if ZSI is enabled and it does NOT see a signal on the ZSI input (pin 21).