Your questions answered: In the Zone of Applying Zone Selective Interlocking (ZSI)

The Aug. 30, 2018, “In the Zone of Applying Zone Selective Interlocking (ZSI)” webcast presenter addressed questions not covered during the live event.


Zone selective interlocking (ZSI) has existed for more than 40 years, yet so many people in the electrical field have little knowledge about what it is and how it works. ZSI is a network of two or more interconnected circuit breakers that communicate with each other so that a short circuit or ground fault will be cleared by the breaker closest to the fault in the minimum time possible. The primary goal of ZSI is to switch off the fault current within the shortest possible time under consideration of full selectivity, regardless of the fault location in an electrical distribution system.

ZSI provides the complete range of selectivity with an extremely short delay time of 50 milliseconds minimum typically regardless of the number of fault zones and the location of the short circuit or ground fault in the power distribution system. The benefits of ZSI become more apparent with a higher number of fault zones typically found in large systems. By shortening the time to clear the fault, ZSI significantly reduces stress and damage in the event of a short circuit in the electrical apparatus. As described in NFPA 70-2017: National Electrical Code (NEC) Article 240.87, ZSI also is a method suitable for compliance with that section of the code.

Key topics include:

  • Applying ZSI in arc reduction and selective coordination circuits.
  • How breakers communicate and connect in a ZSI network.
  • Illustrations and comparisons of ZSI with time current curves.
  • Understanding the fault protection sequences of ZSI.
  • General cost impact.

Presenters during the webcast were:

  • Rick Pinchard, business development manager, Siemens
  • Roy Hicks IV, business developer, Siemens
  • Robert W. Jensen, P.E., business development manager, Siemens.

Roy Hicks IV, SiemensPresenter Roy Hicks IV, business developer, Siemens, responded to questions not answered during the live In the Zone of Applying Zone Selective Interlocking (ZSI) webcast on Aug. 30, 2018.

Question: Time delay options may include instantaneous, inverse, or very inverse. Would time delay increase result in slower clearing time and greater incident energy?

Roy Hicks: Yes. Time delays increase incident energy level. Zone selective interlocking (ZSI) is dedicated to a consistent ZSI delay that is slower than instantaneous but faster than short time delay.

Q: How does the cost of ZSI installation compare to the cost of a maintenance switch installation?

Hicks: That’s somewhat hard to say. If the maintenance switch is remote, touch, proximity, etc., and if the ZSI input/output (I/O) is all factory wired inside the gear or if portions of it will be wired between gear.

Q: Explain again why you want instantaneous under the available arcing current (AAC).

Hicks: You may want a non-ZSI downstream breaker to trip before the 50 millisecond ZSI upstream breaker for coordination. If you do, then lowering the instantaneous below the AAC will allow coordination with an upstream ZSI breaker.

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