In a power distribution system, selective coordination dictates when each overcurrent protection device (OCPD) will trip, delaying those that are further upstream to allow downstream equipment time to clear the fault and minimize system disruption. However, the farther upstream a fault is located, the longer the trip time. This increases energy flow into the fault, which increases equipment damage and incident arc flash energy.
Zone Selective Interlocking (ZSI) is one of several options for arc energy reduction required by NEC 2017 (article 240.87) for OCPDs rated 1200 amps or higher. ZSI is a hard-wired add-on solution that limits the impact of an upstream fault by automatically overriding these time delay settings; it causes the closest breaker to the fault to immediately trip without altering the selective coordination of the breakers. ZSI capability is available in many low-voltage circuit breaker systems and medium-voltage relaying systems.
While ZSI may be an effective way to mitigate risks of injury and damage to electrical equipment in the event of a fault, it must be specified, applied, and installed properly – and thoroughly commissioned. All circuit breakers within the protected zone must be equipped with ZSI capability. ZSI installation depends on proper interconnect wiring between circuit breakers, and often, different switchgear lineups.
Additionally, each ZSI solution may have different manufacturer-specific requirements, such as requiring that the circuit breaker at the end of the zone be terminated with a self-restraining jumper so that during a fault, it doesn’t look for a downstream restraint signal. There may also be limits to the number of levels of breakers (or zones of protection) available within a particular ZSI system, which vary by manufacturer.
Installing a ZSI solution as a retrofit or upgrade project presents even more complications. ZSI functionality is not standard on most molded case circuit breakers, and ZSI systems are only compatible with the ZSI manufacturer’s own equipment. Another potential issue is that a communications network is often required for ZSI, but in the case of an existing distribution system, that network may not exist.
Unfortunately, improper application and installation of ZSI is very common, which significantly reduces distribution system reliability. In the past year alone, I’ve seen three critical facilities where the ZSI network was not properly terminated within the main distribution protected zone. At each of these facilities, if a fault had occurred anywhere within the electrical distribution system, one of the main distribution breakers would likely have tripped instantly, ignoring the selective coordination settings and causing wide-spread system disruption. In one instance, design documents specified that only the main utility breaker be equipped with ZSI; any high-magnitude fault, regardless of its location, would result in de-energizing the entire system.
Whether a new installation or a retrofit or upgrade, it’s impossible to know whether the ZSI system will function properly without rigorous testing and commissioning. If ZSI is on your radar, depend on your special teams – hire a qualified independent specialist to conduct design review, verify proper installation, and thoroughly commission your ZSI system functionality and complete protection system. A poorly executed zone defense can cost you the game, putting your equipment, facility, and personnel in jeopardy.
