Protection
Zone selective interlocking lets a downstream breaker tell the breaker upstream that a fault is in its zone, so the upstream device waits, and clears a fault in its own zone with no intentional delay. Testing proves the signal path, the restraint and the fast trip.
Zone selective interlocking, often shortened to ZSI, is a coordination method in which protective devices in a power system communicate by a low-level signal. When a downstream device detects a fault in its own zone, it sends a restraint signal to the device immediately upstream, which then holds its normal time delay and lets the downstream device clear the fault. If the upstream device sees the same fault and receives no restraint, the fault must be in its own zone, so it trips with minimal delay instead of waiting out the usual coordination time. The method is used on low-voltage breakers with electronic trip units and on relay-controlled switchgear, for short-circuit and ground fault protection. Testing confirms that each device sends and receives the signal correctly, that the wiring between zones is sound, and that the system trips quickly or waits as it should in each situation.
Conventional coordination relies on time delays: the farther upstream a device is, the longer it waits, so that the device nearest the fault clears it first. That works for selectivity but means a fault close to the source is cleared slowly, with high fault energy and, in the case of arcing faults, a high incident energy for anyone working nearby. ZSI keeps selectivity while shortening clearing time for faults that are close to the source. It only works if the interlock does. A broken wire, a reversed polarity, a wrong restraint setting or a device that fails to send its signal either leaves the upstream breaker tripping too fast, so a downstream fault takes out a larger part of the system than necessary, or too slowly, so the protection gives none of the energy reduction that was relied on in the arc flash study. Because ZSI is only exercised during a fault, defects can remain unnoticed for years.
Each device in the scheme has a restraint output and a restraint input. A device that detects a fault above its own pickup sends a signal on its output at once. The device above it reads that signal at its input and, while the signal is present, applies its configured short-time or ground fault delay. When the signal is absent, the device acts on a much shorter delay, close to instantaneous, because the fault is then taken to lie in its zone. The signal travels on a pair of low-voltage control conductors, commonly shielded, daisy-chained from zone to zone, and in some products a small current loop supervises the circuit. The logic therefore depends on three things being right: the output condition of the downstream device, the integrity and polarity of the wiring, and the response of the upstream device in the presence and absence of the signal. A fault on the line side of the main breaker, which lies outside any zone, is by design not cleared by ZSI and is left to upstream protection.
The system is de-energized and the breakers are isolated under the owner's safety plan, with the trip outputs and the restraint wiring identified from the drawings. The technician records the breaker and trip unit identification, the ZSI settings and the delay settings for the short-time and ground fault functions, and verifies the continuity of the restraint conductors and the integrity of the shield between zones, with the wiring disconnected from the electronics where the manufacturer says so. The test starts at the lowest zone: a test set injects a fault signal into the downstream device above its pickup, and the restraint output is checked at the terminals or at the upstream input. With the restraint signal present, the upstream device is given a fault signal and its trip time is observed, to confirm it holds its normal delay. The restraint is then removed, by lifting the wire or disabling the output, and the upstream device is tested again to confirm that it trips quickly. The sequence is repeated for ground fault ZSI where present, and for each zone in the chain. Finally, the settings and the wiring are restored, any test jumpers are removed, and a last check confirms that the signal path is intact before the breakers are returned to service.
The record describes the scheme tested and its zone arrangement, the condition of the restraint wiring, how each upstream device behaved with and without restraint, and the test equipment used. As-found and as-left conditions of the wiring and settings are kept separately, so that a later reviewer can see what the scheme did before and after the work and can repeat the test the same way.
Results are compared with the design of the scheme and the manufacturer's instructions. Each downstream device should send its restraint signal when it sees a fault above pickup, each upstream device should hold its delay while restraint is present and trip quickly when it is absent, and the trip times should fall within the tolerance of the trip unit for the delay selected. Wiring should be continuous, correctly polarized and, where specified, shielded and grounded at one point. Any device that does not send, ignore or honor the signal as designed is a finding. Results are compared with earlier tests of the same scheme. The interlocking test is described in ANSI/NETA ATS-2025 for acceptance and ANSI/NETA MTS-2023 for maintenance, within the protection and ground fault sections; refer to your licensed copies of both, and to the product literature, for the values and criteria that apply to your equipment.
Acceptance testing proves each link of a new scheme and its wiring before energization, and it often goes further than maintenance testing by exercising every zone and every ground fault path. Maintenance testing checks that the interlock still works and that nobody has bypassed it, including the common case in which a restraint wire was lifted during work and never replaced. Schemes built on low-voltage power breakers with electronic trip units use a dedicated restraint pair, whereas relay-based schemes in medium-voltage switchgear may use contacts or digital messaging between relays and need a different method of simulating the signal. Where ZSI is used as part of arc energy reduction together with a maintenance mode switch, both the interlock and the mode are tested and the results are recorded together. Mains, ties and feeders each play a different role in the chain, and tie breakers need particular care because the direction of restraint can change with the system configuration.
The equipment classes and standard sections that call for this test, and the UltraDb forms that carry it.
It lets protective devices stay selective without forcing the main breaker to wait out a long delay for every fault. A downstream device restrains the one above it only when the fault is in its zone, so faults near the source clear quickly, lowering fault energy and arc flash exposure.
The upstream device sees no restraint and treats every fault as its own, tripping with minimal delay. Selectivity is lost, and a downstream fault can drop a larger part of the system. A tested scheme confirms the wiring is continuous and the restraint is received.
No. A fault on the line side of the main is outside every zone of the scheme, so it is left to the utility or upstream protection. ZSI works only on faults on the load side of the devices taking part in it.
The scheme has two behaviors, hold the delay when restrained and trip fast when not. Testing only one leaves the other unproven. A device can respect the restraint but never trip quickly, or trip quickly but ignore the restraint, and each is a defect.
UltraDb references ASTM, ANSI, IEEE, NETA, NFPA, NEMA, FAA, TIA, and IEC standards descriptively, to identify the published methods its forms are built from. UltraDb and Verlecta are not affiliated with, endorsed by, certified by, or licensed by NETA, ASTM, ANSI, IEEE, NFPA, NEMA, FAA, TIA, IEC, or Megger. Standard names and section numbers are the property of their respective organizations. Always test to the edition your contract specifies. Where a standard is sold through the ANSI webstore, Verlecta may earn a commission on a purchase made through our link, at no extra cost to you. A commission never decides which standards we cite or how a form is built.