Zone Selective Interlocking (ZSI)

Presentation

Zone-selective interlocking (ZSI), also called zone restraint, is a system designed to reduce the stress on electrical distribution equipment during short-circuit or ground-fault conditions.

ZSI works with a previously coordinated distribution system to limit stress on the system by reducing the time it takes to clear the electrical fault while maintaining system coordination between overcurrent and ground-fault protective devices.

ZSI allows MicroLogic trip units to communicate with each other so that a short-circuit or ground-fault can be isolated and cleared by the nearest upstream circuit breaker with no intentional time delay. Devices in all other areas of the system (including upstream) remain closed to maintain service to unaffected loads.

Without ZSI, a coordinated system results in the circuit breaker closest to the electrical fault clearing it, usually with an intentional delay. With ZSI, the device closest to the electrical fault ignores its preset short-time and ground-fault delays and clears the electrical fault with no intentional delay.

Zone-selective interlocking eliminates intentional delay without sacrificing coordination and it results in faster tripping times. This limits stress on the system by reducing the amount of let-through energy the system is subjected to during an overcurrent.

The coordination of the system must be correctly set up for zone-selective interlocking to work.

Operating Principle

A pilot wire interconnects a number of circuit breakers equipped with MicroLogic trip units, as illustrated in the following diagram.

The trip unit detecting an electrical fault sends a signal upstream and checks for a signal arriving from downstream. If there is a signal from downstream, the circuit breaker remains closed for the full duration of its time delay. If there is no signal from downstream, the circuit breaker opens immediately, regardless of the time delay setting.

An electrical fault occurs at point A.

Downstream device (2) clears the electrical fault and sends a signal to upstream device (1), which maintains the short-time time delay tsd or the ground-fault time delay tg to which it is set.
An electrical fault occurs at point B.

Upstream device (1) detects the electrical fault. In the absence of a signal from a downstream device, the set time delay is not taken into account and the device trips according to the zero setting. If it is connected to a device further upstream, it sends a signal to that device, which delays tripping according to its tsd or tg setting.

NOTE: On device (1), the tsd and tg time delays must not be set to zero because this would make selectivity impossible.

Connections Between Trip Units

A logic signal (0 or 5 V) can be used for zone selective interlocking between the upstream and downstream circuit breakers equipped with:

MicroLogic 5.0 A, 6.0 A, 7.0 A.
MicroLogic 5.0 E, 6.0 E.
MicroLogic 5.0 P, 6.0 P, 7.0 P.
MicroLogic 5.0 H, 6.0 H, 7.0 H.

An interface is available for connection to previous generations of trip units.

Wiring

Technical characteristics of wires:

Maximum impedance: 2.7 Ω / 300 m (1 000 ft)
Capacity of connectors: 0.4 to 2.5 mm² (AWG 22 to 14)
Wires: single or multicore
Maximum length: 3000 m (10 000 ft)
Limits to device interconnection:
- The common ZSI - OUT (Z1) and the output ZSI - OUT (Z2) can be connected to a maximum of 10 upstream devices.
- Maximum of 100 downstream devices may be connected to the common ZSI - IN (Z3) and to an input ZSI - IN CR (Z4) or GF (Z5).

NOTE: Terminals Z1 to Z5 correspond to the identical indications on the circuit-breaker terminal blocks.

NOTE: If the protection function is not used on circuit breakers equipped for ZSI protection, a jumper must be installed to short terminals Z3, Z4 and Z5. If the jumper is not installed, the short-time and ground-fault time delays are set to zero, whatever the position of the adjustment dial.

Test

Check the wiring and operation of zone selective interlocking between a number of circuit breakers by using EcoStruxure Power Commission software installed on a PC and connected to the MicroLogic trip unit through the Service Interface.

For more information, refer to Testing the MicroLogic A/E Trip Unit.