Zone selective interlocking (ZSI) — faster tripping without losing selectivity between circuit-breakers

The guide on LSI(G) settings covers how selectivity between an upstream and a downstream circuit-breaker is achieved by setting the upstream breaker's short-time delay (tsd) longer than the total trip time of the downstream breaker. This article covers an additional technique that makes this fixed delay smarter: zone selective interlocking (ZSI).

The downside of ordinary time-graded selectivity

With a classic, time-graded selectivity setting, the tsd delay of an upstream breaker is a fixed value, regardless of exactly where the fault is located within the zone that breaker protects. This means that, even for a fault occurring right beneath the upstream breaker itself — where no downstream breaker needs to be waited for at all — the upstream breaker still holds the full delay designed for the deepest level of the network. For a severe fault close to the upstream breaker, this unnecessarily long waiting time means more energy is fed into the fault, and therefore higher arc-flash energy at precisely the location in the installation where the fault is most severe (see the guide on arc-flash risk and PPE).

How ZSI works: a restraint signal between successive breaker levels

ZSI links the electronic trip units of successive breaker levels with a dedicated signal connection (hardwired, or via a communication protocol on more modern digital relays). The principle:

If a downstream breaker sees a fault within its own short-time or ground-fault range, it immediately sends a restraint signal to the upstream breaker(s) in its chain.
If an upstream breaker receives this restraint signal, it holds its normal, longer tsd delay — exactly as with ordinary time-graded selectivity — giving the downstream breaker time to clear the fault itself.
If the upstream breaker receives no restraint signal within a short response time, it concludes that the fault is within its own zone (i.e. not going to be cleared by any downstream breaker) and trips almost instantaneously, instead of holding the full delay intended for the deepest level.

The result is that every breaker in the chain trips almost instantaneously for a fault in its own, nearest zone — reducing arc-flash energy — while full selectivity between the levels is preserved for a fault actually located further downstream in the installation.

ZSI supplements, rather than replaces, the LSI(G) curve

A breaker with ZSI retains its fully configured LSI(G) curve (see the related guide) as a backup: if the ZSI signal connection is not functioning for whatever reason, or is not connected (for example on a breaker not (yet) included in the ZSI network), the breaker falls back to its normal, time-graded tsd setting. ZSI therefore does not replace the underlying LSI(G) setting, but makes it dynamic: short where possible, long where needed for selectivity.

Note: the exact wiring, the signal protocol used, and the extent to which breakers from different manufacturers are ZSI-compatible with each other follow from the breaker manufacturer's specifications; this article covers the principle, not a ready-made wiring instruction for every breaker make or type.

Practical relevance

When designing or assessing a switchboard with multiple levels of circuit-breakers, it is worthwhile investigating whether ZSI is available and applicable, especially at locations where the arc-flash energy risk to personnel (for example at the main distribution board) is high: ZSI can deliver a significant reduction in incident energy there without giving up selectivity with downstream breakers — unlike simply shortening the upstream breaker's tsd setting, which would undermine selectivity instead.

Common mistakes

Assuming ZSI removes the need for correctly configured LSI(G) curves — ZSI works on top of the underlying tsd setting, as a dynamic shortening of it when no restraint signal is received, not as a replacement.
Not including the ZSI signal connection in the commissioning test — a non-functioning or incorrectly connected signal connection falls back to the normal, longer delay, which is safe for selectivity but defeats the expected arc-flash energy reduction.
Wiring breakers from different manufacturers together for ZSI without verification — not every protocol or signal implementation is mutually compatible.
Applying ZSI as a substitute for a proper selectivity calculation — ZSI improves the response time for a fault close to the breaker, but the underlying tsd grading must still be correctly calculated for the situation where no restraint signal is present.