Yes, Zone Selective Interlocking (ZSI) can be used to meet selective coordination requirements between an upstream and a downstream breaker. In fact, ZSI is a method of achieving selective coordination.

Here's a breakdown of why and how: Selective Coordination Explained

Selective coordination means that only the breaker closest to a fault trips, isolating the faulted circuit while leaving the rest of the system operational. This requires that downstream breakers trip before upstream breakers for faults within their respective zones.

How ZSI Works



Communication

ZSI relies on communication between the upstream and downstream breakers. Downstream breakers signal to upstream breakers when they detect a fault within their zone.

Restraint Signal

The downstream breaker sends a "restraint" or "blocking" signal to the upstream breaker.

Time Delay

The upstream breaker, upon receiving the restraint signal, introduces a time delay before it trips. This allows the downstream breaker to clear the fault first.

Escalation

If the downstream breaker fails to clear the fault within a specific time, it removes the restraint signal. The upstream breaker then trips after its normal (longer) time delay, providing backup protection.

ZSI's Role in Selective Coordination



Achieves Selectivity

By delaying the upstream breaker's response, ZSI ensures the downstream breaker trips first for faults in its zone, guaranteeing selectivity.

Maintains Speed

Without ZSI, upstream breakers often have longer intentional time delays to ensure selectivity. ZSI allows the upstream breaker to have faster instantaneous or short-time trip settings for faults further upstream (outside the downstream breaker's zone), improving system protection speed for those events. The delay is only activated when the downstream breaker detects a fault in its zone.

Coordination is Still Essential



Settings Matter

ZSI doesn't magically guarantee coordination. The specific pickup and time delay settings of the breakers must be carefully selected and coordinated with each other, considering the ZSI function. You need to perform a coordination study, taking the ZSI scheme into account.

Beyond ZSI

ZSI typically only works for short-time or instantaneous tripping functions. Overcurrent coordination (e.g., using time-current curves) is still necessary to coordinate long-time overloads and longer duration faults.

In summary

ZSI is a powerful tool for achieving selective coordination, particularly for short-time and instantaneous trip functions. It doesn't replace the need for a proper coordination study, but it enables faster tripping speeds upstream while maintaining selectivity.

Therefore, the answer is YES, Zone Selective Interlocking (ZSI) can be used to meet selective coordination requirements between an upstream and a downstream breaker. They have to coordinate regardless of ZSI, and the ZSI is an important part of that coordination scheme. Flag for review