Lockout relay (ANSI 86) — why a protection trip does not clear itself but requires a manual reset

The guides on transformer differential protection (87T), motor differential protection (87M), and busbar differential protection (87B) cover how an internal fault in the protected unit is detected. This article covers a function that does not itself detect a fault, but determines what happens after a detection: the lockout relay, also called a master trip relay, designated by ANSI code 86.

The lockout relay's role: intermediary, not detector

An 86 relay contains no measuring function for current, voltage, or impedance itself — it responds solely to the trip command from one or more protection relays (for example 87T, 87M, 87B, or an overcurrent or earth-fault relay) and then performs two tasks simultaneously:

Multiplying the command: a single trip signal from a protection relay is converted, via the 86 relay, into multiple, independent output contacts with a high current rating — needed to simultaneously trip the breaker, activate an alarm, send a SCADA notification, activate a close-interlock on the same or an adjacent breaker, and drive any other interconnected equipment.
Latching the state: the 86 relay remains in the tripped/locked-out state, even after the original protection signal has already cleared, until it is deliberately reset.

Why a manual reset, and not automatic recovery

For a protection function indicating an internal fault in a major or expensive unit — an internal winding fault in a transformer or motor, or a busbar fault — automatic recovery after tripping is undesirable: unlike a transient fault on an overhead line (see the guide on automatic reclosing (79)), an internal differential fault is almost always permanent, physical damage to the protected unit. An 86 relay that only releases after a deliberate, manual reset forces a person to first investigate the cause of the trip — for example by inspecting the transformer, motor, or busbar and, if needed, performing an insulation-resistance measurement — before the installation is re-energized. Without this manual step, an installation with a persistent internal fault could potentially be repeatedly switched on and off, risking further, cumulative damage.

Electromechanical versus digital lockout relay

Traditionally, an 86 relay is an electromechanical device with a physical, hand-operated reset lever or button, which visibly shows the locked-out state and only returns to the normal position after physical intervention. Modern, digital protection relays can implement a comparable lockout function in software, but the underlying principle — latching the command until a authorized person deliberately resets it — remains the same, whether implemented electromechanically or digitally.

Note: the exact assignment of which protection functions are wired to which lockout relay, and which reset procedure applies, follows from the design of the specific switchgear or distribution equipment; this article covers the principle, not a ready-made wiring table for every installation.

Practical relevance

When investigating a tripped breaker, it is important to first check which lockout relay was operated and which underlying protection function caused it, before proceeding to a reset — the lockout relay itself typically only shows that a trip occurred, not necessarily the precise cause; that information must be retrieved separately from the connected protection relay or the event log.

Common mistakes

Resetting a lockout relay without first investigating the underlying cause of the trip — this can re-energize an installation with a persistent internal fault before the fault has actually been resolved.
Wiring multiple critical protection functions to the same lockout relay without sufficient, independent output contacts — this can limit the required simultaneous actions (breaker trip, alarm, interlocking of adjacent breakers).
Not periodically functionally testing an electromechanical lockout relay — a relay that has not operated for years may fail to latch or reset correctly when a genuine fault occurs.
Applying automatic reclosing (79) to a breaker controlled through a lockout relay without correctly separating the two functions — a permanent internal fault must always remain locked out, even if the breaker is otherwise configured for automatic reclosing elsewhere.