Lockout Relay 86: What It Is and How It Works

What the lockout relay 86 is

The 86 is a bistable auxiliary trip and lockout relay. When it receives the command from a protection, it changes state and stays there: it trips the breakers of the faulted zone and blocks their closing until someone resets it. It measures no current or voltage; its job is to make a serious protection decision permanent until a person reviews it.

• ANSI/IEEE C37.2: 86, lockout relay.
• IEC 61850: there is no logical node dedicated to this function; in numerical relays it is done with the trip logic and latches.

It is used when re-energizing without an inspection would be dangerous: an internal transformer fault, a bus fault or a breaker failure. In those cases a reclose or a hasty manual close would increase the damage.

Where it is applied

• Power transformers: tripping from the transformer differential 87T, restricted earth fault and the mechanical protections (Buchholz, sudden pressure, temperature).
• Buses: the bus differential 87B trips every breaker connected to the bus through an 86.
• Breaker failure: the 50BF operates an 86 that trips the adjacent breakers and sometimes the remote terminal.
• Generators and large motors: differentials and internal protections, where the machine is also shut down and de-excited.
• Industry: main switchgear and supply transformers.

How it works

An electromechanical 86 has an operating coil, a latching mechanism and many contacts (normally open and normally closed). When the coil is energized:

1. The mechanism changes position and latches.
2. The contacts that close trip the breaker trip coils.
3. The contacts that open break the close circuits of those breakers, including the reclosing circuit.
4. Other contacts send indication and alarms, and block functions such as reclosing 79.
5. A contact of its own cuts the coil current once it has operated, so the coil does not burn out from staying energized.

The reset can be manual (a handle on the front) or electrical, through a reset coil controlled from the panel or the control system. Many utilities require local reset to force an inspection on site.

In numerical relays the same function is achieved with latched logic whose state is retained even if the relay loses supply. It can be the only solution or coexist with the physical 86.

Supervision

Since the 86 only acts on rare events, an open coil or a broken circuit can go unnoticed for years. That is why the 86 coil should be supervised, together with the trip circuits, so that a failure raises an alarm.

Selection and integration criteria

The 86 is not set like a protection, but it is selected and integrated with care:

• Coil rated voltage equal to the substation DC voltage, with margin for its variations.
• Number and type of contacts according to the breakers, blocks and signals.
• Make and break rating of the contacts for the trip coils it feeds.
• Operating time, which adds to the protection time.
• Immunity to DC grounds: inputs and coils with enough operating energy not to act on coupling or on capacitance discharge from long cables.
• Reset philosophy and which protections operate it, according to the owner’s design.
• Single or duplicated 86 scheme when there are independent main and backup protections.

These decisions are part of the panel engineering and the owner’s protection philosophy.

How each manufacturer implements it

In numerical relays, each manufacturer handles the lockout differently:

• Siemens SIPROTEC offers trip and output latching in its configurable logic in DIGSI.
• SEL uses latch bits in its logic equations, which retain their state through a loss of supply, configured in AcSELerator QuickSet.
• Relion (Hitachi Energy 670/650; ABB 615/620) offers trip latching functions and bistable logic in PCM600.
• GE Vernova (Multilin) allows latched outputs and lockouts built with its programmable logic in EnerVista.
• Schneider MiCOM / Easergy offers latched outputs and programmable logic in MiCOM S1 or Easergy Studio.

What changes is where the state is stored, how it is reset (relay front panel, binary input, remote command or IEC 61850) and what happens when the device restarts. If the logical lockout and the physical 86 coexist, it must be clear which one governs.

Why it trips or blocks

• Genuine trip of an internal protection: start by reading the relay events and targets to find out which one operated.
• Ground on the DC system: with long cables, the discharge of their capacitance into a sensitive input or coil can operate the 86 without a command from any protection.
• Transformer mechanical protection: the Buchholz or sudden pressure relay sometimes operates because of air in the oil after maintenance, or a wet contact in the terminal box.
• Errors during testing: forgotten jumpers, incomplete isolation of the trip circuit or tests on the wrong panel.
• The breaker does not close after the reset: the 86 is half reset, a contact is damaged or another interlock is still active.
• Burnt coil: the contact that cuts its supply did not act.

Before resetting, download the event record, check the condition of the protected equipment and check the DC insulation.

How it is tested

During secondary injection the whole chain is checked: the protection operates, the 86 changes state and the breakers trip. With an Omicron CMC 356, the protection time and the 86 time are measured through its binary inputs. The following are also checked:

• the minimum operating voltage of the coil;
• the state of each contact before and after operation;
• that closing of each breaker is actually blocked;
• manual and electrical reset;
• coil supervision and alarms;
• indication to the control system.

Did your relay trip or block on this function and the cause is not clear? At Tiza Group we review the events and settings remotely. See remote relay configuration or contact us.

References

• IEEE C37.2, device function numbers.
• IEEE C37.91, Guide for Protecting Power Transformers.
• IEEE C37.234, Guide for Protective Relay Applications to Power System Buses.
• IEC 60255-1, common requirements for measuring relays and protection equipment.
• J. L. Blackburn and T. J. Domin, Protective Relaying: Principles and Applications.

Frequently asked questions