Kirk and Castell trapped key interlocking: how it works

The trapped key principle

A trapped key interlock is built from locks mounted on each piece of equipment and a key that travels between them. Each lock has a bolt that blocks the equipment mechanism: the operating mechanism of a breaker, the handle of a disconnector or the door of a compartment. The rule is always the same:

• The key can only be removed when the bolt is thrown, that is, when the equipment is locked in its safe position.
• While the key is out, the equipment cannot leave that position.
• To operate it again, the key must be brought back, inserted and turned, which withdraws the bolt and traps the key.

With a single key per sequence, step three cannot be done without steps one and two. The system does not depend on the control supply, needs no wiring between equipment and works the same with switchgear from one manufacturer, a transformer from another and a panel from a third. Kirk and Castell, now part of the same industrial group, are the names used generically, but other manufacturers apply the same principle.

Typical sequences

MV switchgear, transformer and door. This is the classic sequence of a distribution substation or an industrial electrical room:

To re-energize, the sequence runs in reverse: door closed, key B to the earthing switch, earthing switch open, key A to the panel, panel closed. If the sequence includes the earthing switch, the transformer stays earthed while work is done.

Transformer low-voltage side. When the transformer can be energized from its low-voltage side, for example from a standby generator or a tie with another transformer, the sequence must include the low-voltage breaker. Otherwise someone can open the MV panel, enter the transformer enclosure and find it energized from the other side.

Low-voltage transfer. In a switchboard with two incomers and a tie, a classic way to prevent paralleling is to use two keys for three breakers. Each breaker can only close with a key inserted, and since there are two keys, only two of the three can be closed. The same logic can be solved with cable or rod mechanical interlocks, available on low-voltage air circuit breakers.

Utility and standby generator. A key between the utility breaker and the generator breaker prevents both from closing at the same time when there is no automatic transfer scheme designed for paralleling.

Yard and switchgear access. Fences, compartment doors and terminal covers can be part of the sequence, so that access is only possible with the equipment isolated and earthed.

Key exchange boxes

When a sequence needs more than one key, a key exchange box is used. The keys of all the equipment that must be isolated are inserted into it and, only when all of them are in, the access keys are released. While any access key is out, the equipment keys stay trapped in the box. It is the way to require several sources to be isolated before work is allowed.

Advantages and limits

Advantages. It works with no control supply, has no wiring that can be damaged, the sequence is visible to the operator, and it can be added to existing equipment. That is why it is common in medium and low voltage and in industry, where equipment comes from different manufacturers and does not share a control system.

Limits:

• It measures nothing. The key confirms the mechanical position of the equipment, not the absence of voltage. Equipment energized from a source not included in the sequence goes undetected.
• Copies and master keys. A duplicate key breaks the logic, because it releases two locks at once. Manufacturers use codes unique to each installation and keys that are hard to copy, but controlling copies and any master key is a matter of procedure.
• Changes to the installation. If a source is added, such as a generator or a tie, and the sequence is not redone, the interlocking no longer protects what it used to.
• Maintenance. A seized lock leads to forcing or bypassing it. That is the origin of many defeated interlocks that nobody restores.

Where it fits among other interlocks

In MV switchgear, IEC 62271-200 requires internal interlocks between the switching device, the earthing switch and access to the compartments; the trapped key is used above all to link the panel with equipment that is not part of it, such as the transformer or the low-voltage switchboard. In high voltage it is used for access and for equipment without a motor operator, combined with the logic interlocking of the bay controller. The conditions inside the panel are covered in MV switchgear and RMU interlocking.

How it is verified

The test consists of trying every forbidden operation with the key in every possible position and checking that the mechanism prevents it. Each key must come out only in the safe position, the number of keys must match the design, there must be no uncontrolled copies, and the sequence must cover every source, including those on the low-voltage side. The key register and the location of each key are part of the installation documentation.

We design your sequence

Our team reviews and designs trapped key interlocking sequences and their integration with electrical interlocks, for substations, industrial electrical rooms and solar plants. See secondary engineering or contact us with your installation single-line diagram.

Related guides

• MV switchgear and RMU interlocking
• Low-voltage breaker and tie interlocking
• Interlocking at EHV, HV, MV and LV: what changes

References

• IEC 62271-200, AC metal-enclosed switchgear and controlgear for rated voltages above 1 kV and up to and including 52 kV (interlocks).
• IEC 61936-1:2021, Power installations exceeding 1 kV AC, 6.2.16 (interlocking).
• EN 50110-1, Operation of electrical installations, 6.2.
• Kirk Key Interlock, data sheets for type B locks and key exchange boxes (as a manufacturer example).
• Castell Safety, trapped key interlocking documentation (as a manufacturer example).

Frequently asked questions