GuideSwitchgear and motor control8 min read
Kirk key interlocks
A Kirk key interlock is a mechanical lock whose key can only be removed when the equipment it’s fitted to is in a safe state: a breaker open, a switch grounded, a door shut. Because one key can only be in one lock at a time, a set of locks enforces a sequence: you can’t open a transformer’s cable compartment until the breaker feeding it is open and locked, and you can’t close that breaker while the compartment is open. Kirk is the best-known maker, so the name is used for trapped-key interlocks in general.
The short version
- A trapped-key lock holds its key until the equipment is put in the safe state; turning the key then bolts the equipment in that state and frees the key.
- The freed key is the only thing that opens the next lock in the sequence, where it’s trapped again until that equipment is restored.
- Common schemes: feeder breaker to transformer compartment; main-tie-main switchgear (two of three closed); generator transfer; grounding switches before access.
- Each key is cut to a code; one code, one key. A spare key defeats the scheme, so keys are controlled and duplicates come only from the maker, to the code.
- On used switchgear, locks are often present and keys are gone. Locks can be re-keyed to a new scheme; the drawings for the scheme are what you need.
What a Kirk key interlock is
The lock is a heavy steel cylinder lock with a bolt, mounted on a breaker mechanism, a switch handle, a door or a cover. In one position the bolt blocks the equipment (the breaker can’t close, the door can’t open) and the key is free. In the other the equipment is released and the key is trapped. The key can only be turned, and so only removed, when the equipment is in the state the lock was mounted to guard.
That’s the whole mechanism. What makes it useful is giving the key somewhere else to go: a second lock, on a second piece of equipment, that will only release that equipment when this key is in it. The locks don’t talk to each other and need no wiring; the key is the logic.
How a trapped-key sequence works
The scheme in nearly every plant: a 15 kV feeder breaker and the pad-mount transformer it feeds. The transformer’s cable compartment must not be opened with the feeder live, and the feeder must not be closed with the compartment open. One key, two locks.
- With the breaker closed, the key is trapped in the lock on the breaker. The transformer’s door lock has no key, so its bolt holds the door shut.
- The operator opens the breaker and turns the key. The lock’s bolt now blocks the closing mechanism, and the key comes free. The breaker can’t be closed while the key is out.
- The key is carried to the transformer and turned in the door lock, which withdraws the bolt. The door opens, and the key is trapped there until the door is shut and bolted again.
- Shut the door, turn the key, carry it back, and the breaker is released to close. There is no order of operations that leaves the compartment open with the feeder able to close.
Add a grounding switch between the two, with its own lock, and the sequence becomes open the breaker, close the ground, then open the door, each step releasing the key for the next.
Where they’re used
| Scheme | What it prevents | Keys and locks |
|---|---|---|
| Feeder breaker to transformer or switch compartment | Opening a medium-voltage compartment with its feeder live; re-energizing with it open | One key: breaker lock and door lock |
| Main-tie-main (double-ended substation) | Closing both mains and the tie at once, which parallels two sources or two transformers | Two keys, three locks: each main’s lock frees its key when the main is open; the tie needs a key to close. At most two of three closed |
| Generator transfer | Paralleling a standby generator with the utility without a synchronizing scheme | One key shared between the utility breaker and the generator breaker |
| Grounding switch before access | Entering a cell or a cable vault before the circuit is grounded | Breaker open releases a key; the key closes the ground switch; the ground switch closed releases the access key |
| Capacitor banks | Opening a bank before it has discharged | A time-delay lock holds the key for five minutes after de-energizing |
| Medium-voltage motor starters | Opening the starter door with the isolating switch closed | A lock on the isolator and one on the door |
On metal-clad switchgear the breakers already carry mechanical racking interlocks; the trapped-key locks are added where the hazard reaches beyond one piece of equipment, which is nearly always the case with a transformer at the far end of a feeder.
The parts of a scheme
- Bolt lock
- The basic lock: turning the key extends or withdraws a bolt, which blocks a door, a cover, a handle or a mechanism.
- Breaker lock
- A lock fitted to a breaker’s mechanism, usually blocking closing (or racking) when the key is out. Switchgear makers fit these to order.
- Access lock
- A lock on a door or removable panel. The key is trapped while the door is open.
- Key exchange unit (transfer block)
- A box where one key goes in and several come out, or several must go in before one comes out. It turns a single released key into access to many compartments, or demands several conditions before one operation.
- Solenoid key release
- A lock that only releases its key when an electrical condition is met: a voltage detector reading dead, or a timer run out.
- Time-delay lock
- Holds the key for a set time after it’s inserted, for capacitor discharge or a motor to spin down.
- Key cabinet
- Where the free keys live, with their codes labeled. The scheme drawing is kept with it.
Each key is cut to a code, usually a few letters and numbers stamped on it and on the lock. Locks sharing a code accept the same key; different codes don’t. A scheme is designed as a set of codes and a drawing showing which lock on which equipment carries which code.
Specifying and ordering
- Draw the sequence first. What must be true before each door opens or each breaker closes. The maker’s application engineers will turn a one-page sequence into a scheme of locks and codes; this is a free service at the main makers.
- Name the equipment and mounting. Breaker make and model (the lock kit is specific), the door or cover, and whether the lock mounts on the switchgear maker’s bracket or on a new one.
- One key per code. Order no spares. If a key must be in two places (a key that both releases a door and sits in a cabinet for records), the scheme needs an exchange unit, not a copy.
- Match existing codes where a new unit joins an existing scheme: a replacement transformer’s door lock must take the key the old one took.
- Stainless and weatherproof for pad-mounts and outdoor gear; the locks last decades, but the cheap ones don’t.
Kirk keys on used switchgear
Used switchgear and pad-mount transformers often arrive with their trapped-key locks still fitted and no keys: the keys stayed with the old plant’s scheme, which is right. The locks are worth having. The maker will re-key a lock to a new code, or supply a new cylinder for it, so the used gear joins your scheme rather than a stranger’s.
What to check on a listing or at inspection:
- Which locks are fitted, on which breakers, doors and switches, and their codes (stamped on the lock face).
- Whether any keys come with the equipment, and whether the seller has the scheme drawing. A key alone is no use without it.
- That the breaker locks are the right kit for the breaker model; a lock from a different frame won’t block the mechanism.
- That bolts move freely. A seized outdoor lock is a sign of how the gear was stored.
Looking for gear with a particular arrangement? Describe the lineup to Magic Search, interlocks included, and it reads each seller’s page for them.
Questions people ask
What is a Kirk key?
A Kirk key is the key of a trapped-key interlock made by the Kirk Key Interlock Company, and by extension any trapped-key interlock. The lock is fitted to a breaker, switch or door and holds its key until the equipment is in the safe position; the key then opens the next lock in a sequence. The system is purely mechanical, needs no power, and can’t be bypassed without defeating a lock.
How does a Kirk key interlock work?
Each lock has two states. With the key in it, the equipment is free to operate and the key can’t be withdrawn. Putting the equipment into the safe state (opening a breaker, say) lets the key turn and come out, and with the key out the lock’s bolt holds the equipment in that state. The key then fits only the next lock in the scheme, where the same thing happens in reverse. One key per code means only one piece of equipment in the sequence can be in the unsafe state at a time.
Is a Kirk key interlock the same as lockout/tagout?
No. Lockout/tagout is a procedure: a worker puts a personal padlock and tag on an isolating device before working. A Kirk key interlock is an engineered safeguard built into the equipment that makes the wrong order of operations physically impossible. They work together; an interlock enforces the sequence, and lockout protects the person doing the work.
Can you get a spare Kirk key?
Only from the maker, cut to the lock’s code, and only with authorization, because a second key in circulation defeats the interlock: with two keys, two breakers in a two-of-three scheme could be closed at once. Sites keep keys in a controlled cabinet and treat a lost key as a safety incident; the usual remedy is to re-key the scheme.
Who makes trapped-key interlocks?
Kirk Key Interlock Company (the Kirk brand), Castell, Superior Interlock, Fortress Interlocks and Trapped Key Interlock are the main makers in North America and Europe. The products are similar; the keys and codes are not interchangeable between makers.
Are Kirk key interlocks required by code?
Not by name. The switchgear standards (IEEE C37.20.1 for low-voltage switchgear, C37.20.2 for metal-clad) require mechanical interlocks on breaker racking and compartment access, and utility and plant specifications commonly call for trapped-key interlocks on transformer compartments, tie breakers and grounding switches. NFPA 70E’s requirement for an electrically safe work condition is what the schemes are built to guarantee.
Sources: IEEE C37.20.1, C37.20.2 and C37.20.3 (switchgear interlocks); NFPA 70E (electrical safety in the workplace); makers’ scheme catalogs (Kirk, Castell, Superior).