Circuit Breaker Safety Interlock Systems Explained

Photo: Square D Masterpact (Insulated Case)

Drawout circuit breakers are equipped with safety interlock devices that are required by various Industry Standards and Certifying Authorities. Interlock systems are an integral part of keeping qualified personnel safe and protecting equipment from catastrophic damage.

When servicing circuit breakers during routine maintenance, all interlocks should be operated to confirm that they function as required. Caution must be taken to ensure that any interlock lever is not bent and caused not to function.

The standard interlock devices described below are used only on drawout breakers, as stationary or fixed breakers have no required interlocks. Interlock devices for special circuit breaker applications are also available as options.


1. Drawout Cradle Rejection Hardware

Generally speaking, drawout circuit breakers of the same type and rating are interchangeable in their equipment compartments but circuit breakers of different frame sizes are not interchangeable.

To prevent inserting the wrong type of circuit breaker into a drawout compartment, suitable “rejection hardware” is affixed to each breaker and its compartment. When the wrong type of breaker is inserted into a compartment, the hardware prevents the breaker from seating itself into the drawout rails or cubicle.

Rejection hardware works to prevent the insertion of circuit breakers with an inadequate interrupting capability, physically incompatible primary disconnects or an incompatible phase sequence, rejection interlock key plates are provided on both the circuit breaker and cassette Photo: Square D.

The rejection interlocks are steel pins affixed on the circuit breaker frame. As the breaker is pushed into the structure, the mating pins on the circuit breaker move past a set of corresponding pins in the compartment, if the circuit breaker and compartment are compatible. If the circuit breaker and the cassette are mismatched, the rejection pins will block the insertion of the circuit breaker into the cassette before the racking mechanism is engaged.

Within any one physical frame size drawout circuit breakers come in a variety of continuous current and interruption ratings, some of which are incompatible with others. Doublewide circuit breakers may also come with several phase sequence options, which are also incompatible.

2. Racking Mechanism Interlock

The function of the racking mechanism interlock is to prevent the breaker from moving from the connection position before the breaker is in the open position. This interlock is usually a simple cover that must be moved aside to gain access to the drive shaft.

Racking interlocks prevent a circuit breaker from being connected to energized bus in the closed position. Photo: EATON Magnum DS.

The racking interlock cover can be linked to the trip button to ensure the circuit breaker is open before attempting to rack. When the racking screw cover is open, it holds the trip button in.

With the trip button held in, the breaker will be kept in a trip-free state so a mechanism closing cycle will not cause contact movement especially when the breaker is being racked in or out. Always use the correct racking mechanism wrench for racking a circuit breaker in or out, otherwise the trip-free interlock feature may not function.

3. Disconnect Position Interlock

The function of the Disconnect Position Interlock is to block the racking screw cover open when the racking mechanism is in the disconnected position.

When the cover is held open, the TRIP button is depressed. The mechanism is held trip-free and there is no contact arm movement when the closing spring is charged by the Closing Spring Interlock.

4. Closing Spring Interlock

The function of the closing spring interlock is to discharge the closing spring as the breaker is being racked out of its housing. This eliminates the hazard of a completely charged breaker being discharged after the breaker is removed from its compartment.

The racking mechanism arms and the crank are connected to a common shaft. As the breaker is racked out, a pin attached to the crank moves through a slot in the mechanism linkage. The linkage is connected to a lever that engages with a pin on the closing solenoid armature linkage.

When the racking mechanism approaches the disconnected position, the crank pin reaches the end of the slot in the linkage. Continued motion of the racking mechanism causes the linkage to rotate the lever that moves the closing solenoid armature forward. The armature linkage then releases the prop, discharging the closing spring.

Important: Unnecessary force on the racking handle at the fully racked out position may cause the lever to bind up the overall interlock. Under these conditions, continued application of this force will deform the linkage assembly.

5. Positive Interlock

The positive interlock is located on the circuit breaker frame. The function of the positive interlock is to keep the breaker trip-free while it is being racked in or out between the connected and test positions.

As the breaker moves between the connected and test positions, the positive interlock engaged with a ramp cam located in the breaker cubicle. This cam raises the interlock lever assembly causing the trip shaft to rotate and prevent the trip latch from engaging with the secondary latch assembly roller. The breaker is held trip-free and cannot be closed during this interval.

6. Padlocks

Provisions are made on all circuit breakers to use padlocks to prevent the breaker from being closed. Padlock shackles are usually found through the trip button or racking screw cover. In either case, the shackle holds the trip button keeping the mechanism trip-free.

Circuit Breaker Padlock Example
Padlock shackles are usually found through the trip button or racking screw cover. Photo: Youtube.

7. Key and Door Interlock

Optional interlocks include key interlocks and door interlocks. The function of the key interlock is to prevent an open breaker from being closed when the lock bolt is extended and its key is removed.

Photo: Key interlocks prevent an open breaker from being closed. Photo: Square D Masterpact.

Door interlocks prevents the breaker compartment door from being opened. On drawout breakers, these devices are mounted in the equipment and are part of the circuit breaker enclosure.

8. Mechanical Interlock

Families of mechanical interlocks are available to interlock the closing of two or three circuit breakers. The mechanical interlock holds one or more circuit breakers tripped (prevents closure) when the others are closed.

A lever assembly is mounted on each breaker that interfaces with the pole shaft and the tripper bar. The lever assemblies are interconnected with either cables or rods, depending upon the relative orientation of the breakers.

Rods can be used only when the circuit breakers to be interlocked are vertically stacked. Cables can be used for any orientation of the breakers. Mechanical interlocks are available for both fixed and drawout circuit breakers and in both 2-way and 3-way versions.

9. Blown Fuse Interlock

Some circuit breakers have integrally mounted current limiters on the drawout breaker element. On overloads and faults within the circuit breaker interrupting rating, the circuit breaker protects the limiters. On higher fault currents exceeding the circuit breaker rating, the limiters protect the circuit breaker.

Example of a circuit breaker blown fuse indicator. Photo: General Electric

Interlock arrangements trip the circuit breaker whenever any limiter blows (open circuits). The circuit breaker cannot be reclosed on a live source unless there are only unblown limiters on the circuit.

The blown fuse indicator, located on the front of the circuit breaker, provides a visual indication when a current limiter in any phase has interrupted a short circuit. In addition, a blown limiter sensing circuit insures that a circuit breaker will be tripped when any current limiter has blown, preventing single phasing.

10. Bell Alarm with Lockout

The Bell Alarm with Lockout module prevents reclosing of a circuit breaker after a trip until the Bell Alarm with Lockout is reset. It can only be reset by pressing the reset button on the module itself.

This module also provides a switch to remotely indicate that the circuit breaker has tripped. In addition to activation by protection trips, the Bell Alarm with Lockout accessory can be set up to interact with other accessories, such as a Shunt Trip or Undervoltage Release coils.

Circuit breaker bell alarm with lockout explained
Circuit breaker bell alarm with lockout. Photo: GE Powerbreak.

In addition, the Bell Alarm with Lockout can provide normally open (NO) and normally closed (NC) alarm outputs available at the secondary terminal block located on the circuit breaker frame. These outputs are returned to their normal state when the Bell Alarm with Lockout reset button is pressed.