The four positions circuit breaker must go through as they are being inserted or removed from their cubicles include connected, test, disconnected, and removed (withdrawn). See Figure 1.
- Connected-The breaker is connected to the bus, carries load current, and operates electrically (on electrically operated circuit breakers).
- Test-The primary stabs are disconnected. The secondary stabs, which carry control voltage, remain connected. The breaker does not carry load current, but operates electrically, if so equipped. On medium-voltage circuit breakers, it is common to be required to connect a separate secondary contact block in order to put the breaker in the test position. See Figure 2. Note: To engage a medium-voltage circuit breaker into the test position, grasp the T-handle, straighten, and push in firmly while the circuit breaker is in the disconnected position.
- Disconnected-Both primary and secondary stabs are disconnected. The circuit breaker does not carry load current or operate electrically. The circuit breaker remains racked within the cubicle and the door can be closed.
- Removed (withdrawn) -The circuit breaker is removed from its cubicle and placed either on extension arms or on the floor.
Figure 1. The four positions of the circuit breaker as they are being inserted or removed from their cubicles include connected, test, disconnected, and removed (withdrawn).
The following procedure is a general guideline for racking most low-voltage draw-out circuit breakers. Note: This procedure may require adjustment, depending on the circuit breaker OEM, circuit breaker design, and enclosure. OEM instructions must always be followed.
To rack a low-voltage draw-out circuit breaker, employ the following procedure:
- Obtain the proper clearances before attempting to take a power circuit breaker out of service. Clearances are notification to facility departments that certain circuits will be de-energized and that plans must be made to accommodate the outage.
- Follow proper lockout/tag out procedures as specified by the facility where work is to be performed. Use only facility-supplied locks and tags, and be certain to mark the required information on them before commencing with work.
- Verify that the circuit breaker to be operated is the correct one. De-energizing the wrong circuit breaker can cause major, possibly fatal, accidents. Emergency systems, life-support systems, lighting, and ventilation for hazardous locations can be affected by de-energizing the wrong circuit breaker.
Figure 2. On medium-voltage circuit breakers, it is common to connect a separate secondary contact block in order to put the breaker in the test position, which is often accomplished with a T-handle located on the circuit breaker.
- Disconnect load fed by the circuit breaker before operating. This reduces the chance of failure. Circuit breakers are switches and are designed to interrupt load current. However, circuit breakers are primarily a mechanical device, and mechanical devices fail. The best safe work practice is to de-energize the load before opening the circuit breaker.
- Note the position of the open/close indicator (it should indicate “OPEN”). Note: Even if the indicator shows that the circuit breaker is open, it does not mean that it is. Many less modern models of circuit breakers have indicators that are spring-loaded to the open position. If the linkage comes apart or there is some other mechanical failure, it could indicate the circuit breaker was open when it is actually closed. See Figure 4.
Figure 3. The PPE for performing work on circuit breakers must be resistant to arc flashes
Figure 4. Circuit breakers typically have an open/close indicator located on the front of the circuit breaker panel.
- Stand to the hinge-side of the cubicle, extend an arm and manually press the trip button (even if indicator shows “OPEN”). If there is a failure, the handle side of cubicle should open, while the hinge side remains connected to the cubicle. The door can provide some amount of shielding from an arc flash, although not much if the door has louvers or vents. It should still divert some heat of the arc flash away from the operator.
- Rack circuit breaker with the door closed. Note: On some less modern circuit breakers, the cubicle door can be modified by using a metal saw and circular or rectangular metal template to cut a hole in the door to allow access to the racking screw.
- Insert the racking handle while observing that the safety interlock device depresses the trip button. This is a common feature of all but the oldest circuit breakers. A safety interlock device is a device used to ensure that a circuit breaker does not get racked while the circuit breaker is in the closed position. See Figure 5.
- Slowly rotate the racking handle counterclockwise to remove the circuit breaker. Perform this task slowly. It is best practice to listen for any arcing that may occur when the primary disconnects on circuit breaker just begin to part from the bus. WARNING: If the circuit breaker does not trip open and an arcing sound is heard, discontinue racking out immediately and rack circuit breaker back in.
- Once the circuit breaker stabs are clear of the bus, the arc flash hazard is almost nonexistent. Rack the circuit breaker through the test position, unless there is an operational problem with the circuit breaker. The test position is used to check for proper functioning of the circuit breaker. Continue racking until the circuit breaker is in the disconnected position. The door may be opened at this point and extension arms (LVPCBs and some medium- voltage vacuum circuit breakers) installed in or extended from the cubicle.
Figure 5. A safety interlock device is used to ensure that a circuit breaker is not racked while the breaker is in the closed position.
- Use a flashlight or similar lighting device to effectively view the circuit breaker rollers to verify that they are resting squarely on the extension arms with all the components properly in place.
- Slowly pull the circuit breaker out of the cubicle and onto extension arms. Pull until the circuit breaker rear rollers can be seen by using a lighting device. Verify that the rollers are resting squarely on the extension arms.
- Slowly pull the circuit breaker fully out of the cubicle to the stops on the extension arms.
- A spreader bar is a rigging device that is suspended from a crane hook to support lifting lines at various locations. Spreader bars are used to evenly lift long or wide loads. When removing a circuit breaker from extension arms, use the OEM’s spreader bar and verify that the spreader bar is properly attached to the circuit breaker frame. See Figure 6. Often, one side will come loose when the cable is tightened, which can cause the circuit breaker to fall to the floor.
Do not use rope or wire slings strung through the circuit breaker frame. They will push in the sides of the frame and possibly damage the arc chutes and/or operating mechanism. If using a cable and pulley mechanism, be certain to check the cable reel before lifting the circuit breaker for any crossed turns. Crossed cables on the reel can cause a circuit breaker to “bounce” when lowering and cause the cable to snap under the instantaneous weight/force increase. See Figure 7.
- If using a lift platform to remove a circuit breaker, verify that it is completely positioned under the frame before lifting the circuit breaker.
- Slowly lower the circuit breaker onto the floor and remove the spreader bar.
Figure 6. When removing a circuit breaker from extension arms, a spreader bar must be properly attached to the circuit breaker frame.
Figure 7. Improper lifting and rigging techniques can cause extensive damage to circuit breakers and related components.
On medium-voltage circuit breakers, it is not uncommon for a slight buzzing sound to be heard when the primary stabs part. This is much different from an arcing sound, which indicates the breaker is still closed. If in doubt, stop racking and reinsert the breaker to the bus. If the circuit breaker cannot be removed from its cubicle, the entire section of switchgear must be de-energized before the problem can be corrected. Never try to remove a circuit breaker if there is any doubt that it has actually opened. An interlock device is commonly installed on most circuit breakers to prevent them from being inserted or removed from a cubicle while they are closed. As the circuit breaker is being racked in, the interlock device is designed to open the circuit breaker before the bus connections (stabs or primary disconnects) make contact. When a circuit breaker is racked out, an interlock device causes the circuit breaker to trip before the bus connections separate. Unfortunately, it is common for circuit breakers to have interlock devices that do not function properly. Always verify that the interlock devices function properly prior to performing any work.
To re-insert a circuit breaker into its cubicle, apply the following procedure:
- When replacing a circuit breaker onto extension arms, verify that the spreader bar is secure prior to attempting a lift.
- Raise the circuit breaker in a slow, smooth manner and position it so that rollers contact extension arms squarely. Often, the circuit breaker can rotate due to cable tension and must be continuously guided until it is completely resting on the extension arms.
- Once the circuit breaker is firmly on the extension arms, remove the lifting device and spreader bar.
- Push the circuit breaker into the cell until it is in the disconnected position. Manually trip the circuit breaker using the trip button, even if circuit breaker indicator shows that it is open.
- Close and secure the cubicle door.
- Rack the circuit breaker through the test position (unless it is necessary to test operate the circuit breaker). Be certain to depress the trip button before continuing to rack the circuit breaker in from this point.
- Rack the circuit breaker into the connected position. Do not over-torque the racking mechanism. Once firmly seated onto the bus connections, discontinue racking.
An over-torqued racking mechanism can fail mechanically. Over-torquing the racking mechanism can bend and eventually shear the stop pin. If the stop pin is sheared off, the circuit breaker cannot position itself correctly into the connected position and will rack through the connected position and begin to rack out again. This condition can create an extremely hazardous condition.
- Firmly secure all fasteners on the cubicle door when finished. With the door closed and properly secured, there are no perceived hazards. This does not mean that the equipment will not fail, but failures in normally operating equipment are rare.