Recognizing and controlling hazards is essential when working on energized equipment
Every year we read about electricians, engineers, and supervisors who were seriously burned or killed while working in or near a circuit breaker (CB) compartment. I've personally investigated accidents where workers inadvertently contacted energized equipment inside a CB compartment, which initiated a tremendous arc flash that ignited their clothing and resulted in serious burn injuries. Some of the arc blasts extended 50 feet beyond the compartment and burned everything and everyone within that range.
An arc flash generates extreme heat (as high as 35,000°F) that can vaporize copper, aluminum, and steel. These flashes can also generate dangerous pressure and noise levels, and their magnitude of current can easily exceed 20,000A. Recently, an entire town was without electricity for several hours because four workers made a mistake inside a CB compartment that resulted in a powerful arc flash incident. Although they were seriously injured, and had to be evacuated by helicopter to local burn centers, they were lucky enough to live through the ordeal.
Most large (high-energy) CBs (i.e. 480V to 13kV) are installed in metal compartments and can be racked-in, racked-out, racked to test position, and completely removed from the compartment (Photo 1). These CBs typically control high-energy equipment and are capable of containing an arc flash if operated and maintained properly. If performed improperly, however, racking-in or racking-out a CB can be a very dangerous task.
CBs can be racked-in horizontally or vertically. The racking mechanism usually activates a built-in shutter/barrier that exposes the ports in the CB compartment while the CB is racked-in and covers the ports while the CB is racked-out. You should always follow the manufacturer's instructions for racking-in, racking-out, or removing a CB from its compartment. Remember, working in a CB compartment with the unit completely removed can result in catastrophic consequences if approved safe work practices are not followed.
So how do you protect yourself from the dangers associated with this type of work? By following a strict set of work procedures and safety practices.
Typical 3-phase CB compartments contain six ports. When the CB is removed from the compartment, you shouldn't have access to any of the ports because the shutter mechanism is designed to cover all of them (Photo 2). But how do you know which ports are bus, line or load? I've worked in facilities that used a variety of terms to identify these ports, including “bus and line,” “line and load,” “bus and load,” “upper and lower,” and “front and rear.” In some facilities, many CB ports have no identification at all. When faced with this situation, it's up to you to determine the meaning of these terms.
When working in a CB compartment you should adhere to the following requirements:
Follow approved written procedures.
Isolate the compartment from all sources of energy.
Implement an approved lock-out/tag-out procedure.
Use insulating barriers.
Wear flame retardant clothing, rubber insulating gloves with leather protectors, a face shield, and hearing protection.
Because complete isolation usually involves shutting down the entire bus section, many electricians, engineers, and supervisors chose to work in the CB compartment while the bus is energized, but this can have fatal results. The quick and easy way to conduct tests on the power distribution system components is to remove the CB from the compartment and take measurements through the appropriate ports. But how do you know which ports are the appropriate ones? Is it the bus, line, load, front, rear, upper, lower, or unmarked port?
Keep in mind that it's not only the main power feed that can harm you. In addition to the primary power inside the CB compartment, the control circuit power and power that supplies compartment alarms, relays, and heaters could also injure or electrocute you.
As noted earlier, when you remove a CB from its compartment, the shutter/barrier mechanism covers all of the ports to prevent access to those that are still energized. The manufacturer had good reason for such a design. However, you've probably also learned how to expose the ports by operating the CB racking mechanism without installing the CB, which will expose six ports; three are energized and three are de-energized. Do you know with certainty which ports are de-energized? If you're working in a tie CB compartment all six ports are energized.
As discussed earlier, if the ports are marked (many have no identification), you must first determine the meaning of the identification. Your life and the lives of those in the vicinity of the compartment are at risk if you make a mistake. If you wear appropriate personal protective equipment (PPE), you markedly increase your chances of avoiding serious injury — even if you make a mistake.
If you know that three ports are energized and three are de-energized, you should use an appropriately rated voltage detector to check for the presence of voltage. It may be difficult to find a high-voltage detector that includes a probe that will make sufficient contact with the recessed conductive parts inside the ports of a CB compartment, but it may be possible to work with a manufacturer to develop a custom unit.
Once you have properly identified the de-energized ports you must obtain a “safe” probe for your insulation tester or other test equipment to make contact with the de-energized conductive material that's inside the de-energized ports. Remember, if you inadvertently insert the probe into the energized ports, there's a high probability that you will be electrocuted or seriously burned.
Years ago, I investigated an incident where three workers were attempting to test a 4kV motor from a CB compartment. One worker contacted the energized ports with a probe while it was connected to a grounded insulation tester. Although the test leads were 16 AWG (relatively small conductors), they didn't open or isolate the fault current. Instead, as the test leads began to melt, the air ionized and the resultant arc flash ignited the clothing of all three workers and caused serious burn injuries and catastrophic equipment damage. The worker in the compartment was wearing insulating gloves and leather protectors on one hand. When I visited him in the burn center, the doctor indicated that he would lose the arm on the side that didn't have any PPE, while the gloved arm wasn't injured at all.
Some CB manufacturers offer ground and test devices that are designed to fit into the CB compartment after the CB is removed. When the ground and test device is fully racked-in it basically extends the bus and line ports to the front of the CB compartment. These devices incorporate key interlock systems that help ensure proper operation. They contain shutters/barriers that expose the selected ports — when the correct keys are inserted and operated — and allow testing through the ports without entering the CB compartment. However, many of the hazards described above must still be recognized and controlled when using ground and test devices.
In summary, if you must perform testing from inside a CB compartment, consider including these basic steps in your training program and written procedures:
Communicate your intentions and receive permission to perform the task from the authorized representative.
Take appropriate steps to have the equipment “shut-down.”
Implement available approved procedures.
Obtain necessary tools, test equipment, and PPE.
Wear proper class rubber insulating gloves with leather protectors, appropriate face shield, hardhat, hearing protection, and flame retardant clothing.
Secure the area to prevent access by unauthorized personnel.
Ensure the correct CB is open.
Remove the CB from the compartment.
Install an insulating barrier to cover the energized ports (Photo 3 and Photo 4 above).
Expose the de-energized ports.
Obtain an approved high-voltage detector and prove that it indicates properly (use a built-in or handheld tester to prove operation of the high-voltage detector).
Test to prove no-voltage in the de-energized ports.
Retest your high-voltage detector to prove it indicates properly.
Obtain an approved test probe for the test equipment to be used.
Complete the necessary testing.
Restore the equipment to normal condition.
Close and latch the CB compartment doors.
Report completion of the project to authorized personnel.
Company-approved work procedures, safe work practices, training programs, and the proper selection and use of tools and test equipment are meant to prevent accidents. The use of PPE is designed to prevent injury when you make a mistake.
Kaldon is president of Broad Run Consultants Ltd., Coatesville, Pa.