With precautions, utilities can mitigate the risks of arc flash injuries involving switchgear equipment
by Jerry Earl, ABB
Within today’s medium-voltage metal-clad switchgear, the reduction of arcing incident energy—resulting in a reduction of arc flash calories per centimeter squared—can be achieved through a combination of personnel preventative action and switchgear design.
Mitigation in arc flash-prone electrical equipment has become a power industry safety focus in recent years. With the increase of this kind of equipment has come an avalanche of information from companies to articles to products. As a result, calculating arc flash energy is easier.
These calculations yield the energy level and boundary distance. The correct personal protective equipment (PPE) then can be determined using calculation results and the National Fire Protection Agency tables. Arc fault reduction, however, must occur through design and practice, as well.
Following are some practical tips to reduce arc flash risks. Three concepts should be followed:
- 1. Move all people away from the switchgear in distance and space.
- 2. Redirect arc flash energy away from any personnel.
- 3. Reduce arcing time.
When looking to reduce arc flash energy levels, realize that safety options are different for existing equipment and new or newly installed equipment.
|Arc-resistant switchgear redirects arc Flash|
Safety With Existing Electrical Equipment
“- Move personnel far from the equipment when practical. Use remote racking devices to install and remove electrical devices such as breakers and PT/CPT fuse trucks from the compartments. Most manufacturers have options for racking breakers with a motorized device, which allows operators to be 20-50 feet away. Extended racking handles also should be used. Most racking handles are 2 to 3 feet long. Most suppliers can provide a 4- to 10-foot-long extended handle that moves personnel further from dangerous equipment.
“- Upgrade relay systems and reduce arc flash time through relays and controls. Consider upgrading your system with an arc flash mitigation relay system such as ABB’s REA101 series. This type of relay uses a fiber-optic sensor and can detect a trip in 2.5 milliseconds and 80 percent reduction in arc flash energy when compared with the 200-800 millisecond time frame for a main breaker to trip on overcurrent. Install a maintenance mode switch in your equipment if you have microprocessor-based relays. Changing all relay settings to instantaneous overcurrent trip reduces arc flash energy.
“- Update or replace the equipment. The No. 1 failure in switchgear results from cable termination failures followed closely by breaker failure. Replacing older breakers can reduce art fault risk significantly. Having a certified provider fully inspect the switchgear, including the bushings, insulators, bus insulation and all grounding connections, ensures the integrity of older equipment. To reduce cable termination failures, one must observe the failure mode of this type of failure, which typically shows evidence of significant heat buildup before failure. New infrared technologies can be used on buses and connections. Wireless and wired sensors are easy to install and can detect loose connections and failing insulation before an arc fault.
Safety When Purchasing New Equipment
When purchasing new electrical equipment, ensure it protects personnel better.
Customize your switchgear:
- Specify automatic secondary disconnects, which eliminate opening doors during racking.
- If avoiding remote racking units, order racking motors mounted on each breaker. This allows racking of breakers in and out of the gear without personnel at the equipment.
- Specify magnetic actuated breakers, which limit the amount of service required and limit personnel time near the switchgear.
- Building infrared viewing ports into the switchgear allows routine inspection for hot spots.
Check out relay options in new switchgear. Many new microprocessor relays have arc flash detection built in as an option. Installing a high-speed differential relay system will help minimize arc flash levels.
|The arc-resistant switchgear pictured is installed with a roof-mounted plenum that vents exhaust gases outside the building.|
Choose arc-resistant switchgear. There are accessibility levels of arc-resistant switchgear according to IEEE C37.20.7, so pick one appropriate for your needs.
- Type 2: Provides personnel protection in front, rear and to the sides of the switchgear lineup. As long as the switchgear doors stay closed, no PPE is required to operate or monitor equipment.
- Type 2B: In addition to Type 2 protection, Type 2B isolates the low-voltage instrument compartment from the rest of the switchgear. This reduces arc flash levels to the control circuits only.
- Type 2C: This provides compartmental protection. If a fault occurs in one compartment, only that compartment will be damaged, which allows for a faster system recovery.
Nothing eliminates arc flash risk. Operators should use proper PPE and work procedures. Engineers and operators must minimize risks when possible to reduce the risk and extent of personnel injuries.
Jerry Earl is a product manager of medium-voltage switchgear for ABB. He is based in Lake Mary, Fla. Reach him at firstname.lastname@example.org.