Designing for safety: Simple steps for reducing electrical hazards
Incorporating simple steps into the design process is a cost-effective way to stay ahead of the curve.
By Rick Schlobohm, PE GE Consumer & Industrial, Houston
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All workers have a right to a safe work environment. Indeed, the O ccupational Safety and Health Administration (OSHA) of 1970 was put in place to foster worker safety as a top priority on the job. A safer environment is a more productive environment. By providing workplaces free from serious recognized hazards, workers can concentrate on the tasks at hand.
MAIN DEVICES VS. SIX-HANDLE RULE
The concern with worker safety continues to increase. This has resulted in new OSHA requirements and an NFPA 70E standard on arc flash hazards. These documents cover arc flash studies, which calculate the arc flash hazard present and offer recommendations for appropriate personal protective equipment. Arc flash studies are performed to determine the level of hazards and protocol for working on energized electrical equipment, and to provide information to educate the worker on these issues. The key point to consider is the safety of those who work on energized equipment. The designer can increase worker safety by specifying equipment with features that can eliminate the need for workers to work on such equipment.
Article 230.71 of the National Electric Code allows the use of not more than six switches or circuit breakers (known as the six-handle rule) per service for the disconnection of power at the service entrance location. Many times services are small enough that an overcurrent protective device (OPD) of sufficient amperage is available to provide a single point disconnect for the service.
One advantage of a switchboard using the six-handle rule is reduced cost because a large device is not purchased. Smaller footprints are achieved in many instances because the large main device is not provided. Also, many people feel that continuity of service is better with the six-handle service boards than with a single main device. The reasoning: A fault including ground faults downstream will not have the opportunity to disconnect the entire service by causing the main OPD to trip. Generally, with a six-handle board, only the single main serving the faulted load will trip.
Although many services installed across the country use a six-handle service entrance switchboard, there are numerous downsides to this type of arrangement.
If one of the OPDs that are part of the six-service disconnects needs to be removed for service, the electrician may have to work on an energized piece of equipment. He also may be tempted to work on energized equipment if a space for one of the six disconnects was designed into the project and is being added at a later date. The electrician may consider working on the energized equipment because of the hassle involved with getting a shutdown. The only way to de-energize the equipment is through coordination with the local utility. Many times, the electrician will not want to wait for the utility to get to the site or not want to pay the utility's disconnect/reconnect fee. The electrician might decide to work on the equipment in an energized state despite the manufacturer's instructions and safety notices.
If the service entrance equipment is provided with a main device, the electrician is at least given a readily available option to de-energize the equipment. As an added safety feature, the main device should also be located in a separate switchboard section without load-side cables coming anywhere near line-side conductors. The main device offers additional benefits. As the facility grows, more breakers or switches can be added to the main service entrance switchboard. If all of the six disconnects were used in the initial design, the owner is faced with an expensive upgrade instead of a simple additional device (Figure 1).
If during the design process cost and space become a concern to the owner or the architect, the designer should adequately inform the decision makers of the possible hazards that could exist with the six-handle service equipment in lieu of a single main device. Regardless of the service entrance design chosen, the user should always be encouraged to follow the manufacturer's instructions, recommended industry safety standards, and all posted safety notices. The worker's safety should always be of the utmost concern.
DRAW-OUT VS. FIXED MOUNTED BREAKERS
This article is aimed toward electrical distribution systems that use services with overcurrent devices rated greater than 1,200 A. To enhance a worker's safety, the designer must consider designs that minimize hazardous conditions for the technician.
Typically, any device over 1,200 A is individually mounted vs. group-mounted or panel-mounted. Many projects use switchboards that have an individually mounted main device and possibly some feeders, which are also individually mounted and fixed (bolted on) in place.
Draw-out breakers are standard for low-voltage switchgear construction. Switchboards are also available with all draw-out insulated case breakers with features similar to those available in switchgear. The specifier also has the option to provide the end customer with draw-out breakers for the larger frame sizes in many switchboard models.
There are many advantages to draw-out construction. Ease of removal for replacement, service, or testing is the key feature. However, there are other advantages. If multiple breakers of the same frame size are present on a project, then devices can be easily moved from one location to another in case of an emergency. Inspection of the breaker for routine maintenance is simplified with draw-out breakers vs. bolted devices. Continuity of service is always desired, and a draw-out device allows the technician to safely remove the breaker for maintenance.
Lock-out and tag-out is a routine practice for most maintenance procedures. Most fixed-mounted breakers are provided with a device for the technician to attach his lock-out device. However, with draw-out devices, additional safeguards can be implemented with the lock-out procedure. The breaker can be withdrawn from the cubicle and locked in the disconnected position to provide a positive open circuit. The breaker can also be removed from the cubicle and the draw-out rails locked to prevent insertion of a breaker.
When specifying draw-out devices, the designer must take into consideration that the switchboard will typically be 5 to 10 in. deeper than the same board with fixed-mounted devices. The actual cost percentage to add the draw-out devices will vary with the size and the total number of devices within a switchboard. For a simple switchboard with a 3,000 A main, a 1,600 A feeder, a variety of smaller feeder breakers, a meter, and TVSS, the cost adder would be about 10% of the price of the switchboard. A misconception exists that all switchboards with draw-out devices must have rear access. While this is true for low-voltage switchgear and high-end Class 3 switchboards, front connections can be attained with the simplest switchboards when only a single device is in the section.
Another item for the specifier to consider when specifying low-voltage switchgear, which has draw-out devices as standard, is a remote racking mechanism. A remote racking mechanism is an electric motor that is attached to the breaker face and is provided with a cord connected to the control station. The operator then stands as far as 30 ft. away from, instead of directly in front of, the switchgear (Figure 2). This typically will take the technician out of the arc flash zone in front of the gear while performing a critical live bus racking operation.
While group-mounted molded case devices are standard for typical switchboards for amperages 1,200 A and below, individually mounted devices are available for insulated case frames for amperages of 800 A and larger. The draw-out devices provide the end user with ease of maintenance and allow a technician to perform critical maintenance operations and a positive circuit disconnect that is not available with a fixed, bolted-in device. Additional safety features such as remote racking can also be specified when switchgear designs are implemented.
Providing a safe and secure work environment is a must for any company wishing to compete in this global economy. Incorporating simple steps into the design process is a cost-effective way to stay ahead of the curve.
Schlobohm has been a senior specification engineer at GE Consumer & Industrial in Houston since 1993. He provides layout assistance, equipment selection, and application and technical support for power distribution and control equipment to central and south Texas consulting engineers. Schlobohm holds a BSEE from Texas A&M University and has been a licensed professional engineer in the state of Texas since 1992.
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