Integrating electrical safety with design

03/08/2013


Functional and operational considerations

Designing an electrical system for a facility requires several considerations in order to make the electrical system functional, address the operations of the facility from a production and safety standpoint, and make it cost-effective. This includes how the electrical equipment is maintained, operated, and modified. Another important consideration would be continuity of electrical power service. This could include the facility’s tolerance to power shutdowns and interruptions in service, and frequency of modifications to the electrical system to support company operations.

Continuity of service: A primary concern of many facilities is providing an uninterrupted source of electrical power to the entire facility. The designer should solicit input from the operations and maintenance groups to understand the critical functions that must be protected from scheduled and/or unscheduled power outages. Under the OSHA general industry rules, infeasibility is not justified based on economic considerations such as production schedules, interrupting a manufacturing process, or data processing operations. The feeders and branch circuits of a facility’s electrical system may require additional design steps to maintain continuity of service and provide worker safety.

Maintenance: Workers that maintain electrical systems encounter electrical exposures on a daily basis in the course of their work. A review of their assigned work tasks could lead to enhanced design to eliminate or minimize the worker’s exposure. The properly designed electrical system could provide increased safety with a minimum amount of interruption to the operation of the facility.

If a company performs infrared testing on electrical equipment as part of a preventive maintenance program, specifying and installing site window(s) on the equipment to perform the infrared task is a cost-effective design consideration. This site window would remove the hazard to the worker. This would also reduce maintenance time for removing covers and wearing electrical PPE. If installing site windows is not a practical solution, specifying hinged covers on electrical panelboards versus bolted covers is another consideration. The worker would still require the electrical PPE to open the cover, but it would reduce the potential exposure to an arc flash.

Modifications and additions

Modifications and additions to electrical systems and equipment are a major obstacle to the operation of a facility. Taking a proactive approach to consider the operation of the facility and taking steps to design the electrical hazards out of the work task could result in actual cost savings for operations and increased safety for the worker.

Some facilities have production equipment that requires the equipment’s computer program to be modified to accommodate production processes. To reprogram or modify a program requires opening the control panel door, exposing the worker to electrical hazards. A resolution to this hazard is to relocate the computer port to the exterior of the control panel. This would permit the equipment to be programmed by the worker without wearing electrical PPE and without specialized electrical safety training, thus eliminating the worker’s exposure to a hazard.

Another design technique to facilitate maintenance, repair, and electrical installation is to install an overprotection protective device adjacent to or upstream of the power or control panel. This changes the design specification from a main circuit breaker type panel to a main lug only panel. This provides the capability for the worker to disconnect power to the power or control panel, providing a safe environment to make modifications on install new circuitry or components. It also is cost effective and facilitates a lockout/tagout procedure.

Figure 3: Wireways used in electrical power and control panel installations can improve efficiency of operation and worker safety. This design concept provides numerous benefits to the facility for both efficiency and safety. Courtesy: Schneider ElectricAn installation technique that provides both safety and efficiency is to incorporate the use of wireways with electrical power and control panel installations. This design concept provides numerous benefits to the facility for both efficiency and safety. Figure 3 shows a typical design that could be used. The basic design would be to install the power of control panels as shown. The wireway would be installed above the panelboard at a distance not to exceed 24 in. Conduit nipples would be installed between the power or control panels to facilitate installing the conductors into the electrical panel. These conduits would be of an adequate size to accommodate the installation of the number and size of the wires to the power panel. Limiting the length of the conduit nipples to 24 in. between the wireway and the panel(s) would permit the installation of a large number of conductors without having to de-rate them. This is permitted in section 310.15(B)(3)(a)(2) of the 2011 NEC. The space above the wireway would be used to run the individual feeder or branch circuit cables or conductors and conduit to feed substation equipment.

The most significant benefit of using wireways is after the electrical system has been commissioned and energized. When an additional feeder or branch circuit has to be installed, the worker can remove the wireway cover and ensure that there is adequate clearance to enter the conduit or cable in the top of the wireway with the equipment energized. The worker can then install the cable or conduit, and pull in the wire from the equipment to the wireway, leaving the conductors long enough to be installed and terminated in the panel. The worker can then schedule an outage and make the final terminations of the conductors. This would minimize the disruption to the continuity of service for the facility.

This design technique could also be used for data centers. If the installation has a raised floor, the wireway would be located in the plenum space below the raised floor. Many data installations are designed with an ample amount of spare circuit capacity in the power distribution units (PDU). If the majority of the circuits in the PDUs are 120 V/20-amp circuits, it provides an opportunity to proactively prewire the PDU. The strategy would be to install the spare branch circuit wiring from the PDUs to the wireway. The conductors would be marked in the wireway and designated as a spare circuit on the panel directory. The ends of the conductors in the wireway would be made electrically safe.

When an additional circuit is required in the data center, the worker would remove the cover, ensure that there is adequate clearance to enter the conduit or cable in the wireway, and install the branch circuit to the substation equipment. The worker would then determine spare circuit number in the PDU that would be used. The applicable circuit breaker could be locked out and verified, de-energizing the circuit in the wireway. The conductors in the wireway could then be re-energized. The entire task can be performed with no interruption to the operation and electrical power in the data center. It also prevents the worker from being exposed to electrical hazards.



No comments