Controls Design Considerations for Compliance with NFPA 70E
Worker safety is of most importance. And in this competitive manufacturing world, operational efficiency may be the second most important priority. Compliance with NFPA 70E (The Standard for Electrical Safety in the Workplace) will certainly improve the safety of your operation. This newsletter will explore some different approaches to controls design that will help you maintain or improve your operational efficiency. [While not necessary to grasp the concepts presented here, a review of some NFPA 70E Articles will provide greater clarity. See Article 110.2(D)(1) concerning Qualified Persons. See Article 120.1 concerning the verification of an electrically safe work condition. See Article 130.4(C) and Table 130.4(D)(a) for information concerning approach boundaries for unqualified persons.]
Device Reset Considerations It is a best practice that only electrically qualified personnel are allowed access to the inside of electrical enclosures. While this is practiced in many operations, it is often the unqualified operational personnel and mechanics that routinely enter these energized enclosures to perform the necessary resets. Maybe it is deemed less efficient to wait for the electrician or there are no qualified electrical personnel on-site. If that is true, then the reset operation must be designed for safety. After all, most would not allow a chain and sprocket on a conveyor to operate without guarding. So why expose unqualified personnel to electrical hazards (arc flash and shock) while resetting circuit breakers, VFDs, servo drives, and motor overloads. Consider resetting VFDs and servos via a push button on the outside of the enclosure or from an HMI screen. Most of these devices allow for an input to be configured for a system/fault reset. Use MCC buckets that have motor overload resets accessible from the outside of the MCC door. Or replace your current motor overload with one that is remotely resettable. Locate circuit breakers in a load center outside the main electrical enclosure where the operator is not exposed to the terminations. On a limited scale these solutions are very economical, but probably too expensive to retrofit everything. So a good place to start is with the two or three items that most often trip in your factory or mill. Do not worry about the ones that trip infrequently. These can be reset after establishing an electrically safe work condition or sometimes through a power-off-cycle depending on your process. For new equipment, specify remote resets prior to the equipment being built. Enclosure Disconnects A current design practice is to install a main power disconnect in a control enclosure. These disconnects are then mechanically interlocked with the enclosure door. The concern here is that while power is shut off to the machine operating devices, the enclosure is not in an electrically safe work state. Another concern is that there is a false sense of security, because all the power is thought to be turned-off. This is because there is still power on the line-side of the disconnect which is located in the enclosure, usually with exposed terminations. Consider replacing these integral disconnects with ones mounted on the outside of the electrical enclosure. So when the disconnect is off, there is no power in the control enclosure. This will certainly aid in the establishment of an electrically safe work condition when it is needed. Retrofits will be very economical and there should be no cost-adder for new equipment being built. 24VDC Control Circuits The use of 24VDC control circuits verses 120VAC adds a substantial safety factor. The standard considers voltages source less than 50 volts to have ‘no increased exposure to electrical burns or to explosion due to electric arcs’. 24VDC circuits fly under the standard’s ‘radar’ as they are safe even when their exposed conductors are energized. Unfortunately, these can be costly to retrofit, so specify for new equipment or controls upgrades. Separate Enclosures for 3-phase and Control Circuits When specifying new equipment, ask for separate enclosures for the 3-phase devices (servos, VFDs, and motor starters, etc.) and for the 24VDC controls. Since a portion of troubleshooting is on the control circuit, this will reduce exposure to the more dangerous 3-phase power. It is helpful to reduce risk for even your qualified electrical persons. Diagnostics via HMI Thorough diagnostic information is helpful as it reduces the chances of unqualified persons rooting around for something to reset in an electrical enclosure when their equipment has failed. AC Receptacles in Control Enclosures And finally, provide adequate power outlets on the manufacturing floor such that operators do not have to plug their fan or radio into the service outlet in the enclosure.