Designing for Workplace Electrical Safety
The design of a safe electrical work environment starts with an arc flash assessment.
- By Tony Locker
- Sep 01, 2012
As more and more employers perform arc flash assessments of their facilities to keep workers safe and meet OSHA and NFPA 70E requirements, they usually discover that they have a lot of dangerous equipment or areas where workers must use heavy and bulky PPE. Many employers are seeking ways to eliminate or minimize these hazards to keep workers safe, minimize downtime, and increase productivity.
What's more, as plant staffs grow leaner, younger workers often don't have the benefit of working side by side with experienced maintenance workers and electricians who deeply understand electrical safety. To help keep workers safe, managers can design safety into the facility's electrical system. This article covers seven considerations of designing safety into the workplace, ranging from fuses and grounding to relays and arc flash assessments.
1. Switch to Resistance Grounding
More plants and commercial facilities are switching from traditional solidly grounded electrical systems to resistance grounded systems, in which a large resistor is placed between the wye transformer and ground. Such resistors are designed to limit the current that can flow to ground to about 5A, too low to allow an arc flash to develop in a phase-to-ground short. In the "Industrial Power System Grounding Design Handbook," Dunki-Jacobs, et al., the authors estimate that 95 percent of all electrical faults are phase-to-ground faults. Therefore resistance grounding has a dramatic effect on electrical safety.
There are some special considerations. In resistance-grounded systems it is possible to lose ground and not even know it, so monitor the health of the resistance grounding equipment with a special relay made for this purpose. Also, realize that equipment with on-board ground fault detection will probably never trip in a resistance-grounded environment. Most variable frequency drives, for example, are set by the manufacturer to trip at ground fault currents above 5A, so the user should consider adding ancillary ground fault relays.
2. Enable Maintenance Mode on Motor Protection Relay
Some digital relays have a maintenance mode feature that enables a worker to limit the available fault energy when working on the equipment. This reduced overcurrent mode minimizes arc flash hazards during maintenance. When working around a motor, it's a good practice to use this feature so that overcurrent and ground fault conditions are stopped before they create safety hazards.
3. Use Current Limiting Devices
Fuses and circuit breakers that are "current limiting" will limit the amount of let-through current during a fault or arc flash event. That's because a UL Listed current-limiting device opens very quickly -- within the first half cycle of a fault (8.3 msec). Its fast action limits the amount of current that can flow through the circuit and reduces the destructive energy of a short or arc flash. Most standard non-current-limiting circuit breakers can take up to 6 AC cycles (0.1 second) to open under short circuit conditions -- a minimum of 12 times longer than a typical current-limiting fuse or current-limiting circuit breaker.
4. Use Indicating Fuses and Fuse Holders
Typically, finding which fuse has opened requires poking around an energized electrical panel with a tester. Avoid this unnecessary risk by using indicating fuses or fuse holders. Indicating fuses have a dark spot that appears when the fuse opens, and indicating fuse holders point out open fuses with a bright neon lamp or with a signal to a PLC. These are available from a number of suppliers. In addition, using touch-safe devices or fuse holders with covers also reduces exposure to live parts.
5. Conduct an Arc Flash Assessment
The design of a safe electrical work environment starts with an arc flash assessment. NFPA 70E standards require that arc flash hazards be identified at each electrical panel or piece of equipment that may be worked on while energized, indicating the category of arc flash hazard. Because the arc flash hazard is based on the available fault current at a particular piece of equipment, hazards can be decreased by changes to the electrical system, such as adding a current-limiting fuse or circuit breaker. The design changes are relatively easy; the tricky part is the assessment.
A reliable arc flash assessment almost always involves engineering calculations by an expert. NFPA 70E provides a table for determining Hazard Risk Categories, but the table's footnotes state it may be used only if the available fault current and the overcurrent protective device clearing time are known to fall within certain limits. Many people overlook these footnotes and use the Table Method as a shortcut without doing the required analysis. For example, for panelboards or switchboards operating at 240 or 480V, the available fault current cannot exceed 25kA and the clearing time of the overcurrent protective device cannot exceed 2 cycles or 0.03 second. These values must be calculated and verified in order to use the table.
Another common mistake is assuming that if equipment is determined to be a Hazard Risk Category 0, then equipment fed downstream is also a Hazard Risk Category 0. Because impedance in wiring reduces available fault current, a fuse or circuit breaker may open more slowly in equipment located downstream. This actually will increase the amount of potential incident energy, sometimes very significantly.
Some managers stop arc flash assessment at the first 480V drop off the bus because they wrongly assume there is no risk of arc flash below that point. One example of this mistake is stopping the assessment at the main switchgear or a motor control center (MCC). This not only compromises worker safety, it is a violation of OSHA regulations and the NFPA 70E standard that require all equipment to be assessed down to 50V for shock and arc flash hazards.
Coming changes to NFPA 70E include more explicit requirements for arc flash warning labels, including what types of equipment need them and what information they should include. The label is the point where the worker interacts with the hazard assessment on a daily basis. For example, flash protection boundaries should be included on arc flash warning labels. Using information from an arc flash assessment, the distance can be calculated; only qualified workers wearing the proper PPE are allowed to go beyond this boundary.
6. Install Arc Flash Relays
An arc flash relay provides an easy way to design safety into the electrical workplace. It can be retrofitted into any electrical panel where the available fault current is sufficient to allow an arc flash. These relays detect the light of an emerging arc flash and send a trip signal to the existing circuit breaker in less than 1 ms.
The amount of energy released during an arc flash is a function of time and current. During a 50 kA bolted fault on a 480V system, there is little damage in the initial stages; however, in those stages the arcing current is too low to instantaneously trip a circuit breaker. Around 100ms the cable catches fire, and at 150 ms the copper conductors catch fire. At this point, a cloud of ionized copper gas could form -– a near-perfect conductor that allows an explosion of heat and light. An arc flash relay detects the light and increased current of an emerging arc extremely quickly. By making the breaker trip much sooner, the total clearing time is reduced to around 35 to 60 ms, which dramatically reduces the amount of energy released during an arc flash event.
To prevent nuisance tripping -– from intense sunlight or a nearby arc welder, for example -– most arc flash relays don't depend on light detection alone. The relay compares input from the light sensor with input from current transformers on each phase. The relay won't trip unless there is both strong light and quickly rising current. In environments with high amounts of ambient light, it is possible to increase the trip threshold to prevent nuisance tripping; however, ambient light is not an issue when cabinets are closed.
Installation of an arc flash relay is straightforward, involving the placement of light-detecting sensors throughout the cabinet. The user may choose to use a long fiber-optic sensor or multiple point sensors. Then the relay's control output is wired to the existing circuit breaker, which must be capable of signal trip. As an option, the installer may wire other relay signal outputs to factory systems, in order to communicate status and alarms.
7. Monitor in Real Time to Detect Problems Before They Occur
If knowledge is power, then data from equipment is a potent partner in reducing costs and increasing safety. Modern digital motor-protection relays have made it possible for leaner staffs to remotely monitor the health of pumps and motors and then to trend that information to make smarter decisions about maintenance. By catching problems sooner, such as worn insulation, high temperatures, and ground faults, protection relays prevent malfunctions that can expose workers to electrical safety hazards.
This article originally appeared in the September 2012 issue of Occupational Health & Safety.
Tony Locker, P.E., is product manager for Littelfuse Protection Relays. Founded in 1927, Littelfuse offers circuit protection products and solutions.