Providing circuit protection for safety

When correctly selected and applied, circuit breakers, fuses, and other protection components can help keep workers safe from injuries and machines from damage.

05/30/2018


Figure 1: Regulations and methods to protect personnel and machines from shock, burns, and fires can be found in NFPA 70—2017: National Electrical Code (NEC). Courtesy: AutomationDirectMost engineers, technicians, and factory workers have a story about industrial injuries, such as shock or burns, that occurred on the manufacturing floor. While most incidents happen during construction, maintenance, or testing, they can occur during normal operations as well. There are a variety of scenarios where shock or burn injuries can occur, but most can be prevented by using best design practices and following safe operating procedures.

There are many causes for electrical fires and other incidents, with various prevention methods for each type of problem found in the National Fire Protection Association (NFPA) document, NFPA 70-2017: National Electrical Code (NEC). A big part of electrical safety involves proper circuit protection, which can mitigate problems and reduce their impact.

Electrical protection in all forms is critical for the protection of personnel, but it's often overlooked. Properly protecting a system or equipment is a key step in reducing costly downtime and equipment damages. This article examines common electrical safety scenarios, along with ways to protect personnel and machines (see Figure 1). 

Problem scenarios

Proper short-circuit current and overload protection is required to protect against electrical system damages. During construction, an overload could shut down power. During maintenance, incorrect wiring or a misplaced tool could cause these current-limiting devices to activate.

For example, during a construction project, a large circuit breaker in a plant tripped during operation. The breaker did its job, quickly ending a short-circuit condition, but it cut off power completely to an entire building. At the time, it was not known which of the many building loads caused the breaker to trip, so it was difficult to find the source of the problem.

Eventually, the root cause was found to be a shorted brake on a large motor. What made the problem difficult to locate was the load-shedding breakers used to safely limit generator loads by dropping some of the load. These load shedding breakers re-close with a delay, complicating troubleshooting the issue.

While it contributed to the outage, this breaker continued to operate and protected the people and equipment during the troubleshooting and diagnostics process. After the shorted brake was found and repaired, the long-term solution was to coordinate operation of all the breakers so that downstream breakers operate prior to upstream breakers during low-level faults.

Another incident occurred near the end of a week-long, plant-wide outage to perform preventive maintenance on switchgear. As part of the re-energization procedure, final safety testing was done to confirm all the busses and feeder lines were electrically isolated, and not grounded or shorted together. The breakers also needed to be tested to confirm proper insulation integrity.

Figure 2: Overcurrents, short circuits, and ground faults are some of many causes of electrical fires. Courtesy: AutomationDirectEven with all this testing, when the main site breaker was closed, it shorted out the switchgear. Later testing revealed someone inadvertently had left a tool in a spot where it vibrated down onto the main breaker and was not visible. Fortunately, a fast, current-limiting breaker activated and contained the incident with only minor damage. Without the current-limiting breaker, the short likely would have resulted in an explosive arc within the switchgear, and possibly personnel injuries from fire or arc flash. 

Safeguards from shock and fire

To mitigate incidents like the two described above and others, NFPA 70E-2018: Standard for Electrical Safety in the Workplace provides detailed information regarding electrical safety and many other regulations for electrical wiring and overcurrent protection. In addition, NFPA 79-2018: Electrical Standard for Industrial Machinery, discusses protection of equipment as well.

There are many requirements and guidelines to follow to protect personnel and machines from shock, fire, and other damaging events due to the presence of electrical energy or electrical failures. An overview of some of the many electrical safeguards include:

  • Provide a lockable disconnect means
  • Interlock doors to disconnect power
  • Include safety signs
  • Provide overcurrent protection
  • Provide surge protection.

This article is a general safety discussion, and it's important to note that there are exceptions to many requirements, with some important details not covered due to space constraints.

Though often overlooked, disconnects on industrial control systems perform the important function of ensuring the electrical feed circuit to a machine or system is completely de-energized to protect maintenance and operations personnel from electrical shock. Regulations require all power to the equipment must be shut off, locked out, and tagged out before servicing. Disconnects provide this functionality.

NFPA 79 requires a disconnecting means for isolating the supply of power to a machine. While this can be as simple as a plug and a safety sign, most machines use other methods. Typical disconnect methods are UL 98-rated fused/non-fused switches or a UL 489 circuit breaker. Usually there is just one machine electrical supply circuit and all power should be switched off by opening the disconnect for this supply. This should be labeled as the "Machine Power Supply Disconnect." If multiple sources of power are present, signs must clearly state the exceptions and the proper procedure to remove power from a machine.

This disconnect should be mounted in or next to the main control system enclosure. If the main control system enclosure contains common ac voltages such as 120 Vac or 480 Vac, or if any voltage greater than or equal to 50 Vac or 60 Vdc is present, the door should be interlocked to the disconnect. To reduce the risk of electrical shock, the door shouldn't open if the disconnect is on unless it is defeated by qualified personnel using a specialized tool.

While a disconnect with a door interlock is the best-practice way to remove the electrical feed from a control panel, other methods such as door locks and keys, and guards protecting personnel from direct contact with hazardous voltages, are allowed. Regardless of the method used to disconnect power or protect personnel, safety signs on the enclosure should define proper procedures for removing power. 

Branch circuit versus supplementary protection

Circuit protection is critical to protect a machine from currents greater than the machine's or device's current-carrying capacity. Proper electrical protection is the key to safely removing the effects of dangerous overcurrent due to short circuit, overloads, ground faults, voltage transients from switching surges, and other abnormal conditions (see Figure 2).

To provide this protection, it's important to understand branch circuit protection versus supplementary protection. In general, branch circuit-rated devices protect the wires, and supplementary devices provide additional protection, but they are not sufficient to protect the equipment or load exclusively. Supplementary devices often are used for lower load equipment, internal loads, or as a simple additional disconnecting means.

Figure 3: A wide variety of UL 489- and UL 1077-rated circuit breakers and fuses is available to protect wiring and electrical equipment. Courtesy: AutomationDirectThere are many general requirements, wiring practices, and grounding and bonding techniques to protect machines and personnel. To provide overcurrent protection, branch-circuit short-circuit, and ground-fault protective devices, supplemental overcurrent protective devices are needed.

In general, UL 489-labeled devices provide branch circuit protection, and UL 1077-labeled devices provide supplementary protection, with NEC Sections 100, 430, and 409 providing detailed definitions (see Figure 3). Outside the U.S., there are different, but related, standards.

Most electrical circuits start with branch circuit devices, such as an appropriately labeled circuit breaker or fuse. These devices protect against fire and electrical shock by limiting current flowing through wires and provide a means to remove electrical power during equipment service.

Branch circuit protection does not necessarily protect a load such as a power supply, a PC, or a programmable logic controller (PLC). To provide this protection, supplementary protection fuses and circuit breakers are used. Supplementary protection provides additional equipment protection where branch circuit protection is already provided or not required. 


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