Electrical safety from the ground up
Risks associated with shock and electrocutions from inadvertent contact with energized parts have long been recognized as a threat to electrical workers. In recent years, OSHA and industry associations recognized the severity and urgency of the situation which resulted in the development of new standards for electrical workplace safety. In essence, they mandate that work on electrical equipment must be performed in a manner that does not expose the worker to undue risk of injury.
While arc flash awareness has been growing (as well it should), the dangers of shock and electrocution should not be overlooked. In fact, electrocution is the second leading cause of construction site fatalities in the US. In an average eight-hour day, 16 workers require time off the job to recover due to electrically induced injuries.
Dangers of ground faults and arc flash
Ground faults are unintentional current paths to ground, which turn into arc flash events if not appropriately addressed. One way to address ground faults is through ground fault protection systems applied on circuit breakers.
Arc-resistant switchgear is designed to contain the energy created by an arc flash event and to channel the energy away from personnel operating the switchgear. New innovations in circuit breaker and switchgear manufacturing are constantly advancing the state of the art in arc flash mitigation toward the goal of worker protection.
How shock occurs
Electrical current travels the path of least resistance; it either finds a path to ground or another conductor to complete its circuit. Keep in mind that electricity disperses in various ways across different surfaces. If there is a voltage difference between two conductive objects, and a third conductive object bridges the gap to complete the circuit, the current will flow across. If the third conductor is a person, he will receive an electric shock. Electricity will flow through his body as he becomes part of the current’s path. A person can receive a shock by being in contact with:
- Both wires of an electric circuit
- One wire of an energized circuit and the ground
- A metal part that accidentally becomes energized, for example, a break in its insulation.
Even low-voltage electrical shock injuries can manifest in the days or weeks following an event. The injured person may experience side effects that include numbness, muscle weakness, general or localized fatigue, and cognitive dysfunction. The medical community has begun research to understand this phenomenon.
Protective devices (circuit breakers and fuses) are installed in electrical systems to protect against a short circuit or a major fault current. Unfortunately, a person can be electrocuted below the point at which the protective device would operate. Bonding of the electrical system components and equipment helps reduce this type of hazard.
Most everyone has 3-prong electrical outlets in their homes, with the third prong being called the “ground.” Its purpose is to provide a low impedance path for any unwanted electrical leakage (fault current) in your electrical system. When an electrical fault occurs inside of equipment, metal or conductive surfaces such as the body of an appliance become energized and pose a shock hazard to anyone who would touch them.
Without grounded outlets, if a fault current existed in your refrigerator, opening the door could cause a shock. Due to the “grounding” conductor in the outlet, the fault current can flow back to the electrical source, causing the overcurrent protective device (circuit breaker) to trip.
In an industrial setting, equipment and wiring systems are bonded to each other by a wire or other conductor. Proper bonding mitigates the risk of metallic parts remaining energized and becoming a shock hazard.
In addition to lessening the risk of shock or electrocution, a properly bonded electrical system helps to promote a more efficient electrical workplace overall. Lack of proper bonding translates into an increased chance of fire where combustible materials are present (due to the accumulation of hazardous static electrical charges). Other issues caused by improper bonding include data and equipment losses, process anomalies, and drive and sensitive electronic equipment malfunction.
While bonding interconnects the devices with a conductive wire or cable, grounding connects the bonded electrical equipment to the earth. Should a fault condition exist, it is the bonding wire/cable that equalizes the stray electrical current so that one device or component will not have a greater voltage than another. Connecting the bonding conductors to the earth serves an entirely different purpose.
System grounding pertains to connecting one of the current carrying conductors—typically the neutral conductor—of the power system to the earth. This concept is quite different from bonding (connecting metallic enclosures together). System grounding (earthing) is performed to assure that transient surges—and temporary high voltages—entering the building from the utility distribution line are safely shunted to the earth and are not imposed on the building’s electrical distribution system.
NEC: Article 250
Grounding and bonding support the overall purpose of the National Electric Code (NEC), that is, the practical safeguarding of persons and property from electrical hazards. Article 250 of the NEC provides comprehensive guidelines on grounding and bonding.
Article 250.4(A)(1) and (B)(1) provide the reasons for grounding (earthing) of the bonded equipment:
- Limit voltages due to lightning, line surges, or unintentional contact with higher voltage lines
- Stabilize voltage to earth during normal operation (for grounded electrical systems) or limit the voltages on electrical enclosures to ground (for ungrounded electrical system).
Note that even in an ungrounded electrical system the bonded equipment enclosures must still be connected to the earth.
The remainder of the NEC article provides guidelines on topics such as installation rules, sizing rules, and so on. Due to the complexity of the topic, it is highly recommended to have an understanding of the terminology. Article 100 of the NEC contains the definitions of terms used.
Much the same as the NEC, the OSHA standards (29 CFR 1910) recognize two types of grounds:
- System or service ground: One of the current carrying conductors (typically the neutral conductor) is grounded at the service entrance to the building. This is primarily designed to protect machines, tools, and insulation against damage due to surges and high voltages on the utility line.
- Equipment ground (bond): This is intended to offer enhanced protection to workers. If a malfunction causes the metal frame of a tool to become energized, the “equipment grounding conductor” provides another path for the current to flow through the tool to the electrical power source. Further, the equipment grounding conductors are connected to the earth at the service point.
Under certain conditions OSHA permits the power system to be ungrounded. In this case, none of the current carrying conductors is connected to the earth. However, equipment grounding conductors must be provided and must be connected to the earth at the service entrance point.
Grounding installation and maintenance
The selection and sizing of grounding components should be done prior to equipment installation, along with a short-circuit study, which calculates fault-current levels throughout the electrical distribution system. These activities should be performed by a professional engineer and also in accordance with the authority having jurisdiction. Once installed, a resistance test is needed to confirm electrical continuity to ground.
Components of a grounding system are subject to corrosion due to electrochemical, electrolytic, or chemical reactions. In fact, if the system has been in place long enough, a ground grid can be completely consumed. Facilities that have sensitive electronic equipment are particularly vulnerable to disruptions. Qualified field service personnel should inspect a facility’s grounding on a routine basis.
The goal of any company’s workplace safety program is to enable workers to be able to go home safely at the end of their day. Likewise, being mindful of electrical safety at home will help enable us to return to work the next day. Don’t overlook the importance of grounding; safeguard against electric shock at work and home.
Reza Tajali, a registered electrical engineer in several states, is a manager of engineering for Schneider Electric Engineering Services in Nashville, Tenn.
See articles on arc flash myths and GFCIs below.