Equipment Grounding for Safety
Where would we be without electricity? From the time we get up in the morning to the time we go to bed, we’re flipping switches on and off without giving it a thought. But electricity is one of the most dangerous elements we use in our daily lives. To use it safely we need to take precautions.
A Grounding System to Create a Safe Path
For personal and equipment safety, all electrical systems must be ground. We ground electrical systems to limit extra voltage imposed on them by lightening, line surges, contact with higher-voltage lines or ground faults. A grounding system helps to efficiently direct electrical currents through electrical systems and stabilize voltage levels so circuits don’t overload or blow. With the use of low voltage wiring and grounding systems, we are able to prevent further issues from occurring later on.
Excess or stray electricity always takes the path of least resistance, and the earth is the perfect conductor or recipient of that electricity. According to the National Electrical Code, a “ground” is defined as a conducting connection, whether intentional or accidental, between an electrical circuit or equipment and earth or some conducting body that serves in place of the earth. Equipment that is “grounded” is connected to earth or to some conducting body that serves in place of the earth.”
NEC Section 150-51 states that an effective electrical grounding path must accomplish four things. It must be permanent and continuous, have the capacity to safely conduct any likely fault currents, have sufficiently low impedance, and have an additional electrical equipment grounding conductor that serves the same function as the earth. Equipment grounding conductors, grounding electrode conductors and grounded conductors are all conductive objects that extend the ground connection.
An equipment grounding conductor performs three very important functions when it comes to the electrical safety system. It creates a path for electricity to follow, bonds equipment together and keeps abnormal electrical events in check. An electrical grounding conductor is a metal wire, metal bar, or similar item that performs as a conductor that connects equipment to the earth via a grounding electrode. To ground equipment, connect the metal parts on each piece that don’t carry current together and, then, connect these to the system grounded conductor, the grounding electrode conductor, or both. The live wire, which carries the current during normal conditions, is typically connected to the earth so the electricity will dissipate into the ground, effectively grounding the equipment.
Bonding for Zero Electrical Potential
Besides grounding, equipment grounding conductors also bond equipment. Bonding refers to the act of joining two conductive parts such as two pieces of electronic equipment together. Bonding is very important in data, telecommunications or process-control systems. Equipment cabinets, enclosures and structural steel all need to be bonded. If they aren’t, voltage differences among them can disrupt data-flow quality, and that can bring the network to a total halt.
Bonding is accomplished by connecting all the metal parts that are not supposed to carry current (under normal operating conditions) in the two items that are to be bonded. This process evens out their electrical potential so they operate at the same electrical ground-reference voltage. When bonded, no current will flow between them, so no discharge can occur. Reducing the current flow between two pieces of equipment at different potentials protects both equipment and people.
One thing the bonding process does not do, however, is protect either item from a build-up of electrical energy. That type of protection comes from the grounding process. But, if one of the items has been grounded so it has zero electrical potential, the item it is bonded to will also be grounded.
Bonding electrical equipment also helps ensure the safety and protection of employees who may operate or be around the equipment. For instance, if two items of equipment are bonded and an employee touches the equipment enclosures of both items at the same time, the employee won’t receive a shock. If the two items are not bonded, the employee could become the path of equalization for the electricity and receive a nasty shock.
Another reason bonding is so important is that it helps to create a low-impedance path back to the source. When electricity is on a path of low opposition, current can flow freely. These large amounts of current can trip the circuit breaker and terminate the fault.
The best means of equipment bonding is to route a grounding conductor along the same route as the power and neutral conductors, from source to machine.
Keeping Abnormal Events in Check
The primary purpose of grounding electrical systems is to provide protection against electrical faults. An electrical fault is an imperfection in the electrical system that deflects or interrupts the normal flow of the electric current from its intended path. Left unchecked, it can potentially damage electrical equipment.
Various types of electrical faults such as a ground fault can cause damage. Ninety-five percent of faults are ground faults. A ground fault occurs when stray electrical currents bypass the circuit wiring and flow directly to the earth. Ground faults are frequently caused by deterioration of mechanical insulation, which can occur in damp, wet and dusty environments. An irregular or arcing ground fault can build up voltage on the electrical system, deteriorating the insulation and producing voltages six times higher than the nominal system voltage. An effective equipment grounding system ensures all the parts will stay in operation when a ground fault occurs.
Keeping the Terminology Straight
Confusion often arises surrounding “neutral” wires or conductors, “grounded” wires or conductors and “grounding” wires or conductors. Grounded wires or conductors are actually the same thing as neutral wires or conductors. Grounding wires are very different, but the terms “grounding wire” and “ground wire” are often used interchangeably.
It’s easy to distinguish a grounding wire from a neutral wire based on the color. The National Electrical Code (NFPA 70 NEC) requires the grounding wire to be a bare wire. If it’s an insulated wire, it should be green or green with a yellow stripe of insulation. Neutral wires are white or gray. Standard colors help to simplify the installation of the wiring and to increase safety.
A neutral (grounded) wire or conductor has two important functions. It serves as a zero-voltage reference point in an electrical circuit and it provides a return path for the current supplied through the live conductor.
Like the neutral wire or conductor, the grounding wire or conductor also operates with zero voltage. However, its main function is to provide a grounded connection for all the equipment. The neutral conductor carries all current returns, but under normal conditions the grounding conductor carries no electric current. When a line fault (short circuit conditions or other potentially dangerous situations) occurs, however, the grounding wire or conductor serves as an alternate path for the fault current to safely flow back to the source.
What happens if you don’t use a grounding wire? The fault isn’t disconnected and the equipment can become “energized” if a live conductor touches it. That means anyone who touches the energized equipment will receive an electric shock.
Since both grounding and neutral conductors operate with zero voltage, most devices will operate correctly if the wires are switched, however, the job will be in violation of electrical codes.
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