Some of us have felt that tingling sensation at one time or another when coming in contact with metallic equipment. But for an electrician, it is a sign that all is not right, especially when the sensation becomes more like a vibration or an intense jolt. The problem arises from electrons wanting to find a return path to ground. A common culprit is a stray wire inadvertently touching the metal casing via a high-resistance path that is not of sufficient magnitude to trip a breaker.
When the stray voltage induces a current via a household appliance, it is usually a mild tingling sensation when exposed to it. But when a high or medium voltage substation is not designed properly, the results can be catastrophic. Some simple approaches to mitigate induced stray voltages can be incorporated into a design to prevent injury or death to personnel.
Avoiding Injury or Death
In industry, the problem becomes serious when electrical substations are not properly designed to protect personnel from injury or death. Even when overhead wires are as high as 30 feet high, the electrical field is still attempting to find a way to establish a ground path. If an energized wire is relatively close to ground, an induced voltage may exist. For the professional engineer designing a proper electrical facility, he/she can consider suitable approaches to mitigate the hazard.
Gradient control consists of using equipotential planes to maintain safe step and touch potential levels. With this approach, all conductive objects that could inadvertently come into contact with an energized conductor, are bonded to the same electrical grounding system at a common reference plane. With a common reference plane, the magnitude of an induced stray voltage can be reduced to a near equipotential level.
Active suppression consists of a nulling device to counter the stray voltage by delivering a controlled current to the earth. This is accomplished by measuring the stray voltage between a given point and an isolated reference ground, which is then used as an input to a differential amplifier to deliver a specified amount of current to a remote grounding electrode to null out the measured stray voltage.
In specific electrical distribution systems, there may be unintentional current flowing from the utility company’s neutral grounding system to the customer’s neutral grounding system. To protect the integrity of the utility distribution systems and ensure their power reliability, the utility companies sometimes intentionally isolate their grounding reference planes from the customer. Thus, to reduce the stray voltage impact of this, impedance isolation methods such as a spark gap, a saturable reactor, or a solid-state switch can be used. When properly applied, this technique creates an intentional higher impedance to restrict the magnitude of the stray voltage on the customer side.
If you would like to know more about reducing your stray voltage shock hazard risk by using a properly designed electrical system, feel free to call me at (240) 582-3900 — Priyan De Silva