|NEC Article 250 - Grounding and Bonding
|By Mike Holt for EC&M magazine
Why is grounding difficult to understand?
Grounding is difficult to understand for many because they do not realize that Article 250 contains the requirements for both Grounding and Bonding. In addition, the proper definitions of, and differences between, important terms such as "bond, bonded, bonding, ground, grounded, grounding, and effectively grounded," and their intended application are often misunderstood, or worst yet, they are improperly used. Let’s review the differences between Grounding and Bonding.
"Bond, bonded or bonding" means the permanent joining of metallic parts to form an electrically conductive path that provides the capacity to conduct safely any fault current likely to be imposed. Proper bonding creates an effective, low impedance, ground-fault current path for the purpose of removing dangerous voltage from a ground fault by quickly opening the circuit’s overcurrent protection device.
Grounding of metal parts" of electrical equipment is the intentional connection of the equipment to earth. Failure to properly ground the metal parts to the earth could result in high voltage being applied to metal parts if lightning enters the building or structure via metal raceways or cables. Lightning does not necessarily strike only grounded items when seeking a path to the earth. If the metal parts are not effectively grounded, much of the high energy from the lightning strike will be dissipated in the structure, which can result in
electric shock or fires inside the premises.
In addition, metal parts are grounded to the earth to help prevent the build-up of high-voltage static charges where the discharge (arcing) could cause failure of electronic equipment that either is being assembled on a production line or is in actual use. Grounding can also prevent an explosion and fire in a hazardous-classified area. Furthermore, sensitive electronics are
sometimes grounded to an isolated electrode, in accordance with the manufacturer’s instructions, to reduce Radio Frequency (RF) interference.
"System grounding" is the intentional connection of one terminal of the power supply to the earth for the purpose of stabilizing the system line-to-ground voltage during normal operations. According to IEEE Std. 142 (Green Book) "arcing, restriking, or vibrating ground faults on ungrounded systems can, under certain conditions, produce surge voltages as high as six times normal."
Effectively grounded" means intentionally connected to the earth via a low impedance path.
CAUTION: The term "ground" or "grounded" is sometimes improperly used in the NEC as well as in the trade. For example 404.9(B) specifies that "snap switches shall be effectively grounded." Naturally we are not to intentionally connect a switch’s metal yoke to the earth via a ground rod. What is required is bonding the metal switch yoke to a low impedance, effective, ground-fault current path so that dangerous voltage from a ground-fault can be removed by opening the circuit protection device.
The key point in understanding the difference between "grounding" and "bonding" is to remember that "grounding" is a connection to the earth for lightning discharge, system voltage stabilization, or reducing static voltage or RF interference. Grounding is not intended to, and will not, remove dangerous voltage from a ground fault.
Bonding" is intended to provide the low-impedance, ground-fault current path to the power supply that is necessary to remove dangerous voltages on metal parts from a ground fault by opening the circuit’s protection device.
The above material is an edited excerpt from Mike Holt’s Illustrated Guide to Understanding
Grounding and Bonding.