Electrical Bonding

By Clarence Klassen 

Ground, earth, bonding, neutral. All these words are used around electrical equipment, including drives.

Bonding is connecting or joining all metallic parts near electrical equipment and wiring. This includes conduits, wiring trays, motor frames, transformer frames, drive frames, electrical boxes, and cabinets. Don’t forget cabinet doors and metal fences and gates around equipment. There must be electrical continuity between all the metal parts. Most equipment provides a bonding stud marked Protective Earth (PE), GND, or the ground symbol shown below. The bonding conductor must connect to the ground or earth. Your local electrical code lists the minimum size of bonding conductors. The bond conductor must be nearly the size of the largest current-carrying conductors. There is also a minimum size for bonding conductors.

The bonding circuit is important for electrical safety and the reduction of Electro-Magnetic Interference.

Regarding safety, it is important to keep all conductors (all metal?) at the same potential. If someone were to be holding the door of an electrical cabinet and leaning against a building beam when the building was struck by lightning, we want to ensure that energy flows through bonding conductors rather than the person. If a short circuit occurs with a conductor touching metal, the fault current should safely flow through bonding conductors until protection removes the fault current.

Shielded or screened cables are connected to metal in cabinets and through these to the ground over the bonding conductors. The shields must be connected at one end of the cable only. Connecting the shield at both ends permits a circulating (ground loop) current to flow through the shield. The ground loop current can be induced into signal wiring in the cable, thus introducing noise, rather than shielding from noise.

These voltages are induced into all cables in a building by current flowing elsewhere in the building. Check this the next time you pull a new cable for a project. Before connecting either end of the cable, measure volts between the unconnected cable and ground with a digital multimeter. You may be surprised to read up to 70 or 80 volts. This induced voltage usually carries little energy but can corrupt a critical measurement.

Note: ground, earth, bonding, and PE refer to the same thing. Neutral is not the same thing.