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Don’t Bet Your Life: Get System AC Grounding Right

Although hum and other problems are often blamed on “improper grounding,” in most cases there is actually nothing improper at all.

By Bill Whitlock January 3, 2013

After teaching seminars for more than 15 years now, it still amazes me how many otherwise competent professionals don’t understand the importance of proper equipment safety grounding.

Even more shocking (pun intended), many routinely and casually disconnect safety grounds to solve noise problems!

Generally speaking, the purpose of grounding is to electrically interconnect conductive objects, such as equipment, in order to minimize voltage differences between them.

National Electric Code (NEC) requires that 120-volt AC power distribution in homes and other buildings must be a three-wire system.

Figure 1 shows how AC power is typically delivered from the utility company to the load at an outlet. For simplicity, only two of the three main utility connections are shown in the drawing.

Figure 1: A look at how AC power is typically delivered.

One of these incoming utility wires, which is often un-insulated, is the grounded or “neutral” conductor.

Note that both neutral (white) and line (black) wires are part of the normal load current circuit shown by the arrows.

Code requires that the neutral (white) and safety ground (green) wires of each branch circuit be tied or “bonded” to each other and to an earth ground rod at the service entrance.

Any AC line powered device with exposed conductive parts (that includes signal connectors) can become a shock or electrocution hazard if it develops certain internal defects.

Insulation is used in power transformers, switches, motors and other internal parts to keep electricity where it belongs.

However, for various reasons, the insulation can fail – effectively connecting “live” power to exposed metal as shown in Figure 2. Such a defect is called a fault.

Figure 2: Watch out for faults… They can be mighty unpleasant!

For example, if the motor in a washing machine overheated and its insulation failed, the full line voltage could energize the housing of the machine!

Anyone who accidentally touched the machine and anything grounded, such as a water faucet, at the same time could be seriously shocked or electrocuted. 

Remember: current will always return to its source, whether the path is intentional or accidental. Electrons don’t care – they can’t read schematics! 

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