Study Hall

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.

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.

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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.

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!

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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! 

Trip The Breaker
To return this fault current directly to its source, many devices have a third wire connecting exposed metal to the safety ground pin of their plugs. The outlet safety ground is routed, either via the green wire or metallic conduit, to the neutral conductor at the main breaker panel.

This low-impedance connection to neutral causes a high fault current to flow, quickly tripping the circuit breaker that removes power from the circuit. To function properly, the safety ground must return to neutral. (Note that the EARTH connection had NOTHING to do with this process!)

NEVER, EVER use devices such as three- to two-prong AC plug adapters, a.k.a. “ground lifters,” to solve a noise problem! (Figure 3) Such an adapter is intended to provide a safety ground (see the fine print) in cases where three-prong plugs must be connected to two-prong receptacles.

If a proper safety ground isn’t available, always use a ground-fault circuit interrupter (GFCI). A GFCI works by sensing the difference in current between the line and neutral conductors.

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This difference represents current in the live conductor that is not returning in the neutral - the assumption is that the missing current is flowing through a person.

If the difference reaches about 5 mA (milliamps), an internal circuit breaker is tripped, removing power from the circuit. The GFCI shown in Figure 3 is unusual because it has a retractable ground pin that allows it to be used with a two-prong outlet.

Also consider two devices connected by a signal cable, each device having a three-prong AC plug. One device has a ground “lifter” on its AC plug and the other doesn’t.

If a fault occurs in the “lifted” device, the fault current flows through the signal cable to get to the grounded device. It’s very likely that the cable will melt and burn. Defeating safety grounding not only is both dangerous and illegal, it also makes you legally liable!