The NEC allows replacing non-grounding-type (2-prong) receptacles with grounding-type (3-prong) receptacles by adding GFCI protection at the breaker or first outlet.

What reasons might there be for wanting to upgrade the circuit wiring to include an equipment grounding conductor (EGC) instead of, or in addition to, adding GFCI protection? (alluded to in this answer)

Specifically, I'm curious about:

  1. Safety - In what scenarios does an EGC provide protection (from electric shock, fires, etc.) where a GFCI does not?
  2. Can an EGC help prevent damage to computer or audio equipement?
  3. Could an EGC reduce noise/static from audio equipment?

3 Answers 3

  1. A GFCI is the best protection against shock you can reasonably put on a 120V mains circuit, grounded or not.

  2. Surge suppressors absorb surge energy as well as attempting to divert some of it back to the source—even without a ground, they can function just fine.

  3. The mains ground is ineffective at RF as it is long enough to be highly inductive (or even a transmission line!) at typical EMI frequencies. Local (i.e. as part of the computer, AV, et al installation) bonding is what provides effective RF suppression due to the equipment chassis being equipotential to both AC and RF relative to the other chassis it is connected to—cable shielding is an extension of the chassis when terminated to the chassis correctly.

Note: Leave the ungrounded outlets ungrounded if you cannot put a new home run in! In other words, don't run individual EGC wires to outlets or ground outlets to water pipes. Bootleg or otherwise improper grounding can be very dangerous to life, limb, and equipment when combined with reversed polarity due to 120VAC flowing through equipment chassis and cable shields.

  • I moved the part about improper grounding to the bottom since that wasn't part of my original question. I understand that the only way to add grounding is to run all new wiring from the main panel. Commented Jan 30, 2015 at 16:51
  • This is a good answer, but #3 is way over my head. Could you try summarizing in lay terms? What does "ineffective at RF" mean and how does it relate to reducing noise in audio equipment? Commented Jan 30, 2015 at 16:53
  • @littleturtle -- the impedance of a piece of wire goes up with frequency, and quickly reaches a point where the length of an electrical circuit has a significant enough impedance (AC resistance) that it no longer can be treated as a wire at those higher frequencies. Commented Jan 30, 2015 at 23:04
  • 1
    Strictly speaking, a bootleg ground is when somebody connects a ground wire to the neutral somewhere downstream from the main service panel, yeah? Doing that BY DEFINITION energizes the equipment chassis and cable shields every time a piece of equipment is switched on, regardless of any polarity issues. Or, energizes said chassis and/or cable housing 100% of the time if the "polarity" is reversed and the device's power switch is only a single pole switch. Commented Aug 17, 2015 at 2:08

Points one and two, I can't make a definitive answer but generally when a ground is not provided a GFCI provides acceptable protection.

As for point 3, grounding issues are a major source of noise/static. If the item is not grounded the metal case around the device will be very ineffective in shielding the device from EM interference. Likewise, any EM interference the device creates will be poorly contained within the case.

Another source of grounding on audio/video equipment is referred to as "ground loops". Pretty much all audio/video cables use a grounded jacket around a central wire to shield it from EM interference and this jacket also connects different pieces of equipment together on the ground. If two pieces of equipment happen to be grounded to two different sources, a small electrical current can be transmitted across the grounding jackets causing a lot of noise on the lines.


Safety - In what scenarios does an EGC provide protection (from electric shock, fires, etc.) where a GFCI does not?

GFCIs are a poor substitute for grounding. In the absense of grounding a GFCI will only disconnect a fault to the case after your electric shock starts. Hopefully it will do it before the shock kills you but there are no gaurantees.

Having some equipment grounded and some ungrounded is often worse than having nothing grounded. Current flows in circuits so to give yourself a shock

Can an EGC help prevent damage to computer or audio equipement?

Without a proper grounding path fault and leakage currents may flow down routes they were not intended to such as signal interconnections. This may cause damage

Again the worst case is when some equipment is properly grounded and other connected equipment is not.

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