A question on Audiogon about using completely independent ground for audio gear. The question I was asked was:
Can you explain to me why an isolated ground is prohibited by the electrical code?
To be clear, in this conversation the question was about an audiophile who used a grounding system which connected to a ground rod which was driven into the earth but never bonded to the house's electrical system. This is correctly called an earth-only ground.
Isolated Ground
An isolated ground outlet has a little green triangle on it, a feature usually only found together with hospital grade outlets. The isolation here means the ground pins in the receptacle and the strap across the back are not bonded together. If this is used in a metal outlet the outlet must have some other means of grounding. All totally safe when done correctly.
Earth-Only Ground
What we do want to talk about is why you can't just use an arbitrary new grounding system to replace the ground conductor in an outlet. In other words, why is an earth-only ground expressly prohibited?
The answer boils down to "what is the ground conductor in a receptacle meant to do?"
A ground pin on a receptacle should provide a high quality connection back to the service panel where the ground is bonded to the house neutral. This means low resistance. The idea is that should a fault occur from the electricity flowing through your appliance to the outer chassis the ground should whisk that voltage away and cause a breaker trip.
Audiophiles get very tunnel visioned when it comes to ground wires, never before thinking about them until they start chasing some real or imagined problem with their gear. To these audiophiles it’s like "Aha!! Thomas Edison himself along with the American electro-industro-capitalist cabal are conspiring to keep me from my ideal sound!!" And having realized this undeniable truth then set about to undermine it at every opportunity!
So, back to reality and why it’s bad/not up to code. The quick answer is that this earth-only grounding conductor won't function as intended. It won’t trip the breaker during a fault and tripping the breaker and keeping the voltage off the chassis is exactly what it should do.
Without getting into math, just consider that the dirt around your home is a poor conductor, or at least, incredibly variable quality of conductor. If it were a good conductor we’d probably have speaker cables made of dirt, but I digress. Point is, you can’t use the dirt around your home as a guaranteed high quality, low resistance path back to your service entrance ground point.
The Washing Machine Example
A surprising number of people have been shocked at one point or another by a washing machine and lived to tell about it so we'll use one as an example.
Imagine you have a washing machine, with a fault. Meaning, the hot wire (black or red) is conducting to the outer metal case, the chassis, of the washer. This is a type of short circuit. The quality of this short can vary, sometimes it’s very slight and sometimes it’s lethal. If it's really bad we call it a "hard" fault.
If that washer's chassis is attached to an excellent conductor back to the ground at the service entrance then 100% of that current is conducted through it, and the voltage at the outer chassis must stay near 0. Meaning when you touch it you touch the voltage at the ground.
If the fault is "hard" and the ground conductors are of high quality then this short to ground will also trip the breaker, exactly as intended.
Now lets imagine this same faulty washing machine relies on an earth-only ground. In the diagram above we put in several hypothetical isolated ground rods to attach the washing machine's grounding conductor to. As you move that new ground rod physically away from the house ground rod the voltage on the chassis rises, and the likelihood of the breaker tripping before you are shocked to death decreases. Of course the worst case is you have infinite distance to ground with a lifted ground and the full AC voltage could appear at the metal case of the washer.
Keep in mind that a typical home circuit breaker is 15 Amps, but 0.008 Amps in the right circumstances can be lethal. A typical breaker won't trip before a lethal injury occurs.
Also, 15 Amps through a delicate audio signal carrying RCA connector can start a fire when it melts explosively. This can happen if the RCA ground becomes energized and the appliance it is connected to is not properly grounded. I’ve seen very similar situations described on Audiogon, smoking an expensive phono stage.
To summarize, an earth-only ground is expressly forbidden by the NEC and for exceedingly good reasons. I hope my explanation helps you understand the whys of that. As a lifelong audiophile I 100% agree with the NEC and encourage you to hire a qualified electrician if in doubt.
If you absolutely cannot run a ground wire to your outlets, the NEC does allow you to use a GFCI outlet, properly labelled. The issue with this is that it is to protect people, not equipment and therefore most surge protectors won't work properly.
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