I disagree with this.
If one side is tied to ground (and it is), then tying the chassis of a piece of equipment to that same line is the same as tying it to ground...a ground that you're already standing on.
Am I missing something?
> “Occasionally an electrician will accidentally wire an outlet backwards. This can't be helped, because Perfect Electricians are far more expensive than the normal human variety. And so we cannot intentionally wire appliance cases to the Wide Prong of the plug, since it would cause a lethal hazard if the appliance was plugged into a miswired wall outlet. Miswired outlets look exactly the same as the normal ones.
> “The solution? Why, add a Third Prong! Connect this prong to the neutral side of the network, but do it only in one place in the circuit, and run a new third wire out to all of the wall-outlets. Give this wire a new color, one which is different from the other two. Give this this third prong a very different shape as well, so even Highly Imperfect Electricians will rarely connect the special prong to the wrong wire. Inside metal-cased appliances, insist that manufacturers connect this third wire to the case.”
This is true. Even though the neutral conductor is grounded at the panel box, loading elsewhere in the circuit can cause the neutral conductor to have a potential above ground.
That is, there is a small resistance of the copper wire as it goes from the electrical outlet back to the panel box. If something in the circuit is drawing significant current, this small resistance can cause a voltage to appear in the grounded ("neutral") conductor.
If your home uses "shared neutral" wiring (two opposite-phase 120V hot lines sharing a single neutral wire for the return), then even throwing the circuit breaker on a given circuit can leave a potential in the neutral wire. This is because the paired hot conductor can be carrying current, causing the shared neutral wire to have potential.
This can be surprising.
So, once we switched the polarized plugs, some of the appliances would always have a hot chassis; unless we made sure appliance-makers always attached neutral to the chassis.