On Tue, 11 Jun 2019 06:32:15 -0700 (PDT), wrote:




Why is the ground rod at the service any better than the one at the
shop? I agree 4 is better than 2 but they are already bonded together
via the neutral in a 3 wire feeder. I doubt he wants to dig up the
yard. This was done this way for 90 years and we didn't pile up a lot
of bodies. The code change was mostly just to make the language
consistent with the rest of the code. Same with ranges and dryers. The
grandfather clause is still there for things done pre 96 adoption.
It does mean you have to be more careful with your bonding and
grounding tho.

the ground connection serves 2 purposes

1) a path for lightning
the ground rod is to provide a path for lightning etc.
this NEEDS to be a path to actual Earth ground because that is where lightning will go.

2) protection from shock due to equipment faults
this is to protect you from a shock if the hot wire shorts to the metal case inside an appliance. Think of a motor with all those windings of enameled wire. If the insulation should fail and connect the power to the metal case of the appliance, you can be shocked by touching the appliance. If the case is grounded however, when the short happens, a large fault current will flow and blow the breaker. For a LARGE fault current to flow, the ground must be low resistance. So you want a BOND WIRE between the neutral and ground in the system. This provides a path for the fault current to flow sufficient to blow the breaker if there is a ground fault. Often a ground rod alone is too high resistance.

#2 is an important safety feature, I would not skimp on this.


So if have only 3 wires between the buildings and you can't add the 4th wire, I think bonding the outbuilding neutral and ground together at the entrance to the outbuilding is the safer option compared to relying on a ground rod alone.

You can also add a ground rod there to help for lightning.

If you don't need 240V and need only 120V in the outbuilding, you can use the 3 wires as hot neutral and ground. That is 100% safe and legal but you give up having 240V.



mark

With a 3 wire feeder you are REQUIRED to bond the EGC (ground) to the
neutral.
The earth is NEVER a fault path.
You are also required to provide a ground electrode system.
If you drive a rod and if you can't demonstrate that it has 25 ohms
to ground, you need the second rod more than 6' from the first one.
That has been code since Thomas Edison was around.