By code the Neutral is only permitted to be connected to ground at one point and that one point should be in the main electrical service panel.
Now the code reuuires that the sub panel cabinet be connected to ground via the ground wire from the main panel and also via a #6 or larger copper wire to the ground rod. It wouldn't hurt to also connect a #6 ground wire from the panel ground to a clamp on any nearby metal water pipe, if one exists. Telephone, cable TV, and other electrical services should also be ground connected together, at least at one point on the property.
Before the code required that a sub panel in an out building had to be grounded via a separate ground rod, a 60' pine tree next to my shop was struck by lightning (3' away). It was such a large strike that the top 16' of this pine tree was blown off and fell to the ground. The lightning also jumped from the tree trunk into the metal face of the door and then up from the top corners of the door through the 1/2" plywood sofitting above the door into the armored electrical cabling above the door (blew 1"X6" slots through the plywood). Although the electrical panel in the shop was grounded via the service cable from the shop to the main panel in the house, at the time I didn't have a separate ground rod installed at the shop, as it wasn't required by code. The ground connection via the electrical cable to the house aparently wasn't sufficient to carry the lightning load because the lightning jumped between the ground and neutral in the sub panel in the shop and went back to the main panel in the house that way too. This resulted in a large burn mark in the sub panel and burns between neutral and ground in many of the electrical outlet boxes both inside the shop and the house.
Four days later, while I was just sitting in the shop looking at some plans on my workbench, the receptacle directly behind my radial arm saw suddenly threw sparks and set fire to the wall around it. Nothing electrical in the shop was running at the time, except for the overhead lights, which are on a separate circuit. It's a good thing that I was there. I was able to quickly shut off the power and use an extinguisher to put out the fire on/in the wall. If I hadn't been sitting there when this happened I would have lost my whole shop.
After this incident I replaced many of the outlets in my shop and in my house, personally inspected all of them, and added a ground rod to my shop panel wiring. I now frequently take the time (at least once every spring) to check to be sure that my phone wiring, my cable TV wiring, and my house electrical panels are all ground connected to each other and to the ground rods. When I make this check I remove, clean, and replace any connection that appears not to be perfect.
I have always had a significant respect for lightning, but this incident enhanced this respect significantly.
<DIV><FONT face=Arial size=2>I recently built a new detached shop (the kind of
shop that my wife parks her van in). The electric service to the shop is
fed from the house via 240V 50A breaker. When I had the electrical
inspected, the inspector noted that since I had more than one circuit in the
shop subpanel, the subpanel needed its own ground rod. The electrian
came back and put the ground rod in and connected the wire from the ground rod
to the neutral bus. The inspector came back again and said that the wire
from the ground rod needed to be connected to the panel case instead of the
neutral. I figured I could take care of that quickly myself, but now I
have a question. There is a ground from the main panel that currently
connects to the panel case. Do I leave that in place when I connect the
wire from the new ground rod? In case a pic would help, here it is
before I moved the new ground wire off the neutral bus.</FONT></DIV>
<P><FONT face=Arial size=2>The bottom left conduit is the service from the
house with two hots, a neutral, and the ground wire. The conduit to the
right of that is the wire that connects to the ground rod (the wire is black,
but is marked as ground with green tape). I moved this wire to the
ground connection in the bottom middle of the box, but I don't know what to do
with the ground wire that is there now that connects back to the main
<P><FONT face=Arial size=2>todd</FONT></P></BLOCKQUOTE></BODY></HTML>