Grounding the metal control panel box. O Gauge

It depends on the purpose of grounding the panel.  If the box contains exposed or potentially exposed line voltage circuits there should be a three wire power cord with the green (ground or earth wire) connected to the enclosure.  This prevents the box becoming energized in a fault situation.  If there is a removable metal panel the panel should have its own ground wire.  Also, there should be protection from the line voltage wiring having a fault to any low voltage wiring.  An alternative to grounding the case is double insulation as you would see with a power drill.

An example of a fault is a high voltage wire into a terminal strip.  Even if the wire has no exposed copper and the terminal strip is the enclosed type, the wire could come loose and contact the inside of the box.  An example of double insulation is where the terminal strip is inside an insulated box within the enclosure.

Transformers with a two wire plug are using the double insulation approach.  If they are UL certified they have this.  Older transformers may not be up to snuff in this regard.  A transformer with a two wire plug should not have an exposed ground post since there is no ground present.  I don't know of any train transformers with a external protective ground post.

If the ground is not protective and is only for signal purposes you can run one wire to the ground terminal of a three wire plug and plug this into an outlet.  Sometimes a water pipe can be used but that doesn't work if you have plastic pipes.  A water pipe would never be used for a protective ground.

Of course, if the box is hardwired to the house wiring (not via a plug/outlet) it comes under the requirements of the electrical code.

Technically, you are not “grounding”, you are “bonding” the control panel box to eliminate differences in potential of electrical components. 

This is to be done so that if you get between two items and there is a difference in voltage, you do not become the easiest path to ground. 

Unless there is another electrically controller item, that you can touch as you are touching your control box, the danger of you becoming a conductor is minimal. 
But, if there is, then make your electrical connections common to each one. 

With out seeing your set up, it’s difficult to give a 100% satisfactory answer that will get you all the results you seek. 

What other metal objects are near your control box location?

Here is all I am worried about.

If a loose wire inside the box, or part of an uninsulated control  switch wire mounted to the box,  happens to contact the metal box, I don't want to get shocked when I touch the box.

So, in case that happens, I just want to screw a wire to the side of the box, and have it run to a ground. 

I know that AC transformers only put out about 2 amps and 140 watts, but that may still be an unpleasant shock.

I have will have an electric wall outlet underneath the train table, so I guess I will be running the grounding wire from the control box to the ground wire inside of the outlet box.  There are metal support posts in my basement, so if one of those is close by, I may run the wire to one of those.  But since those sit on 8 inches of concrete, I am not sure that they would really qualify as a grounding rod.

I was just hoping that larger transformers would have a simple post on the side, for purpose of running these types of grounding wires to it, which would then run to the ground wire inside of its power supply cord, and then to the wall outlet.  It sounds like none of them do.  Good info to have.

Thanks for all replies.

Mannyrock

OK.  You are dealing with the secondary side of the transformer.  You are not going to get a shock from 18 VAC.  That is why it is OK for the track to sit at this voltage.  A greater concern is the current that would flow in a short, a fire hazard, but the circuit breaker in the transformer is intended to take care of this.

Another reason is the secondary side of the transformer may not be referenced to earth ground so a fault to outlet ground would not have any effect.  However, if you have a TMCC base it will connect earth ground to the track so there would be some current flow if the hot side of the transformer contacts your grounded case.  Again this would appear as a short and the circuit breaker in the transformer should protect from this.

The metal posts in your basement are not suitable for a ground connection.  The highest quality ground you have available is from the outlet unless you run special wiring.  A water pipe ground should not be used for protection.  For example, I have used a water pipe for the ground connection to a shortwave radio.

To accomplish what you want you can connect the transformer common to your box with the intent to trip the transformer breaker if a hot wire contacts the box.  Unfortunately, the common terminal varies from transformer to transformer.  Depending on your overall wiring scheme may complicate where to make the connection.

I am assuming all the line voltage connections to transformers, outlet strips, wall warts, etc. are not modified from from there out-of-box condition.  So, getting a line voltage shock is not a concern.

If you have an outlet on the train table it should be connected to the house by a plug/outlet.  If you hard wire to the house wiring you train table would be required to comply with the electrical codes.

There is a case to be made to not connect the box to the track ground (common).  If you are working with a train or accessory and you make contact with the case you could trip the transformer breaker.  This is no different from shorting to the track.

If you have a over the air signal that needs to get in or out of the box the metal box would be a problem whether it is grounded or not.  So don't put a command base inside the metal box.

OK.  You have all given me excellent reasons for not doing it. So, . . . I won't

Thanks,

Mannyrock

Now, it is possible to get a small shock from the secondary, but normally you don't even notice. You can handle it (5-25v) pretty freely as you've likely noticed. But lean on powered track  with a sweaty forearm and you'll likely be looking for something sharp that just poked you, before you even realize it was a tiny little voltage bite (like a short needle, or wire poke, or mabe a bunch of little ones).    Don't panic, dry your arm.   It's similar to a 9v battery. OK on fingers, bites the tounge if "tasted" for power.

That raises another point.  If you have a separate power source for each loop they will have a common connection via the command signal wire.  If they are out of phase you will have the sum of the outputs between the center rail of one loop and the center rail of the other loop.  This could be up to 40V as some transformers go up to 20V (or more if you have high line voltage to your house).

So it is a good idea to phase all your transformers even if the two loops are not connected by track.  I'm sure this has been covered on the forum.  Search for Phasing Transformers to see how this is done.

OOPS! - I mixed threads in this reply.  Phasing transformers is always a good idea but the loops I was referring to are part of another discussion.