Grounds are electrical circuits.
There are two types of grounds: Signal Ground and Earth Ground.
Earth ground is the "earth" itself. A long metal rod is driven into the ground to complete the circuit with the generator. Certain circuits, like a Wye configured transformer, must be grounded this way to reduce harmonic currents.
The third pin in an outlet is connected via a green wire back the the load center, which is "earthed" at some point nearby. The Neutral and Ground conductors will be bonded at this point.
This is a protection system, designed to conduct any currents that may appear on metal casework, etc., safely to ground instead of passing through you on the way to ground. Ohm's Law Says: Current will always take the easiest path, which may be you.
For example, a transformer in a power supply is connected to the Line and Neutral wires. If the case is metal, the ground wire would be connected to the case (chassis ground.) Otherwise it would be connected to the frame of the transformer. The purpose is, that should a short to Line develop, the case would be energized, and touching it would result in an electric shock. The ground would conduct the energy to the load center and possibly blow a fuse in the power supply or the breaker/fuse for that circuit. This way the chassis would not become energized and float at 120 or 240V. Many years ago, when many household appliances, fixtures, and electronic devices had metal knobs and trim, this was very important. A Hi-Pot test was done at the factory to insure that no external surface or knob that you could touch was connected in any way that current could flow between it and chassis. Very important when three prong plugs were not used, and polarized plugs were not common. It was very possible to have 120 volts on the metal chassis within the cabinet just waiting to bite you.
Sometimes referred to as Common.
This is a common circuit designed to tie all the components to the same level. This is often needed in digital circuits to create a common reference for the Zero signal level. An example is the Digitrax PM42 which must be connected to the command station common for proper operation. A lack of a common reference voltage can result in erratic or undesired operation.
- Connecting a heavy gauge wire between power management devices and boosters will allow any mismatches in polarity to have a path for current returning to the source. It also allows quicker operation of the auto reverse function when a locomotive crosses between two booster districts.
This should not be connected to the safety ground on the high voltage side of the system. Connecting it to the high voltage side's earth ground could create a situation where a defective ungrounded electric tool, should it touch the rails, completes the circuit. Or you might be energized and not know it, until you touch the track or a metal case such as that of the command station or booster.
- Many direct and alternating current power supplies have a GND or ground terminal on the Low Voltage side. This is usually developed internally on the low voltage side and is not connected to the safety ground. It is used to develop a reference point for measuring the voltage outputs. The transformer inside will have a centre tap that can be used as a ground or common point if needed.
- Digitrax tends to contradict themselves in their manuals. They indicate that boosters and other devices should be "grounded", but in no way should they be connected to earth ground as implied. A common return bus between various devices is needed between them, as your LocoNet cable's ground wires are not up to the task of handling high currents between power management devices or boosters. Again, for your safety, nothing on the low voltage side of your DCC system should be connected to the high voltage side of your power source.
Remember: Electricity is like a dog. Treat it with respect. It is always watching you, get too close, it will bite.
If you do not understand electricity, ask someone who does.
Do not make modifications to electrical equipment unless you are qualified to do so. Seek qualified advice before doing electrical work, and it is best to contract a qualified professional, especially if it involves high voltages (anything above 48 Volts).
This ariticle is general in nature. Consult a qualified technician and follow all applicable Electrical Codes.