DCC Wiring: Grounds
Summary: Grounds are one of the most confused issues with Digital Command Control wiring. Grounds are electrical circuits, including: Signal, Chassis and Earth Ground.
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 contact a qualified professional, especially if it involves high voltages (anything above 48 Volts).
This article is general in nature. Consult a qualified technician and follow all applicable Electrical Codes for your jurisdiction.
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 "ground" pin in an outlet is connected via a green wire back 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, knobs, switches, 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 or chassis. Otherwise, it would be connected to the frame of the transformer. Should a short from the case to Line develop, the case would be energized, and touching it would result in an electric shock. If the casework is connected to earth ground, the current flowing to ground should blow a fuse on the line side of the transformer (if so equipped). The open circuit caused by the blown fuse protects you from getting a shock. Should that mechanism fail, the fuse at the load center may blow.
This way the case 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 ensure 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.
Many devices lack a ground on their line cord. These devices are designed to very strict safety standards to prevent electrocution.
A device which should, but does not have a ground connection, is considered to be floating.
Sometimes referred to as Common. This is a common circuit designed to tie all the components to the same level. Often needed in digital circuits to create a common reference for the Zero signal level. There may be several "grounds" in an electronic device, all with specific purposes, such as reference levels, returns, or for noise reduction.
An example is the Digitrax PM42 which must be connected to the command station/booster common for proper operation. A lack of a common reference voltage can result in erratic or undesired operation. It also functions as a return path for excess currents, which would otherwise use the LocoNet cables to return to the booster. Unfortunately, Digitrax confuses this by labelling that connection as Ground.
This common circuit should not be connected to the safety/earth 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.
Common Connection between Devices
A number of DCC components should have a common connection between them. Boosters (standalone and those built into the command station) and Power Management devices need a common return between them for several reasons.
- Establish a reference 0V level
- Provide a return path for current between devices
By connecting all the devices together with a common return wire, all devices see the same 0V level, allowing more consistent operation. Connecting a heavy gauge wire between power management devices and boosters will allow any mismatches in phase to have a path for current returning to the source. It also permits quicker operation of the auto reverse function.
The common return allows current to flow between boosters when vehicles cross boundaries between booster and power districts. With all-wheel pickup, if the wheels and rails are perfectly clean, there will be little issue when crossing into another district. If the pickup is not perfect, it will behave as an offset pickup would.
With offset pickup vehicles (one truck connects to the right rail, and the other to the left), when crossing over into another district an unbalance will occur, and current will blow between boosters until the vehicle is completely across the boundary. Without a common between the boosters, a vehicle may stall or lights could flicker.
When a short circuit occurs, the common return will carry the current until the booster shuts down. This is preferable to having your throttle network carry large currents in their light gauge wires, which were never intended to do that.
Grounding Your DCC System
Connecting your DCC system to earth ground defeats the purpose of the SELV power supply, and is a safety issue. Your DCC system should float, meaning there is no connection between it and the earth ground.
Most DCC systems are powered using a SELV designated power supply. The term means Safety (or Separated) Extra Low Voltage. This type of power supply is designed to completely isolate the High Voltage side from the Low Voltage device being powered. The purpose is to protect the user from any contact with high voltages. These power supplies may or may not have an earth ground on the high voltage side, depending on the design. Direct current SELV power supplies may 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 earth ground. It is used to develop a reference point for measuring the voltage outputs.
Digitrax and Grounding
Digitrax tends to contradict themselves in their manuals and the labelling on their DCC components. The warning labels on their devices such as command stations/boosters will state that it must be powered by an approved SELV power supply to avoid electric shock and other hazards.
The labelling indicates 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, 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, the low voltage side of your DCC system should be completely isolated from the high voltage side of your power source. Your DCC system should be floating, with its own signal grounds for its purposes.