It is a fact of life. Ground current happens. And where it flows makes all the difference.
In a perfect world there would be no ground current on the AC power line. AC would enter on the LINE, and return on the NEUTRAL, and the earth ground would just sit there. In the real world there is inductive and capacitive coupling that creates ground current. Ground current can also be created by various electrical and electronic equipment, or it can be created by the pick up RF energy, especially when the ground forms loops (loops can increase the amount of RF energy that is picked up).
Before we get too deep into ground current let’s take a look at how a typical house is wired.
Power comes from an electric substation to the pole outside your house. The distribution voltage is approximately 13kV, but higher voltages are used. The power then goes to a transformer which takes the high voltage AC and steps it down to 120V for your house. Usually a few neighbors share a transformer. The center tapped secondary of the transformer gives you two “hot” wires, each with 120V, and a neutral wire. The voltage between the two “hot” wires is 240V which is the sum of 120V+120V, which are 180° out of phase. Even though there are two “hot” leads entering your house, and they are 180° out of phase, technically this is still a single phase system.
SIDEBAR: 13,000 Volts eh? Yes, so don’t be tempted to touch it, not even with a 10 foot pole. The moisture in a wooden broom handle will conduct enough current to light a 100W light bulb (meaning it could kill you). Linesmen use fiberglass poles.
Victims of electrocution at this level will have a burn where they contacted the power line and their feet are blown off (or whatever part of their body was touching the ground at the time). Not cool.
Really super cool: These are linesmen working on LIVE high tension wires (100kV-500kV). Yes those are electrically conductive suits. Talk about a “bird on a wire!” http://www.chopperworx.co.za/powerline.html A helicopter drops them off and picks them up. http://infao5501.ag5.mpi-sb.mpg.de:8080/topx/archive?link=Wikipedia-Lip6-2/1536956.xml&style#3 for more info on live line work.
OK, back to the blog… In your house the power goes to your breaker panel and to the “main breaker” (the main breaker is omitted in my sketch for clarity). The main breaker feeds the bus bars. Opening the main breaker disconnects the electricity to the entire house. Each bus bar carries one of the two “phases,” and where you install the individual circuit breakers determines which “phase” you get. Installing a 240V breaker connects to both “phases” (the breaker is a “double” wide). These circuits are used for HVAC, dryer, and range. There is also a neutral bus bar. The house earth ground is attached here also. (Notice the earth ground symbol is a capital “E” turned on its side) If you cut open an AC wire you will see three wires as shown. All of the neutral wires (WHITE insulation) from all of the house circuits are attached to the neutral bus bar, as are all of the circuit ground wires (BARE copper wire, can also be GREEN for 240V). The “hot” wires (BLACK for 120V, BLACK and RED for 240V) from each circuit are connected to the circuit breaker.
Did you notice in the diagram how both the “N” and the “G” go back to the same bus bar in the breaker panel? The electric current could care less whether it gets back to the bus bar on the “N” wire or the “G” wire. The insulation prevents the current from flowing on the “G” wire… of course it’s this ground current that we’ll focus on here.
More next week…