The electrical grid transmits power over wires to our houses, and our Bryan Cockfield has covered it very well in his Electrical Grid Demystified series, but what part does the earth ground play? It’s commonly known to be used for safety, but did you know that in some cases it’s also used for power transmission?
Typical House Grounding System
A pretty typical diagram for the grounding system for a house is shown here, along with a few of the current carrying conductors commonly called live and neutral. On the far left is the transformer outside the house and on the far right is an appliance that’s plugged in. In between them is a breaker panel and a wall socket of the style found in North America. The green dashed line shows the normal path for current to flow.
Notice the grounding electrodes for making an electrical connection with the earth ground. To use the US National Electrical Code (NEC) as an example, article 250.52 lists eight types of grounding electrodes. One very good type is an electrode encased in concrete since concrete continues to draw moisture from the ground and makes good physical contact due to its weight. Another is a grounding rod or pipe at least eight feet long and inserted deep enough into the ground. By deep enough, we mean to include factors such as the fact that the frost line doesn’t count as a good ground since it has a high resistance. You have to be careful of using metal water pipes that seemingly go into the ground, as sections of these are often replaced with non-metallic pipes during regular maintenance.
Notice also in the diagram that there are places where the various metal cases are connected to the grounding system. This is called bonding.
Now, how does all this system grounding help us? Let’s start with handling a fault.
Handling A Fault
One purpose of the grounding system is to cause a breaker in the breaker panel to trip if there’s a short circuit somewhere. That happens if there’s an appliance with a metal case and the insulation on the live wire in the appliance is damaged, causing the copper wire inside to touch the metal case. The case becomes an extension of that live wire. This is called a fault.
But the metal case is connected to an electrical path consisting of the ground wire in the power cord plugged into the wall socket, as well as the wire from the wall socket to the breaker panel. In the US National Electrical Code (NEC) these are referred to as the equipment grounding conductors.
In North America at least, in the box where the service first enters the house, the equipment grounding conductors are connected to the neutral wire. In this case that box is the main breaker panel. In most breaker panels this connection is made by having both the wires go to metal bars that are screwed, or bonded, to the panel’s case, thereby making the electrical connection through the case.
Following the red dashed line from the fault, a high current now runs through the live wire, through the appliance’s case, and uses the equipment grounding wires as the return path to the breaker panel. From there the current goes through the panel’s case to the neutral bar and neutral wire back to the transformer. Along the way, the live wire runs through a breaker in the breaker panel and the current is high enough to trip that, opening the circuit and making it safe again.
But where does the earth ground come into it? Often it doesn’t. Sometimes, however, as shown by the blue dashed line, a little current will flow through a parallel path including the grounding electrodes and the earth ground.
Discharging Stray Charge
There’s a purpose for earth ground that many here on Hackaday are quite familiar with, and that’s stray charge and problems with electrostatic discharge to electrostatic-sensitive devices and components such as MOSFETs, CMOS ICs, and TTL chips. Methods for handling this are to wear an antistatic strap or to work on an antistatic mat. These usually have a clip or a dedicated socket for connecting to earth ground.
The charge on your body will put you at a different potential than the ground and so current will flow between you and ground. The earth ground is largely electrically neutral and will easily absorb the charge, leaving the combination of the ground and you neutral.
Not all electrostatic discharge is accidental. We previously covered experiments by [Kevin Darrah] where he deliberately tested the effect of it on various components as well as tried out circuitry for protecting against it.
Metal cases can also become energized by indirect lightning strikes and any charge build-up will drain to earth ground in the same manner.
Single-Wire Earth Return (SWER)
To save on costs, mostly in rural areas or for remote, isolated dwellings, sometimes only a single wire is used for transmission. This eliminates the cost of the neutral return line, provided the cost savings makes up for the decrease in efficiency. The decrease in efficiency is due to the use of the higher resistance earth ground as the return path. This turns out to be quite safe and there are over 200,000 kilometers of transmission lines done this way in Australia and New Zealand. In the US, it’s used in portions of the Midwest and Alaska.
Power is first supplied by the grid to the primary of an isolation transformer, isolating the grid from the earth. Here, the voltage is typically stepped down from 22 kV to 19 kV. One side of the secondary is the single transmission wire and the other side goes to earth ground.
Distribution transformers are then used at customer sites where the 19 kV is converted to voltage suitable for the customer, 240 volts for example. One side of the primary is the single wire and the other side goes to earth ground for the return, which eventually makes its way back to the earth ground side of the isolation transformer.
Soil resistance is an issue. Dry soil is less conductive than wet, and in Alaska the ground rods have to extend to below the permafrost, as the ice is also less conductive. In addition, this higher resistance also causes the primary’s voltage to float higher and make it difficult for self-resetting breakers to reset, as they rely on a potential difference.
Additional phases can be added by adding second and third transmission lines.
Have you had experience trying to create a good earth ground? Perhaps you’ve encountered soil that was too dry, and had to work around it? Or, as happens all too often, you work in a place where you’re certain some wall sockets just don’t have ground connections at all. Let us know in the comments below.
Or step over to the live side of electrical distribution and check out Bryan’s Electrical Grid Demystified series.