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Yes, Grasshopper, ground is not always the nice stable place you think it is. Here we'll post some examples of how you can get in trouble.
What is ground?
And why do Brits call it "earth?" You can imagine that dirt and rocks are pretty bad conductors, but if you put 6x10^24 kilograms of material in one big clump, you get the ultimate electrical terminal that can withstand more juice than mankind can generate. Think about all the lightning that hits the earth, and unless you are standing very close to the strike, you have nothing to fear from electrical current hitting your wet shoes. "Ground," ultimately, is "Earth".
Let's follow the power from your local power company (here in Tucson, Tucson Electric Power), from a neighborhood substation to your home. 64 kV comes in on 60 foot tall steel towers The power is three-phase, without a neutral conductor. The phases are 120 degrees apart. Think of it as a helicopter rotor with three blades, rotating 60 times per second. The analogy probably sucks but pretend that all of the power is transmitted from the blades, and none of it is transferred through the blade hub. If that doesn't work for you, you need to up your dose from the local dispensary.
Note that there is a ground wire at the top of the masts, it is much smaller conductor than the three-phase power. It is there to try to attract any lightning that might strike the power line (and send it to the Earth), to reduce the severity of power surges. The ground wire is hard-attached (no insulator) to the metal power poles and therefore is self-grounded by the metal towers.
Here in Tucson, once the power company starts talking about upgrading the distribution system to 128kV on 100 foot poles, all of the neighborhood hippies, combined with NIMBY homeowners, get bent out of shape and start to protest. Seriously, everyone, we will need more power in the future. You can't make an omelet without breaking some eggs.
The 64kV power is delivered to a neighborhood substation, which is protected by very high walls. You don't want to learn what "Muy Peligroso" means by jumping this fence. The output of the substation is three-phase power at 14kV.
14kV power is distributed around this urban neighborhood using a combination of metal and wooden poles, providing a cool third-world vibe. In some cases, a dummy ground wire is run above the three-phase lines, like in this photo, for lightning protection.
Homes run on 240V two-phase here in the US. It is possible to transform 14kV three-phase directly to 240V two-phase, but in the photo below we see a 14kV to 4kV three-phase transformer. That's a lot of weight on a wooden pole, you might not want to park under it during a thunderstorm in case there is a microburst. On this pole, 14kV comes in on the left and 4kV exits on the right. How do you know the voltages? The cross bars are labelled!
Finally, three-phase power (14kV or 4 kV) is transformed to two-phase 240 volts. Here is where the "neutral" wire is actually connected to Mother Earth. There are three wires for 240V two-phase, the middle one is a neutral. The neutral wire returns any out-of-balance current to the center-tap of the final 240V transformer. A good electrician will try to ensure that your loads are close to equal oin the two phases when she installs the breaker box.
Two-phase 220 volt power continues on ancient wooden poles, just like you would expect in Tucson's original African-American neighborhood. This is the only downtown neighborhood where the city never installed sidewalks, care to guess why?
Each 240V wooden pole, in theory, is grounded to Earth with a fat copper wire, attached to a metal stake that is hammered deep into the ground. Copper is like currency to local drug addicts, in many cases the copper ground has been cut off and brought to the recycle yard. If you look closely on this pole, the wire has been cut about as high as someone could reach.
Is that safety issue? Maybe, maybe not. Earth ground is only really a factor in power surges which could be caused by lightning strikes, downed poles or other disasters. There should be many redundant Earth grounds in the system, each home (and power pole) is supposed to have one, right? In an older neighborhood where a lot of homeowners did do-it-yourself upgrades to plumbing and electrical systems without permitting, there are a lot of homes that are no longer grounded. In the old days the circuit-breaker panel (40 ampere fuse-box in the really old days) was grounded with a wire that was attached to the nearest metallic water pipe, That pipe was presumably connected to metallic water pipes buried underground, all the way to the metallic street connection. These days, chances are that water pipes are completely insulated from ground, as water is delivered in plastic pipe. You might want to check your ground connection at your power panel...
Continuing our grounding journey, 240V is connected to a home owned by a couple of modest means who hold a signed-off electrical upgrade permit. Behold in jealousy, a 400 amp panel... the conductors from the mast into the panel are 600 mils of copper, which costs roughly $10 per foot. That power line weighs hundreds of pounds, so it has been heavily strapped to an exterior wall, 12 feet above the ground where no fool can reach it. The top of the wall of this one-story casita is 16 feet, but that's another story...
Finally, here is the Earth connection, all up to electrical code. Those fat copper wires are bonded to all of the rebar inside the monolithic stem-wall/slab under the house, a 700-square-foot steel grid embedded in 30 yards of concrete.
Apologies to people outside the US who have different construction standards that were not covered here. And tio all people who live in subdivisions where the electric is buried, you might have an advantage in violent storms, but it is just a matter of time before some idiot digs up your street and shorts the whole neighborhood out!
Once inside the home, most equipment runs on single phase power, at 120V. Heavy-duty equipment like electric washers, dryers and water heaters use both phases (240V). The two phases of power are distributed randomly among the outlets in your home, to try to equalize the power they deliver, and minimize current in the neutral line. In Europe, single-phase wall outlets are usually 240 volts. 240V can be deadly, so why is it used? Could it be that much of Europe's copper was used up in two world wars? 240V requires half as much cross-section to deliver the same power, which amounts to quite a savings. Number one in copper production in the 1950s, the US has fallen to number five, with Chile is now the number one producer. Copper is infinitely recyclable, as all street drug addicts all know.
Grounded electrical equipment
Let's look how ground is supposed connect out in your lab. Somewhere there is a circuit breaker panel, where two-phase power ties in from the street (shown on the right). Going into the breaker panel are two phases of 120V (180 degrees apart) and a neutral wire. The neutral wire is attached to two busbars... a neutral bus and a "safety ground" bus. The ONLY place these two conductors should be tied together is in the breaker box. Note the difference between chassis ground and Earth ground. Earth ground is created by driving one or more electrodes into the dirt. In the old days, you could attach Earth ground to a nearby copper water pipe. Today, even if you find some copper plumbing nearby, chances are the water supply coming into the building is done using PVC plastic pipe, which is a non-conductor.
The room is wired up with three-conductor copper: a hot wire (usually black), a neutral (usually white) and safety ground (usually green, sometimes just a bare wire.) At each 120V outlet, three prongs must be wired correctly: The two vertical prongs are neutral and hot... the narrower one is hot, as you'd rather have your two-year-old kid insert a fork into the wider neutral conductor. The roundish conductor is the safety ground.
The cord to your test equipment (here, a power supply) has three conductors. The safety ground is connected internally to the chassis. If somehow a bare wire carrying electrical current contacts the chassis, the breaker will pop as the safety ground is more than capable of exceeding the breaker's trip current. 12 gauge wire is used to for 20 ampere branches, and 14 gauge is used for 15 amperes.
Th guts of the power supply are on a circuit card, which we called the "power conditioning circuit". It can contain voltage rectifiers, filter capacitors, DC-DC converters, linear regulators, etc. That card is also tied to chassis ground.
A single-voltage power supply (as shown) has a plus and a minus terminal. These are typically NOT referenced to chassis ground. The outputs of the power supply is these two voltage polarities, plus a connection to chassis ground. You should NEVER connect either one of these voltages to safety ground at the power supply.
Dual power supplies sometimes tie V+ of one supply to V- of the other one, and only a single "neutral" wire is provided. That configuration creates a ground loop which can cause misery, which you can learn about here. Do NOT connect the neutral to safety ground.
In our example, three wires are used to connect the power supply to the device under test (DUT): the V+, V- and safety ground. Once again, the safety ground is tied to the chassis of the DUT, internally. Wait, why did we waste a wire to run the safety ground between the two boxes, when we could have tied the safety ground to one of the voltages back at the power supply? (i.e., tie it to V- to create a positive voltage, or tie it to V+ to create a negative voltage?) Here's two reasons: the safety ground is usually very noisy, it can pick up a lot of stray signals. And you want to avoid ground loops.
Inside the DUT you could have additional power conditioning, creating additional voltages and applying additional filtering. Then these cleaned-up power waveforms are applied to whatever active circuitry you are powering up. In the case of a solid-state power amplifier, you will need high-current drain voltage which will be positive polarity, and a stable (but low current) gate voltage (which will be negative polarity). The power conditioning and active circuitry at some point will be tied to chassis ground. Thus, your safety is handled redundantly.
Now, go back and observe that none of the conductors (power and return) that carry AC or DC current from the power panel, through the power supply, to the active circuits in the DUT, are connected to safety ground. Now you know how to ground your equipment properly.
Ground loops
Content originally appearing on this page has been moved here.
Two parallel DC-blocked paths to ground
This is an interesting topic, we hope to cover it soon.