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SAFETY GROUNDING, a long explanation.
The following is supposed to be known, but here it is for just in case.

SAFETY ground, and SIGNAL ground is not the same. Though in many equipment you will find these electrically connected, when testing with an ohms meter, it is the WAY they are connected which makes the difference. This difference is not in electrical contact, but in safety, in case a contact gets loose, or bad. Like this, signal ground can be kept apart from the safety ground wire. Even use another color. Though they are electrically connected too, we just do as if they are not. Like this, the signal ground can be broken or messed up by somebody, and yet likely this person did not mess up safety ground wiring, because there was probably no reason.

Electrical safety doesn't care about signal, but only wants to connect the chassis to the mains safety ground wire of the building. Electricity is provided to a building as 3-phasis system, for food reasons. First, the energy flow is not sine wave pulsed (as opposed with a 1-phasis system). The energy flow with in a 3-phasis system is a constant! Same as with DC. This makes transformers and cables more efficient and more silent too. Then, to the building itself, the 3 phases lines are presented, together with a "zero" the "cold" line. Before the building, so coming from the community 3-phases transformer, the zero carries almost no current. Then, inside the build ling the three phases are randomly distributed. Normally one to each room. Each line represents now an AC voltage against it's zero line. This zero line now becomes the return, and it is kept close to ground by the electricity supplier. Inside the house, that works more or less by itself, because all zero-currents of each phase is 120° out of phase with the next one, and even each other out. This is difficult matter. If you already knew it, you will remember it, but if not, this explanation will not really explain it well.

So why all of this...Well because safety ground is really important. What happens for instance when you have a defective apparatus, and would have the hot line connected to some metal parts of a swimming pool. That would give more current to ground, as the "hot" line fuse can take, and it will blow out, ending the danger. This is the lowest possible protection level, but it least it works, because the electricity company (more or less....) grounds the zero.

A higher level of protection comes, when the apparatus has a metal case which is safety grounded. Now, you would not need to touch the metal part of the swimming pool with it. The fuse would already blow, just be plugging in the defective apparatus in a mains connector with safety ground.

Is there a higher level? Yes there is! Many electric apparatus have a plastic case. So the safety ground is not working. When you use your Iphone with mains power supply in the bath tub, and it drops in the water, it might still kill you, provided it has the right kind of defect making this possible. What can be done against this? The answer is a ground leakage detector. This is now obligated on all new buildings. The ground current can be directly measured, and if all apparatus are good, no phono falls in the bath tube, and no electric lamp fell into the aquarium, there should be no ground current. So a detector on this (with emergency switch-off relay) will increase the safety very much.

Today, the ground wire, in addition of being able to blow up the group fuse, is now also a sense wire for the fault current. It measures if some piece of equipment is returning a fault current into the safety ground, larger than 10...30mA. If so, this activates an relay, which shuts down the electricity even though there was no short.

Such a fault current would for instance happen if you touch with your one hand something with high voltage .(whatever it is) and via your other hand it flows to the chassis. So then, because the chassis is grounded via this separate green-yellow wire, of the mains cable, this small current reaches the security switch, and latest above 30mA it activates, and switches off the mains voltage.

So the "Earth" comes in at the mains connection of an apparatus, but it is also where leakage current comes OUT of faulty equipment, and from there the earth leakage switch is activated.

At the mains inlet plug, is the ground connection. Best is a soldered version. This is connected with a shortest possible piece of wire to the chassis. This wire may be used only for this one connection, nothing else, and it must be screwed to the chassis. Like this the chassis becomes a monkey proof reference.

In this condition, the leakage fault current switch can already detect leakage to ground. Reasons for this can be many, and this kind of leakage often appears before a short. If water drips in equipment, first comes leakage, and then comes a short. Also a transformer can become leaky. For this reason a mains transformer must be grounded to the chassis, in order to allow the ground safety switch of your house, to do it's work.

The equipment designer adds an earth symbol at every position where he wants to see a dedicated (yellow-green wire) earth connection. As a minimum, there will be two such connections: One at the mains inlet, and in the circuit diagram.

Like in this picture, the signal ground is called GND, and has the symbols called "Ground" and "Earth" to it. The ground symbol stands for signal ground, this is one wire throughout the whole circuit diagram.

So make good note of this:

  1. the GND Symbol can appear multiple times, and only shows it is "GROUND" and it's the same electrical connection. Even so, in many schematics these inter-connections between GND symbols are not drawn, to make the circuit easier to read.
  2. The "Earth" symbol is something different. It tells exactly at THIS point, the signal ground is connected to the chassis with a dedicated wire of it's own. Also, when the chassis is made of multiple metal parts, the designer needs to define one, which is the master. To this, all safety ground wires must be connected. So when you remove for instance the bottom panel of a washing machine, you will find it was connected with a safety ground wire, to the back panel, using copper hardware. Useually at the place where the mains enters. So in case the bottom screws stood in the water, and got rusty, there is always the earth wire, grounding it, and in case of emergency be able to take many amperes.