Hello,

I just started to read the e-book. There are a few things I am not sure to understand about the "Electrical Safety" chapter.

1) If a few different devices with different voltages are all connected to the ground in the same area, isn't this going to create a circuit between the devices?
2) The book says that connecting a portion of the circuit to the ground transforms that portion into a "neutral conductor". But the ground has a pretty high resistance, and as we know a resistor creates voltage potential. So if someone touches the "neutral conductor" a few meters away from the point where the circuit is in contact with the ground, shouldn't this person be subject to the voltage difference?
3) The book advises to connect at least one part of the circuit just in order to have a safe part of the circuit. But isn't the ground going to "absorb" a fraction of the current, therefore making the system less efficient?

Thanks by advance for your answers!

 

There can be high resistance at between ground points, but if and when a potential or supply is referenced to earth ground it is necessary to establish an acceptable ground resistance in order for it to be safe.
This is done by taking qualified readings.

Early developers of electrical systems theorized that the earth was an electrically neutral body, i.e. an equal number of negative and positive charges are distributed throughout the earth at any given time. Being electrically neutral, earth is considered to be at zero potential and establishes a convenient reference frame for voltage measurements. Noting that voltmeters read only the difference in potential between two points, absolute measurements can be made by using earth as a reference.

http://www.ese.upenn.edu/rca/instruments/misctutorials/Ground/grd.html
Max.

 

The ground doesn't "absorb" current unless there's a compete circuit from a voltage in the circuit to the ground. Otherwise all an earth ground does is provide a path for stray voltages and charges or accidental short circuits.

 

I made this little simulation:



I will let you answer your own questions with respect to what the simulation shows: Look at the various node voltages and branch currents.

Note that V(g1), V(g2),I(R7) and I(R8) are zero within the numeric accuracy of the simulator... With that in mind, do you see any interaction between the two circuits?

Note that this configuration is what is referred to as a "single point ground", where each sub-circuit has only one path to the common ground point. All sorts of weird things happen if each sub-circuit has a path to ground in more than one place. That is called a ground-loop.

 

1) When many circuits connected = separate pieces of equipment then each piece will have its own power supply, so they will not interact.
If several circuits are powered from the same supply then there may be interaction.

You can really only compare the supply voltages if you connect one terminal of each supply together and call it 'common'.
Then one supply may be +30 volts, another may be +20 volts, another -20 volts, yet anotherr may be 120 volts alternating, all with respect to this one common connection.
volts
If you don't establish a common connection then you cannot say the +30 volt supply is 10 volts higher than the +20 volt one, the term is meaningless.

This establishes one major function of grounds - that of providing a stable voltage reference point or zero.

2)The stable bit brings us to your second question.
Firstly resistors don't create voltages, they have voltage differences imposed upon them. Power supplies AKA voltage sources provide voltages.
The point about the 'neutral' is that in this context neutral really means undefined (or nearly zero) voltage, as distinct from a hot mains supply line that would carry 120 volts, above earth.
Stable means that it will not change almost whatever you do to it.
Most circuits do not possess any points with that quality so we connect some part of the circuit to something that does - the ground.

3)This brings us to the second use of grounds - the safety ground.
In normal circuit operation the circuit does not pass any current into or out of the safety ground.
If, however a safety fault develops, the circuit is arranged so that a large current will flow to ground and rupture a fuse or trip a breaker.

 

Thank you for all your answers. I think I have enough elements to understand the concept