Electronic – How does the power station compensates for the current that flows down to earth due to domestic body-current faults

In theory, you cannot get shocked and current will not go to ground if you touch one of the leads because the ground is not connected to the source which is the alternator.

That's correct but both in theory and in practice we have to remember about capacitance between the generator and ground and between the human body and ground.

Figure 1. Capacitance can close the loop in an isolated system.

In Figure 1 we can see that the effect of capacitance between the generator and ground could lead to a significant (mA) current flow.

In Europe we use isolation transformers for portable tools in the workplace.

Figure 2. A 110 V tool safety transformer with centre-tapped secondary.

^{simulate this circuit – Schematic created using CircuitLab}

Figure 3. In the tool transformer the 110 V secondary centre-tap is deliberately grounded to provide a 55-0-55 output voltage.

• The tool is powered from the two outer pins. This limits the output voltage to 55 V AC maximum in the event of a fault.
• Having a definite ground point means that in the event of a single fault one of the output fuses will blow and this will alert the operator to the fault. Without this earthing feature one side could become grounded and if the operator were to come in contact with the other wire s/he would receive a 110 V AC shock.

You will find that many portable generators feature a 55-0-55 winding or transformer built in to provide this safety function. It's safety will depend on a proper grounding of the winding centre-tap.

There are four reasons for grounding the neutral.

1. Grounding neutral provides a common reference for all things plugged into the power system. That makes connections between devices safe(r).

2. Without a ground, static electricity will build up to the point where arcing will occur in the switchgear causing significant loss in transmitted power, overheating, fires etc.

3. With a floating system it is possible to have a short between both in-house and neighboring systems via the ground path as shown below. Turning a light on in your house can cause a light to go on in your neighbors house too. This characteristic is highly unpredictable.

^{simulate this circuit – Schematic created using CircuitLab}

4. Finally, by giving ground a return path to neutral, a short to the grounded chassis of an appliance causes a predictable outcome in terms of a fuse or breaker response. This provides a great deal of preemptive protection to the user.

In Summary

In a simple model it appears that not tying ground back to the neutral would be safer. However, in reality, in a distributed power system there is no guarantee of this since you have no way to know if there is some other path back to the transformer via a different route. That is, in point 3 above, you may be in danger of being electrocuted just as much as if your neutral was grounded.

In the end the other benefits of tying ground back to neutral outweigh the one possible, but unreliable, isolation benefit.

NOTE: From point 4 there is a paradigm shift in the way you need to think about the neutral-ground connection. Do not think of neutral connected to ground, but instead think of ground being connected to the neutral to allow the current from a short to ground to return to the transformer.