Line to ground faults

Question

I am having a trouble understanding the concept of line to ground faults,and ground in general.

In this simple single phase circuit, does current reach the load and the circuit operates normally? Line touches ground, and neutral is not earthed.

According to what I understand, the voltage in line wire will be zero and voltage source will make neutral wire voltage oscillate between +220 v and -220 v relative to voltage of line wire, and so, the circuit will operate normally.

Does this mean that line to ground fault does not happen in case of unearthed neutral?

In my understanding, when line to ground fault happens, huge short circuit current flows, and that only makes sense if the neutral is earthed because then the line wire and neutral wire will have equal voltages and that,somehow, equates a short circuit across the load, like this:

However is the ground really considered a single node, circuits wise, for us to say that circuits 2 and 3 are the same?

And according to what I researched, line to ground fault can happen even if the neutral is not earthed, how so?

So, in summary, my question(s):

1- Will circuit 1 operate normally? if no, why not?

2- Are circuits 2 and 3 equivalent? how?

3- Can line to ground fault occur in unearthed neutral? if yes, how?

Answer 1

In this simple single phase circuit, does current reach the load and the circuit operates normally? Line touches ground, and neutral is not earthed.

If it's an isolated supply then yes, but you have labelled one of the wires as "neutral". If it has been "neutralised" then that means it is connected to earth already and in that case you have a short circuit on your supply. The fuse should blow and the load will receive no power.

According to what I understand, the voltage in line wire will be zero and voltage source will make neutral wire voltage oscillate between +220 v and -220 v relative to voltage of line wire, and so, the circuit will operate normally.

Again, that will be true if the neutral is floating and not neutralised. The peak voltages will be \$ \sqrt 2 V_{RMS} = 220\sqrt 2 \ \text V\$.

Does this mean that line to ground fault does not happen in case of unearthed neutral?

Not quite. You have a fault but it doesn't cause a trip. Some systems use isolated supplies for this reason but generally incorporate a fault sensing circuit to alert maintenance to fix it before a second fault occurs and shuts down the system. Medium voltage 3-phase distribution systems use this approach.

In my understanding, when line to ground fault happens, huge short circuit current flows, and that only makes sense if the neutral is earthed because then the line wire and neutral wire will have equal voltages and that,somehow, equates a short circuit across the load, like this:

However is the ground really considered a single node, circuits wise, for us to say that circuits 2 and 3 are the same?

Correct although ground resistance can vary.

And according to what I researched, line to ground fault can happen even if the neutral is not earthed, how so?

It means your once isolated system is now no longer isolated and has an unexpected ground reference.

---

Tip: Crop your photos.

Answer 2

You are correct in your assessment that there will be no fault current flowing in situation #1.

In the first diagram you have drawn, the hot is referenced to ground, while the "neutral" is floating. Consider that this is effectively the same situation as simply reversing the leads on the transformer in a grounded neutral situation. In your diagram, the hot and neutral are effectively just re-labeled. The "hot" is now pegged to zero volts relative to the ground reference, and the "neutral" will read 220 Vrms relative to the ground reference.

Note that while no fault current flows, this is still an undesirable situation. Generally, you want the neutral to be affixed to some sort of ground reference, so if the neutral is floating, it is likely for some sort of isolation purpose (for example, patient safety inside a medical device). So, the fact that the hot is shorted to ground, while it will not immediately cause a fault current to flow, may have created a hazardous situation by defeating the isolation system.

You are also correct about #2 and #3 being equivalent - the ground is a single node that is usually considered to provide a low-impedance path, so when both hot and neutral are connected to ground in a fault, you have a short-circuit situation. Fault current will flow, and if your circuit protection is not up to snuff - kaboom!

Now, as for the question if fault current can flow in the case of an ungrounded neutral situation - usually, no, as you have surmised. There are two mechanisms where you might see fault current:

1. If one of the two conductors (either hot or neutral) is strongly capacitively coupled to the ground, you might find a situation where fault current can flow if the other conductor is shorted to ground. Generally, not much current will flow due the high impedance presented by the small capacitance at line frequency, but even a few mA could still be enough to injure or kill somebody if the "short" happens to pass through their heart.

^{simulate this circuit – Schematic created using CircuitLab}

2. If your circuit is fed from an autotransformer (not an isolation transformer), you are not actually galvanically isolated - the "neutral" on the output is directly connected to the neutral on the input, so you are connected to ground through the transformer. Autotransformers tend to be very popular in power networks as they require less winding material.

(Image source: Wikimedia Commons)

Answer 3

You have already got many facts, but you should notice that practical power distribution systems generally have three phases. That lifts up the importance of grounding of the common neutral of the three phase lines. If it wasn't grounded and one day one of the lines touch the ground, then the voltage stress over the insulation between other lines and ground would jump up to 170% of normal. That's because the voltage between two lines is about 1,7 times as high as the voltage between a line and the neutral. If the neutral is grounded, the protection circuitry will disconnect the ground touching line in a second and a long time stress will be avoided (see NOTE1). That means substantial savings in insulation costs inside electric devices.

Another good reason for grounding is the weather. Lightning can generate very high voltage pulses to power lines. A direct hit to a line is not needed, it's well enough that a cloud somewhere up loses its charge via a blitz between that cloud and the earth. That induces to all lines and the neutral wire under that cloud a fast rising high voltage pulse. When that pulse enters to your house via the power lines, it can well find that the weakest point between the wires and the ground happens to be your electric shaver. The pulse jumps at first to your chin and continues its travel to the ground from your feet and bathroom floor. A stressful moment, but surely soon forgotten. Forever, I guess.

The pulse comes equally along all lines and the neutral wire. If the neutral was grounded near your house, that connection is a short circuit for the incoming pulse and the hit on your chin doesn't happen. As well you can thinh the grounding forces also local ground potential to rise along the pulse, so there's no H.V. pulse between local ground and the power lines.

NOTE1: Actually there's always some leakage, at least capacitively. If there's not grounded neutral, the voltages between the ground and the phase lines can vary indefinitely and all insulations must be designed for continuous 1,7 x line voltage + the normal overvoltage reserve.