# What happens when you get a fault to earth without an MEN link on a 3 phase generator? [closed]

For context, an isolated type single phase generator (no earth stake or neutral-earth link) had a residual current device connected to it and then a fault between active and earth was placed on the generator. The RCD did not trip, and I worked out why it didn't based on the single phase generator schematic below. What happens in a three phase system?

Single Phase:

simulate this circuit – Schematic created using CircuitLab

With no Earth - Neutral bond. If there is a connection (fault) between earth and active, then the active conductor will be bonded to earth (0V) and the Neutral line will oscillate between +/- 230 v.

Three Phase:

simulate this circuit

What happens now if there is a connection (fault) between earth and one of the lines?

1. A dead short and the Circuit Breakers trip?
2. The line is bonded to zero volts and the other two lines and neutral oscillate around it?

Thoughts?

• Please have a read at en.m.wikipedia.org/wiki/Earthing_system. You have only shown the generator side and not the load side, so I can’t tell if you have an IT grounding system or not. Dec 9, 2023 at 7:50
• This adds no value, I have included everything relevant.
– SS1
Dec 9, 2023 at 7:53
• You have not. A floating system is illegal in most countries. Dec 9, 2023 at 12:15

If the generator circuit is entirely isolated, including minimal capacitance, then no current can flow in the case you give, as there is no circuit. (Your case 2.)

However, the reason this can be dangerous is that secondary contacts (or preexisting shorts) can lead to currents which are high enough to be dangerous, but not so high as to trip breakers.

Neutral earthing is primarily intended to force the ground reference, giving predictable results, and quick breaker opening.

This is the norm, but in limited cases ( IT systems) the isolated approach is better when supported by regular testing to ensure the isolation is maintained.

• Thank you for this. Any insight on what the voltage does on the shorted line? Do you think it forces to 0 V and the others rotate around it ?
– SS1
Dec 9, 2023 at 9:27
• If there is minimal capacitance, and true isolation, then yes, the grounded line will be at ground potential, and the rest will rotate round it. Very similar to what happens if you have an isolation transformer, a bridge rectifier, and ground one of the DC terminals - the transformer secondary potentials move relative to earth depending on which diodes are conducting. Dec 9, 2023 at 10:04
• Thanks @colintd. What do you think the neutral will do ? Rotate as well, somewhere in there?
– SS1
Dec 9, 2023 at 10:28
• If you draw the whole phasor diagram, you'll see neutral rotate round the grounded line, and the other lines go round the rotating neutral as normal Dec 9, 2023 at 21:13
• legend, that's exactly what I suspected.
– SS1
Dec 9, 2023 at 22:22

Nothing happens if one of the phase lines, say L2 takes contact to earth in a system which contains nothing else, but the drawn items. That's because no new ways are opened to the current. I'm afraid such system would not be considered to have any long term usefulness, so there must be something connected to the wires L1, L2, L3, N, E.

Anyway, a contact between L2 and E causes there's full one phase AC voltage between the earth and N.

If it happens that the undrawn part of the system is designed by assuming that N and E are connected, the missing N-E link + the unwanted L2-E link show their appearance as soon as something is connected between N and E.

It can be for ex. you when you stand on the ground and touch with your hand a device which is grounded in the easy way - by connecting the protective earth to the N in the AC outlet (see NOTE1). If L2 has not a sensitive enough circuit breaker it's well possible that a gravedigger gets some work. Other possible employed: The Police, lawyers, insurance inspector.

Hopefully the whole system - the drawn part and the rest are constructed properly by obeying the local code. Anything else is breaking the law. And it's more, if it's done by design, it's breaking the law intentionally.

NOTE1: Such groundings without no separate PE wire and connection to the real ground are forbidden in many countries, but there are surely areas in the world, where nobody cares.

In the case of the three-phase generator, with its neutral and enclosure earthed, a line-to-earth fault will result in a blown fuse or a tripped circuit breaker ensuring the safety of personnel and equipment.

With the neutral and enclosure not earthed, the line-to-earth fault would remain undetected and thereby expose personnel to the danger of electrocution at line voltage.

Earthing the neutral and enclosure of the single-phase generator is also a must for the same reason.

• That's not correct, there are such things as isolated generators that don't have Neutral-Earth bond. You also said that there is a danger of electrocution at line voltage, under what scenario? You mean if they are standing bare foot on the earth that is shorted to the line and touch another separate line?
– SS1
Dec 9, 2023 at 10:25
• I will add this: If you mean that earthing the frame is a must I agree, if you mean that the neutral-earth link is a must then that's wrong.
– SS1
Dec 9, 2023 at 10:34
• A portable generator with floating neutral is common in industrial / residential settings where its neutral gets automatically bonded to earth when connected to the existing electrical system. However, in the case of a portable generator used with an RV (or any other system that doesn’t have neutral / earth bonding), a separate neutral / earth bonding is a must. Dec 9, 2023 at 11:33
• Whatever the scenario, that danger of electrocution is real. Dec 9, 2023 at 11:47
• yes, I agree I just need to articulate it.
– SS1
Dec 9, 2023 at 20:06