If one phase is short-circuited to ground, there will flow a ground fault current. Its magnitude depends heavily on
the method of system grounding, which are:
Ungrounded
Ground fault currents will still flow due to capacitive coupling between healthy phases and ground, typically in the order of a few amps in a ship installation.
Low resistance grounding
Ground fault currents will flow through the ground fault, with a low resistance ground resistor limiting the fault current to not less than 100A.
High resistance grounding
Ground fault currents will flow through the ground fault, with a high resistance ground resistor, limiting the fault current typically less than 20A.
Bolted ground
Create a high ground fault current, in the order of a short circuit current.
Coil grounding
With a proper tuned coil grounding, the fault current is theoretically very low. Not commonly used in ships, mainly since the network configuration varies, and tuning of the coil is impractical.
System grounding in ships are normally either low or high resistance grounding of system neutral point or, or ungrounded. It might also be a combination of these in different parts of the distribution system.
Ground fault calculations are done to ensure that the fault current is low enough to reduce the risk of damaging equipment, and to determine the levels needed to adjust ground fault detection relays.
For some ships (especially tankers), there might be desired or required to operate with isolated neutral point in order to reduce fault currents to a minimum.
Due to system capacitance, a fault current with a typical magnitude of some Ampere will still flow through the ground fault. The power system may be allowed to continue operation with such low fault currents until it is possible to disconnect and repair faulty parts without large disturbances in
operations.
The disadvantages with this grounding method is that one can experience high overvoltages due to resonance in the fault current circuits, and it might also be difficult to identify in which branch the fault current
occurred.
A high impedance neutral point grounding limits the ground fault currents typically to less than 20A.
Also with this fault current it might be allowed to continue operation with a fault for a limited period of time.
The grounding resistor will reduce the risk of resonance oscillations and it is easier to detect and disconnect the faulty branch of the power system. High impedance neutral point grounding is normally the preferred method in medium voltage systems.
Low impedance neutral point grounding gives high ground fault currents and a ground fault must be cleared by disconnecting faulty parts immediately (typically <200ms).
Compared to isolated and high impedance grounded neutral point, this method will reduce the voltage stress on the healthy phases during the fault.
