Question

With reference to shipboard electrical distribution systems:

(a) describe the meaning of the term earth fault;

(b) explain why an insulated neutral is preferred for low voltage systems;

(c) sketch a circuit diagram of one arrangement for detecting phase to earth faults for a star neutral earthing resistor (NER)

(d) How is the ohmic value of a NER calculated to limit the earth fault current to the full load rating of a three-phase neutral earthed a.c. generator.

Answer 1

Part (a)

Meaning of the term earth fault:

An earth fault is an unintentional connection between a live conductor (phase) and the earth (hull, frame, or earthed metalwork). It occurs when insulation fails, a cable is damaged, or moisture enters a terminal box, allowing current to leak from the live conductor to earth.
In a shipboard system, an earth fault can be a single phase-to-earth fault (which in an insulated system does not immediately cause a large fault current but leaves the system in a dangerous condition) or a more serious fault involving two phases.

Part (b)

Why an insulated neutral is preferred for low voltage systems:

In an insulated (unearthed) neutral system, a single phase-to-earth fault does not cause a large fault current to flow, because there is no direct earth return path. The system can continue to operate, and the fault is indicated by an earth fault alarm so it can be located and cleared at a convenient time.
This improves continuity of supply, which is important on a ship where loss of power could be dangerous.
It reduces the risk of electric shock and fire from a single earth fault, and prevents the large fault currents and arcing that would occur in an earthed system.
It allows the use of earth fault monitoring (insulation monitoring) to detect deterioration of insulation before it becomes a serious fault.

Part (c)

Circuit diagram for detecting phase-to-earth faults with a star neutral earthing resistor (NER):

The generator is star-connected with the neutral connected to earth through a neutral earthing resistor (NER).
The three phases are connected to the busbars through current transformers.
An earth fault relay is connected to the residual circuit of the three current transformers (the secondary windings are connected so that the relay sees the vector sum of the three phase currents, which is zero under balanced conditions).
On a phase-to-earth fault, the fault current flows through the NER to earth and returns through the faulted phase, producing an unbalanced current in the current transformers which operates the earth fault relay.
Alternatively, a core-balance (zero-sequence) current transformer surrounds the three phase conductors; on an earth fault the unbalanced current induces a signal that operates the relay.
The relay gives an alarm and/or trips the generator circuit breaker.

Part (d)

Calculation of the ohmic value of the NER:

The NER is chosen to limit the earth fault current to the full-load rating of the generator.
Full-load current of the generator: I_fl = S / (root 3 x V_line), where S is the rated kVA and V_line the line voltage.
The earth fault current is limited by the NER. For a star-connected generator, the phase-to-earth voltage is V_phase = V_line / root 3.
The NER resistance R = V_phase / I_fl = (V_line / root 3) / I_fl.
Substituting I_fl = S/(root 3 V_line): R = (V_line/root 3) / (S/(root 3 V_line)) = V_line^2 / S.
So the ohmic value of the NER = V_line^2 / S, where V_line is in volts and S in volt-amperes (or V_line^2 in kV and S in kVA gives R in ohms directly: R = (V_line in kV)^2 / (S in MVA) x 1000).
Example: for a 440 V, 1000 kVA generator, R = 440^2 / 1,000,000 = 0.1936 ohm.

✓ Verified Model Answer (Text Solution)

Meaning of the term earth fault:

Why an insulated neutral is preferred for low voltage systems:

Circuit diagram for detecting phase-to-earth faults with a star neutral earthing resistor (NER):

Calculation of the ohmic value of the NER: