Power Distribution and Electrical Work in Ships and Factories

What is power distribution?

Power distribution means carrying electricity from where it is made or received to where it is used. The main parts are:

• Source: a generator (on a ship or for a standby supply) or the supply from the utility grid.
• Main switchboard: a metal cabinet with thick copper bars (busbars) that receive the power and share it.
• Feeders: cables from the board to a machine, a motor, or a smaller distribution board.
• Protection: circuit breakers and fuses on every feeder.
• Loads: motors, lamps, heaters and electronics.

Most plants use three-phase power for large loads. A three-phase supply has three live wires (R, Y, B) whose voltages peak at different times, one third of a cycle apart. Motors run smoothly on it, and the same cable size carries more power. Single-phase 230 V is taken between one phase and the neutral for lights and small devices. In a 400 V (line-to-line) system, each phase to neutral is 400 / √3 ≈ 230 V.

Shipboard power distribution

A ship is a floating island, so it makes its own electricity. Typical layout:

1. Two or three diesel generators make three-phase AC (commonly 440 V, 60 Hz on many ships; other ships use 400 V, 50 Hz). More than one runs, so a failure does not stop the ship.
2. The main switchboard in the engine control room joins them together (this is called paralleling) and sends power to feeders.
3. A step-down transformer gives 230 V or 115 V for lighting, sockets and navigation equipment through an emergency or lighting switchboard.
4. The emergency switchboard has its own small generator and batteries, so the steering gear and emergency lights work if the main supply fails.
5. When the ship is in port, shore supply can replace the generators through a shore connection box.

Special care on ships: salty damp air attacks metal and insulation, so cables are armoured and sealed, and the system is checked often for insulation resistance. Most ship systems are insulated neutral, so a single earth fault gives an alarm instead of a trip, and the crew can repair it before a second fault occurs.

Factory power distribution

A factory takes high-voltage supply (for example 11 kV) and steps it down with a transformer substation to about 400 V three-phase. Then:

• The main LT (low tension) panel has an incoming breaker and outgoing feeders.
• Feeders go to motor control centres (MCC) for big machines and to distribution boards for lights and sockets.
• Each motor has a starter (to limit starting current) and an overload relay (to protect the motor).
• Capacitor banks improve the power factor, so less current is wasted. A standby diesel generator with a changeover switch covers cuts.

Selective protection: the breaker nearest the fault should trip first, so only the faulty part loses power. Breakers closer to the source have bigger ratings and trip later.

Power for a three-phase load: P = √3 × V × I × cos φ (V = line voltage, I = line current, cos φ = power factor).

Electrical work: installing, testing and staying safe

Electrical work includes laying cables, fixing switches and boards, joining wires, fitting equipment and testing. Good practice:

• Plan first: read the drawing and add up the load. Choose a cable thick enough for the current (a thin cable heats up), and a breaker or fuse a little above the normal current but below the cable limit.
• Wire colours: follow the local colour code, for example red, yellow and blue for phases, black or blue for neutral and green or green-yellow for earth. Do not mix codes in one building.
• Joints: strip, twist or crimp, and insulate fully. Loose joints get hot and cause fires. Use proper connectors and tighten terminals to the correct torque.
• Earthing: join every metal body to the earth wire and earth stake. If a live wire touches the case, a large current flows to earth, the breaker trips, and nobody gets a shock.
• Testing: with the supply off, check continuity (a complete path), insulation resistance (a megger, very high value) and earth resistance (low). Then switch on and check voltage.
• Safety: switch off and lock out the supply, tag it, prove it dead with a tester, wear insulated gloves and shoes, use insulated tools, and never work alone on live equipment. Never use water on an electrical fire; use a CO₂ or dry powder extinguisher.

Try it: the load budget

Write the current of every appliance in your room (power in watts ÷ 230 V). For example a 2000 W heater takes 8.7 A and a 60 W fan takes 0.26 A. Add them up and compare with the room breaker (often 16 A). In the 3D, switch on the motor and lights, then add the heater and watch the total pass 25 A and the breaker trip. Next, switch on the earth fault with the earth wire removed and read what happens to the casing.

Key formulas and definitions

• Three-phase power P = √3 × V_L × I_L × cos φ
• Phase voltage (star) = line voltage / √3, so 400 V line gives about 230 V
• Total current = I₁ + I₂ + I₃ + … for loads on one feeder
• Current of a single-phase load I = P / V (resistive loads)
• Cable voltage drop = I × R; power lost in the cable = I² × R
• Energy (kWh) = power (kW) × time (h); cost = kWh × price per unit

Worked examples

Common mistakes

• Using a thin cable because the load looks small. The cable heats if the current is more than it can carry. Choose the cable first, then the breaker.
• Choosing a breaker bigger than the cable can carry. The breaker must protect the cable, so it should trip before the cable overheats.
• Leaving out the earth wire, or joining it to the neutral. A fault then makes the metal case live.
• Working on a circuit that is only switched off. Always lock out, tag and prove it dead with a tester.