Protective Devices in an Electrical Distribution System

Question 1

List FIVE protective devices fitted to a main electrical switchboard

Answer 1

• Fuses
• Circuit Breakers
• Overload Protection
• Preferential Trip
• Reverse Power Relay
• Under/Over Frequency Trips
• Under/Over Voltage Trips
• Short Circuit Protection

Question 2

Explain the meaning of the term earth fault and state why it is undesirable in electrical installations

Answer 2

WHAT IS IT
- An Earth Fault is when the live conductor part of a machine comes into contact with an earth point (casing, hull)
WHY UNDESIRABLE
- To have an Earth Fault means that for example the casing, would become ‘live’ meaning its possible to receive an electric shock if the device is touched.
- There will also be a loss of performance as well as the device potentially becoming damaged.

Question 3

with reference to earth faults on a ships electrical system
State how they are indicated

Answer 3

Three-Lamp Indication Method
- Three lamps are connected between each phase and earth through equal resistors.
NORMAL - All lamps glow equally
EARTH FAULT - The lamp connected to the fault goes dim or off, while the other two get brighter.

Instrument Type (Insulation Resistance Tester)
- Disconnect power, then use a tester between each phase and earth.
The phase showing low insulation resistance is the faulty one.

Question 4

with reference to emergency generators
Describe the routine testing

Answer 4

1), Inform Bridge that EMG is going to be tested.
2), Set EMG to test mode on the starter panel.
BATTERY START
3), Start the EMG using battery start, allow engine to run up.
4), Check parameters, (L.O pressure, CFW temperature)
5), Check for any leaks.
6), Check for unusual vibration or noise.
7), Check exhaust system for leaks or smoke colour.
TO STOP GENERATOR
8), Switch to manual and depress stop button.
9), Reset to automatic when EMG has stopped.
BLACKOUT TEST
10), Switch over all equipment that normally runs on the emergency switchboard to their redundant (alternate) power sources.
11), Isolate the emergency switch board from normal power supply.
12), De-energise the automatic start sequence of the EMG.
13), Observe the automatic start sequence of the EMG.
14), The EMG should start within 45seconds.
15), The EMG should automatically energise the emergency switchboard.