A detailed TST crib sheet covering marine main switchboards, protection discrimination, reverse power, preferential trips, synchronising, HV/LV breaker technology, synchronous compensation, survey testing and thermography.
Module 24SwitchboardsProtectionSurvey & Testing
Golden rule
Isolate only the faulted section. Good switchboard design and protection coordination keep the rest of the vessel powered.
Oral exam theme
Say what the device senses. Then state what it trips, what it protects and the operational consequence.
Survey theme
Prove the breaker works. A clean cubicle is not enough—mechanical, electrical and thermal condition must be verified.
How to use this Module 24 crib sheet
Use the sections below to structure oral answers. Start with the board layout, then explain the protection principle, then move into specific devices such as reverse power relays, preferential trips, breaker types and survey checks.
Define protection discrimination, overload and short-circuit response.
Explain reverse power relay operation and typical purpose.
Differentiate LV insulated-neutral systems from HV resistance-earthed systems.
Compare MCCBs, ACBs, vacuum and SF6 breakers.
Explain what a synchronous compensator is and how it improves power factor.
Describe switchboard survey proving tests and the role of thermography.
Read a high-level main-system and emergency-board distribution overview.
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01 • Switchboard Role
What the Main Switchboard Does
Main function
The main switchboard receives generated power, sections and distributes it, allows generators to run in parallel, and provides measurement, control, protection and safe isolation.
Typical layout
Generator incomers, busbar sections, bus-tie, feeder breakers, service transformer feeders, PMS interfaces and links to the emergency switchboard.
Design aim
Maintain continuity of essential services while isolating only the faulty section through discrimination and coordinated protection.
Enhanced TST single-line: realistic breaker and isolator symbols show how incomers and outgoing feeders connect to the busbar, while the transformer and emergency interconnector demonstrate the path from HV generation to LV distribution and emergency supply.
02 • Protection Philosophy
Protection, Discrimination & Key Relays
Discrimination
The device nearest the fault should operate first, so a feeder fault trips a feeder breaker rather than blacking out the entire board.
Overload vs short-circuit
Overload is sustained overcurrent from excessive load and normally uses inverse-time protection. Short-circuit is a very high fault current and requires instantaneous or high-set protection.
Key devices
Overcurrent, short-circuit, reverse power, under-voltage, earth-fault and differential/zone protections all work together to protect the switchboard and prime movers.
03 • Reverse Power & Preferential Trips
Why the Board Protects Both Machinery and Continuity
Reverse power relay
This relay senses real power flowing from the busbar back into a generator. If the prime mover fails or loses fuel torque, the alternator can motor and drive the engine, so the breaker is tripped.
Typical setting
Often a small percentage of generator rated kW, just high enough to avoid nuisance tripping but low enough to protect the diesel or turbine quickly.
Preferential trips
Non-essential loads are shed automatically when frequency, bus voltage or loading reaches unsafe limits, preserving power to essential auxiliaries such as steering, cooling, control air and navigation services.
Chief Engineer oral line
“A reverse power relay protects the prime mover, not just the alternator. If fuel is lost or the engine fails, the live busbar can motor the set, so the breaker trips before the engine is driven backwards or overheated.”
04 • HV, LV & Earthing
How LV and HV Systems Differ
LV practice
The common 440 V shipboard system is usually insulated-neutral. A first earth fault does not immediately trip, which supports continuity, but it must be found and cleared promptly.
HV practice
High-voltage systems are usually resistance-earthed through a Neutral Earthing Resistor. This limits earth-fault current to a controlled value so protection can detect and clear the fault safely.
Exam point
The first earth fault on an insulated-neutral LV board is an alarm condition, not an automatic blackout. The danger comes if a second earth fault occurs on another phase.
05 • Breakers & Switchgear
Breaker Types, Interrupting Media and Interlocks
MCCB vs ACB
MCCBs are compact moulded-case devices typically used on smaller feeders. ACBs are larger low-voltage air circuit breakers used as generator incomers, bus-ties or major feeders with adjustable trip functions and draw-out construction.
HV breakers
Marine HV boards commonly use vacuum circuit breakers, and some designs use SF6 interrupter switchgear. Both are housed in metal-clad cubicles with withdrawable breakers and dedicated relay protection.
Interlocks
Mechanical and electrical interlocks prevent unsafe switching, such as paralleling two dead sections incorrectly or closing onto an earth or incompatible supply. Draw-out positions normally include disconnected, test and service.
Air interruption, draw-out, adjustable protection, interlocks
440 V generator incomers and major feeders
Vacuum breaker
HV switching and fault interruption
Vacuum interrupter bottle, high endurance, compact metal-clad design
6.6 kV main switchboards and large HV motors
SF6 breaker
HV interruption where fitted
SF6 gas provides excellent dielectric strength and arc quenching
Some HV switchgear designs and specialist installations
06 • Synchronising & Bus Operations
Before You Close a Breaker
Four conditions
Match voltage, frequency, phase sequence and phase angle before closing an incoming generator to a live bus.
Incoming slightly fast
The incoming machine is normally trimmed slightly fast so the synchroscope approaches 12 o’clock and the breaker is closed just before the in-phase position.
Bus-tie purpose
The bus-tie joins or splits board sections. It supports maintenance flexibility and fault management, but interlocks and synchronising discipline are critical.
07 • Synchronous Compensator
Reactive Power, Voltage Support and Power Factor Improvement
Definition
A synchronous compensator is a synchronous machine running without a shaft load and used purely to control reactive power.
How it works
By changing field excitation, the machine can either absorb lagging kVAr (under-excited) or supply leading kVAr (over-excited).
Benefit
Supplying leading kVAr improves overall power factor, reduces total current for the same kW, supports bus voltage and eases loading on alternators, cables and switchgear.
Memory line
Over-excite a synchronous compensator and it supplies leading kVAr. That offsets inductive lagging kVAr, improves power factor and reduces current for the same real power.
08 • PMS, Emergency Board & Blackout
Automatic Logic That Keeps the Ship Alive
PMS duties
The Power Management System monitors generator loading, frequency and bus status; auto-starts standby sets; synchronises them; shares load; and sheds heavy or non-essential consumers when required.
Emergency switchboard
Under normal conditions it is fed from the main switchboard via the interconnector. During blackout or main-board failure, the interconnector opens and the emergency source supplies the emergency board.
Blackout sequence
The system detects voltage collapse, trips or opens the right breakers, starts the emergency source, sheds selected loads, and restores essential services in a controlled order.
Enhanced TST emergency single-line: the main-board interconnector, emergency generator breaker, emergency transformers, UPS section boards and essential/SOLAS outgoing feeders are shown using more realistic protection and switching symbols.
09 • Survey & Testing
What a Good Switchboard Survey Should Prove
Breaker functionality
Operate breakers locally and remotely; prove close/open, spring charging, trip-free operation, under-voltage trip, shunt trip, auxiliary contacts, racking and interlocks.
Electrical proving
Relay functional checks, secondary injection, control-circuit proving, insulation resistance where appropriate, contact/ joint inspection and confirmation of indications and alarms.
Thermography
Live thermal imaging highlights loose joints, overloaded contacts, high-resistance busbar connections and imbalance before they develop into faults or fire.
Survey check
What you do
What it proves / reveals
Mechanical operation
Close/open locally and remotely, charge springs, confirm trip-free action
Mechanism is free, breaker can operate on command and will not defeat protection
Interlocks & racking
Prove disconnected, test and service positions and all key interlocks
Breaker cannot be closed or withdrawn unsafely
Relay / trip proving
Secondary injection and proving of UV, shunt and protection trips
Protection circuits actually trip the breaker, not just raise alarms
Thermography
Scan live joints, busbars, breaker contacts and cable terminations
Finds hot spots from looseness, wear, overload or unbalance before failure
10 • Oral Exam Matrix
Say This First, Then Expand
Protection
“The switchboard must isolate the faulty section while keeping the healthy section alive, so coordinated discrimination is essential.”
Reverse power
“Reverse power means real power is flowing back into a generator because the prime mover has lost torque; the breaker trips to prevent motoring.”
Breaker types
“At LV we commonly see MCCBs and ACBs; at HV we commonly see withdrawable vacuum or SF6 breakers with relay protection and interlocks.”
Discrimination: “The protective device nearest the fault must operate first to preserve healthy sections of the board.”
Preferential trip: “Preferential trips automatically shed non-essential loads when the board is overloaded or frequency/voltage collapse is developing.”
HV/LV difference: “LV boards commonly use insulated neutral and ACB/MCCB protection; HV boards are commonly resistance-earthed and use withdrawable vacuum or SF6 breaker technology.”
Survey line: “A switchboard survey should prove breaker function mechanically, electrically and thermally—not just visually.”
