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1. SWITCHGEAR

2. Definiton Switch Gear and Device
• Is the combination of switches,fuses or circuti breaker use to control,
protect and isolate electrical equipment.
Fuse
Circuit Breaker
Molded Case Circuit Breaker
Miniature Circuit Breaker
Relay
Air Circuit Breaker

3. Fuse
Protect circuit from damage – faults & over current.
Designed to blow rapidly before circuit damage takes place.
Many types and sizes, marked with size of steady current can carry
without blowing - fuse rating
Fuse Rating :
* Important – correct rating for normal current flowing in circuit
it protects
* Lower rating – every time switch on, fuse will blow
Higher rating – promoting positive dangerous
– circuit with over current flowing without
blowing fuse - overheat & can cause fire
Position - between supply and the circuit – fuses removal means total isolation for
the circuit
Two main types : (i) Catridge Fuses
(ii) High Rpturing Capacity (HRC) Fuse

4. Cartridge fuses
Suitable up to 100 amps
High rupturing capacity (HRC) fuse
Used for higher ratings up to several
hundred amperes
Several strands silver wire passing
through ducts in earthenware casing,
packed in Quartz powder
 When excessive current flows, wire & quartz
powder melt - quartz is good insulator, thus no
arcing occurs

5. Fuse selection factors
 Lighting / heating circuit – continuous current (e.g. 30A fuse for 30A circuit),
unless for motor circuit
 Motor circuit – large enough to carry starting current for certain period plus
necessary margin
 Interrupting capacity – fuse should > short circuit fault current at particular
point in circuit, otherwise may exploded causing damage to equipment
 Ambient temperature – BS 88 for ambient temperature 35ºC
• but 45ºC for shipboard use
 HRC fuses - insensitive to small over current, will blow at 25% over current
only after 4 hrs
 Fuses - give protection against short circuit
 For protection against small overloads - over current relay

6. Circuit Breaker
 Safely & interrupt prospective short circuit fault current expected in circuit
 Will trips but can be reset & reused
 Link mechanism provided, closes main contacts under spring pressure & wipes
the surface of fixed contact points - ensuring good electrical contact
 Main contact open rapidly with snap action
 Resulting arc transferred to special arcing contacts above the main contact
 Arc chutes with arc ‘splitter’ quickly stretch & cool the arc till it ‘snaps’
 Circuit breaker is ‘open’ when the arc quenched
 Two main group – MCCB & ABCB

7. 
MCB
(Miniature Circuit Breaker)
MCCB
(Molded Case Circuit Breaker)
 MCCB & MCB - used for feeder
& distribution circuits
 Have lower breaking capacity &
lower current rating
 30~1500A for MCCB & 5~100A
for MCB
 Non adjustable & provides
overload protection for motors,
control equipment, lighting
circuits, conductors etc

8. Main Circuit Breaker (ABCB)
• A main circuit breaker is an
equipment that breaks a
circuit either manually or
automatically under all
condition of no load, full
load or short circuit.

9. ACB ( Air Circuit Breaker)
 Alternator circuit breaker – connect / disconnect alternator safely to busbar,
with undervoltage, overcurrent, short circuit, reverse power, low & high
frequency take into consideration
 600~6000A, air break type - means CB contacts / separate in air
 Operating handle is “trip free” – closing CB handle safely
becomes free if tripping mechanism actuated
 Thus, CB contacts won’t close if operator tries to close CB - handle may in
close position, but main contacts remains open
 Main contacts kept closed & latched against high spring force -
quick acting & sensitive tripping action provided
 Main contacts closing pressure is kept high – limits temperature rise at
contacts when carrying current

11. Maintenance & Inspection
 Enclosure Starter panel box - dirt & dust accumulation
Cleaned & repainted corroded parts
Starter fixing bolts & earth bonding connection & vibration
 Contactors & relays
Sign of overheating & loose connections
Dust & grease from insulation - surface tracking
Magnet faces - kept clean & oil/grease free (fine emery cloth)
Moving parts - free & without undue wear at pivot &
magnets bedding properly
 Contacts
Check for “wipe” & “follow through” - i.e. if fixed contact removed, the
moving contact will move in further
Silver contact for excessive mechanical wear
Do not file silver alloy contacts / remove silver sulphide - good conductor

12. Maintenance & inspection (cont/…)
 Copper contacts
provide cleaning action, remove any oxide formed, thus ensuring
good metal to metal contact
Close first before main contact & open last after main contacts
 Connections
All power & control connections – tightness & overheating
Flexible cables - brittleness and bad insulation
 Over current relays
Proper sizes & rating
Dirt, grease, corrosion & free movement
Thorough OCR test – using calibrated current
 Control operation
Operation sequence - start up, control & shut down
Sign for excessive sparking

13. Instrumentation / Devices Fitted to
Main Switchboard
Ammeter
Voltmeter
Frequency meter
Watt meter
Synchronizing devices
Power factor Meter

14. DME Sem 6 / MEP / March 2013/ 27J. AC Switchgear
To Be A World Class Maritime Academy

16. Reverse Power Relay
 Generator which operate in parallel mode must have reverse power protection.
 Monitors direction of power flowing between generator & switchboard
 If prime mover failure, generator will act as synchronous motor.
 Reverse power relay will detect this fault & trips the generator breaker.
 Mandatory by Lloyds but may replaced by electrical interlocks / contacts which
respond to various conditions - closing of fuel/steam admission valve, failure of LO
pressure, excessive back pressure in condenser etc.
Reverse power setting
Steam turbine – 2 ~ 3% of full load
Diesel engine – 10 ~ 15% of full load
Both sets with 3 ~ 5 sec delays
Testing:
> Checked during generator change over.
> Outgoing generator is throttled back to
become a ‘motor’
> This will simply causing reverse
power relay to trip its generator circuit
breaker.

17. Under Voltage Trip
 Fitted to all generator CB
 Main function – trip the CB when severe
voltage dip occurs (<70%)
 When release, it also prevent it being
closed when generator voltage very low or
absent – during parallel operation
 If this CB closed, dead generator = short
circuit fault on busbars and cause blackout.
 Also provide back up to short-circuit
protection
 Provide tripped on generator CB if feeder
fault occurs
 Time delay incorporated - voltage transients load switching / synchronising
 Motors are disconnected during temporary supply loss,otherwise over current
may result

18. Under Voltage Protection
for Motor
Also require for motor starters – when lost/reduced supply voltage, huge
current possibly drawn in
These won’t allow motor to re-start after power restored except special
automatic re-starting facilities provided
Checking & calibration - by voltage injection
For generators - usually ‘slugged’ to prevent spurious tripping during
transient voltage dips (typically 15%)

19. Preferential Tripping

20. Preferential Tripping
 When generator subjected to overload, supporting generator must paralleled or
non-essential load must be dropped
 If overload is allowed, greater overload occurs to bus bar machines, cascade
tripping & complete power loss may resulted
 If overload builds up too sudden, best preventive measure – to drop non
essential loads preferably in stages
When generator overload, preference overload trip timing relay will energise and
drop the non-essential services in sequence:
1st trip – air conditioning and ventilation – 5 sec
2nd trip – refrigerated cargo plant – 10 sec
3rd trip – deck equipment – 15 sec
When sufficient, then preferences overload trip resets & no further load
disconnected
 Normally incorporated with combined electric relay - monitors over current &
reverse power

22. The end