Power Distribution Boards
Some of the Equipments that consume very little power are not connected to the MSB Directly
(440 V or 220 V – Small Motors, Lightings, Sockets)
(i.e. Individual Equipments are not provided with individual cable from MSB)
Instead –
These Equipments are grouped together(many such groups are formed)
Each group of equipments will be connected to a Power Distribution Board (PDB)
i.e. each group of equipments will get power supply form each PDB
Power supply to the PDB is provided by MSB
PDB has small bus bars.
From these small bus bars, the supply is provided to these Equipments via Circuit Breakers
In this way, the protection for the Equipments is precise and a small fault (in small equipment) will not operate the protection devices on the MSB
Circuit Breakers (CB)
CB is similar to a Switch
It is used to make and break electrical contacts/circuits carrying large currents
CB connects the Incoming Feeder (Electrical Cables) to Bus Bars
(i) Generator (Alternator) to MSB
(ii) MSB to PDB
CB connects the Outgoing Feeder to Loads
(i) MSB to Motors/Motor Switch Board,
(ii) PDB to Motors/Lighting
(iii) Motor Switch Boards to Motors
Safeties provided in a Circuit Breaker
(i) When a CB makes or breaks electrical contacts/circuits, carrying large currents – Arc is formed
CB Hinders (Obstructs) the Arc Formation & Extinguishes the Arc by the following arrangements:
Arc Contacts Makes – Before the Main Contacts
Arc Contacts Breaks – After the Main Contacts
Main + Arc Contacts together provide a long travel path for the Arc (Production/Formation and Travel of the Arc is hindered/obstructed)
Arc is extinguished as it rises into the Splitter (Arc Chute)
Fast Opening/Tripping/Breaking of Contacts (by powerful spring mechanism) also obstructs the arc formation
(ii) CB also protects the following equipments (against various faults / fault currents)
Generator (Alternator), MSB, Incoming/Outgoing Feeder, Bus Bars, Power Distribution Switch Boards (PDB), Motor Switch Boards, Lighting Switch Boards, Battery Switch Boards
Closing (Making of Contact) = Manual or Electrical (solenoid/motor)
A mechanical latch holds the Circuit Breaker in closed position
Opening / Tripping (Breaking of Contact) = Manual or Automatic
Relays are fitted to detect the following Faults:
Short Circuit / Over Load / Low Frequency / Under Voltage / Phase Imbalance / Reverse Power / Over Speed of prime mover
If fault occurs – the Relays will produce a signal
Signal is fed to the Circuit Breaker
Circuit Breaker will Open / Trip [by releasing the hold-on latch]
Maintenance of CB
CB can be removed totally or racked out on rails & tilted (draw out Type) (for inspection & maintenance)
Handling & Maintenance should be always carried out with the springs discharged
Monthly
(i) Operate Circuit Breaker (Closing & Opening/Tripping a few times) (Done to free the mechanism, clean the contacts check for contact welding)
(ii) Measure Insulation resistance (should be minimum 5 MΩ) (i) Between Poles (ii) Each Pole and Earth
Annually
(i) Check Tightness of all Electrical Connections – Tighten
(ii) Check Condition of Arc Chute (Splitter) – Clean/Replace
(ii) Check Condition of Main Contact & Arc Contacts – Clean/Replace
(iv) Check Operation of Breaker – Opening, Closing & Charging
(v) Lubricate Opening & Closing Mechanism
After a Fault Trip
(i) Check Main Contact and Arc Contact
(ii) Check Arc chute
(iii) Check Operation of Closing and Opening/Tripping Mechanism