Power Distribution Boards


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


(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


(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