Six Important Circuit Breakers to Use in Underground Mining Applications

Written by Jeson Pitt at D&F Liquidators

The requirements of the circuit breaker box for underground mining are different from home or industrial requirements.  In mining, the power center is engineered to transform and distribute electrical power brought underground.  The user must be aware of the specific application issues to make sure these devices operate in the right manner. This is essential for miners’ safety and reliability of operations.

Six Circuit Breakers Used for Safety in Mines

1. Break Oil Circuit Breaker (BOCB)

Oil is used as a dielectric or the arc-quenching medium in BOCB.  The main contact separation takes place in steel tanks that are filled with oil up to a certain level as per the design.  When the circuit on load breaks during the arc-extinction process, gases are formed due to the heat of the extended arc, which then sets up the flow in the oil.  This gas under pressure has a high dielectric strength. 

2. Minimum Oil Circuit Breaker (MOCB)

BOCB requires a large quantity of oil for clearance between the tank and the contacts.  In MOCB, only a small quantity of oil is required.  In addition to this feature, no steel tank is required.  

There are two chambers of oil that are separated from each other.  The upper chamber acts as the arc quenching chamber whereas the lower chamber acts as the dielectric support.  The arc quenching chamber is fixed to the fixed contact that is fitted at the top inside the upper chamber.  The lower fixed contact, of ring shape, is fitted at the lower end of the upper chamber.  The moving contact is designed to make sliding contact with the lower fixed contact.

3. Air Circuit Breaker (ACB)

In ACB, the contact separation and arc extension take place in the air at atmospheric pressure.  There are two sets of contact – main contacts and arcing contacts.  The main contact is designed to conduct the current in a closed position of the breaker, as well as have low contact resistance.  These arcing contacts are hard and heat resistant.  At the time of breaking operation, the main contacts open first and the current is shifted to the arching contacts.  The arching contact opens within the designed time and the arc is drawn between them. 

4. Air Blast Circuit Breaker (ABCB)

In ABCB, high-pressure air is forced on the arc at the time of control separation.  Due to the blasting of high-pressure air in the arcing chamber, the ionized medium between the contacts separates the main moving and fixed contacts.  Because of this, the arc that is developed due to the contact separation will be extinguished. 

5. Vacuum Circuit Breaker (VCB)

A typical vacuum interrupter has an outer wall that is made of ceramic material and the vacuum chamber is made of urethane foam.  A glass fibre pull rod of high mechanical strength is used for contact separation.  This pull rod is used to separate the contact by lifting the moving contact upwards.  The electromagnetic forces coupled with gravitational effect force the moving contact downwards during the normal closing operation.

6. SF6 Circuit Breaker 

SF6 circuit breaker uses Sulphur HexaFlouride (SF6) as the arc-extinction medium.  The SF6 gas from a high-pressure tank is released in the arc-quenching chamber at the instant of contact separation.  At the time of arcing, SF6 gas decomposes slowly-to SF4 and SF2 and metallic fluorides which are very good dielectrics. So, the dielectric strength increases at the time of opening.