MCC/PCC/DB/BC/ES/VCB/SF6

MCC – MOTOR CONTROL CENTRES: 

MCC is a panel where all motor starters of particular are assembled in one single cabinet. The parts of MCCs are cable chamber, start compartment, bus bar chamber and an incoming feeder. The starter compartment can be rack out type or fixed one.

PCC – POWER CONTROL CENTRES: 

PCC is a panel where power is being distributed to different M<CCs, DB s and large motors. The parts of PCC are an incoming feeder and outgoing feeder of different current ratings according to the connected load Normally LT PCCs will be having OCBs or VCBs.

DB – DISTRIBUTION BOARD: 

Normally all DBs will be having fuse distribution. DBs used for lighting will be having MCBs.

BUS – COUPLER: 

Where two different bus sections connected through an breaker (ACB or VCB or OCB) so that both bus section supply can have an alternative feeding arrangement.

EARTH SWITCH:  

Where the connected equipment transformer, motor or bus section can be earthed during maintenance. This will provide perfect safety to the person working. If earth switch is not there, that equipment has to earthed by using earth rod.

VCB – VACUUM CIRCUIT BREAKER:  

Vacuum is the dielectric used in the breakers.

Advantage:      Non flammable, non poisonous medium

    Sealed interrupter, sealed for life

    No maintenance is required for interrupter

    Very long switch life

    Suitable foe repeated switching

    Used for high voltages from 3.3 KV to 11 KV

    Self life vacuum interrupter is about 15 to 20 years

MCB Miniature Circuit Breaker: 

This isused for small loads such as lighting

MCCB MOULDED CASE CIRCUIT BREAKER: 

This is normally used for electronic controls. This can avoid short circuit current very fast. They are replacing fuses.

ACB AIR CIRCUIT BREAKER: 

Where air used for ARC extinction normally used for low voltage switch gear.

OIL CIRCUIT BREAKER:  

In these types of breaker transformer oil is used as quenching medium. Now – a – days VCB is replacing OCB as OCB requires maintenance and oil is easily inflammable.

BOCB – Bulk oil circuit breaker

MOCB – Minimum oil circuit breaker

In BOCB all three phase contacts assemble in a single tank.

In MOCB all three phase contacts assembled in three different tanks.

SF- 6 Breaker – Sulphur Hexa Fluoride Breaker: 

Where SF6 used as a dielectric and arc quenching medium. Used for high voltage from 33KV to 760KV. SF6 is an inert gas.

Maintenance requirement of SF6 breakers are very much less due to long contact life and inert operating medium.

Cost of the circuit breaker is less if cumulative maintenance cost is also considered.

                SF6 CB is explosion proof, hence safe.

                Normally SF6 gas pressure is 5 kg / cm²

                At 5 kg / cm² the gas becomes liquid at 4°C

                Mostly all SF6 breakers use pressurized air for tripping.

HRC FUSE – HIGH RUPTURING CAPACITY CARTRIDGE FUSE:

This fuse consists of a ceramic body having metal caps to which are welded fuse-able silver or bimetallic current carrying elements. The body surrounding the elements is completely packed with non inflammable power or other special material like Quartz sand.

HRC fuses are used from low voltage to up to 33 KV with short circuit capacities of about 8760 Amps at that voltage. Normally HRC Fuse offers inverse time characteristic; if the fault current is light the fusing time will be less.

P.T. – PONTENTIALTRANSFORMER:  

Voltage transformer or potential transformers are used foe measurement and protection. They may be either single phase or 3 phases. In case of HT systems this transformer stepping down the voltage for measuring and protection as high voltage cannot directly be used for this purpose. The raring will normally be in VA (Volt Ampere) and this will be around 200VA.

C.T. CURRENT TRANSFORMERS: 

These are used for reducing / stepping down A. C. current from higher value to lower value for measurement / protection.

The rating of C. T. will be in VA Volt Ampere and that will be between 10 VA to 30 VA.

Both P. T. & C. T. used for the following

For Measuring – Voltage, current, KVA, KW, frequency, P. F. (cosѲ).

For protection – Over current, earth fault, differential, reverse power, etc. Normally CT’s are mentioned as 10 P 10.

                10P                         Accuracy class percentage                               3%

         10                           Composite error                                              10%

OPEN CIRCUITED SECONDARY OF CT:

CT secondary never be opened circuited. An important aspect in CT operation is the voltage appearing across open circuited secondary. Normal voltage 15 / 5 = 3 V. If by mistake secondary is open circuited the voltage across the secondary rises to some kilovolts. The increased voltage is because the following flux increases due to reduced back EMF in the secondary (as 0 current in the secondary)

Due to the high voltage in the secondary core of the C. T. and Primary gets overheated and finally bursts off. Voltage are induced in the secondary by electromagnetic induction. The peak value may rise several kilovolts in the secondary. This may cause danger to personnel working on secondary side. Therefore C. T. secondary should never be disconnected.

TRIP CIRCUIT HEALTHY INDICATION

For knowing the trip circuit health a lamp connected in series with the trip coil of breaker / lockout relay, the connection shown in the diagram.

As the bulb resistance is higher than the trip coil, the drop across the lamp will be higher than the trip coil, so bulb only glows.

ALARM ANNOUNCIATION PANEL

Normally Alarm Annunciation panels will be window type visual and audible bell or speaker.

When any one of the alarm initiated visual window lighting will blink with audible sound. When accepted the audible alarm will stop. The visual alarm can be resettled only after the fault clears away.

Each window will be connected to individual equipments where announciation is required. The visual window will be around 50 to 100.