MCC Questions & Answers

MCC

1. What are the advantages of MCC?

a. Starters and contactors all will be a standard size.
b. Spares inventory will be less.
c. Cost of cables will be less because same size of cable is used for all capacity and
gland holes can be provided before hand.
d. Maintenance and trouble shooting is easier in MCC.

2. What type of motor starting adopted in MCC?

DOL (direct on line) starter.

3. What is the purpose of grounding secondary of the control transformer?

To protect the operating personnel from high induced voltage.

4. Based on what factors will you select rating of components for a starter cell?

Factors for selecting rating of components are,
a. Capacity of load.
b. Type of starting.
c. Duty (continuous or intermittent).
d. Type of protection.
e. Nature of starting (acceleration time is slow or fast).

5. What maintenance checks you will do for an MCC cells and MCC panel?

Maintenance checks on MCC cells
a. Ensure that the load is tripped from control room and switch is in off position.
Switch off the isolator at MCC cell.
b. Open the door and rack out the cell into isolation position.
c. Check the tightness of terminal of contactor, 3C, control transformer, control
fuses, wipe in contacts, power cables, etc.
d. Check the tightness of component mounted.
e. Look for any charred components or terminals.
f. Check the IR value of 3C, contactor, control transformer, isolator etc.
g. Check the isolator double switch feature.
h. Check the OLR and calibrate the OLR.
i. Check the pick and drop out value of contactor, 3C.
j. Check the fuses for healthiness and fuse carriers for proper contacts.
k. Clean the arc-chutes of the contactor and clean all the components of the MCC
cell properly.
l. Check the resistances of control transformer, contactor, 3C etc.
m. Check the tightness of control cable at main TB compartment.

Maintenance on MCC panel.
a. Ensure the permit and all isolations.
b. Open the bus bar chamber and discharge the bus bar.
c. Disconnect the cables connected to bus bar and take the IR value of bus bar and
cable individually. Connect it properly and tight it to proper torque.
d. Check the tightness of nut and bolts and cables connected to buses.
e. Open the main TB compartment and check the tightness of all cables and clean it.
f. See the tightness of power terminal compartment and clean it.
g. Check that cables are supported properly.
h. Do checks on CT, PT used for indication purposes.
i. Clean the entire MCC panel properly and take the IR value.
j. Carry the checks on relays, which are used in the MCC panel.
k. See for proper earthing connection and tightness of the earthing connections.
l. See for proper house keeping.

6. What is the difference between an auto reset and manual reset overload relay?
a. Auto reset relay closes its contacts when the bimetallic strip gets cooled. In
manual reset relay we have to manually reset the relay because even though
bimetallic strip cools its contacts are not closing without manual reset. L & T type
OLR have only manual reset and siemens type has both manual and auto facility.
b. Auto reset over load relay is reset by switching OFF the respective had switch
and again switching it ON.
c. Manual over load relay is reseted by pushing the reset button provided on the
MCC cell.

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