The design of the MCC (motor control centre) is as important as the starters it contains. A MCC is an assembly of cubicles physically and electrically joined where each cubicle contains a number of compartments and each compartment contains a starter or other functional unit.
For ease of maintenance with maximum personnel safety MCCs should be constructed to BS EN 60439-1 Form 4a type 7 which allows any single starter, designated a functional unit, to be electrically isolated for maintenance.
Form 4 type 7 is defined - Where the busbars are separated from the functional units by rigid metallic barriers, the functional units are separated from each other by metallic barriers, the external cable terminals; both main and auxiliary terminals; are within their own sub-compartment complete with integral glanding facilities separated from their associated functional unit by rigid metallic barriers.
The MCC must have one or more incoming sections and a busbar system that distributes power to the individual starters and functional units.
The MCC must also contain the control system transformer and its’ distribution system to the starters, also the SCADA sub-ring system for the starters; and the power distribution system for the SCADA system.
The MCC busbar system must have a current rating for the main busbars equal to the current rating of the incomer, or where there are two or more incomers, the rating of the largest incomer. The down drop busbars for each cubicle of the MCC must have a current rating equal to the full load of the cubicle.
The busbars both main and down drop and their fixings within the MCC must be rated for the fault level at the MCC’s point of incorporation into the electrical distribution system network.
The busbars both main and down drop should also be sized for a maximum temperature rise of 40oC when operating at their full rated load.
The incoming sections may be one, two or three incomers depending on the level of supply security required, where a MCC has more than one supply the supplies should have a preferred supply with one or more levels of standby. The change-over system between the supplies should be as described in the change-over article of this web site. (in preparation)
The number of starters per cubicle will depend on the current rating and starting method of the starter. It is also important that each compartment containing a starter is of sufficient size for ease of component maintenance and replacement.
An important point to make is that a designer must have a variety of skills one of them is the ability to listen, this also differentiates designers from politicians who have never mastered the skill of listening. If the opportunity arises when the factory testing of the MCC takes place, ask the opinions of those building, assembling, wiring, testing and installing the starters/MCCs, but not the directors, managers or salesmen, they will say anything to flatter, which means their opinions are worthless. Ask the electricians, fitters and panel builders and make it clear you will be considering their comments in any amendments to the design; but be prepared to be humiliated as they will tell you exactly what they think of your designs without consideration of your feelings. It is however a great way of improving both the designs and your abilities.