Many of the projects we deal with depend on a reliable source of power. When the normal supply fails, either an alternate mains supply or a standby generator can be used, which requires a switching system.
For large projects using Air Circuit Breaker incoming devices, they will usually be placed side-by-side and interlocked to ensure that only one can be closed at any one time. For smaller systems (below 800A for instance) an ACB may not be necessary, yet we still need to carry out automatic switching.
Below are some of the ways this can be achieved, depending on the detailed project requirements.
Contactor changeover (2 x interlocked 400A contactors), with dual isolators below. Cables are bottom entry. This has the advantage that it will fail safe, that is the contactors will drop out automatically when the supply fails. This also means that a failure in the control system will remove the power, maybe not what you want in a hospital.
MCCB auto changeover, mounted on a common plate. In this case the cables are top entry, and the space below the devices is used for the automatic control system. These MCCB's have motor operators, which can also be switched manually if required. In the event of a control system failure, the MCCB's will usually stay in the same position and supply power if available.
Another MCCB example in a common compartment, cabling from below. The motor operators are not yet fitted. Rolla arrange the busbars to connect together at the rear.
ASCO auto-changeover unit, with dual MCCB isolators above (top front cable entry). These units are expensive but can switch very fast so may be appropriate for critical supplies (in this case the banking industry).
Dual supply, auto-changeover with bypass. This is the most complicated system, allowing the incoming supplies to bypass the autochangeover. Cabling is top front, switch handles at the bottom are the bypass devices.
Now we open up the doors we can see the complexity of the busbars and the level of segregation achieved to allow maintenance on one side, or on the changeover devices, whilst the other side provides power. The top centre section will house the controls, bottom centre are dual interlocked MCCB's.
1000A rated unit (using 1250A nominal devices) which includes fuseswitch protection for the mains supply, automatic transfer to generator supply, full bypass (manual override). Shown with doors removed, ready for small wiring to commence.