Commercial buildings, residential complexes and industries usually install a back up generator as an alternative power supply. Power from the generator and power from the utility mains supply line is routed to points of consumption through a control panel.
Automatic transfer switch function
The control panel, usually known as the automatic transfer switch, serves a vital function. It has electrical components and circuitry to sense mains supply. Should the mains supply fail the switch automatically sends a signal to control panel of the generator to activate it and transfers the consumption line to the generator supply line. In ideal conditions, it takes only a few seconds for the changeover to take place. If and when mains power supply resumes, the switch senses it and switches off the generator while transferring load to the mains supply. The control panel may have only a voltage sensor or a frequency sensor or both. Variation in any of the two outside the prescribed limit may trigger the switch.
Types of automatic load transfer switch
An automatic load transfer switch is available in different configurations. Make before break is a common type of switch used in power transfer panels. Another type is the make before break type, which is safe assuming that the mains power does not resume while the changeover is effected and it assures seamless switching with no break in supply, particularly suited for sensitive electronic devices like computers.
The safest type should be the delayed type where the switch automatically delays changeover to the generator for a few seconds in the event mains power resumes. The sensing circuitry, components used and design as well as manufacture have a bearing on the speed and performance of these changeover units. Normal types of switches include mechanical components like sensors and relays.
One can also buy theorist based solid state switches that are recommended for seamless switches with no imperceptible break. The generator can supply only a limited amount of power and it is desirable that the switch should have a load sensing unit that will trip the generator if the load exceeds the generator capacity.
The features to look for in these devices are automatic cut off, per-settable delays for the changeover, current capacity to handle the load and fail safe features. Advanced systems may be wired to more than one generator to assure seamless supply of power or to handle extra load demands, typical in industrial settings. In any case, it is important to specify a switch that can handle the load of the building. There are refinements, such as voltage and frequency sensors attached to the generator line that will transfer power only when the right parameters are achieved after the engine has achieved full speed.
Proper installation and maintenance is necessary
While the transfer switch control panel must be made to stringent specifications and designed as well as manufactured with care, its installation is equally important. A qualified and experienced electrician conversant with the process as well as with laws and electrical codes is the one who should be entrusted with the task. Improper wiring and connection can result in fire. If mains power goes to the generator, it may even result in an explosion.
Regular maintenance is absolutely necessary to ensure that the device performs as expected when required. Preventive checks can disclose components that are overheated or corroded or have a layer of dust or dirt that will hinder performance or will result in sparks or breakdown. In fact, regular maintenance of these devices is a complicated task to be performed by a qualified electrician. The steps include DE-energizing the switch gear, remove covers, test and re-calibrate all sensors and clean dust from the switch gear. Visual inspection is part of the routine process to detect moisture, grime, cracks, discoloration due to heat and damaged relays as well as contacts with excessive corrosion due to arcing. Inspection and maintenance will be followed by tests to ensure the device performs as specified.
When one buys this type of equipment, it deserves critical consideration for durability, safety and performance and also the service support offered by the manufacturer.