A building automation system (BAS) is the software, and the network of small controllers, that runs a building's cooling, lights, lifts, fire alarms and security from one place. It is also called a building management system (BMS), which is the name most Indian engineers use. The building's own engineers use it from a control room, and more and more from a web browser or a phone.
A BAS has three levels. At the bottom are the sensors and switches: thermometers in the rooms, meters on the electricity supply, valves on the cold-water pipes and relays that switch the lights. Each sensor reading or switch is called a point. In the middle are controllers, small computers fixed in panels on each floor. A controller reads its points every few seconds and follows rules, such as "if the room is above 25°C, open the cold-water valve further". At the top is the software. Its screens show every floor, every alarm and every reading over time. Most equipment sends its readings in an open standard format called BACnet. So a chiller from one maker and a lift from another can appear on the same screen.
Take, for example, an IT campus in Kochi with four office blocks and about 6,000 points. At 7 a.m. on a weekday, the BAS starts the HVAC plant, so that the floors are at 24°C when employees arrive at 9. At 7 p.m., it turns off the lights and the cooling on every floor where the access control system shows that nobody has tapped a card since lunch. On Sundays, it keeps only the server rooms cool. Schedules like these often cut a building's electricity bill by a tenth or more.
The BAS also links systems that would otherwise act alone. Say a smoke detector on the sixth floor of one block goes off. The fire alarm panel reports the alarm to the BAS. Within seconds, the BAS stops the air handlers on that floor, so that the fans do not spread the smoke. It sends the lifts to the ground floor and keeps them there. It releases the electronic door locks on the escape routes. If the mains power fails, it starts the standby generators.
When a reading goes wrong, such as a pump that vibrates more than usual, the BAS can open a repair job in the computerised maintenance management system (CMMS) by itself. That job is a work order.
How is a BAS different from industrial energy management software? A BAS controls the equipment in one building, second by second. Energy management software reads the meters of many buildings or plants, reports where energy is wasted and suggests changes. It usually takes its data from the BAS. Because a BAS is connected to the network, attackers can target it too. Protecting it is covered in OT and IoT security.