According to NFPA 496 (item 5-4.9)"The electrical power for the positive pressure air system shall be taken off the main power line ahead of any service disconnects to the control room or shall be supplied from a separate power source." What does exactly mean? In my case, I have a Control Building along with a Motor Control Center Room (class 1, div 2, G2). Do I have to take the corresponding MCC out? Do I have to make the corresponding control and switch panel appropiate for Class I, Div 2, Group D?

 

In short, you must detect positive pressure in the building so the MCC, UPS, computers or any other non-classified equipment can receive
power. When positive pressure is lost, then all non-classified equipment must be powered down through contactors or shunt-trips.

Also review the standards for detecting explosive vapor with gas sensors in the room and the required purges for room changes per hour.

In many applications, when gas is detected or purge pressure is lost, then all main power is disconnected to everything. It is then up to
the user to bring in hand-held detectors and certify that no gas is present sometime later to restart the purge fans. This is not the best
way, but is often the case. It is better to use CL1Div2 purge and sensor equipment and provide power upstream or from another service
entrance means so that purge and detection remains active.