Siemens SINAMICS S120 Manual page 316

Cabinet design and EMC
7.3 Cabinet air conditioning
Figure 7-5
Devices must not be operated in an "air short-circuit", since this can damage equipment or
cause it to fail.
The drawn air of the fan causes negative pressure to build up at the ventilation openings in
the cabinet doors. The pressure is dependent on the volume flow rate and the hydraulic
cross-section of the openings.
The air, which blows out of the top of the device, accumulates under the top cover/hood,
resulting in overpressure.
The difference between the overpressure at the top of the cabinet and the negative pressure
at the bottom creates a flow of air (air short-circuit). This can vary in strength depending on
the cross-section of the door and cover openings and the volumetric flow of the fan.
314
Air guidance for Basic Line Module, frame sizes FB, GB, GD
Manual, (GH3), 01/2013, 6SL3097-4AE00-0BP3
Chassis power units