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D-83
Additional information
Thermal characteristics of
switchboards
Switchboard ventilation
The air enters the lower section via the fans and exits the upper section:
bb
through a ventilated roof
bb
or through a ventilation opening.
The air throughput of the fans is determined by the equation:
P
D
T
D = 3.1 x
- KS
(
)
The chart below can be used to determine the necessary throughput, based on the
dissipated power, the difference in temperature (internal - external) and the exposed
surface area of the enclosure.
Example
Consider an IP3X cubicle, 650 mm wide and 400 mm deep, containing components
(devices, connections, busbars, etc.) dissipating 1000 W.
The ambient temperature around the cubicle is 50 °C.
Given that the average temperature at mid-height should not exceed 60 °C, the
difference in temperature
D
T is equal to 60 - 50 = 10 °C.
The exposed surface of the cubicle (non adjacent to a wall or other cubicle) is
4.46 m².
(back = 1.3 m², front = 1.3 m², roof = 0.26 m², side panels = 1.6 m²).
What is the necessary throughput of the ventilation system?
The throughput can be calculated as:
1000
10
D = 3.1 x
- 5.5 x 4.46
(
)
D = 234 m
3
/h.
In the range of Prisma P accessories, select a system with a throughput
of 300 m
3
/h.
Dd381391.eps
Calculation data
P:
power dissipated by the devices, connections and busbars (in Watts)
Pr:
power of the heating resistor (in Watts)
Tm:
maximum internal temperature in the device zone (in °C)
Ti:
average internal temperature (in °C)
Te:
average external temperature (in °C)
D
Tm
= Tm – Te
D
T
= Ti– Te
S:
total free surface area of the enclosure (expressed in m
2
)
K:
thermal-conduction coefficient of the material (W/m
2
°C)
K = 5.5 W/m
2
°C for painted sheet metal
D:
ventilation throughput (in m
3
/h)
Note:
The dissipated power of each device is provided by the manufacturer.
Add approximately 30 % to account for the connections and the busbars.
Thermal management of
switchboards
Ventilation