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n
Overload protection
Busbar overload protection is ensured following the same criteria used
for cables. It will be necessary to check the relationship :
I
b
≤
I
n
≤
I
z
Where :
I
b
= circuit utilisation current
I
n
= switch rated current
I
z
= rating at permanent cable regime
The lb utilisation current in a 3 phase system is calculated based on the
following formula :
I
b =
(A)
Where :
P
t
= sum of the active powers of the loads installed (W)
d
= power supply factor equal to 1 if the trunking is :
•only powered from one side
•powered from the centre or from both ends at the same time
Ue
= operating voltage in (V)
cos
j
m= average power factor of the loads
I
b
= utilisation current (A)
α
= diversity coefficient of the loads (.)
β
= coefficient of utilisation of the loads (.)
P
t
•
α
•
β
•
d
√
3
•
Ue
•
cos
j
medium
Ambient temperature (°C)
15
20
25
30
35
40
45
50
55
60
k
t
thermal correction factor
1·15
1·12
1·08
1·05
1·025
1
0·975
0·95
0·93
0·89
K
t
correction coefficient for ambient temperature other than 40°C
The ambient temperature where the busbar trunking system is installed
impacts on its rating
During the design stages, it is necessary to multiply the rating value
at the reference temperature by a correction coefficient referred to the
final operating temperature
All Zucchini products have been sized and tested for an average
ambient temperature of 40 °C. For installation in environments with
average daily temperatures lower than 40 °C, the rated current of the
busbar must be multiplied by a k
t
factor, which is higher than the unit
for temperatures lower than 40 °C, and lower than the unit if the ambient
temperature is higher than 40 °C
I
z
= I
z
0 · K
t
Where :
• I
z
0 is the current that the busbar trunking system can carry for an
indefinite time at its reference temperature (40 °C)
• K
t
is the correction coefficient for ambient temperature values other
than the reference temperature, as shown in the following table
I
I
b
I
z
1.45 I
z