MV design guide - Schneider Electric

Design rules
Busbar calculation
For the short-time withstand current (Ith)
c We assume that for the whole duration (1 or 3 seconds):
v all the heat that is given off is used to increase the temperature
of the conductor
v radiation effects are negligible.
The equation below can be used to calculate the short-circuit
temperature rise:
∆θcc =
0.24 • ρ 20 • Ith 2 • tk
(n • S) 2 • c • δ
with:
∆θsc : short-circuit temperature rise
c : specific heat of the metal
copper:
0.091 kcal/daN°C
aluminium: 0.23 kcal/daN °C
S : busbar cross section cm 2
n : number of busbar(s) per phase
Ith : is the short-time withstand current:
(maximum short-circuit current, rms value ) A rms
Example:
How can we find the value of Ith
for a different duration?
Knowing: (Ith) 2 • t = constant
c If Ith 2
= 26.16 kA rms. 2 s,
what does Ith 1
correspond to for
t = 1 s?
tk : short-time withstand current duration (1 to 3 s)
δ : density of the metal
copper: 8.9 g/cm 3
aluminium: 2.7 g/cm 3
ρ 20 : resistivity of the conductor at 20°C
copper:
aluminium:
1.83 µΩ cm
2.90 µΩ cm
in s
(Ith 2
) 2 • t = constant
(26.16 • 10 3 ) 2 •2 = 137 • 10 7
(θ - θn) : permissible temperature rise °C
so Ith 1
= ( constant ) = ( 137 • 10 7
)
t
1
Ith 1
= 37 kA rms. for 1 s
c In summary:
∆θsc =
∆θsc = °C
0.24 • 10 -6 • ( ) 2 •
( ) 2 • •
v at 26.16 kA rms. 2 s,
it corresponds to 37 kA rms. 1 s
v at 37 kA rms. 1 s,
it corresponds to 26.16 kA rms. 2 s
The temperature, θt of the conductor after the short-circuit will be:
θt = °C
θt = θn + (θ-θn) + ∆θsc
Check:
θt ≤ maximum admissible temperature by the parts in contact
with the busbars.
Check that this temperature θt is compatible with the maximum
temperature of the parts in contact with the busbars
(especially the insulator).
26 Merlin Gerin MV design guide Schneider Electric