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Home / Resources (video trainings, theorems) / Knowledge / Thermal Short-time Rating

Formulas

If a conductor which is rated to carry full load current Iload continuously is rated to carry a maximum fault current Ilimit for a time tlimit, then a lower fault current Ifault can be carried for a longer time tfault according to:

( Ilimit – Iload )2 tlimit = ( Ifault – Iload )2 tfault

Rearranging for Ifault and tfault:

Ifault = ( Ilimit – Iload ) ( tlimit / tfault )½ + Iload

tfault = tlimit ( Ilimit – Iload )2 / ( Ifault – Iload )2

If Iload is small compared with Ilimit and Ifault, then:

Ilimit2 tlimit » Ifault2 tfault

Ifault » Ilimit ( tlimit / tfault )½

tfault » tlimit ( Ilimit / Ifault )2

Note that if the current Ifault is reduced by a factor of two, then the time tfault is increased by a factor of four.

NOTATION
The symbol font is used for some notation and formulae. If the Greek symbols for alpha beta delta do not appear here [ a b d ] the symbol font needs to be installed for correct display of notation and formulae.
B
C
E
f
G
h
I
j
L
P
Q
susceptance
capacitance
voltage source
frequency
conductance
h-operator
current
j-operator
inductance
active power
reactive power
[siemens, S]
[farads, F]
[volts, V]
[hertz, Hz]
[siemens, S]
[1Ð120°]
[amps, A]
[1Ð90°]
[henrys, H]
[watts, W]
[VAreactive, VArs]
Q
R
S
t
V
W
X
Y
Z
f
w
quality factor
resistance
apparent power
time
voltage drop
energy
reactance
admittance
impedance
phase angle
angular frequency
[number]
[ohms, W]
[volt-amps, VA]
[seconds, s]
[volts, V]
[joules, J]
[ohms, W]
[siemens, S]
[ohms, W]
[degrees, °]
[rad/sec]

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