### Formulas

If a conductor which is rated to carry full load current **I _{load}** continuously is rated to carry a maximum fault current

**I**for a time

_{limit}**t**, then a lower fault current

_{limit}**I**can be carried for a longer time

_{fault}**t**according to:

_{fault}**( I _{limit} – I_{load} )^{2} t_{limit} = ( I_{fault} – I_{load} )^{2} t_{fault}**

Rearranging for **I _{fault}** and

**t**:

_{fault}**I _{fault} = ( I_{limit} – I_{load} ) ( t_{limit} / t_{fault} )^{½} + I_{load}**

**t _{fault} = t_{limit} ( I_{limit} – I_{load} )^{2} / ( I_{fault} – I_{load} )^{2}**

If **I _{load}** is small compared with

**I**and

_{limit}**I**, then:

_{fault}**I _{limit}^{2} t_{limit} » I_{fault}^{2} t_{fault}**

**I _{fault} » I_{limit} ( t_{limit} / t_{fault} )^{½}**

**t _{fault} » t_{limit} ( I_{limit} / I_{fault} )^{2}**

Note that if the current **I _{fault}** is reduced by a factor of two, then the time

**t**is increased by a factor of four.

_{fault}NOTATION |
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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. |
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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] |
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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] |