- Short Circuit Current Definition: Short circuit current is defined as the large current that flows through an electrical system when a fault occurs, causing potential damage to circuit breaker components.
- Thermal Stresses: Thermal stresses in circuit breaker contacts increase with short circuit current, affecting the CB’s performance and safety.
- Temperature Rise Formula: The temperature rise in CB conductors can be calculated using current, cross-sectional area, and resistivity coefficient.
- Electromagnetic Force: Electromagnetic force between parallel conductors during a short circuit is crucial for understanding mechanical stress in circuit breakers.
- How to Calculate Short Circuit Current of Circuit Breaker: Calculating the short circuit current involves specific formulas and understanding material limits, ensuring CB designs can handle faults safely.
Prospective short-circuit current is set by the source and the impedance up to the fault, not by the circuit breaker (CB). While that current still flows through a closed CB, the poles take thermal I2t heating and electrodynamic force. The formulas below estimate those stresses from a known short-circuit current.
If the conducting parts are too small in cross-section, they overheat and the insulation can be damaged.
Contacts heat as well. Heating energy scales as I2Rt. R is contact resistance. I is the short-circuit rms value. t is how long the current lasts.
After the fault starts, current continues until the interrupter opens. So t is the breaking time of the circuit breaker. Break time is only tens of milliseconds on many designs, so the usual model is adiabatic: the conductor absorbs the I2t heat because convection and radiation have almost no time to act.
Temperature rise from that adiabatic heating is written as,
Where, T is the temperature rise per second in degree centigrade.
I is the current (rms symmetrical) in Ampere.
A is the cross-sectional area of the conductor.
ε is the temperature coefficient of resistivity of the conductor at 20oC.
Older aluminium-bus practice often treats about 160°C as a strength ceiling, so short-circuit temperature rise is kept under that figure. Break time and conductor section are the two levers.
Short Circuit Force
Electrodynamic force between two parallel electric current carrying conductors is given as,
Where, L is the parallel length of both conductors in inch.
S is the centre spacing in inch.
I is the current in each of the conductors.
A handbook peak used on this page puts the largest electrodynamic force when I is 1.75 times the initial symmetrical rms value. IEC peak-factor kappa can be higher, up to about 2.5 times Irms, on a fully offset wave.
Stiffer bars, or bars that can resonate, can see still larger mechanical reactions. Tests also show that the shock at the instant force is applied or removed can exceed the force while current is flowing, even on a non-resonating structure.
Designers therefore error toward a higher mechanical load and size for the force from the initial peak of the asymmetrical wave. A common allowance on this page is twice the force from the formula above.
The formula is for circular section. L is only the parallel run, yet the derivation treats each conductor as infinitely long.
Real bars have finite length. Flux density near the ends of a current carrying conductor also differs from the mid-span field.
Used on a short bar, that infinite-length formula therefore overstates the force.
A better finite-length term is,
in place of L/S in the force formula.
The formula then becomes,
Equation (2) is the usual pick when L/S is greater than 20, on this page’s error ranges. When 20 > L/S > 4, formula (3) is the better fit.
If L/S < 4, formula (2) is used again. Both apply to circular section. Rectangular bars need a shape factor K, so the working expression is 
Shape effect falls off as spacing between the conductor grows. K is largest for a thin wide strip, near zero for a square section, and 1 for a round section. The same force check applies to a local breaker and to a remote control circuit breaker.
