When two or more conductors of different phases come in contact with each other, a short circuit can occur. In this situation, the impedance of the circuit is significantly reduced, leading to a large current flow in the un-faulted phases. This is because the shunt created by the short circuit creates a low-impedance path, effectively bypassing the normal resistance and reactance of the circuit.
Important Terms to understand the short circuit Current:
What is impedance & Admittance in electrical Circuit?
Resistance:
Resistance is an electrical property that represents how much a material or component resists the flow of electric current.
It's measured in ohms (Ω).
The formula R = ρl/A defines resistance,
where:
R is the resistance.
ρ (rho) is the resistivity of the material, which depends on the material's properties.
l is the length of the conductor (like a wire).
A is the cross-sectional area of the conductor.
Impedance:
Impedance is a more complex concept and is essentially the resistance in circuits where there are alternating currents (AC) and components like capacitors and inductors.
For simple DC circuits, impedance is the same as resistance.
Resistance is an electrical property that represents how much a material or component resists the flow of electric current.
It's measured in ohms (Ω).
The formula R = ρl/A defines resistance,
where:
R is the resistance.
ρ (rho) is the resistivity of the material, which depends on the material's properties.
l is the length of the conductor (like a wire).
A is the cross-sectional area of the conductor.
Impedance:
Impedance is a more complex concept and is essentially the resistance in circuits where there are alternating currents (AC) and components like capacitors and inductors.
For simple DC circuits, impedance is the same as resistance.
In AC circuits, impedance can have both resistance and reactance components due to the effects of capacitors and inductors.
Admittance:
Admittance is the opposite of resistance. It measures how easily electric current flows in a circuit.
Admittance:
Admittance is the opposite of resistance. It measures how easily electric current flows in a circuit.
It's measured in Siemens (S) or reciprocal ohms (1/Ω).
In simple terms, it tells you how conductive a material or component is.
Capacitance:
Capacitance is an electrical property related to devices called capacitors.
A capacitor stores electrical energy in the form of an electric charge.
The energy stored in a capacitor is given by the formula E =1/2 CV²
E is the energy stored.
C is the capacitance of the capacitor.
V is the voltage across the capacitor.
Think of a capacitor as a device that can store electricity temporarily, like a rechargeable battery for short bursts of power.
Let's Now Understand the short Circuit Current:
The reduction in impedance allows a greater amount of current to flow through the circuit, leading to a rise in current. The magnitude of the current flow depends on several factors, including the system voltage, the impedance of the circuit, and the location of the short circuit. If the current flow is not quickly interrupted, it can cause significant damage to the electrical equipment in the circuit.
It is important to note that short circuits can occur not only between different phases but also between a phase and ground or between two points of different potentials in a circuit. In any case, the result is a significant reduction in impedance and a large flow of current through the circuit, which can lead to equipment damage, safety hazards, and power outages.
Short-circuit currents are dangerous for several reasons, as you mentioned. Firstly, when a short circuit occurs, a very high current flows through the circuit, and this can cause overheating of the equipment. Overheating can cause insulation breakdown, melting of conductors, and other forms of damage to the electrical equipment. In some cases, the equipment can even catch fire due to the heat generated by the short-circuit current.
Secondly, the flow of short-circuit current in the current carrying parts produces a force of electrodynamics interaction, which can cause significant mechanical stresses on the equipment. This force can cause mechanical damage to the equipment or even destroy it altogether.
During a short circuit event, the current undergoes a continuous change, and this is referred to as the transient phenomenon. The magnitude of the current is shown in the graph you mentioned. During the first moment of the fault current, the current attains its maximum value, which is called the first peak short circuit current (Ifp). This current value is reached immediately after the fault occurs and can be many times higher than the steady-state value of the short-circuit current.
After the initial peak, the current decreases rapidly to its steady-state value, which is the sustained short-circuit current. The sustained current is the fault current that will flow in the circuit if it is allowed to persist after the transient process ends.
During the transient phenomenon, the current is very high but falls rapidly, and this zone is called the sub-transient current. The sub-transient current is the current that flows during the first few cycles of the fault and is used to determine the short-circuit current rating of equipment.
The rms value of the periodic component of the short-circuited current is called the Ipc. During the steady-state, the rms value of the short-circuit current remains almost constant.
To protect against the damaging effects of short-circuit currents, protective devices such as fuses, circuit breakers, and relays are used. These devices detect and isolate faults and interrupt the flow of current to prevent damage to the equipment and ensure the safety of personnel.