Why the Reactance and Voltage under Fault Condition is Low and Current is High
Theoretically and ideally, the Voltage and Reactance of a system (under fault condition such as short circuit) becomes low to zero and fault current may rise to the highest dangerous value.
We know that the power in watts is constant from supply source and under fault conditions in ac circuits such as short circuit there is no load and impedance “Z”, reactance i.e. inductive reactance and capacitive reactance which is equal to the overall resistance of ac circuits.
When short circuit occurs, there will be no resistance or reactance and current will be too much high. In this case, when power is constant and current increases, voltage will be decreased.
To understand this basic point, let consider the following example.
Suppose, (In normal condition)
Power = P = 1000 Watts
Voltage = V = 110 Volts
Current = I = 9.09 Amp.
But in Short circuit condition, (When current is too high)
Then,
P = 1000 Watts
I = 1000 A
V = P ÷ I
1000 Watts ÷ 1000A = 1 V.
To know why the system voltage under fault condition is too low, according to the ohm’s law:
V = I × R
As in case of short circuit, there will be no load i.e. resistance and reactances, so if we put resistance as zero in ohm’s law, the voltage will be too low due to high value of current flowing through the conductor as the circuit resistance is almost zero.
So we can say that, in case of short circuit and fault conditions, Inductive reactance is zero, current increases to the higher value and voltage decreases as power is constant from the power house.
Good to know:
- The system reactances ( Inductive reactance and capacitive reactance) is almost zero under fault conditions such as short circuit.
- In case of short circuit, the system resistance is ideally zero.
- The value of voltage between two points is theoretically “0” when short circuit occurs between phase and neutral.
- Current rises to the dangerous value in case of short circuit which leads to damage the connected appliances and installation.
- Proper protection, such as insulation and protective circuits like fuses and circuit breakers, is advised to prevent short circuits and dangerous sparks, which can lead to damage to the overall system or even hazardous fires.
FAQ
Why is current high in short circuit?
Short circuit means there is no voltage difference between the two points in the circuit. It is characterized by infinitely small resistance. Hence by ohms law large current flows through it. So, at the time of the short circuit, the current in the circuit increases heavily
Why current is maximum when voltage is zero?
When V=0 all the stored energy is in the inductor so the current must be at a maximum. A bit later all the stored energy is in the capacitor, the current is 0 and the voltage is at a maximum. Theoretically, this can happen in AC circuits with perfect inductors or capacitors
Why is current high at low voltage?
This is because Power = Voltage x current. And to achieve the same power output from a low voltage device, the current has to be higher. For eg. Two devices of 10volts and 100volts draw 100 watts of power each
What happens to voltage in short circuit?
Detailed Solution. In the case of a short circuit, the voltage becomes zero and the current abnormally high
Why is current zero in a short circuit?
A short circuit implies that the two terminals are externally connected with resistance R=0 , the same as an ideal wire. This means there is zero voltage difference for any current value
Why is there no voltage in a short circuit?
There is no internal resistance and source will deliver a constant magnitude of voltage. Moreover, this voltage supply is also independent of the current flow through the circuit. Therefore, the short circuit accounts for zero voltage
Why is voltage always 0?
Because you’re measuring the voltage at a node with respect to that same node, i.e. with respect to itself. So, the potential difference (a.k.a. voltage) between those nodes is zero because it’s the same node. It’s like measuring the length or distance from a point to that same point: it’s zero
Does higher voltage always mean higher current?
Ohm’s law states that current is directly proportional to voltage but inversely proportional to resistance. At constant resistance, current increases as voltage increases and vice versa
