Inrush current is a surge of energy that occurs when an electronic or magnetic device is turned on, which can cause damage to the device or blow fuses and circuit breakers. Thermistors and active circuits are built into devices to compensate for the large inrush current. Surge protectors can also be installed to limit the current. Measuring the peak current accurately is important to determine the best method to correct problems associated with surge current.
An inrush current is energy that is supplied to an electronic or magnetic device when it is turned on. This can include devices such as computers, copiers or motors. The peak current is usually much greater than the normal current required to operate the device safely. To compensate for the large inrush current, thermistors and active circuits are built into the devices. Some of the effects of inrush current include blowing fuses and circuit breakers.
The inrush current can be 20 times greater than the normal operating current of an electronic device. It usually takes 10 milliseconds for the current to reduce to normal levels. Typically, the number of times the current flows through the circuit before reducing is between 30 and 40. Several things can happen during this time.
When you turn on the main power of a computer, for example, a high inrush current occurs. Components inside the computer called filter capacitors produce the large current and propagate it rapidly. The typical effect of inrush current is to prevent circuit breakers or fuses from operating normally. A more serious problem is that the surge current can damage the switch contacts as it passes, essentially welding the contacts together.
There are several ways to correct problems associated with surge current, including increasing the wire size or installing surge protectors. To determine which method is best, the inrush current must be measured with a meter. Clamp meters are typically used to measure peak current.
The most reliable meters usually have the ability to measure peak capture time and have a peak hold function. These two specifications will ensure that the peak current is measured accurately. Good meters have a peak measurement time of 1 millisecond, while cheaper models have a time of 100 milliseconds. Since most of the peak current dissipates within 10 milliseconds, slower models won’t even capture the peak itself.
The surge protector can also be called the limiting thermistor, which is a heat sensitive resistor that reacts to changes in circuit temperature. As the temperature increases, the resistance of the resistor essentially decreases, allowing rush current to flow through the circuit. This prevents a buildup of the peak current.
Since a surge protector heats up during its operation, it requires a cool down time before the next inrush current. During the recovery time, the resistance of the surge protector is gradually restored. This will effectively suppress the following current.
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