Everything You Need to Know About Transients in Electrical Circuits

Key Takeaways

• The types of transients in electrical circuits.

• The sources of transients in electrical circuits.

• The effects of transients in electrical circuits.

Lightning is an external source that generates transients in electrical circuits

When there is switching action in an electrical circuit, the voltage and current go through a transient state before attaining steady-state conditions. The transients in electrical circuits occur for a short duration immediately after the switching action. The duration of the transients is mostly in the range of microseconds to several milliseconds and depends on circuit parameters such as resistance, inductance, capacitance, etc. The magnitude of the transient voltage and currents in the electrical circuits keep changing quickly until a steady-state is achieved. In any electrical circuit, transients indicate an unstable system, so controlling transients increases system reliability. In this article, we will discuss the types, sources, and effects of transients in electrical circuits.

Transients in Electrical Circuits

Whenever the electrical power supplied to a circuit changes momentarily over a short duration of time, it is called transients. Transients invariably affect the voltage and current. AC and DC circuits are equally vulnerable to transients, and steady-state values are reached after the transient period. According to ANSI standards, transient duration is about 1/16 times the time period of the voltage or current waveform, or about one millisecond. The typical duration of voltage transients is 50 microseconds and current transients occur for about 2 microseconds. The transients generated in a circuit can be unipolar or bipolar.

Transients are the shortest power quality disturbance with a burst of energy in the circuit. The energy of transients can be additive or subtractive to nominal steady-state waveform energy. Generally, additive energy is experienced during transient conditions with a rise in voltage or a current several times greater than the nominal steady-state value. Under transient conditions, it becomes difficult for the components present in a circuit to withstand voltage spikes and the current surges as it is rated for the steady-state voltage and current values. Transients are also associated with abnormal changes in frequency, which can go up to 5MHz.   

The Types of Transients in Electrical Circuits

Transients in electrical circuits can be classified as:

1. Impulsive transients - According to IEEE 1159 standards, impulsive transients are sudden transient disturbances of non-power frequency that are unipolar (either positive or negative) that enter electrical circuits. Lightning induces impulsive type transients in electrical circuits. Impulsive transients are characterized by their rise time, decay time, and spectral content. 
2. Oscillatory transients - Oscillatory transients are similar to impulsive transients, but they are bipolar or bidirectional. The polarity of oscillatory transients can quickly vary from negative to positive and vice versa. Oscillatory transients are characterized by their magnitude, duration, and spectral content. 

Both impulsive and oscillatory transients are further categorized based on frequency. The table below gives the classifications. 

Classification of transients based on frequency

Sources of Transients in Electrical Circuits

There are several sources causing transients in electrical circuits. Sources can be either internal or external to the circuit. The table below outlines these sources.

Internal and external sources of transients

The Effects of Transients in Electrical Circuits

The effects of transients influence not only power lines but also signal and control lines. The effects of transients can be divided into four types:

1. Intermittent interruptions - Results in a loss of data or corrupted data in data lines or control lines.  
2. Chronic degradation - The repetitive occurrence of transient events degrades the components in an electrical circuit and diminishes performance, ultimately causing complete circuit failure.
3. Latent failures - Similar to chronic degradation, except the component continues to operate with derated performance.
4. Catastrophic failures - The component stops working immediately.

The generation of transients in electrical circuits cannot be stopped until there are energy storing elements, semiconductor switches, relays, circuit breakers, transformers, etc. present in the circuit. Cadence software can help you implement transient suppressing techniques in your circuit design. 

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