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What Are Surges? Types & Reasons of Occurrence Explained

What are surges

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What are Surges?

Surges, or transients, are brief overvoltage spikes or disturbances in a power waveform that can damage, degrade, or destroy electronic or electrical equipment. This sudden high voltage is beyond the voltage limit that the circuit is designed to handle. Surges are very short and last only for several microseconds to a few milliseconds. A surge is a kind of overvoltage, but an overvoltage is not always a surge.

Surges are very common, and an electrical network experiences multiple surges in a day. Low-level surges do not cause any damage to the devices in the electrical system. But continuous exposure to such surges can cause malfunctioning and gradual degradation of the electronic components resulting in complete equipment failure over time. Large power surges, on the other hand, can result in instantaneous damage, like blowing up of devices.
Power surges can be widely classified as Internal surges and External surges based on the cause of a surge.

Internal Surge:

Internal surges are those which originate within a facility, building or electrical system. 60 – 80% of power surges originate within facilities. They are usually caused by the switching of large loads. Faulty wiring can also be a reason for internal surges. Short circuits and tripped circuit breakers also create spikes or surges in the network. These internal surges can be powerful enough to damage the electrical and electronic equipment in the system. Common causes of internal surges are listed below:

Switching of electrical loads from:

– Switching of capacitor banks and loads.
– Discharge of inductive devices (motors, transformers, etc.)
– Arcing faults (ground)
– Power system recovery from outages
– DC Battery storage systems
– Faults or bad wiring

Magnetic and inductive coupling from:

– Elevators
– Copy machines
– Computers
– AC and Refrigerators

External Surge:

External surges are usually of higher voltages compared to internal surges. The main reasons for external surges include lightning strikes, switching at the utility end, short circuits in the main power supply line etc. Some of the common causes for external surges are given below:
a) Direct or indirect lightning strikes
b) Grid and capacitor bank switching (utility end)
c) Damage to power lines or transformers

Understanding Surges caused by Lightning Strikes:

Lightning Surges, as the name suggests are caused due to lightning activity. There are four ways that lightning surges can occur during lightning activity.
1) When lightning hits directly a building.
2) When lightning hits near a building.
3) When lightning hits on the power line structure.
4) When lightning hits near the power line structure.

A direct lightning strike on the facility is rare, but the damage it causes is extremely severe. It can damage the structure and all the equipment inside. Transients are induced into the conductors due to lightning strikes.

Lightning Surges and Switching Surges:

Lightning Surges are uncontrollable and can travel long distances through power or communication networks. Even though a lightning protection system is provided for a structure, the surges due to the above conditions can affect the equipment inside it. As per the standards, a lightning surge is usually10/350µs waveform.

Switching surges are the surges developed in a network when heavy loads are switched on and off. These types of surges develop within a building, which has heavy load equipment or due to heavy load anywhere in the network (mostly from the utility ends). As per the Standards, the switching surges are 8/20 µs waveform.
The graphical representation of these surges is shown as waveforms above. The lightning impulse waveform 10/350µs means, that when this surge happens, it will reach a maximum value of 10µs and will deplete to 50 per cent of its highest value in 350µs. Similarly, for switching surges waveform 8/20µs means that it will reach its highest value in 8µs and will deplete to its 50 per cent in 20 µs.

How to Protect Your Devices?

Malfunctions or damage to electrical devices can occur when a surge voltage exceeds a device’s dielectric strength. These kinds of surge voltages with very high amplitudes in the kilovolt range, are usually transient, meaning that they range from a few microseconds to several hundred microseconds. These short-duration surges with high amplitudes cause sudden voltage rises in devices, whose effects can only be protected by surge protection devices.

A Surge Protective Device (SPD) is a device designed to limit the transient voltages and divert surge current from entering the electrical network. SPDs are intended to limit the surge voltage so that the dielectric strength of the device is not exceeded. Thus they are protected from damage. SPDs transition between high and low impedance states. At normal operating voltages the SPDs remain at a high impedance state and do not affect the system. When a transient voltage or surge occurs, the SPD moves into a state of conduction which means a state of low impedance and directs this transient current to the ground. This limits the voltage in the network to a safe level thereby protecting the equipment in the system. After the surge is diverted, the SPD automatically resets to a high impedance state.

To read more about the working and types of SPDs read our blog – Surge Protection Devices – Complete Overview


The occurrence and damage from transient overvoltages are unpredictable. The one way to protect the electrical network or devices from such damages is to install Surge Protection Devices and maintain a safer electrical environment. The increased equipment and devices in our house, commercial spaces and other facilities increase the load in the power network and thereby increase the chances of power surges. To avoid power disturbances, interruptions and equipment failures due to both internal and external surges, the first step is to ensure a healthy electrical network with safety and protection systems.




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