What is Surge Current?
According to Chegg (n.d), surge currents, inrush currents or power surges are the “currents that raise or fall from the normal rated value in the short duration of time”. These are the instantaneous sharp impulse currents. A surge current is a quick and short lived large current flow an electrical system that is not designed to handle such amplitudes might experience. Surges usually occur as short pulses that have very high rates of change and amplitudes (slow varying current or voltage is not considered as a surge). Depending on the type and design of the system, the surge may or may not cause damages. If the surge amplitude is within the pulse current capability of the device, then it might not cause any harm. If it is otherwise, catastrophic failure can happen.
The surge currents damage, partially damage, or not damage the electrical machines based on their magnitude. If the surge current’s magnitude is very large in the short duration of time, then the surge current damages the electrical equipment. If the surge current’s magnitude is a small value, then it does not damage or partially damage the electrical equipment. In general, the surge currents are very high currents (100kA or more than 100kA).
Causes of Surge Currents
Surge currents occur in electrical devices such as domestic electrical appliances, DC electrical machines, industrial machines, transformers, induction machines, synchronous machines and other AC machines for various reasons (Chegg, n.d.). Below are some of the most common reasons for the occurrence of surge currents:
- Lightning strikes
- Trees falling on power lines
- Power restored after a grid black out
- Surge transmitted through power cables to reach the equipment
- Ignition of high-power devices such as motors, heavy equipment
- Poor wiring
- Overloaded circuits
- Grounding faults
- Short circuit
- Lightning Strikes: When lightning strikes near a power line, whether it’s underground, in a building or running along poles, the electrical energy can boost electrical pressure by millions of volts. This causes an extremely large power surge that will overpower almost any surge protector. In a lightning storm, you should never rely on your surge protector to save your electronics. The best protection is to unplug them (Tom & Tolan, 2022).
- Trees Falling on Power Lines: A power surge often occurs when power is restored following service interruptions or outages. High winds during hurricanes (or other powerful storm systems) can lead to outages in a variety of ways. Some of the more common scenarios include falling trees bringing down power lines, lines making contact with one another or with tree limbs, and power poles getting knocked over. Once a power outage occurs, the lines are void of electrical current. Upon restoration of power, the electrical current rushes to fill the “empty space” in the lines (known as an inrush). Electronics and appliances are not designed to handle such power-flow fluctuations, and can be damaged during such events (Reza, 2017).
- Power Restored after a Grid Black Out: Once power is restored after an outage, the sudden jump in current can create a power surge, damaging any previously unpowered appliances (Roman Electric Co., n.d.).
- Surge Transmitted Through Power Cables to reach the Equipment: power surges involves “magnetic coupling”. When electricity flows through a circuit, a magnetic field is created. In some circumstances this field can affect nearby wires, inducing a voltage in them and so leading to an unexpected power surge (Sollatek, n.d.).
- Ignition of High-Power Devices such as Motors, Heavy Equipment: The ignition a two-stroke and a four-stroke engine entails the piston rising during the compression stage to compress the air-fuel mixture in the combustion chamber. Just before the piston reaches the top-dead center (TDC), a spark plug fires in the cylinder and ignites the compressed air-fuel mixture. The ignition of the air-fuel mixture forces the piston down in the cylinder, producing the power stage. The power produced by the ignition of the air-fuel mixture turns the crankshaft, which in turn keeps the piston moving and the engine running. One of the requirements for an efficient engine is the correct amount of heat, delivered at the right time. This requirement is met by the ignition system. The ignition system supplies properly timed, high-voltage surges to the spark plug(s). These voltage surges cause combustion inside the cylinder. It must also be able to deliver a high-voltage surge to each cylinder at the proper time during its compression stroke. How the ignition system does these things depends on the design of the system (Industrial Electronics, n.d)
- Poor Wiring: Wiring is essential for distributing electricity. And if it is damaged or the casing is exposed, a power surge is more likely to happen. Wiring that is damaged or exposed offers little electrical resistance, meaning any conductive material can spike the wire’s current to dangerous levels (Roman Electric Co., n.d.).
- Overloaded Circuits: As its name suggests, electrical overloads are when you attempt to draw too much power from a single circuit. This can happen when you overuse extension cords, plug in too many appliances, or use an appliance above the circuit’s amperage levels. Electrical overloads can lead to power surges as the overwhelmed circuit may receive a massive spike in current due to the excessive power being drawn (Roman Electric Co., n.d.).
- Grounding Faults: A ground fault happens when electricity strays to an unplanned path to the ground. This path is unrestricted as there is no longer resistance and the flow of charge increases dramatically and quickly. Ground faults are most dangerous in areas that tend to experience high moisture, such as bathrooms or garages. Sometimes the chosen path to the ground could be the outside of an appliance, where when someone unknowingly touches it, electric shock occurs. Shock is the most common danger, but fires and burns are also a high risk (Bay Power, 2021).
- Short Circuit: A short circuitis when there is a low resistance connection between two conductors that are supplying electrical power to a circuit. This would generate an excess of voltage streaming and cause excessive flow of current in the power source (D&F Liquidators, n.d.).
Control/Prevention of Surge Currents
The control/prevention of surge current is essential in protecting electrical appliances from preventable damages which may result from the occurrence of surge currents. This control/prevention is carried out by making use of surge protection devices.
According to Axis Electrical Components (2021), surge protection devices play a vital role in voltage fluctuations or variations. These fluctuations are the most common behaviours observed in any electrical system. Voltage change can vary from small voltage fluctuations for a short duration to a complete blackout for an extended period. When the voltage drops below the standard rate (of tolerance) for the short term, it is called sag. When the voltage increases, it is called swell. There are many fluctuations with shorter durations than swell and sag. These very short fluctuations are called transients, impulses, spikes or a notch. An increase in at least 10 per cent of voltage or current lasting for less than a few microseconds is known as a Surge. The amount of electrical energy produced during a surge (for short period) will be significantly high as it can rise to a few thousand volts.
A surge protection device is a device that protects electrical devices from voltage spikes in alternating current (AC) circuits. Surge protection devices safeguard the electrical installation made up of consumer unit, wiring and accessories, from electrical power surges, generally known as transient overvoltages. The surges occurring from transient overvoltages can either result in instant failure or longer damage to the equipment. Different SPDs are used to protect the electrical installation from incoming services such as telephone lines and cable TV within the consumer unit.
Transient voltages are short duration surges of electricity that occur due to the sudden release of energy that is previously stored or induced by other means. These overvoltages can be occurring either naturally or can be man-made.
Man-made transient overvoltage occurs during the switching of motors and transformers. New technologies such as electric vehicles, air/ ground source heat pumps and speed-controlled washing machines are showing more transients even in domestic installations. A natural transient overvoltage occurs due to indirect lightning strikes. The electrical installation/equipment associated with it can be damaged due to a direct lightning strike on an adjacent overhead power transmission line or a telephone line, leading to the transient overvoltage to travel along the transmission lines.
The function of a surge protective device is to protect electrical equipment from the damaging effects of electrical surges. The rating of SPD decides the amount of protection provided by the SPD. An SPD protects an electric circuit by limiting the voltage applied to the protected circuit during the occurrence of a surge. The SPD channels the excess energy (obtained from the surge) away from the protected circuit into the ground. A metal oxide varistor (MOV) is a device in SPD that provides a path or excess energy. There are two main characteristics of a MOV which make them most desirable for surge protection.
- The resistance of a MOV decreases with an increased voltage
- MOVs are fast-acting varistors, that can respond to surge in a few nanoseconds, which help in suppressing the surge before it damages the equipment.
The voltage limiting capability of a MOV can be measured by clamping voltage. When a surge occurs, the MOV limits the excess voltage to the clamping voltage level by diverting the surplus energy to the ground. The peak current rating or impulse current rating is the maximum current dissipated from one surge without failing the operation of an SPD.
A MOV provides a high resistance path under normal conditions. The resistance path limits the currents flowing through the MOV and allows the current to flow through. The load in all electrical devices is connected to power sources in all residential applications. The clamping voltage rating of a MOV is greater than the standard supply voltage. When a surge happens, the clamping voltage exceeds, the MOV switches from a high resistance path to a low resistance path. The excess energy generated passes through the MOV to the ground bypassing the connected load. Along with MOVs, SPDs also have inductors or other filter elements to reduce the effects of the surge and incorporate other protective devices to ensure the safe operation of SPDs.
There are three different categories of Surge Protection Devices:
Type 1 SPD installed at the main distribution board i.e. origin. In this type of arrangement, an SPD is mounted between the service transformer’s secondary terminal and its main over-current protection device called a surge arrester.
Type 1 SPD is characterized by a 10/350 µs current wave.
Type 2 SPD installed at sub-distribution boards. In this type of arrangement, the SPD is mounted at the load side of the service’s main over-current protection device that is known as a Transient Voltage Surge Suppressor (TVSS)
Type 2 SPD is characterized by an 8/20 µs current wave.
Type 3 SPD installed close to the protected load. This type of arrangement is a point of SPD, which includes MOVs and assemblies containing MOVs.
Type 3 SPD is characterized by a combination of voltage waves (1.2/50 μs) and current waves (8/20 μs).
A combination of Type 1 and Type 2 SPDs are also used and usually installed in consumer units. Type 3 SPDs must be used only as a supplement to Type 2 SPD.
The Requirements and Testing of any Surge protection device is as per IEC 6163-1 is to protect the equipment from transient overvoltage until the risk assessment is completed. There are many consequences by the transient overvoltage that include:
- Serious injury or loss of human life.
- Interruption of public services and damage to cultural heritage.
- Interruption of industrial or commercial activity.
- A large number of co-located individuals will be affected.
Before purchasing a surge protection device, one must analyze the safety risk factors and the cost evaluation, as they are expensive than the cost of electrical installation and equipment connected to the SPDs. Appropriate physical space is mandatory for installing it. In case not, an external enclosure adjacent to the existing unit works.
Electrical Appliances Requiring Surge Current Protection
Whether an electrical appliance needs a surge protector or not depends on the devices plugged in. Surge protectors help to extend the lifespan of electric and electronic devices whether it is at home, office or industrial premises. Low-cost, easily replaceable devices do not need surge protection, such as light bulbs or fans. Because surge protection may be more expensive than the equipment itself, in such cases! However, when it is expensive and power-sensitive equipment installed on the premises, it needs to be protected from permanent damage, component failure, and frequent wear and tear. Laptops, telecom equipment, medical electronics instruments, power distribution panels are examples of voltage-sensitive devices, which could be easily damaged with a power surge and cause a supply breakdown. Repeated power surges would cause slow damage to devices and shorten their lifespan, which is not easy to detect. A power surge could shorten the life of a computer or even wipe out all of the data (Anuradha, 2021).
Common Surge Current Suppression Techniques
According to Anuradha (2021), surge protection works by setting up a preventive safety mechanism against sudden electrical excesses, such as a power surge or transient voltage. The increase in voltage may be significantly above the designated voltage ratings of equipment installed in the premises. The surge protection device is first connected to the supply line, only then the electricity is allowed to enter the actual equipment. With the correct voltage, current flows through as normal into the equipment. But with a spike or surge, the surge protection device triggers immediately and redirects the excess. Depending on the equipment under protection, different methods of surge protection are used, ranging from surge suppressor resistors toEMI filter circuits. Some of the common surge protection products and methods are listed below.
- Surge Protection / Pulse-Withstanding Resistors
- Voltage cut-off Devices
- EMI filter circuits
- Lightning Guards
Surge Protection Circuit
- Surge Protection / Pulse-Withstanding Resistors: Most commonly deployed solution. Power resistors are used to dissipate excess energy from high power circuits during transient or overload conditions. Once voltage rises above the expected level, the surge protector comes into play. It suppresses the excess voltage, diverts it safely to the ground and prevents it from causing any harm. When used together with air terminals, grounding rods, these surge arrestors form an industry’s total surge protection system. These arresters are best installed directly before the device to be protected. This can be in a socket or trailing socket (on extension lead) but also in the terminal or junction box of the device itself (Anuradha, 2021).
When surge suppressing resistors are applied to a 3-phase supply:
- The surge resistor between the 2 AC lines is known as a differential mode surge suppressor.
- The surge resistors between one AC line and the ground are known as a common mode surge suppressor.
- Voltage cut-off Devices: Advanced voltage sensor circuits that cut-off power when a surge occurs. Sensors are used to constantly gauge the supply voltage. Controllers and latching circuits can trigger a switch-off as soon a voltage threshold is reached. However, this solution may be complex and expensive (Anuradha, 2021).
- EMI filter circuits: To protect against permanent interference such as “ripples” or “noise” caused by other systems, additional filter circuits are used for the voltage supplies to devices (Anuradha, 2021).
- Lightning Guards: Plug into a mains socket to moderate power from the grid, also into a telephone jack in order to monitor and control power into tele-communication lines and equipment (Anuradha, 2021).
IEC Standards for Surge Current Protection
IEC 62305 is the overall guideline for all applications when dealing with lightning and surge protection. This standard covers all the parameters: risk analysis, external and internal, lightning protection. In most cases, a surge protection device in between AC lines is enough to pass the IEC standard. Occasionally, a surge protection circuit is needed across line and ground also. This is especially at higher surge voltage requirements (> 4kV).
Other related standards for surge protection:
- IEEE 142 Grounding of Industrial and commercial power systems.
- IEEE1100 Recommended Practice for powering and grounding electronic equipment.
- IEEE C62.41.2 Recommended Practice on characterization of Surges in Low-Voltage (1000 V and less) AC Power Circuits.
How Power Resistors Help in Preventing Surge Current
- Pulse-withstanding Resistors: Pulse-withstanding resistorsare used to dissipate excess energy from high power circuits. During transient or overload conditions, these high energy resistors safeguard the equipment from energy surges (up to thousands of Joules) for short durations. Carbon composition resistors, wire wound resistors or thick film resistors are commonly used due to their surge handling capabilities. Materials with low inductance are more suitable. The application must also be able to tolerate a wide range of resistance shifts due to temperature, humidity, or even drift over time (Anuradha, 2021).
- Stator Series Resistors: Stator resistors are used to gradually increase the voltage supplied to the induction motor. Induction motors comprise two main components – a static stator and a rotating rotor. During start-up, since the rotor is initially stationery, the rotating magnetic field from the stator cuts the rotor bars at a very high rate. This causes very high induced EMF and current during start. Stator coils draw huge amounts of current when the motor starts, which could potentially damage the stator. This will trigger a large voltage drop which could affect other devices in the line. As the speed of the rotor increases, the rate at which the stator magnetic field cuts the rotor bars decreases. Stator resistors are connected in series with the supply voltage to the stator. The resistance is gradually decreased and disconnected when the rotor speed reaches an optimal speed. Thus, the stator series resistor prevents excess stator current and high voltage drop in circuits involving induction motors (Anuradha, 2021).
Choosing the Right Surge Current Suppression Resistor
The key factors to assess before choosing a suppression resistor:
- Surge Level (Peak Voltage)
- Peak Current
- Time (Usually it will be in a short duration up to few seconds)
Industry Applications of Surge Current Suppression Resistors
Power & Energy
- Power Generation and Distribution units
- Renewable industry
- Control Panels
- Diesel Generator (DG) sets at incomer
- Power Supplies
Automation and Drives
- Motor Control Unit
- At incoming supply of the control panels
Automotive and Transit
- EV Vehicles chargers and charging stations.
- Protection circuits for battery management.
- Converter Panels for Traction application
- Power Distribution and control boards (Control Cards/PCB).
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