Electrical Surges: Causes, Hazards & Multi-Level Protection

A surge refers to a transient peak in a circuit that exceeds the stable value, also known as a voltage spike or transient, and it encompasses both surge voltage and surge current. Economically speaking, a surge is a violent pulse occurring within just a few millionths of a second. Lightning strikes, short circuits, power switching, and the activation of heavy equipment can all trigger surges.

Surges are ubiquitous within power distribution systems, and their hazards are primarily categorized into two types: catastrophic hazards and degradation hazards. A catastrophic hazard occurs when the surge voltage exceeds the bearing capacity of a piece of equipment, resulting in the complete destruction of the device or a drastic reduction in its lifespan. A degradation hazard refers to the cumulative effect of multiple small surges that degrades the performance of semiconductor components in electronic equipment, causing intermittent device failures, shortening equipment service life, and ultimately leading to production stoppages or decreased productivity.

· Characteristics of Surges

Surges have a short duration, typically lasting from just a few nanoseconds to several milliseconds. When a surge occurs, the amplitude of the voltage and current exceeds normal levels by more than twofold.

· Impact of Surges

Surges are omnipresent. Within power distribution systems, the primary impacts of surges on sensitive electronic equipment manifest as destruction, interference, and premature aging.

· Causes of Surges

The sources of surges in power distribution systems are mainly divided into external (lightning) and internal (equipment startup/shutdown, system faults, etc.).

(1) External Causes:

• Direct Lightning Strikes: Lightning directly striking the power grid instantly generates massive energy, possessing extreme destructive power.
• Induced Lightning: The rapidly changing electromagnetic fields generated by lightning act on conductors, producing high overvoltages that feature a very steep wavefront and decay rapidly.
• Surges Induced by Lightning Transmission: These are transmitted along the line from distant overhead wires. Because the equipment connected to the power network varies in its capability to suppress overvoltage, the energy of the transmitted overvoltage attenuates as the line length increases.
• Oscillatory Surge Overvoltage: A power line is equivalent to an inductor and shares distributed capacitance with the earth and adjacent metallic objects, forming a parallel resonant circuit. In TT and TN power supply systems, at the exact moment a single-phase ground fault occurs, the high-frequency components trigger resonance, generating a very high overvoltage on the line that primarily damages secondary instruments.
  
  

(2) Internal Causes:Internal surges are related to the startup and shutdown of equipment within the power supply system as well as faults in the operations of the power network. Surges originating inside the system are primarily caused by the impact of internal electrical loads, accounting for approximately 80% of all occurrences. The causes of internal overvoltage induced in power systems can be roughly categorized as:

• The switching on and off of large electrical loads, such as air conditioners, compressors, water pumps, or motors.
• The switching on and off of inductive loads.
• The switching on and off of power factor correction capacitors.
• Short-circuit faults.
  
  

· Surge Protection Methods

A Surge Protective Device (SPD) is a highly efficient circuit protector capable of suppressing transient high-voltage interference pulses down to a predetermined voltage, thereby effectively protecting equipment and sensitive components from damage. The common characteristic of different surge protectors is that under normal voltage conditions, they have no impact on circuit operation. However, the moment a high pulse voltage arrives, the impedance of the surge protection component drops rapidly, causing its own current to increase. It swiftly conducts and diverts the current, preventing the surge from damaging other equipment within the loop.

· Coordinated Operation of Multi-Level Surge Protectors

Because the energy of a lightning strike is immense, it must be discharged into the earth stage by stage through a step-by-step dissipation method.

• First-Level Protection (CLASS I): First-level surge protection devices are typically installed at the service entrance of a house or building. They protect all downstream equipment from the entrance wiring against the hazards of surges. Generally, first-level surge protectors feature very large capacities and physical dimensions. This stage of power lightning arresters is typically required to possess a maximum surge capacity of over 100kA per phase, with a clamping voltage restricted to less than 1500V. Known as CLASS I power lightning arresters, they limit surge voltages ranging from tens of thousands to hundreds of thousands of volts down to a range of 2500V to 3000V.
• Second-Level Protection (CLASS II): These should be installed at branch distribution areas that supply power to important or sensitive electrical equipment. The power lightning arresters used at this location require a maximum surge capacity of over 45kA per phase and a clamping voltage of less than 1200V, designated as CLASS II power lightning arresters. The main technical parameters include: a lightning current discharge capacity greater than or equal to 40kA (8/20µs), a residual voltage peak not exceeding 1000V, and a response time of no more than 25ns.
• Third-Level Protection (CLASS III): When acting as the third level of protection at the AC power input terminal of electronic information equipment, the power lightning arrester should be a series-connected, voltage-limiting type. Its lightning current discharge capacity should not be less than 10kA to completely eliminate minute transient overvoltages. Implementing third-level protection is essential for particularly important or highly sensitive electronic equipment, as it simultaneously protects the load equipment from the impacts of transient overvoltages generated internally by the system.
• Fourth-Level Protection: Depending on the voltage withstand level of the protected equipment, if it consists of sensitive electronic devices, a lightning and surge-protective PDU socket or a surge-protective adapter plug can be connected at the equipment input terminal. The lightning current discharge capacity for this fourth level of protection should not be less than 5kA.