Figure 1: SPD is installed in parallel in a network (Source)

Surge Protective Devices (SPDs):

All electronic equipment operates within a specific power range. Excessive voltage can cause permanent damage, making SPDs essential. SPDs cannot prevent voltage surges entirely, but they mitigate their effects by creating a low-resistance path that converts the transient voltage into a current and directs it to the ground. When installed within the consumer unit, SPDs act as a frontline defence by redirecting excess voltage away from vulnerable equipment, thus preventing potential damage. They are designed to sacrificially absorb harmful voltage spikes, prioritizing the protection of the electrical system even in critical situations. It's crucial to ensure that the design is safe so that in rare cases of SPD failure, it won't disrupt processes by triggering circuit breakers, fusing upstream, harming nearby equipment, or endangering people with smoke.

Figure 2: Construction of MOVs (Source)

Metal Oxide Varistors (MOVs):

The MOV is a type of non-ohmic variable resistor and has a non-linear voltage-current characteristic. Unlike ohmic resistors, such as potentiometers, the MOV's resistance changes automatically with voltage. Under normal conditions, when voltage remains within rated limits, the MOV exhibits high resistance, allowing current to flow through the circuit and bypass the MOV. However, during voltage spikes, particularly from the primary power source, the voltage is directly applied across the MOV since it is connected in parallel to the AC mains. This surge causes the MOV's resistance to plummet very low, effectively simulating a short circuit. While MOVs are effective against short surges but unsuitable for prolonged surges.

Composed primarily of zinc oxide and other metal oxides like cobalt and manganese, the MOV is structured with these oxides embedded in ceramic powders between metal electrodes. This setup forms diode junctions between adjacent oxide grains, effectively creating a network of series-linked diodes within the MOV.

Figure 3: Triboelectric charging (Source)

Figure 4: Block diagram of TVS diode operation (Source)

Transient Voltage Suppression Diodes (TVS Diodes):

A Transient Voltage Suppressor (TVS) is an avalanche diode designed to clamp overvoltage and dissipate high transient power surges. It absorbs excess energy when the induced voltage exceeds its avalanche breakdown voltage and then automatically resets after the overvoltage condition passes. The TVS diode is a p-n semiconductor junction that becomes conductive during a transient voltage spike. Under normal conditions, it has high impedance and very low leakage current, effectively acting like an open circuit. When the voltage exceeds its threshold, the avalanche effect in the semiconductor activates, causing the p-n junction to conduct and create a low-impedance path that diverts excessive current away from the protected device.

TVS diodes respond extremely quickly, often in picoseconds, making them effective at diverting intense ESD pulses, even those with fast rise times. Compared to Metal-Oxide Varistors (MOVs), TVS diodes offer several advantages, such as the absence of aging effects, ensuring stability and reliability over time. They are more robust and of higher quality than Zener diodes, rated for low DC and not specified for handling surges.

Figure 5: TVS Diode I/V Curve (Source)

Figure 7: Configuration of GTDs (Source)

Gas Discharge Tubes (GDTs):

These surge protection devices use gas discharge principles to conduct surge currents. Initially, GDTs have a high impedance and small capacitance, functioning as an open circuit. When a surge overvoltage reaches the GDT's pulse breakdown voltage, the electric field strength exceeds the gas's breakdown strength, causing ionization and changing the GDT from an open to a closed state. This allows the surge current to discharge to the ground, protecting subsequent circuit components safely.

GDTs act like voltage-controlled switches. When the overvoltage surpasses the GDT's spark over voltage, a controlled discharge occurs, ionizing the gas and generating an arc. This arc efficiently dissipates the surge energy, maintaining a low arc voltage (approximately 20 to 40V) and preventing further overvoltage buildup. After the surge, the arc extinguishes, and the GDT's internal resistance rapidly increases to over 1000MΩ, restoring its initial high-impedance state.

Figure 8: Operating regimes (Source)

Figure 9: I/V Curve for TSS (Source)

Thyristor Surge Suppressor (TSS):

These overvoltage protection devices based on thyristor principles. They use the breakdown current of a PN structure to trigger conduction and discharge, allowing them to handle large surge pulse currents. When the voltage across a TSS exceeds its breakdown voltage, it short-circuits to maintain low impedance. The TSS reverts to a high-impedance state once the current drops below the holding current. Compared to similarly-sized TVS devices, TSS can tolerate larger surge pulse currents and have lower capacitance, making them suitable for higher surge protection in signal lines.

TSS devices switch from a high-impedance (blocked) state to a low-impedance (conducting) state during transient overloads, protecting circuits against high overload currents and preventing overvoltage damage to circuit terminals. TSS offers several advantages over traditional Gas Discharge Tubes (GDTs), including long-term stability, no aging effects, and turn-on without parasitic oscillation or ringing. They are ideal for telecom network applications such as xDSL, T1/E1, Ethernet, EPON, GPON, and voice band protection in line cards and telephony equipment in infrastructure and residential installations.

Figure 10: Inductive transients in a partially protected LAN (Source)

Applying surge protection to a local area network

Surge protection devices (SPDs) can only be effective when installed correctly. This section provides guidelines for networks where LAN cables run between buildings with structural lightning protection or negligible risk of direct strikes.

• LAN cable routing: Run the cables passing between buildings close to the mains power distribution boards, from which each building's electrical supply earth originates.
• Network SPD installation: Install a network SPD in the LAN cable near the mains power distribution board in each building. Treat all buildings similarly, as the concern is potential differences between their earth grounds.
• SPD earthing: Earth the SPD at the mains power distribution board using the shortest possible cable with a minimum cross-section of 2.5mm². For better performance, use several cables electrically paralleled and spaced apart. Ideally, mount the SPD on earthed metal panels, if available.

Figure 11: Use of SPDs to protect individual items of LAN equipment (Source)