How do surge protection devices protect a CCR and its circuit?
A surge protection device does nothing in normal service. When a lightning or switching surge arrives, it conducts, limits the voltage on the CCR and the circuit, and passes the energy to earth.
A CCR feeds a long series circuit across an open airfield, so it is exposed to surges from several directions. Surge protection devices are fitted where those surges enter, so the equipment only sees a voltage it can withstand.
1Where the surges come from
Lightning near the airfield induces a voltage in the circuit cable, even when it is buried, and can raise the potential of the earth around a strike. Switching on the supply side also produces surges. They reach the CCR by three routes: the mains input, the series circuit output and the control and communication lines.
2Where the devices are fitted
One protects the mains input. Another connects each output conductor to earth, so a surge on the circuit is clamped at the CCR. A signal-type device protects the control and communication lines. Further devices may be fitted at other points the design specifies, such as the ends of long circuits.
3Choosing and rating them
Common technologies are metal oxide varistors, which clamp the voltage, gas discharge tubes, which conduct when a set voltage is reached, and diode-type devices for low-energy signal lines. Hybrids combine them. Three ratings matter most: the maximum continuous operating voltage, the voltage protection level it lets through, and the surge current it can pass. The first must be above the highest voltage the CCR can put on the circuit, including an open circuit.
4Fitting them
Leads are kept short and straight, because lead length adds voltage during a fast surge. The earth side goes to the same earthing network as the CCR, and bonding keeps the equipment at one potential. Where devices are fitted in stages, they are chosen to work together so each takes the share it is built for.
5Ageing and failure
Devices wear with the surges they take, and most carry a status indication. One that has failed short, or leaks too much, can look like an earth fault on the circuit. Devices are checked with the CCR, and after severe storms. The output devices also connect the circuit to earth, so they can distort insulation-resistance readings, and a test voltage above their operating voltage can make them conduct; how they are handled during testing follows the test procedure and the manufacturer.
| Location | What it protects | Device | Key checks |
|---|---|---|---|
| CCR mains input | CCR power stage and controller | Mains SPD | Operating voltage above the supply, protection level within the CCR's withstand, surge current rating |
| CCR output | CCR output and circuit insulation | Arrester from each output conductor to earth | Operating voltage above the highest circuit voltage, including open circuit; leakage low enough not to show as an earth fault |
| Control and comms | Controller and its interface | Signal-line SPD | Matches the signal voltage and data rate |
| Earth side | Path for the surge | Earthing network and bonding | Short leads, low resistance, one bonded system |
Frequently Asked Questions
Q1Why does a buried series circuit need surge protection?
The circuit cable runs for long distances across an open airfield, and a nearby lightning strike can induce a high voltage in it even when it is buried. Surges can also arrive through the mains supply and the control lines. The CCR and the circuit insulation are the equipment at risk.
Q2What is the difference between a surge arrester and an SPD?
The two terms overlap. Both are devices that stay quiet in normal service and conduct when a surge arrives, clamping the voltage and passing the energy to earth. Arrester is often used for devices on higher-voltage circuits and outdoor equipment, and SPD for devices on low-voltage supply and signal lines.
Q3Why must the continuous operating voltage rating be above the circuit voltage?
A device that starts to conduct at the normal voltage of the circuit would overheat and fail. Its maximum continuous operating voltage must therefore be above the highest voltage the CCR can put on the circuit, including the higher voltage that can appear if the circuit goes open.
Q4Can a surge protection device cause a fault indication?
Yes. Devices wear with the surges they absorb, and one that has failed short, or leaks too much, looks like a connection to earth. On a series circuit this can appear as an earth fault, so a suspect device is checked as part of earth-fault finding. Because output devices connect the circuit to earth, they can also distort insulation-resistance readings, and a test voltage above their operating voltage can make them conduct. How they are handled during testing follows the test procedure and the manufacturer.
Q5Why must the leads be short and straight?
During a fast surge the lead itself adds voltage in proportion to its length. That added voltage appears across the equipment on top of the voltage the device lets through, so a long or looped lead reduces the protection the device was chosen to give.
Q6Does a surge protection device replace earthing and bonding?
No. The device limits the voltage, but it can only pass the surge energy to a low-impedance earthing network, and bonding keeps the equipment at one potential. The protection works only as well as the earthing behind it.
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