Surge protection in outdoor lighting: why 10 kV

Where surges come from

A surge is a brief spike of hundreds to thousands of volts on a 230 V supply. Outdoor lighting sees far more of them than a luminaire in an office, for three reasons: the cable runs are long, the columns are metal and they usually stand on the highest, most open point of a site.

The two main sources:

• Thunderstorms. A direct strike is rare, but a strike nearby easily induces several kilovolts in a cable run of a few hundred metres. The longer the cable and the more open the terrain, the higher the spike.
• Switching transients. Switching large motors, compressors, capacitor banks or a whole lighting circuit at once causes spikes from a few hundred volts up to a couple of kilovolts. These occur far more often than lightning and the damage accumulates.

The grid operator adds spikes of its own, for instance when switching over in the medium-voltage network. A luminaire at the far end of a long street lighting circuit sees every one of them pass by.

Why the driver fails first

The LED module itself is robust. The weak point is the driver: the bridge rectifier, the switching MOSFET and the electrolytic capacitors at the input tolerate a spike of a few kilovolts for microseconds at most. One heavy spike kills the driver outright; a series of small ones wears the components down step by step.

Typical signs of surge damage are a luminaire that stays dark after a stormy night, a driver that fails in summer while the fuse is still intact, flicker at switch-on, and several luminaires on the same circuit failing within a few weeks of each other. That last pattern almost always points to a surge.

How the SPD in the luminaire works

An SPD (surge protective device) is a small module between the mains terminal and the driver. It consists of varistors and usually a gas discharge tube. At normal voltage they pass nothing; once the voltage rises above a threshold they become conductive within nanoseconds and divert the spike. Varistors between L and N catch the differential-mode spike, the connection to PE catches the common-mode spike between the supply conductors and earth.

End-of-life behaviour matters. Every diverted spike wears the varistor a little. A good SPD switches the luminaire off when it reaches end of life, so the fault is visible and you replace the module. An SPD that burns out silently lets the next spike straight through to the driver.

A driver without a separate SPD usually has built-in protection of 2 to 4 kV. That is adequate indoors, but not on a column.

4, 6 or 10 kV: which class for which location

The class of an SPD follows from the test to IEC 61000-4-5: a 1.2/50 µs voltage wave with a matching 8/20 µs current wave. Because the test generator has an impedance of 2 ohms, the figures come in pairs: 4 kV/2 kA, 6 kV/3 kA and 10 kV/5 kA.

• 4 kV: indoor lighting in offices, shops and halls on a short, shielded installation. High bays such as ATRIA hang under a roof on a short circuit.
• 6 kV: facade lighting and site lighting close to the building, where the cable is short and a distribution board with a type 2 SPD is nearby.
• 10 kV: street lighting, car parks, industrial estates and sports grounds. Long cable runs, 4 to 6 metre columns in open ground and a distribution board hundreds of metres away.

That is why the street luminaires of VIA are fitted with 10 kV surge protection, rated IP66 and tiltable on a 60 mm column. Floodlights on a column, such as FLUX on a car park or sports pitch, sit in the same conditions; check the specification for the luminaire's class and add an SPD in the column connection box or the distribution board where needed.

Earthing the column and the connection box

An SPD diverts energy, and that energy has to go somewhere. Without a sound earth path only the protection between L and N works. With a class I luminaire on a steel column such as TURRIS, the earth path runs through the PE conductor of the supply cable, the earth braid at the door and the column connection box. Check at installation that these connections are actually made; a loose earth braid is a common mistake.

The column connection boxes of NEXUS come with earth braids and one or two fuses per column. Those fuses protect against short circuit, not against surges: a spike lasting microseconds is long gone before a fuse reacts. The combination that works is a type 2 SPD in the distribution board at the start of the circuit and 10 kV in every luminaire at the end. The energy is then brought down in two stages.

If you work on a class II installation without PE, as in much public lighting, the SPD only protects between L and N. That is still the most common type of spike, so 10 kV is worthwhile there too.

What to do after a surge fault

• Note the date, time and weather. A nearby thunderstorm is easy to look up afterwards and makes the cause plausible.
• Check the whole circuit, not only the luminaire that is dark. A surge usually hits several columns at once.
• Isolate the circuit and measure the insulation resistance of the cable with the luminaires disconnected. Never test at 500 V on a connected driver or SPD.
• Replace a tripped SPD module and check the driver. If the SPD is intact and the driver is dead, the spike was probably too large for the class or came in by another route.
• Check the earth braid and the PE connection in the column connection box before re-energising the circuit.

If a circuit fails at every storm, the protection is not in order. Have the distribution board checked for a type 2 SPD and replace luminaires without 10 kV on the outermost columns first.

Frequently asked questions