Floodlight Mast with a Lightning Rod: Combining Lighting and Protection

What a mast with an air terminal means

It is a mast where an air terminal stands above the floodlight platform and the current runs down through a down-conductor to the earthing. In PMT and PMV floodlight masts the lightning rod is part of the structure, together with the service platform and the stairs with a guard rail. For PMZh we make the steel fittings: the floodlight platform, the ladder and the air terminal, and we do not supply the reinforced concrete.

Read the height carefully. In the PMS, PMZh and PMS-B designations the number is the height to the floodlight platform, and the overall height is greater by the length of the lightning rod.

The table shows what the protection on such a mast consists of.

The protected zone and the class of the system are set by an engineer’s calculation under the standard, not by the mast itself.

Terminal height and the protected zone

How much area a terminal covers depends on its height above the protected area and on the class of the lightning protection system (I–IV; I is the highest level). Under Table 2 of DSTU EN 62305-3 the largest rolling sphere radius is 20, 30, 45 and 60 m for classes I–IV. The higher the protection level, the smaller the radius and the closer the masts stand.

A lighting mast stands where the lighting calculation needs it. Whether its terminal covers the required area is shown by the protection zone calculation. For a large area several masts are usually installed.

The three calculation methods are described in Lightning Rod Protection Zone Calculation, and a quick estimate of the radius by the sphere method comes from the calculator.

The shaft as a down-conductor and section joints

There may be no separate wire along the mast: DSTU EN 62305-3, cl. 5.3.5 a, lets a metal structure serve as a down-conductor if its parts are reliably connected (cl. 5.5.3) and the dimensions are not below Table 6. For hot-dip galvanised steel that is 50 mm² (strip, solid round or stranded conductor; the exact tube wall section is set by the design). An insulating coating does not interfere (note 1).

The clause is written for parts of structures, so for a mast it is read by analogy. The tube cross-section calculation and the comparison with the standard are in Steel Mast as an Air Terminal and Down-Conductor.

The practical conclusion is simple: the weak spot is not the tube but the section joints. On our lightning rods the sections are joined by a telescopic joint. Dirt collects on flanged section joints and corrosion starts, bolts included, while a telescopic joint has no such problems. The drawing of the joints and welds still deserves separate attention.

How to run cables to the floodlights when the mast serves as a lightning rod is described in Floodlight Mast 30 m.

Earthing and the test joint

The standard recommends an earthing resistance of up to 10 ohm (cl. 5.4.1). The earth termination is made as type A (external electrodes, at least two) or type B (a ring or foundation electrode; cl. 5.4.2). Electrode lengths and the PUE requirements for lighting poles are covered in Earthing of Metal Poles.

A test joint is fitted on each down-conductor (cl. 5.3.6). The exception is a natural down-conductor connected to a foundation earth electrode. The joint is opened only with a tool: it exists so that earth resistance can be measured without taking the system apart.

After erection, continuity and earthing resistance are measured. A specialist does this work; do not touch down-conductors and connections at height yourself.

Floodlights and the mobile crown

On PMK masts the floodlights are serviced from the ground: a winch lowers the crown with the equipment and then raises it again. No special machinery is needed. The standard PMK range is 20, 25, 30, 35 and 40 m.

The crown moves, while the air terminal and the down-conductor must stay reliably connected. The air terminal is fixed through a flange joint, on PM and PMV masts as well as on PMK.

All mast ranges are on the Floodlight Masts page, and the designations are compared in PMS, PMZh, PMV and PMK.

What to send for the calculation

To propose a structure we need:

• the mast height;
• the number and model of the floodlights;
• whether an air terminal is needed and how tall;
• whether a ladder and guard rail are needed;
• the city, the wind pressure zone and the terrain type.

The POLYGONAL company makes the mast with an air terminal. The system class and the protected zone are calculated by an engineer under the standard, and erection is carried out by a licensed construction partner company. Send the data through the Lightning Rods page.