Lightning Protection

What are typical surge diverters?

A typical surge diverter contains metal oxide varistors (MOVs) or gas discharge tubes (GDTs). Protection between active phases and Null is provided by MOVs. Protection between Null and Earth where the MEN connection is not made, is provided by GDT which is known as common mode protection.

Is it acceptable to run cables in parallel with lightning down-conductors?

When planning equipment installation it is advisable to avoid runs of power and telephone/data cabling parallel to lightning down-conductors. Placing these cables in metal ducts provide excellent shielding and will overcome this problem.

Does down-conductors’ resistance have impact on potential rise?

Although the resistance of down-conductors contributes to potential rise, it is inductance of down-conductors which ultimately determines the potential rise. This potential rise can never be eliminated, only minimised, by utilizing multiple down-conductors.

What is recommendation for communication towers earthing?

Provide an earthing system for communication masts and towers. For microwave towers containing VHF, UHF and microwave antennas located adjacent to communications buildings, connect each leg of the tower to an earth electrode. Bond the tower earthing system to the building earth via a 25×3 mm copper strap.

Antennas, masts and supports for HV antennas located in an antenna farm may be earth via single earth rod.

What are advantages of surge filters compared to surge diverters?

Surge filters have a far lower let-through voltage than surge diverters. Surge filters are connected in series. They employ three-stage protection consisting of surge diverters at the input and output, and a low-pass LC filter in the middle. The low-pass filter not only suppresses surges, but also provides some filtering against harmonic noise.

Surge filters are connected in series. This means the filter can be design to minimise the length of connecting leads, therefore minimising the let-through voltage of the filter. The performance of the filter is not installation dependant. The let-through voltage experienced by connected equipment in practical situations is always the same as that stated in the technical specifications for the filter.

What is the surge current value specified in the standards?

The largest surge current specified in both the British standard BS6651 and the American standard IEEE C62.41, is only 10 kA. This figure has been reached through years of research and field measurements. The largest surge current specified in any lightning protection standard anywhere in the word is only 70 kA (Australian Standard / AS1768).

If the largest current specified in the standard is 70 kA, why 200 kA surge arresters are recommended?

Surge protectors rated at 200 kA are not designed to withstand 200 kA surges, but instead are designed withstand many surges of less than 70 kA without being damaged.

What is SAD?

Silicon avalanche diodes are very reliable surge arresters which can divert surges for many times without being damaged. They are usually used in third stage of filters.

Should surge arresters be protected by RCDs in circuits?

Depending on local utility regulations, RCDs are connected upstream or downstream of surge arresters. If the RCD is placed upstream, it detects the current surge produced by lightning and may trip. A delayed (type S) or reinforced-immunity RCD is recommended. If the RCD is downstream, a standard RCD may be used.

What are the main criteria for lightening protection system air terminals and down-conductors selection?

Based on AS/NZS 1768, air terminals shall be placed on or close to the most vulnerable parts, if a strip conductor is used, it shall be installed directly on the part it is to protect, if the vertical road is used, its length shall be not less than 500 mm, and it shall preferably be mounted on the part it is to protect or within 1 m or ½ its length, whichever is the smaller. The maximum allowable length of the road terminals is 6 m.

Main conductors shall interconnect all air terminals and shall form one or more paths to earth via down-conductors, such that the spacing between the down-conductors does not exceed 20 m. There shall be one earth termination per down conductor. Low earth resistance is desirable and all practical measures should be taken to achieve 10 Ω or less for the whole interconnected LPS (Lightning Protection System) earth termination network.

For any system incorporating two or more down conductors, it should not be necessary to install a total length of more than 50 m of widely separated horizontal or vertical earthing electrodes per down conductor, regardless of the earthing resistance. Depending upon site and operating conditions, it may be possible to obtain overall protection by using one earthing system. Where it is considered that one common earthing system may be adapted to comply with all the requirements, it is necessary to ensure that the value of earth resistance does not exceed 4 Ω.

The earth termination of the earth protective system should be interconnected by a ring conductor. This ring conductor should preferably be buried to a depth of not less than 500 mm and be not less than 2 m from the walls of the structure unless other considerations, such as the need for bonding other objects to it, testing or risks of corrosion, make in desirable to leave it exposed.

All major metal forming part of the structure, including continuous metal reinforcement and services should be bonded together and connected to the LPS. Such connections should be made in at least two places and should, as far as is possible, be equally spaced round the perimeter of the structure at intervals not exceeding 15 m.

As mentioned above and also concluding from other parts of AS/NZS 1768, consider distances below for conductors and down-conductors.

  • Down-conductors in LPS system preferably installed in corners shall not be spaced more than 20 m.
  • Down-conductors/conductors used as air terminals for building sides shall not be spaced more than 10 m.
  • Connecting steel structure to the LPS round the perimeter shall not be spaced more than 15 m.

What are the restrictions on tall components next to the structures in lightning protection point of view?

Based on AS/NZS 1768 structures with explosive or highly-flammable contents should not be equipped with tall components such as spires and flag staffs or radio antennas on the structure or within 15 m of the structure. This clearance applies also to the planning of new trees. But structures near existing trees should be treated in accordance with section 6.3 of AS/NZS 1768 as below.

The following clearance between the structure and the tree may be considered as safe:

  • For normal structures one third of the height of the structure.
  • For structure with explosive or highly flammable contents the hight of the structure

Otherwise the LPS should be considered for trees.

Testing the surge suppressor (varistor)

A good surge suppressor should have a very high resistance 9more than 100 MΩ in either direction). A faulty surge arrestor will be either open circuit (usually showing signs of burning) or short circuit in both directions.

What size of the conductors is recommended for lightning protection system?

In practice the cross sectional area of the normal lightning conductor is determined primary by mechanical and secondary by thermal considerations. The cross section of the conductors have very little effect on the resistance of the earth connection so that the strip or cable size is not important provided it affords reasonable protection against mechanical damage and corrosion, and is of adequate current surge capacity (below values).

Air terminals Down conductors Earthing electrodes and buried conductors Main bonding conductors
Copper strip mm2 25×3 25×3 25×3 25×3
Rod mm (dia) 10 10 12
Stranded conductor mm2 35 35 70
Cable mm2 35

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