Effective surge protection is not just simply installed. It has to be individually coordinated – to the system that is to be protected and the ambient conditions that are prevalent on site. For this reason, the design and concept must be consistent. This means many details must be taken into account, from considering the standards and stipulations right through to classification according to lightning protection zone.

Lightning and surge protection standards

National and international standards provide a guide to establishing a lightning and surge protection concept as well as the design of the individual protective devices.

Lightning protection according to IEC 62305:

Part 1: Characteristics of lightning strikes

In Part 1 of this standard [1], the characteristic properties of lightning strikes, the likelihood of occurrence, and the potential for hazard are taken into account.

Part 2: Risk analysis

The risk analysis according to Part 2 of this standard [2] describes a process with which, first of all, the need for lightning protection for a physical system is analyzed. Various sources of damage, e.g., a direct lightning strike in the building, come into focus, as do the types of damage resulting from this:

• Impact on health or loss of life

• Loss of technical services for the public

• Loss of irreplaceable objects of cultural significance

• Financial losses

The financial benefits are determined as follows: how does the annual total cost for a lightning protection system compare to the costs of potential damage without a protection system? The cost evaluation is based on the outgoings for the planning, assembly, and maintenance of the lightning protection system.

Parts 3 and 4: Planning aids and specifications

If the risk assessment determines that lightning protection is required and cost-effective, then the type and scope of the specific measures for protection can be planned based on Parts 3 [3] and 4 [4] of this standard. The lightning protection level determined by risk management is decisive for determining the type and scope of the measures.

For physical structures that require an extremely high level of safety, almost all strikes must be captured and conducted away safely. For systems where a higher residual risk is acceptable, strikes with lower amplitudes are not captured.

Surge protection according to IEC 60364-4-44

This standard [5] describes the conditions in which surge protective devices are to be used in low-voltage systems to protect the electrical installation against surge voltages. The area of application is thereby limited to surge voltages caused by atmospheric influences or as a consequence of switching procedures that are transmitted by the power supply system. Direct lightning strikes in a structural system are not considered, only strikes in or in the vicinity of supply lines.

Likewise, structural systems with an explosion risk as well as structural applications that could cause damage to the environment (e.g., petrochemical systems or nuclear power plants) are not included in the application of the standard. For these processes, lightning strike standard IEC 62305 is to be used exclusively.

Surge protective devices should be used if transient overvoltage could have effects on the following:

• Human lives, e.g., safety systems, hospitals

• Public and cultural institutions, e.g., loss of public services, IT centers, museums

• Industrial or business activities, e.g., hotels, banks, production systems, farms.

Basic protective measures and equipment

In order to consistently protect a structural system from lightning strikes and surge voltages, various protective measures or equipment that are tailored to one another are required. A broad division can be made as follows:

• External lightning protection

• Internal lightning protection

• Grounding and equipotential bonding

• Coordinated SPD system

External lightning protection

External lightning protection (Fig. 15) aims to divert strikes which come near to the object to be protected and to transmit the lightning current from the point where it hits to ground. As such, no damage can be caused by means of thermal, magnetic or electrical effects. External lightning protection is systematic: it consists of the air-terminal, the arresters, and the grounding system.

Internal lightning protection

The internal lightning protection system should prevent dangerous spark formation inside the system. Sparks can be caused by lightning current in the external lightning protection system or in other conductive parts of the structural system.

The internal lightning protection system consists of equipotential bonding and the electrical insulation of external lightning protection systems.

Lightning protection equipotential bonding is a combination of measures that prevent potential differences. They mainly connect the lightning protection system to metal installations, internal systems, as well as electrical and electronic systems within the system. This occurs by means of equipotential bonding lines, surge protective devices or isolating spark gaps.

Grounding and equipotential bonding

The grounding system aims to distribute and discharge the captured lightning current to ground. Here, the type of grounding system is more important than the grounding resistance. The lightning current is a very short pulse that behaves like a high-frequency current. Effective equipotential bonding is also important. Equipotential bonding connects all electrically conductive parts with each other via conductors – active conductors are protected by surge protective devices. By doing so, it protects against all types of couplings.

Coordinated SPD system

A coordinated SPD system is understood to be a multi-level system of surge protective devices that are coordinated with each other.

The following steps are recommended in order to achieve a high-performance SPD system.

• Divide the structural system into lightning protection zones

• Incorporate all lines that cross between different zones into the local equipotential bonding using suitable SPDs

• Coordinate different types of SPDs: the devices must address each other selectively in order to prevent individual components from overloading

• Use short supply lines for the parallel connection of SPDs between active conductors and the equipotential bonding

• Lay protected and unprotected lines separately

• Only ground equipment via the respective SPD (recommended)

Lightning protection zones

Deciding where to install surge protective devices within a structural system is based on the lightning protection zone concept explained in Part 4 of the lightning protection standard IEC 62305 [4].

It divides structural systems into lightning protection zones (LPZ), and does so from outside to inside with decreasing lightning protection levels. In external zones only resistant equipment can be used. However, in internal zones, sensitive equipment can also be used. The individual zones are characterized and named as follows:

LPZ 0A

Unprotected area outside a building in which direct lightning strikes is a possibility. Direct coupling of lightning currents in lines, unattenuated magnetic field of the lightning strike

LPZ 0B

Area outside the building that is protected from direct lightning strikes by means of an air-terminal. Unattenuated magnetic field of the lightning strike only induced surge currents on lines.

LPZ 1

Area inside the building which may still be subjected to high-energy surge voltages or surge currents and strong electromagnetic fields

LPZ 2

Area inside a building which may still be subjected to surge voltages or surge currents and electromagnetic fields that have already been significantly weakened.

LPZ 3

Area inside the building which may only be subjected to extremely low or hardly any surge voltages or surge currents and very weak or non-existent electromagnetic fields.

All lines that cross between zones must use coordinated surge protective devices. Their power values are based on the protection class to be achieved, which is determined according to legal specifications or by means of the risk analysis. When it comes to selecting surge protective devices, use the standard as a basis, assuming that 50% of the lightning current will be discharged to ground. The other 50% of the lightning current is directed to the electrical installation via the main equipotential bonding and from there must be conducted away from the SPD system.