EN 62305 Protection against lightning

EN 62305 Protection against lightning

The EN 62305 standard series specifically cover the protection against lightning to structures, their contents, persons and livestock.

EN 62305 accepts that we now live in the electronic age, making LEMP (Lightning Electromagnetic Impulse) protection for electronic and electrical systems an integral part of the standard through EN 62305-4. LEMP is the term given to the overall electromagnetic effects of lightning, including conducted surges (transient overvoltage and currents) and radiated electromagnetic field effects.

EN 62305 – 4 categorizes the source of damage, type of damage and the type of loss.

Sources of damage

Damage that can be caused by lightning is sub-divided into:

• Damage to a structure (including all incoming electrical overhead and buried lines connected to the structure)

• Damage to a service (service in this instance being part of telecommunication, data, electrical supply, water, gas and fuel distribution networks).

Types of damage

Each source of damage may result in one or more of three types of damage. The possible types of damage are identified as follows:

 D1 Injury of persons or livestock due to step and touch voltages

 D2 Physical damage (fire, explosion, mechanical destruction, chemical release) due to lightning current effects including sparking

 D3 Failure of internal systems due to Lightning Electromagnetic Impulse (LEMP).

Types of loss

The following types of loss may result from damage due to lightning:

 L1 Loss of human life

 L2 Loss of service to the public

 L3 Loss of cultural heritage

 L4 Loss of economic value

LEMP damage is so prevalent that it is identified as one of the specific types (D3) against which protection should be provided and can occur from ALL strike points to the structure or connected services – direct or indirect. This extended approach also takes into account the danger of fire or explosion associated with services connected to the structure, e.g. power, telecoms and other metallic lines.

EN 62305 makes it clear that structural lightning protection must no longer be considered in isolation from transient overvoltage/surge protection and given that lightning from all strikes points, direct or indirect, to the structure or connected services creates a risk from transients, SPDs are a vital means of protection whether structural lightning protection is present or not.

Current and voltage waveforms

EN 62305 takes account of protection measures on metallic service lines (typically power, signal and telecom lines) using transient overvoltage or Surge Protection Devices (SPDs) against both direct lightning strikes as well as the more common indirect lightning strikes and switching transients. Standards such as the EN 61643 series define the characteristics of lightning currents and voltages to enable reliable and repeatable testing of SPDs (as well as lightning protection components). Although these waveforms may differ from actual transients, the standardized forms are based upon years of observation and measurement (and in some cases simulation). In general they provide a fair approximation of the real world transient.

Transient waveforms have a fast rising edge and a longer tail. They are described through their peak value (or magnitude), rise time and their duration (or fall time). The duration is measured as the time taken for the test transient to decay to half its peak value.

Direct strikes

Direct lightning can inject partial lightning currents of the 10/350μs waveform into a system where a structure with a structural Lightning Protection System receives a direct strike (Source S1) or where lightning directly strikes an overhead service line (Source S3).

Indirect strikes

Remote or indirect lightning flashes near the structure (Source S2) or near a connected service to the structure (Source S4) of up to 1km radius away (and hence far more common) are represented by the 8/20μs waveform. Induced surges from direct lightning flashes and switching sources are also represented by this waveform. With a much shorter decay or fall time relative to the 10/350μs waveform, the 8/20μs waveform presents significantly less energy (for an equivalent peak current) but is still devastating enough to damage electrical and electronic equipment.

EN 62305-1 recognizes that failure of internal systems (Damage Type D3) due to Lightning Electromagnetic Impulse (LEMP) is possible from all points of strike to the structure or service – direct or indirect (all Sources: S1, S2, S3 and S4).

Surge Protection Measures (SPM)

EN 62305-4 describes a number of measures to minimize the severity of transient overvoltage caused by lightning and electrical switching.

Key and basic protection measures are:

• Earthing and bonding

• Electromagnetic shielding and line routing

• Coordinated Surge Protective Devices

Further additional protection measures include:

• Extensions to the structural LPS

• Equipment location

• Use of fiber optic cables (protection by isolation)

SPMs also have to operate within and withstand the environment in which they are located considering factors such as temperature, humidity, vibration, voltage and current.

Selection of the most suitable SPM is made using the risk assessment in accordance with EN 62305-2 taking into account both technical and economic factors. For example, it may not be practical or cost effective to implement electromagnetic shielding measures in an existing structure so the use of coordinated SPDs may be more suitable. Ideally SPDs are best incorporated at the project design stage, although they can also be readily installed in existing installations.

To ensure continuous operation of critical systems even in the event of a direct strike, SPDs are essential and these must be suitably deployed, based on the source of surge and its intensity using the Lightning Protection Zones (LPZ) concept within EN 62305-4.

The Lightning Protection Zone (LPZ) concept

Protection against LEMP is based on a concept of the Lightning Protection Zone (LPZ) that divides the structure in question into a number of zones according to the level of threat posed by the LEMP. The general idea is to identify or create zones within the structure where there is less exposure to some or all of the effects of lightning and to coordinate these with the immunity characteristics of the electrical or electronic equipment installed within the zone. Successive zones are characterized by significant reductions in LEMP severity as a result of bonding, shielding or the use of SPDs.

External zones:

• LPZ 0A is the area subject to direct lightning strokes and therefore may have to carry up to the full lightning current. This is typically the roof area of a structure without structural lightning protection. The full electromagnetic field occurs here.

• LPZ 0B is the area not subject to direct lightning strokes and is typically the sidewalls of a structure or a roof with structural lightning protection. However the full electromagnetic field still occurs here and conducted partial or induced lightning currents and switching surges can occur here.

Internal zones:

• LPZ 1 is the internal area that is subject to partial lightning currents. The conducted lightning currents and/or switching surges are reduced compared with the external zones LPZ 0A, LPZ 0B as is the electromagnetic field if suitable shielding measures are employed. This is typically the area where services enter the structure or where the main power switchboard is located.

• LPZ 2 is an internal area that is further located inside the structure where the remnants of lightning impulse currents and/or switching surges are reduced compared with LPZ 1. Similarly the electromagnetic field is further reduced if suitable shielding measures are employed. This is typically a screened room or, for mains power at the sub-distribution board area.