A picture is worth thousand words

Capacitive Coupling - Illustration

Capacitive Coupling

Capacitive coupling is the transfer of energy within an electrical network or between distant networks by means of displacement current between circuit(s) nodes, induced by the electric field. This coupling can have an intentional or accidental effect.

Direct Coupling Lightning Strikes - Illustration

Direct Coupling Lightning Strikes

In the event of a direct lightning strike say, to a power pylon, part of the discharge current will be conducted to earth, through the pylon, the remainder will be propagated along the cables, in either direction.In this case, for simplicity, we can assume a third in each direction.

This in reality will be subject to the resistance path to earth. We can see that the sensitive equipment is vulnerable.

Ground Potential Rise - Illustration

Earth Voltage Lightning Strikes

When there is a cloud to ground lightning strike, the strike point is raised to an extremely high voltage, due to large currents being conducted through the ground, and cables within 2km of the lightning strike can experience an induced surge, and therefore damage can be caused to equipment within buildings supplied by these .

HEMP - High Altitude Electromagnectic Pulse illustration

High-Altitude Electromagnetic Pulse (HEMP) is a near-instantaneous electromagnetic energy field that is produced in the atmosphere by the power and radiation of a nuclear explosion, and that is damaging to electronic equipment over a very wide area, depending on power of the nuclear device and altitude of the burst.

High-altitude nuclear explosions are the result of nuclear weapons testing. Several such tests were performed at high altitudes by the United States and the Soviet Union between 1958 and 1962.

Inductive Coupling - Illustration

Inductive Coupling

The amount of inductive coupling between two conductors is measured by their mutual inductance. The coupling between two wires can be increased by winding them into coils and placing them close together on a common axis, so the magnetic field of one coil passes through the other coil.

Types of SPD

Type 1 SPD is characterized by a 10/350 μs current wave.

The Type 1 SPD is recommended in the specific case of service-sector and industrial buildings, protected by a lightning protection system or a meshed cage. It protects electrical installations against direct lightning strokes. It can discharge the back-current from lightning spreading from the earth conductor to the network conductors.

Type 2 SPD is characterized by an 8/20 μs current wave.

The Type 2 SPD is the main protection system for all low voltage electrical installations. Installed in each electrical switchboard,it prevents the spread of overvoltages in the electrical installations and protects the loads.

Type 3 SPD is characterized by a combination of voltage waves(1.2/50 μs) and current waves (8/20 μs).

The Type 3 SPDs have a low discharge capacity. They must therefore mandatorily be installed as a supplement to Type 2 SPD and in the vicinity of sensitive loads.

With Lightning Protection System Present 

An example of high rise residential block with LPS (Lightning Protection System) installed.

The specification of the CLASS I SPD will depend on the LPS installed.eg: Level I, II, III, IV

A TYPE 1 / CLASS I SPD is essential for any system that is required to comply with BS EN 62305.

We recommend a co-ordinated approach where further, down stream protection is installed to prevent damage to sensitive equipment connected to consumer units.

With No Lightning Protection System Present

Single Phase - Domestic Installation in High Rise Flats

This example shows a high rise residential block without LPS (Lightning Protection System) installed and the DNO (Distribution Network Operator) Supplied is via underground.

No LPS means that a TYPE 1 / CLASS I SPD is not required, how ever a TYPE 2 / CLASS II SPD should be installed to prevent damage to any electronics within the building from transient overvoltages created by non atmospheric conditions, eg: switching surges etc.

A secondary TYPE 2 / CLASS II SPD should also be considered due to equipment distance from original source of protection and the risk of damage should the incoming SPD sacrifices itself and leave the installation with no protection.

We would always recommend a co-ordinated approach where further, downstream protection is installed to prevent damage to sensitive equipment connected to consumer units.