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Electrified security fences

OM 2011/04

Author Unit/Section:  Safety Unit

Target Audience:  All Inspectors

Summary

This OM alerts inspectors to the use of electrified fences designed to deter the entry of unauthorised persons into premises, storage compounds and other areas. It provides information on the standards required for the design and installation of the fences.

Introduction

Electrified stock control fences have been in use in this country for many years and generally consist of one or more rigidly mounted strand(s) of wire together with an energiser and control box.

Figure 1

Typical agricultural electric fence

Typical security fence

Military security fence

Typically, all of the fence strands are connected together and the return shock current path is through the ground. Since the output of the fence energiser is pulsed, conventional measurements of voltage and current would be difficult to interpret. As a result, the output is defined as the energy in each pulse measured in joules, where one joule is one watt of power applied for one second. Standards have been developed relating to the maximum acceptable output energy level of the pulses transmitted along the wire, typically 5 joules, 8 joules, or greater than 8 joules.

Recently, electrified fences have been developed which are designed to deter unauthorised persons from entering premises such as car storage compounds, builders merchants, factory yards, unattended electricity substations, etc. The first known of these was installed in the UK in 1992 and the number of installations is expected to increase rapidly as they have, so far, proved to be a very effective deterrent.

Action

Action by inspectors

Inspectors can, if necessary, ask for information to establish whether electrified security fences meet the standards referred to later in this document.

Brief information relating to installations that are not manufactured or installed by one of the supplying companies (see Appendix 1) or their agents , particularly if they do not meet appropriate standards, should be sent to the Product Safety Team and Safety Unit should be informed.

Where installations result in significant risk, either as a result of the energiser exceeding 5 joules, or as a result of the design or installation of the fence creating mantraps, appropriate enforcement action may be necessary. Under such circumstances specialist groups should be consulted at an early stage. When enforcement action is taken Safety Unit should be informed

Background

Design and installation

The fences consist of several horizontally mounted parallel strands of spring steel wire, each approximately 100 mm apart, which can be located on top of, or inside of, an existing or new wall or fence.

Figure 2

Standard post

Cranked post

Each wire runs through plastic insulator bushes mounted on fence posts that are a maximum of 3 m apart. The wires are tensioned so that under normal weather/wind conditions they do not sag and touch, however they can be easily moved by hand, and would sag and short out to each other if used as footholds by a climber.

Figure 3 - How an electrified security system is configured

How an electrified security systemis configured

("Figure 3 is reproduced by permission of Gallagher Security (UK) Ltd")

The insulator bushes are designed to shear off if a climber attempts to scale the fence. However, they would take the weight of a child.

Alternate strands of the fence are connected together and the main shock current return path is intended to be the adjacent fence strand (see Figure 1 ). To receive a shock it may be necessary to touch two or more strands of wire simultaneously, or in some cases, to touch one wire and the ground as with an agricultural stock control type fence. The shock itself is unpleasant but should not cause harm.

Each fence has an energiser and control box. Unlike stock control fences, the security fences are continuously monitored when energised and are designed to activate an alarm system, either on site or elsewhere, to alert someone to the fact that intruders have tried to gain access or cut the wires.

A number of companies supply these types of fence, the components of which will have been selected and manufactured to an acceptable standard. Detailed instructions should be provided covering their installation to ensure their safety.

Risks

The main risks are of an unpleasant electric shock due to contact with the fence, falls and falls from a height after coming into contact with a fence projecting beyond the top of a wall etc. There is also a remote possibility of death due to exhaustion if a person became trapped across or against a fence and could not easily free themselves. In the UK it is known that at least one person has been rendered unconscious due to repeated shocks where the clearances (see Precautions) were infringed. There has also been one reportable accident involving a homemade "fence" energised by direct connection to the mains supply.

There have been deaths involving animals as a direct result of contact with agricultural stock control electric fences.

Precautions

Design and installation standards for electrified security fences at commercial premises should meet the following minimum standards which have been discussed and agreed by the HSE Specialist Inspectors (Electrical Engineering), and Safety Unit:

warning sign

Further references

Electrified Security Fence manufacturers

Current manufacturers are:

There are several suppliers of electrified security fences most of these organisations install fences under license to the above four companies.

Contacts

Safety Unit

Updated 2013-11-27