For security installations we are only interested in cables employed in low or extra low voltage systems, but in all cases there are still many conditions to fulfil. This is because all cable installations, including those for signalling, must comply with the current Codes of Practice as laid down in the IEE Wiring Regulations 7671: Requirements for Electrical Installations.
In practice, the security installer is committed to using a diverse range of different cable types depending on the application. These are normally specified by the manufacturer to link the system component parts, based on historic knowledge of their performance in the field. They must, however, still be installed to meet the particular site conditions.
More ancillary devices
Although the essential cables used in intruder alarms have changed little over the years, we are seeing a much greater need for the addition of ancillary devices using data and communication wiring networks in both these and other security systems. In particular, the diverse nature of access control involves a great range of wiring forms. And with the expansion of CCTV networks, inroads are being made by unshielded twisted pair wiring for high performance video, control data and power. Furthermore, we are seeing the introduction of many specifications calling for low smoke and fume sheathed data cables to give greater safety in the event of a fire.
Perhaps in the first instance we should overview the cables that are currently used in mainstream security systems before going on to look at some overall wiring installation requirements. In doing so we should note that the signal cables in which we are interested operate at extra low voltage, whereas the electricity supply cables we employ are classed as low voltage (LV) because they function at a higher potential than 50v AC to earth but less than 600v AC. These low voltage cables are used for the mains power to our systems and for security lighting. (See Fig 1 showing the type of cable for each application.) In every system the installed cables must be protected from mechanical damage so they are always best run in areas that are not accessible or they are best concealed. But even in these circumstances they may still need to be supported to retain them in their rigid and secure environ-ment. With more specialist cables such as MICC or trademarked FP200 and Firetec, the manu-facturers will provide guidance on fixing distances. In the absence of such information, however, the following table can be used, as it is the general specification for the spacing of cable supports for PVC insulated cables in inaccessible positions.
It may be referred to with confidence by the installer for security installations as the Inspectorates in the main refer to recommended wiring and fixing techniques used throughout the electrical engineering industry via the IEE Wiring Regulations.
Cable spacing
Spacing distances relate to cable supports by clips or saddles (See Fig 2 "Spacing of cable supports for PVC insulated cables in accessible positions"). But if cables are installed in normally inaccessible positions and are resting on a reasonably smooth horizontal surface then no fixing is needed provided they cannot readily be caused to move from that protected position. In all environments, however, fixing is still to be applied to any vertical runs that are greater in length than 5m.
Having accepted the need for supporting cables to hold them in a secure position and particularly in accessible areas, we also know there are many different and varied cable installation techniques required for the various system networks. Nevertheless, there are certain time-honoured practices that have come to apply throughout our industry: * Cables should follow contours, travel in straight lines, never run diagonally across walls and not be closer than 10.5cm to any fixing point such as the corner or a ceiling to a wall.
* Wiring should not pass close to steam or hot water pipes and when, passed through wooden joists, should be 50mm below the top or above the bottom of the joist.
Although cables used in intruder alarms have changed little over the years, we see a greater need to add ancillary devices
If passed through metalwork, the hole is to be bushed to prevent abrasion, and holes through which cables pass should be made good to the full thickness of the floor, wall or ceiling to prevent the spread of fire.
* Security cables are not to be run in lift-shafts as they do not form part of its installation.
* Cables are always to pass through areas of low fire risk, be in protected areas or be supplemented with mechanical protection.
* Data and communication or signal cabling is always to be kept isolated from mains cabling and heavy current apparatus wiring and only cross it at right angles. If the operating conditions of other building services cables are unknown, it is normal to treat them as LV electricity supply cables which includes both single and three phases systems normally found on consumers' premises.
The close proximity of security systems and data wiring to electricity supply cables has always been a problem: they should never be routed in parallel to each other. In fact, there is a requirement to maintain a spacing of at least 50mm at all points. But if this cannot be achieved at the positions at which they must cross, a non-conducting divider should be inserted between the different runs.
However, if the supply cables are in a separate conduit or containment, the 50mm spacing need not apply, but our system cables are never to share the same containment as the supply cables or to pass through the same holes. Indeed, if high voltage supply cables operating above 600v AC are in the same area as the security wiring, the spacing must be further increased to 150mm.
Extra low voltage
Extra low voltage is truly an advantage. Without doubt the running in of the security system is a complex procedure. The ideal is to conceal the cabling, but if this is not possible, containments such as conduit or trunking should be applied. This not only approves the appearance of the final installation but it ensures that we also uphold the time-honoured electrical engineering need for protection of, perhaps, the most vital and significant part of the network.
Of course, the great advantage of using extra low voltage signalling cables is that we do not need to meet the more onerous installation requirements plus the inspection and test criteria of low voltage mains installations.
Source
Security Installer
