Two wires to save

July 2012 Fire & Safety

Two-wire fire alarm systems are the best choice for most small and medium-sized installations, says Alan Locke of Fike Safety Technology.

When asked to provide a fire alarm system for an apartment block or small commercial premises, most electrical contractors would choose a conventional system, where the detectors are connected, via one or more circuits to a simple control panel and separated circuits are used to connect the sounders and beacons to the panel.

Often, however, there is a better choice – a two-wire (or bi-wire) alarm system. The components for two-wire systems cost a little more than those for conventional systems, but there are big savings to be made on installation costs. This is because two-wire systems use simple inexpensive cabling, and because the detectors, manual call points, sounders and beacons all share the same cable.

As a result, the overall cost of a two-wire installation is almost always lower than that of a conventional installation. That is not the only benefit, however, as a good two-wire system will, as we shall see, also offer valuable extra functionality.

Control panels for two-wire systems available with four or eight zones, and a four-zone control panel would be a good choice, for example, in an apartment block with three apartments, as each apartment could then have its own zone, with the forth zone used for common areas. Similarly, an eight-zone panel would be a good choice for premises with up to seven apartments.

With an arrangement like this, if a detector is actuated, the panel will instantly show in which apartment the actuation has occured. With some systems, the panel will also show whether the actuation originates from an automatic detector or a manual call point, and the best two wire systems go even further by allowing selective response to alarms from different sources.

In our apartment block, for example, an alarm from a manual call point or an automatic detector in the common areas could instantly trigger sounders and beacons throughout the premises, whereas an alarm from an automatic detector in an individual apartment could be arranged to initially trigger only the beacons and sounders in that apartment. If the signal from the apartment is still resent after, say, a minute, however, the alarm would be raised throughout the premises.

This arrangement is useful as an automatic alarm from a signal apartment may only mean that someone has burnt their toast, in which case raising a general alarm is unnecessary and disruptive. It is worth noting that this type of selective delayed operation with two-wire systems complies fully with Part 1 and Part 6 of BS5839.

There is not space in this short article to discuss the many other advantages offered by two-wire systems, but hopefully enough has been said to show that for most fire alarm installations in all but the largest premises, two-wire systems are not only the best choice in terms of operational flexibility, but also the best value for money.

For more information contact Geoff Clarke, Okavango Electronics, + 267 391 2626, sales@okavelectron.com, www.okavelectron.com





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