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Hybrid 2-Wire Systems

Hybrid 2-Wire Systems have been developed to simplify the installation of a system compared with conventional equipment and to offer significant cable savings. This has been achieved by integrating the alarm sounders onto the detection zones obviating the need to install the sounder circuit wiring at all. 2-wire fire alarm technology has advanced through three main steps which may be described as:

1. Reverse-Voltage Sounders            2. Increasing-Voltage Sounders            3. Data Controlled Systems.

2-Wire smoke detectors and other devices may or may not be cross compatible with other systems and the manufacturers advice should be sought. Most generations of system are still available and may be identified by a number of terms including bi wire, twin wire, Twinflex, Alarmsense, Savwire, etc. Due to the fact that 2-Wire Systems can often be quite complex (and hence misunderstood) they sometime receive poor press reports but installed correctly they can offer significant savings in overall 'installed-system' costs and give useful technological advantages over conventional systems.

·   Commonly Zoned detection and alarm devices

·   Zonal fire and fault indication

·   Monitored field circuits

·   Limited inter-compatibility of different manufacturers devices between control panels

·   Integrated sounder circuits with detection zones
















Reverse-Voltage Sounders:

With this early type of 2-Wire System the detection devices (including manual call points) utilise largely standard conventional technology, but sounders are modified to sit across the detection circuit in reverse. Hence when the Control Panel wishes to operate the sounders the zone voltage is reversed and the sounders operate. These early 2-wire fire alarms are often very reliable and effective but some types of system suffer from their inability to monitor the circuit during the 'Sounders-On' time making them non-compliant with more recent standards.

Increasing-Voltage Sounders:

Later systems still utilise largely standard conventional detection device technology, but sounders are modified with additional voltage-threshold sensitive circuitry. Thus sounders may connected in a standard (forward) orientation and the control panel increases the zone voltage (e.g. above 27V DC) when it wishes to operate the alarm sounders.

Data Controlled Systems:

Technological advances within 2-Wire Systems utilise full data communications to all devices in a simplified version of addressable system technology. Although these systems appear like a conventional one on the surface it is very different underneath and may easily be misunderstood by the unwary. Usually the control panel will only identify the zone of origin rather than the individual device but it may allow many different types of signal to be identified, ie smoke, heat, manual call point, sounder, beacon, relay etc. Recent developments allow quite complex installations developed around the idea of minimizing unwanted alarms in flats and apartments where unwitting residents are likely to mismanage standard conventional systems.

2-Wire Fire Alarms systems still utilise End-Of-Line circuit monitoring for all radial circuits connected. This EOL is specifically developed by the Control Panel manufacturer and is unlikely to be compatible with other brands of system. Each zone circuit is compliant with the requirement that ‘no more than one area of protection may be lost in the event of a circuit failure’.

The total number of devices allowable per circuit is dependent on both the requirements of BS5839 Part 1 and on manufacturer specifications, commonly 20 or 32 detection devices per zone, which may or may not include sounders.

2-wire circuits are normally installed in 2 core 1.5mm2  cable which should be screened to minimise interference. Older style mineral insulated cable (ie, pyro) are rarely suitable but the system manufacturer’s advice should be sought.

These systems may make use of conventional fire detection principles based on smoke or heat levels passing a set-threshold, often referred to as 2-state detection. They may also include the algorithm based analysis as found in later Analogue-Addressable technology where a degree of decision making is made within the detection head itself.

Ideal in medium size projects where system programming & flexibility is important. Cable saving gives 2-wire systems a very competitive overall installation cost. However, this must be balanced against the simplicity of installation of a conventional system.

The experience of the ‘Installation Contractor’ is not critical although guidance by a suitable experienced commissioning engineer is essential as 2-wire system technology varies a lot.

In assessing the suitability of any type of system for your application, often the practical experience of an established engineer will give a lot more insight into daily practicalities than the manufacturers sales driven ‘Marketing’ information.