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Conventional Systems

The Conventional Systems section describes the earliest form of monitored fire systems which have formed the platform from which the majority of today’s technology has been developed. Monitoring of field circuits gave systems that were inherently more likely to be maintained in an operational state and improved system ‘Control and Indicating Equipment’ (Control Panels) led to reasonably useful levels of user operability. Development around 'BS5839: Part 1: 1988' led to improved functionality and form the basis of the conventional systems still available today.

· Commonly Zoned detection devices

· Zonal fire and fault indication

· Monitored field circuits

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

· Separate conventional sounder circuits using commonly available sounders

· Control panels with standby battery power supply

Conventional systems utilise simple resistance monitored detection devices which are connected to the Control Panel in radial multi-device circuits installed around each area. This allows the Control Panel to identify in which area a fire or fault incident may have occurred, allowing subsequent closer examination of the device status indication in the area in question to identify the individual trigger device. This 'Search Time' governs the size of physical area that a zone may cover in order to reduce the risk of a fire situation becoming established before being identified.

Circuit monitoring is carried out by installing an End-of-Line (EOL) device at the furthest point of the circuit. This EOL is specifically developed by the Control Panel manufacturer and is unlikely to be compatible with other brands of system. Early systems often used a simple resistor as an EOL device, but the requirements changed with the introduction of 'BS5839: Part 1: 1988'.

Prior to 1988:

1. A short-circuit across the detection zone pair was caused by the operation of a

detector or Manual Call Point and is interpreted as a fire situation at the control panel.

2. Removing a detection head will simply break (open circuit) the detection zone causing

the EOL device to be disconnected from the Control Panel and a fault condition to be

indicated. All devices further down the circuit will no longer be operational.

After 1988:

1. A dedicated load (ie, resistance) across the detection zone pair (often 470 to

680 ohm) is caused by the operation of a detector or Manual Call Point and is

interpreted as a fire situation at the control panel. A Short Circuit is now interpreted as

a fault.

2. Removing a detection head must not cause any Manual Call Points further down the

circuit to be non-operational. It is no longer acceptable to simply break (open circuit)

the detection zone when a detection head is removed.

For a short time after the 1988 update, installers were to ensure that Manual Call Points were installed on the circuit before detection devices to overcome this weakness. In light of this manufacturers developed improved zone monitoring technology. Systems developed to be compliant with this new requirements usually utilise some form of pulse based monitoring. These usually require a dioded detection base where the removal of the detection head causes either the positive or the 0 volt line to be split and a diode to be switched into circuit to repress the EOL monitoring pulse. Early systems usually use a small active EOL module mounted on a PCB, whilst more recent systems have utilised a variety of EOL devices including simple capacitors and diodes. It is important to note that the manufacturer specified EOL device must be used with each control panel and that they are not usually interchangeable between brands and sometime even between panel ranges from the same manufacturer. Likewise, detection bases may be dioded to different parameters depending of the system type and they may not be interchangeable.

The total number of devices allowable per circuit is dependent on both the requirements of 'BS5839: Part 1', equipment standards and on manufacturer specifications: commonly 20 or 32 detection devices per zone.

These systems utilise a radial based circuit ,connecting devices together until reaching the EOL device at the far end. This means that 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’.

Detection circuits are normally installed in 2 core 1.5mm2 cable but may not have been screened to the same standard required by more modern systems. Likewise, prior to BS5839 Part 1 1988 it was not required to use a fire resistant cable for detection circuits. It also is quite common to see older systems that have been installed in unsheathed mineral insulated cable (ie, bare copper pyro), which is rarely suitable for upgrade to modern addressable systems. Even if considering a more current conventional system upgrade on the same cable, the Control Panel manufacturer’s advice should be sought.

This generation of conventional system made use of quite basic fire detection principles based on smoke or heat levels passing simple set-thresholds within the detector processor. This is often referred to as 2-state detection as it does not include the algorithm based analysis found in later Analogue-Addressable technology. However, there are now some Analogue-Conventional detectors available on the market where a degree of decision making is made within the detection head itself.

Conventional Control Panels provide a minimum of two traditionally resistance monitored sounder (or alarm) circuits which would be installed independently of the detection zones. These circuits should be installed in fire resistance cable and of csa between 1.5mm2 and 2.5mm2 depending on alarm current loading and circuit length.

These systems are ideal in smaller buildings and where system programming and flexibility is not required. As building sizes becomes larger, the amount of cable required to install both the detection zones and the sounder circuits (and the associated installation costs) will give a conventional system a very high overall installation cost. However, this must be balanced against the simplicity of installation, and the ease of interfacing a conventional system with many different types of requirement (ie, simple resistance monitoring using volt-free contacts).

The experience of the ‘Installation Contractor’ is not critical although guidance by a suitable experienced commissioning engineer is beneficial.

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 ‘Marketing’ information.