EN-54-22-LDT

EN54 Part 22: A Guide to the European Draft Standard for Resettable Line-Type Heat Detection

The EN54 family of standards covers many different aspects of fire detection and fire alarm systems.

EN54 part 22 is the European draft standard designed to harmonise the minimum acceptable quality and performance of line-type heat detectors.

There are four main types of line-type heat detector, all of which will be discussed in this article. While these resettable line-type heat detectors operate in significantly divergent ways, they do share common operating characteristics between them, such as the ability to trigger an alarm at a specific temperature which is comparable to that of point-type heat detectors covered by the harmonised standard, EN54 part 5.

The EN54 part 22 draft standard recognises that many resettable line-type heat detectors consist of a controller and detection cable or tube but will be connected to control or indicating equipment which forms part of a fire alarm system.

What is a resettable line-type heat detector?

Cable-based sensing, formally known as line-type detection, increases the possible coverage from a single sensor over and above a ‘spot’ detector. Spot detection consists of a device in a discrete location and linked to a control panel, possibly via other spot detectors, by an inert cable.

The linking cable is a necessity but provides no additional benefits to the sensitivity or performance of the system. Conversely, a cable-based sensor (line-type), inherently senses along its entire length, maximising sensitivity and minimising the need for ancillary equipment.

Other advantages of line-type heat detectors are small size and flexibility, both the ability to be formed to the area or equipment requiring protecting but also the areas and situations which can be covered by a linear detector. The detector may be placed in the immediate vicinity of the identified risk, for example a gearbox or transformer or in a cable tray.

In its simplest form, whilst remaining extremely effective, a linear heat detector consists of a pair of conductors which are mechanically held apart. Once the cable reaches a pre-determined temperature, the conductors come into contact, triggering an alert.

This is known as a digital detector since the two possible states are off (normal) and on (alert). The reliability of such a linear heat detector means it has a huge range of uses, from motors to tunnels, cable trays and even protecting nuclear power stations.

The range of possible activation temperatures, the temperature at which the cable switches from off to on, further expand the range of applications a linear heat detector is suited to, including an abnormal overheat situation, which may be at a much lower temperature than that required to ignite a fire.

One consideration with this type of detector is once it has activated – that is an overheat or fire has occurred – the process of the two conductors coming into contact is irreversible. In this sense the detector is known as non-resettable. However, it should be noted that only the section which has been subject to the activation temperature need be replaced.

For example, on a 1000m installation, if three metres is subject to an overheat condition, only that three-metre section requires replacement. In many applications this apparent inconvenience is mitigated by the early warning the sensor provides but more importantly remembering that in a fire incident, much more substantial damage has likely occurred to the surroundings in any case.

A resettable line-type heat detector therefore, is one which does not require replacing or repairing after an alert has been triggered. It can either be reset manually or is ‘self-restoring’, requiring no intervention to continue operating. Clearly these types of detectors work in a different manner to the ‘digital’ line-type heat detectors.

analogue lhd cable

Analogue LHD cable

There are four main types of resettable line-type heat detectors. A multi-point heat detector (although this should be considered a hybrid between a spot and line-type detector), an analogue line-type heat detector, a pneumatic line-type heat detector and fibre optic line-type detection.

Multi-point heat detectors consist of discrete spot-type heat detectors embedded within a cable and spaced apart at regular intervals. As such, while the detection is part of the cable and can be installed in one continuous length, it is somewhat of a misnomer to refer to it as a line-type heat detector since the detection is not continuous along the entire length of cable.

Typical spacings between the detection points may be 2m, 3m or 5m with the compromise being that closer spacings increase the overall cost of the detector. Nevertheless, this type of detector is very flexible in its configuration as each detection point may be set to alarm at a different temperature and providing the spacing between detection points is regular, also allows the distance along the cable to the incident to be calculated.

An analogue line-type heat detector uses temperature-dependant properties of polymers and conductors within the cable to determine the current temperature in the surroundings. A controller interprets the feedback from the cable and can provide, for example, alerts on abnormal rapid changes in temperature, such as a sudden drop in temperature, or can be programmed to detect a variety of temperatures, even providing multiple alerts as the temperature increases, giving an early warning of potentially damaging situations.

Because these temperature-dependant properties, inherent to the specially chosen polymers and conductors, are not only predictable but repeatable, the system is self-restoring after an incident. Furthermore, it is the controller which determines whether or not to trigger an alarm allowing the alarm temperature to be programmable by the end user.

Pneumatic line-type heat detection triggers an alarm upon a pressure change within a sealed tube. For example, as the pressure within the tube increases due to the temperature around the tube increasing, this may operate a pressure switch at one end of the tube, triggering an alarm.

These types of linear heat detector have a natural immunity to radiated or conducted electromagnetic interference which could affect other types of detector and are suited to particularly harsh environments. They can also self-monitor their operation through the use of a test pump which, at regular intervals, generates a known pressure, simulating a rise in temperature.

Advanced analogue heat detector

Finally, fibre-optic line-type heat detection measures temperature-dependant properties related to the scattering of light in a fibre-optic cable to determine the temperature surrounding the cable. Similarly to an analogue line-type heat detector the sensing is continuous along the entire cable length but additionally, the temperature profile along the cable may also be attained.

There are some caveats however, such as the more accurate the temperature profile required, the more time required to sample the cable. Notable advantages are the built-in ability to use the sensing cable in hazardous areas since there is no risk of a spark to cause ignition and the range over which a single detector may operate – tens of kilometres.

In relation to the previous two types of line-type heat detector, the control units for a fibre-optic detection systems are typically significantly more costly.

Overview of the standard

Perhaps the most significant change brought by the EN54 part 22 standard, is the size of the heat tunnel required to test the detector. A heat tunnel is used to simulate different types of fire or overheat incidents to which the line-type detector may be subject to.

The performance and more importantly consistency of the cross-sectional temperature of the heat tunnel is paramount to ensure not only like-for-like testing on a particular detector but fair comparison and testing between any two resettable line-type heat detectors.

EN54 part 5, the standard used to evaluate point type heat detectors, detailed a heat tunnel which is now a common sight in many approval houses and manufacturers of such detectors across the world. By comparison, there are few EN54 part 22 heat tunnels in existence. The operation and performance of the heat tunnel is described in Annexes C through G in part 22.

While many of the performance characteristics are similar to those of a part 5 heat tunnel, the test frame to which the detection element of the resettable line-type heat detector is attached is substantially larger, too large to fit in a part 5 heat tunnel.

The frame is conical in shape, intending to expose all parts of the detection element equally to the air-flow in the heat tunnel, although the standard allows for the orientation of the frame to be chosen arbitrarily.

tunnel linear heat

Typical tunnel were linear heat is used

Typically, 10 metres of detection element is wrapped around the frame, spaced equally apart, but there is some provision for the length to be defined by the manufacturer to suit the product operating parameters.

As per point heat detectors in the EN54 part 5 standard, EN54 part 22 uses a classification system to determine the response temperature range, maximum application temperature range and response time for line-type heat detectors. These range from Class A through to G, with class A line-type heat detectors being further separated into two groups, 1 or 2, where group 1 indicates the heat detector is quickest at responding to an overheat or fire incident.

Class A heat detectors trigger an alarm at lower temperatures but correspondingly have lower maximum application temperatures. Class G heat detectors trigger at higher temperatures, approximately 150°C, but must be designed to operate continuously at ambient temperatures up to 115°C.

What this means for Europe

Underwriters Laboratory (UL) is a recognised approval body which tests and certifies products primarily for the US market. There are many UL standards covering a huge range of products including fire alarm systems and components.

UL521, ’Heat detectors for fire protective signalling systems’, first published in 1961 and now in its seventh edition, covers in its scope linear heat detectors. This has led to line-type heat detectors becoming established in the fire detection marketplace across the US and other places where UL is recognised, such as the Middle East.

Europe has had no such harmonised standard for line-type heat detectors, the best being those approval bodies which use EN54 part 5, a point heat detector standard, to certify a line-type heat detector, certainly not ideal.

The introduction of EN54 part 22 will settle the debate on whether line-type heat detectors in Europe are as qualified as point-type heat detectors and allow them to be used in applications without affecting the overall system certification and perhaps more importantly, in situations where point-type detectors simply cannot provide adequate protection.

Future EN54 part 22 technology

Having an established standard in place allows for research and development to be directed and focused and therefore promotes innovation in the sector. Future resettable line-type heat detectors will most likely concentrate on providing rapid and accurate distance location to the incident, self-checking capabilities to reduce maintenance and servicing costs and improving on-site reconfigurability.

Achieving these advances should not come at significant increases in cost either as modern digital signal processing techniques and manufacturing processes can be taken advantage of.

EN54 part 22 provides a platform for line-type heat detection to become a qualified means of detecting overheat and fire incidents for applications throughout Europe. Full harmonised standard status makes the production of a Declaration of Performance to comply with the EU Construction Products Directive a straightforward process and removes any ambiguity in compliance or certification.

This will see line-type heat detection become an established market sector within Europe, in a regulated and controlled manner, vital for the fire protection industry.