Devising a cost-effective fire alarm system for houses of multiple occupancy (HMOs) that combines reliability with minimum disruption from unwanted alarms has, until now, been an impossible challenge. However, as Mark Kendall of Fike explains, recent developments in two-wire alarm systems offer a complete solution.
If there is one common factor that applies to virtually all HMOs, it is that the budget for the fire alarm installation is invariably tight. This often leads to the conclusion that a conventional or mixed system (a system comprising of a mix of conventional and mains powered detectors) is the only choice. The chances of the budget running to a sophisticated addressable fire alarm system are, for all practical purposes, virtually zero.
However, conventional systems offer very limited functionality and, particularly in HMOs, this often means that they produce many unnecessary alarms – for instance from burning toast or cigarette smoke. This is a problem, not only because unnecessary alarms are disruptive, if they occur often enough, but there is also a high probability that all alarms will be assumed as unnecessary, and genuine alarms will be ignored.
Due to the issues experienced with conventional systems, BS5839 Part 6 was devised to cover the installation and maintenance of fire detection and fire alarm systems in dwellings. This document defines the type, operation and coverage required of systems in HMO applications. It is a common misconception that only BS5839-6 applies in HMO applications, in reality Part 6 must be used in conjunction with BS5839 Part 1 (The general code of practise for fire alarm installations). BS5839-6 describes the minimum system level required for HMO’s. It states that the communal areas of the HMO require a system complying to Grade A (in effect a BS5839-1 compliant system) be fitted to cover all common areas as defined in BS5839-1 for level L2. The dwellings themselves are then given coverage specification within BS5839-6 that depends on the age of the building (new or existing) and physical fire protection installed. Generally this allows for mains interlinked devices to be fitted within the dwelling itself and this has resulted in the uptake of what are commonly defined as, “Mixed Systems”.
Mixed systems reduce the effect of unwanted alarms by fitting mains interlinked detectors throughout the flat or apartment. A single heat detector and sounder connected to a conventional fire alarm system are then fitted within the entrance to the apartment. The theory behind this is that the effect of any unwanted alarm is contained within the apartment while a true fire condition will grow to activate the conventional heat detector and evacuate the building. While mixed systems go some way to mitigating the risk of unwanted alarms they do introduce new considerations; a real fire for instance is likely to result in the loss of a large area of the apartment before the main system will activate.
But what if there was a fire alarm system more versatile than a conventional system that offered the same unwanted alarm mitigation as mixed systems and was far less costly for small installations, than an addressable system? In fact, there is – the two-wire or bi-wire system. Components for two-wire fire alarm systems cost only a little more than those for conventional systems, and there are big savings to be made on installation costs. This is because two-wire systems use inexpensive cabling and the detectors, manual call points, sounders and beacons all share the same cable.
This means that the overall cost of a two-wire installation is almost invariably lower than that of a conventional system for the same application. The benefits don’t end with the price; the best two-wire systems also offer extra functionality that is particularly useful in HMOs.
First of all, the control panels for two-wire systems are typically available with four or eight zones. A four-zone panel is a good choice, for example, in an HMO with three apartments, since each apartment can have its own zone, while the fourth zone is allocated to the hallways, stairways and other communal areas. Clearly, an eight-zone panel could be used for a similar property with up to seven apartments.
While it won’t show which individual detector has been operated, this arrangement will show in which apartment the actuation has taken place and, in the best systems, it will also show whether the actuation originates from an automatic detector or a manual call point. In fact, the best two-wire systems can go even further by responding selectively to alarm signals.
For example, in an HMO, one of these sophisticated two-wire systems could be configured so that a fire signal from any manual call point or from an automatic detector in the common areas instantly triggers sounders and beacons throughout the premises, whereas a fire signal from an automatic detector in one of the apartments triggers the sounders and beacons in that apartment only. If, however, the fire signal from the apartment persists for, say, one minute, the alarm is then raised throughout the premises.
This is a huge benefit, as a fire signal from a single apartment may well indicate nothing more than someone has burned their supper or lit up a cigarette. In such cases, raising a general alarm is unnecessary and disruptive. Incidentally, this mode of delayed alarm operation complies fully with BS5839-6 Section 9.1.5 Mixed Systems and also allows the communal areas to, at the same time, comply with BS5839-1.
Another valuable feature of good two-wire alarm systems is their use of dual-function detectors that incorporate both smoke and heat sensors. These are usually factory set to give a balance between the detection functions that, in typical situations, provides a fast response to real fires while minimising the risk of false operation.
However, not all situations are typical, and the best of these dual function detectors allow the installer to adjust the balance between heat and smoke detection to suit the operating environment. Once again, this can be very useful in HMOs, where a reduced sensitivity to smoke – and hence steam – in cooking areas can minimise the risk of false alarms without compromising overall fire detection efficiency.
In summary, the latest two-wire fire alarm systems, such as Fike’s TwinflexPro, offer important benefits for applications in HMOs. They’re versatile, they can respond selectively to alarm signals, and they use detectors that can be fine-tuned to suit their operating environment. As well as all of these benefits, in most cases they actually cost less to install than a basic conventional system.