Sketch and describe the following detectors, explaining their mode of operation and stating where they would be fitted in the machinery spaces:

Sketch: A heat detector consists of a sensing element (a bimetallic strip, a fusible element, or a thermistor/thermocouple) connected to the fire alarm circuit. The detector is mounted on the ceiling/overhead of the protected space, with the sensing element exposed to the air.

Mode of operation: The heat detector responds to a rise in temperature. There are two types:

• Fixed-temperature detector: contains a fusible element (e.g. a eutectic alloy) or a bimetallic strip that operates when the temperature reaches a set value (e.g. 68-72 C), closing the circuit and actuating the alarm.
• Rate-of-rise detector: responds to a rapid rise in temperature (e.g. a rise of more than a set rate per minute), using a pneumatic or electronic element; it detects a fire that produces a rapid temperature increase.

The detector is connected to the fire alarm panel; when it operates, it closes the circuit and raises the alarm for the zone.

Where fitted: Heat detectors are fitted in machinery spaces where smoke detectors would give false alarms (e.g. in the engine room, boiler room, and other spaces with heat/smoke from normal operation). They are placed on the overheads, distributed so that the heat from a fire reaches at least one detector.

Sketch: A smoke detector (ionisation or optical) consists of a sensing chamber with a radioactive source (ionisation) or a light source and photocell (optical), connected to the alarm circuit. It is mounted on the overhead.

Mode of operation:

• Ionisation detector: contains a small radioactive source that ionises the air in the chamber, producing a small current between two electrodes. When smoke particles enter the chamber, they absorb the ions, reducing the current, which is detected and actuates the alarm.
• Optical (photoelectric) detector: contains a light source and a photocell arranged so that the light does not normally fall on the photocell. When smoke enters, it scatters the light onto the photocell, producing a current that actuates the alarm.

The detector is connected to the fire alarm panel; when smoke is detected, the alarm is raised for the zone.

Where fitted: Smoke detectors are fitted in spaces where a fire produces smoke early (e.g. in the accommodation, corridors, and in machinery spaces where appropriate, such as the control room, and in cargo holds via the sampling system). In machinery spaces, smoke detectors may be used in the control room and other areas, while heat detectors are used in the main engine room to avoid false alarms.

Regulations governing the fixing of these detectors:

• The fixing (installation) of fire detectors and the fire alarm system is governed by SOLAS Chapter II-2 (Regulation 7 - Detection and alarm) and the FSS Code (Chapter 9 - Fixed fire detection and fire alarm systems). These require that the detectors be of an approved type, be arranged so that a fire in the protected space is detected, be connected to a control panel that identifies the zone, and be powered from the main and emergency supplies. The spacing, location and the type of detector (heat or smoke) are specified for the different spaces. The detectors must be tested and maintained per the FSS Code and the manufacturer's instructions.