11.4 Alarm, Emergency Lighting, Data & Control Circuits
Key Takeaways
BS 5839-1 fire alarm systems use detectors, manual call points, sounders and a control panel wired in fire-resistant cable; conventional systems report by zone and addressable systems by individual device.
Emergency lighting is maintained (always on), non-maintained (on only when the normal supply fails) or combined, and self-contained luminaires take an unswitched feed from the local lighting circuit.
Intruder alarm and data circuits are Band I extra-low voltage circuits that must be segregated from Band II power circuits.
A copper Ethernet channel is limited to 100 m, and data cables must keep their pairs twisted to the termination and respect the bend radius.
In a direct-on-line starter, the retaining contact in parallel with the start button keeps the coil energised, while the stop button and overload contact in series give no-volt and overload protection.
11.4 Alarm, Emergency Lighting, Data & Control Circuits
Unit 203 learning outcome 3 (15 of the 40 test questions) asks you to describe the principles of operation of different circuit types. Alongside lighting, power, heating, ring final and radial circuits, the range lists alarm and emergency systems, data communications and control circuits.
Fire Detection and Fire Alarm Systems
In non-domestic buildings, fire alarm systems are designed and installed to BS 5839-1, which defines categories of system: L (life protection, L1 to L5), P (property protection, P1 and P2) and M (manual systems with call points only).
| Component | Function |
|---|---|
| Control and indicating equipment (the panel) | Monitors the system, shows fire and fault conditions, and holds the standby batteries |
| Automatic detectors | Optical smoke detectors; heat detectors (fixed temperature or rate-of-rise) where smoke detectors would cause false alarms, such as kitchens; multi-sensor detectors |
| Manual call points | "Break glass" points on escape routes and exits so people can raise the alarm |
| Sounders and visual alarm devices | Bells, electronic sounders and flashing beacons to warn occupants |
| Interfaces | Release door holders, shut down ventilation plant, and send signals to an alarm receiving centre |
- Conventional systems wire detectors and call points in radial circuits, one per zone, so the panel shows which zone is in alarm.
- Addressable systems connect devices on a loop, each with its own address, so the panel shows the exact device.
The wiring uses fire-resistant cable (standard or enhanced, Section 10.2), kept separate from other circuits. The mains supply is taken from a dedicated circuit through a double-pole isolating device, labelled so that it is not switched off by mistake (for example "FIRE ALARM: DO NOT SWITCH OFF").
In dwellings, BS 5839-6 applies. Approved Document B recommends at least a Grade D2, Category LD3 system in a new house: mains-powered smoke alarms with a standby supply, interconnected, in the circulation spaces (Section 9.1).
Intruder (Security) Alarms
A typical intruder alarm has a control panel with a rechargeable standby battery, passive infrared (PIR) movement detectors, magnetic reed contacts on doors and windows, a keypad, and an external sounder and strobe with its own battery. Anti-tamper circuits are monitored at all times. The detection wiring runs at extra-low voltage (typically 12 V d.c.), so it is a Band I circuit that must be segregated from Band II power cables. The panel's mains supply is usually an unswitched fused connection unit with a low-rated fuse, as the manufacturer specifies.
Emergency Lighting
Emergency escape lighting is designed to BS 5266-1 so that people can leave safely if the normal lighting fails.
| Type | Operation | Typical use |
|---|---|---|
| Maintained | The lamps are on all the time, and stay on from the battery when the supply fails | Exit signs, cinemas, theatres and other places of entertainment |
| Non-maintained | The lamps light only when the normal supply fails | Corridors, stairways and offices |
| Combined (sustained) | Contains two or more lamps, at least one powered by the emergency supply | Signs and luminaires needing both normal and emergency operation |
- Self-contained luminaires have their own battery and charger. Each one is fed from an unswitched line from the local lighting circuit, so that it lights if that lighting circuit fails.
- Central battery systems supply many luminaires from one battery bank through fire-resistant cable.
- A rated duration of 3 hours is common. Systems are given a short monthly functional test and an annual full-duration test, with results logged.
Data Communication Circuits
Modern buildings carry structured cabling for computer networks, telephones, CCTV and building controls:
- Copper twisted-pair cables, Category 5e, 6 or 6A, unscreened (U/UTP) or screened (F/UTP), terminated on RJ45 outlets and patch panels.
- Fibre-optic cables for backbone links between floors and buildings and for long distances.
- An Ethernet channel over copper is limited to 100 m (the cable run plus patch leads).
- Power over Ethernet (PoE) sends power along data cables to devices such as cameras and wireless access points; Amendment 4 added a new Section 716 to BS 7671 for PoE installations.
Good practice: segregate data cables from power cables (separate trunking compartments or adequate spacing), follow the manufacturer's minimum bend radius, avoid kinks and over-tightened cable ties, keep the pairs twisted right up to the termination, and test and label every link.
Control Circuits
Control circuits use small currents to switch larger loads safely.
Contactors and relays
A relay or contactor has a coil that, when energised, closes (or opens) a set of contacts. A low-current control circuit (often at 230 V, 110 V or 24 V) can therefore switch a heavy three-phase motor or heating load.
The direct-on-line (DOL) starter
- Pressing the start button (normally open) energises the contactor coil, which closes the main contacts and an auxiliary retaining (hold-in) contact wired in parallel with the start button. The coil stays energised when the start button is released.
- Pressing the stop button (normally closed, in series) breaks the coil circuit and the contactor drops out.
- The overload relay contact (normally closed, in series) opens if the motor draws too much current for too long.
- No-volt protection: if the supply fails, the contactor drops out and the motor will not restart by itself when the supply returns; someone must press start again.
- Emergency stop buttons (red mushroom heads on a yellow background, latching until reset) are wired in series with the stop circuit.
Other control circuits
Time switches, photocells and PIR occupancy sensors control lighting; heating systems use programmers, room thermostats, cylinder thermostats and motorised zone valves, linked at a wiring centre; and interlocks prevent machinery running while guards or doors are open.
Tip
In the test, remember stop in series, start in parallel with the retaining contact, and that no-volt protection stops a motor restarting unexpectedly after a power cut.
Which type of emergency luminaire lights only when the normal lighting supply fails?
Maintained
Non-maintained
Combined (sustained)
Dimmed decorative
In a direct-on-line starter control circuit, what is the purpose of the auxiliary (retaining) contact wired in parallel with the start button?
To keep the contactor coil energised after the start button is released
To trip the motor automatically if it draws too much current for too long
To reverse the direction of rotation when the start button is pressed again
To reduce the motor's starting current
What is the maximum channel length normally allowed for a copper twisted-pair Ethernet link?
10 m
50 m
300 m
100 m
Which cable is appropriate for wiring between a fire alarm control panel, its detectors and its sounders?
Standard flat twin-and-earth cable
Category 6 data cable
Fire-resistant cable such as FP200 Gold or MICC
Arctic-grade PVC flexible cable
Sections you finish are checked off in the contents.