Visual notification: strobes and candela

A student at Willowbank who is deaf has joined the school this year, and her parents have asked how she will know when the fire alarm sounds. The answer, in most of the building, is the strobes: bright white flashes that tell anyone who can't hear the alarm, or can't hear it over noise, that the building is in alarm. Ravi asks Sasha to check whether the strobes in the 1968 wing meet current requirements, since that wing will be connected to the new panel during the summer work. 'A strobe isn't a decoration,' he says. 'It has to be bright enough for the size of the room, mounted where people can see it, and flashing in step with every other strobe in view. And it can't flash so fast that it triggers a seizure.' This lesson covers visual notification appliances: how they are rated, where they go and how they are made to work together.

Why visual signals

Visual signals serve people who are deaf or hard of hearing, and people in places so noisy that audible signals can't be relied on: gyms during games, workshops, mechanical rooms and loading docks. They are also used where an audible signal is deliberately quiet or private, such as in hospitals where staff respond to chimes and strobes. Accessibility laws and building codes have driven their spread since the 1990s, and most new fire alarm systems put visual appliances in all public and common-use spaces, washrooms, assembly areas and many work areas. In sleeping rooms used by people who are deaf, strobes bright enough to wake people, and tactile devices such as bed shakers, are added.

The National Building Code requires visual signal devices in specified spaces, such as assembly areas, and in other places where the ambient noise is high or people with hearing loss are expected, and CAN/ULC-S526 is the product standard for them. CAN/ULC-S524 sets installation, and the provincial accessibility rules may add requirements, such as for suites designed for people with hearing loss. The fire alarm designer checks the edition in force.

Candela and the light from a strobe

A strobe's brightness is rated in candela, the unit of luminous intensity. The rating is the effective intensity of the flash, measured along the strobe's axis; off-axis, the intensity falls, and the listing defines how it falls with angle. Typical ratings are 15, 30, 75, 110, 135 and 185 candela, and many appliances let the installer select the rating with a switch, which also changes the current the appliance draws: a 185 cd setting draws several times the current of 15 cd. The light falls off with the square of the distance, so a person twice as far from a strobe sees a quarter of the illumination.

Strobes flash between once and twice per second, with a very short flash. The upper limit is set because flashing lights at higher rates can trigger seizures in people with photosensitive epilepsy, and multiple unsynchronized strobes in the same view can combine into a higher effective flash rate. That is why every strobe visible from one place must be synchronized, flashing at the same instant, so the combined rate stays within the safe range. Synchronization is done by the panel's notification circuits, by a sync module, or by a sync protocol built into the appliances, and strobes from different manufacturers usually can't be synchronized together.

Sizing strobes for a room

The installation standards give tables of minimum candela for one strobe, or for two or four strobes, mounted on the walls or ceiling of square rooms of given sizes. A single wall strobe at the centre of one wall of a room up to about 6 m square needs only 15 cd; one up to about 21 m square needs 185 cd. Rectangular rooms are treated as squares based on their longer dimension, unless the strobe is on a long wall and the standard's rules allow otherwise. Larger rooms, and rooms where furniture or partitions block the view, use several lower-candela strobes spread around the room rather than one very bright one, so that anyone anywhere can see a flash without it being blinding up close.

# Wall strobe selection for square rooms (one strobe, centre of a wall; values as commonly tabulated)
table = [  # (maximum room side in m, minimum candela)
    (6.1, 15), (8.5, 30), (12.2, 60), (13.4, 75), (16.5, 95), (18.0, 110), (19.2, 135), (20.7, 185),
]
rooms = [  # name, length, width (m)
    ("Office 112", 4.0, 3.5),
    ("Classroom 204", 9.0, 8.0),
    ("Library", 15.0, 12.0),
    ("Gym", 30.0, 20.0),
]
for name, L, W in rooms:
    side = max(L, W)
    cd = next((c for s, c in table if side <= s), None)
    if cd is None:
        print(f"{name:14s} {side:4.1f} m: larger than the table for one strobe -> use several strobes")
    else:
        print(f"{name:14s} {side:4.1f} m: one wall strobe of at least {cd} cd")

Office 112      4.0 m: one wall strobe of at least 15 cd
Classroom 204   9.0 m: one wall strobe of at least 60 cd
Library        15.0 m: one wall strobe of at least 95 cd
Gym            30.0 m: larger than the table for one strobe -> use several strobes

The table values follow the widely used wall-mounted table for one strobe in a square room, converted to metres; designers use the table in the edition of the standard that applies. The classroom needs a 60 cd strobe because it is just over the 30 cd size; a second 30 cd strobe on the opposite wall, if the multi-strobe table allows it, is another option. The gym is too large for one strobe and gets several, as Ravi's design in Lesson 9 showed.

Mounting height and location

Wall-mounted strobes are mounted so that the entire lens is between 2000 and 2400 mm above the floor, under CAN/ULC-S524, high enough to be seen over people and furniture and low enough to light the room below. Where the ceiling is too low for that, the strobe goes as close to the ceiling as allowed, and ceiling-mounted strobes, listed for ceiling mounting and selected from the ceiling tables, are used in rooms with high ceilings. Strobes are placed so that their light is seen from everywhere in the space, including around corners and partitions.

In corridors, strobes are spaced along the length so that no point is too far from one; the standards give a maximum distance from the corridor's ends and between strobes, and corridors wider than a set width are treated as rooms. In washrooms, a strobe is needed where the public or staff use the room. In large open offices with workstation partitions, strobes are placed so that someone seated in a cubicle can see a flash, which may mean more strobes than the open-room table suggests.

Sleeping rooms

In sleeping rooms, a strobe must be bright enough to wake a sleeping person, which takes far more light than alerting someone who is awake. The requirements for visual signals in sleeping rooms of suites designed for people with hearing loss, and for smoke alarms with visual components, follow the code and its referenced standards; one Canadian code text requires at least 175 cd for the visual component in a sleeping room. Tactile appliances such as bed shakers and pillow vibrators are used with them for people who are deaf.

Other visual signals

Not every visual signal is a white strobe. Amber or other coloured strobes may be used for mass notification messages, so people can tell a non-fire emergency from a fire. Textual displays and digital signs can show instructions during an emergency. Annunciators and graphic panels show firefighters the location of alarms, but those are for responders, not occupants. Status lights on detectors are not notification appliances. A technician installing coloured strobes or signs checks that they are part of the approved design and that the white fire strobes remain distinct.

Power and testing

Strobes draw much more current than horns, and a large current surge when they all charge at once. That makes notification circuit voltage drop and power supply sizing critical, which FAT-130 covers in detail. At installation, every strobe's candela setting is set to the design value, recorded and checked, and synchronization is checked by watching all strobes in view flash together. During verification and inspection, each strobe is confirmed to flash, at the correct rate, with no visible lag between strobes in the same view.

# Strobe current on one notification circuit at its candela settings (illustrative currents)
current_ma = {15: 45, 30: 65, 75: 120, 110: 155, 185: 245}   # mA at the circuit's rated voltage, from a data sheet
strobes = [15, 15, 30, 30, 75, 75, 110, 110, 185]
total = sum(current_ma[c] for c in strobes)
print(f"{len(strobes)} strobes, total {total} mA = {total / 1000:.2f} A")
for c in sorted(set(strobes)):
    n = strobes.count(c)
    print(f"  {n} x {c:3d} cd at {current_ma[c]} mA = {n * current_ma[c]} mA")
print("the circuit and power supply must carry this with margin, at the lowest battery voltage")

9 strobes, total 1015 mA = 1.01 A
  2 x  15 cd at 45 mA = 90 mA
  2 x  30 cd at 65 mA = 130 mA
  2 x  75 cd at 120 mA = 240 mA
  2 x 110 cd at 155 mA = 310 mA
  1 x 185 cd at 245 mA = 245 mA
the circuit and power supply must carry this with margin, at the lowest battery voltage

Visual signals in existing buildings

Many existing buildings have strobes installed before the current rules, with low candela ratings, unsynchronized flashes or mounting heights that are now outside the range. The rules for existing systems generally let those strobes stay until the system is altered, but an alteration that affects an area, as at Willowbank, usually brings that area's appliances up to the current requirements. Owners are often surprised by the cost, because brighter strobes also need more power and sometimes new notification circuits and power supplies. A survey that lists every strobe's rating, height and synchronization before the work is priced avoids surprises later, and it is a job Sasha is now often sent to do.

Survey results feed the power design. Replacing twelve 15 cd strobes with 60 cd strobes more than doubles their current, so the survey also records which notification circuit each strobe is on and how loaded each circuit already is. Ravi uses those numbers in FAT-130's voltage drop and battery calculations to decide whether the wing needs a new power supply as well as new appliances.