Fire detectors for the home

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McGuire, J. H.

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~ - l b 8 no. 9

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I

FIE3 DETECTORS FOR THE HOME

J.H. McGuire

Divisfon of B u i l d i n g Research, National Research Council of Canada

FIRE DETECTORS FOR THE HOME by

J-H, McGuire

In

prepared as a response t o these inquiries. The question and answer

format has been chosen as the best way of presenting the necessary information.

WHAT TYPES OF DETECTOR ARE 0d THE MARKET?

Several types involving a considerable number of different detection

techniques are available. Response might be, for example, to flame

(ultra-violet

,

,

ate-of -rise, ultra-sonic wave. pattern)

,

however, only heat detectors (fixed temperature, rate-of-rtse, and rate

compensated) a-nd smoke detectors (photo-electric and ioniz,atim) need t o be considered here,

WHAT IS THE FUNCTION OF A

HEAT

Being overcome by smoke is the usual mechanism resulting in death by fire

in

home,

in

a home fire detection system. Their function is to complemnt smoke

detectors, where a high level of safety i s desired, by sensing fire in

closed compartments remote from, far e-e, the sleeping householder.

They are thus apPropriate-Tor c G s e t s iGvolving some specFa1 risk

of ignition, f o r garages and for basement and attic areas that are reasonably well closed o f f f ~ o m the rest of the house. An alarm could

then be expected

be

fore

upaccept*

ions

adequate protection, howeverI f o r the basement o f a - b u s e that is air-conditioned or heated by a forced wam-air system.

Heat detectors may prove the b e s t choice in a domestic kitchen

largely because moderate fires capable of activating them (e.g., fire

in

a frying pan)

can develop

use is by default, that smoke detectors (particularly the h n i z a t i o n type)

ARE THERE DIFFERENT TYPES

OF

Yes, there are numerous types. One detectar responds at a 'Ifixed temperaturef1 and depends, for operation, on the movement of a bimetal strip ox snap-disc or on the melting of a eutectic .metal alloy. Bursting glass bulbs are also used and t h e m e l e c t r i c detectors exist, depending

on the generation of very small voltages by thermocouples.

Another fom of heat detector operates when a specific rate-of-rise

of temperature is exceeded; it is usually pneumatic, involving a chamber

with a pressure sensor and a small calibrated hole.

Detectors operating on both the fixed-temperature and the rate-of-rise

principle are available and are either a combination of the first two

described types, k n m as a "combination1' heat detector, or of the type

known as rate-compensated fixed-temperature detectors. These latter usually consist of an exterior cylinder of one metal containing interior components of a metal with a different coefficient of expansion. The

temperature at which the device operates

is

it would be with slower heating. -

Other factors must be considered in choosing a heat detectur, For

example, two of the detectors described (the eutectic metal alloy and

bursting glass bulb fixed-temperature typqs) a r e obviously destroyed on

operating. They cannot therefore be rested.

Most heat detectars will have normally-open contacts but some

might have'normally-closed contacts for use in a sophisticated household alarm system involving continuous monitoring of the components to ensure that they are functional and t h a t no essential connections or wires have

been destroyed. Such a system is known as a supervised system.

HOW DOES THE PERFORMANCE OF SMOKE AND HEAT DETECTORS COMPARE?

Heat detectors will only operate following the development of a

vigorous fire. They are therefore quite unacceptable (from the life-safety

point of view') for installation in the living areas and escape routes of

a home. Smoke detectors should be considered essential even for the

basement application, discussed earlier, if a cunventional slowly-developing

fire can be anticipated. Smoke detectors will i n practice invariably

respond much earlier than heat detectors.

HOW DO

_THE

PRINCIPAL

SMDKE

A domestic photo-electric detector has a small source of lfght shining into a dark space and a photo-electric sensing cell ltlookingt' into that

space. It is intended that the photo-cell should not wfseeil any l i g h t

under normal circumstances.

When

scatters the light and some falls on the photo-cell which actuates the

alarm a t a preset level. A recent development _{- -} is the use of a pulsed

light-emitting diode (LED) as a source to economise in power consumption

The

smoke

in a chamber and permits a flow of current between two electrodes

[having a potential between them). When aerosols ox particulate matter

enter the chamber, a lower ion mobility results and increased ion

recombination reduces the wer-all current through the c h d e r ,

An appropriate reduction triggers am alarm circuit. Some ionization

type detectors use a second reference chamber to balance some of the

possible effects, on ion current, of variation of ambient temperature,

humidity and barometric pressure.

DO ION CHAMBER

AND

Pnm-~mcmIc

CHARACTERISTICS?

Their behaviour is often markedly different. Obviously domestic photo-electric detectors operate when v i s i b l e smoke reaches an

appropriate concentration. Ion chamber detectors, on the other hand,

substantially respond at a certain concentration of particles, size not being as significant as might be expected.

If

is burned

cleanly, for example, a detector can alarm before smoke becomes visible.

A characteristic of smoke i s that as it ages (over a matter of ten minutes or so) agglomeration occurs; in other words, a number of

particles come together t o

form

detector will be much less responsive to such aged smtlke. To a crude

approximatian, therefore, it can be said that an ion chamber detector

responds when a particular mass rate of burning is attained regardless

of

nature

As

ion

photo-electric detectors in responding to smouldering fires as

i n i t i a t e d by cigarettes in chesterfields, for example, Unfortunately,

when periods af mare than half an hour or so are involved, such

smouldering fires can give smoke levels that will render an escape

route untenable before an ion chamber detector will respond.

On the

chamber

most

feature makes ion chamber detectors prone t o false alarms when they are

located near kitchens.

HOW MANY _{SmKE DETECTORS} ARE

_{REQUTRED AND}

_BE

Although the installation of a siqgle smoke detector between the

living and sleeping areas of

a

located at least at every level in a house or an apartment, particularly

where it is air-conditioned

or

addition, it can be further recommended that each bedroom include a

television set in a particular bedroom. Detectors shoufd be located

on

t h e ceiling or high on an inside wall. Location at the bottom of a

stairwell (on the adjacent ceiling) is considered to be slightly more

appropriate than mounthg at the top. Installation near an air supply

register (within 1 m for example] is to be avoided and

if

be

considered in choosing suitable locations.

IS BATTERY OPERATION SATISFACTORY?

Other aspects of the design heing the same, a battery operated

detector can be expected to be as sensitive as the corresponding model operated from the household power supply and usually gives just as loud

an alarm. An audible low battery indication is given for at least a week before the battery becomes ineffective. A f t e ~ a long continuous absence from the home it

is

IS THEW A NEED TO INTERCONNECT DETECTORS?

It has already been mentioned that authorities are currently

recommending smoke detectors at each level in a home to give a reasonable chance of escape. There is, however, l i t t l e point in installing a smoke dete.ceor in the basement, for example, if i t s alarm cannot be heard in

the sleeping qua~ters. If t h i s is t h e case the detector in question must

be interconnected with the ane near the s l e e p i n g area. (Alternatively all the detectors must be incorporated in a system - yet another issue

t o be discussed).

A n unfortunate feature of most detecto~s on the m a k e t is that,

generally, only identical detectors can be interconnected because the

interconnection is associated with a triggering circuit that is unique

t o each model. However, there are now some models of the smoke detectors so far discussed (having self-contained alarm devices and referred t o

as

'vSmoke Alarmsu] that include radio or tone generator transmitters which can send a signal t o remote receivers without the necessity of

interconnecting wiring, Any detector t h a t alarms will send a coded signal t o operate a horn in the ~eceivex which can be installed in any

bedroom.

Tn a home, it is usually desirable t o have both a photo-electric

smoke detector and an

iw

will be discussed later, this will involve choosing a model of detector that does not include its own _{alarm u n i t .}

Until recently it was assumed that if a householder were prepared

to interconnect detectors he would also want to rvire them permanently to

the main supply. One manufacturer, however, is now ofgering a battery

Tt has also been thought that Section 32 ["Fire Alarm Systems") of

the Canadian Electrical Code required

heavy

cable. It can be argued howe.vex that two or three interconnected

detectors do not constitute a system and that the section does not apply. It is further hoped that the wiring requirements of Section 32 will be

relaxed to correspond with Section 16 [low voltage and low energy power circuits e t c . )

.

WHAT XS

THE DIFFERENCE BETWEEN INTERCONNECTION OF

self-contained. When two or more smoke alarms are interconnected they

w i l l . all sound

if

indicating lamp (usually an LED

-

A fire detection and alarm system has numerous component parts and the detectors do not usually include alarm devices; these are located

separately at the discretion of the designer, A central panel incorporates

several features, indicating, for example, the detector involved in t h e

event of an alarm. Various circuits will usually be supervised

(automatically} and faults indicated visually and audibly.

WHAT

Smoke alarms east from $20 to $40; heat detectors are appreciably

less expensive unless they include a self-contained alarm. Installation

of a battery-operated smoke alarm merely involves securing it to a

ceiling or wall and l i t t l e more is required for a model operated on the

household current if the lead incorporated proves to be an appropriate

length. No lead will be provided with a model intended to be wired in,

and installation (including possible interconnection] will involve an

electrician's services.

Substantial expense can be incurred if a system is chosen, because the control box alone might cost $200. The t o t a l c o s t of s system, including wiring, i s l i k e l y to be in t h e range of $500 t o $1,000.

WHAT

Most authorities recommend mmthly testing of smoke alarms. Some

have a test button provided and the owner should enquire as to i t s

function. In one photo-electric model, the test button mechanically inserts a short length

of

propo~tion to the photo-electric sensor, Such a test button obviously

offers a remarkably effective and realistic t e s t . On the other hand,

other types might do little more than check whether the sounding device

will operate. In this case blowing smoke into the unit is necessary.

If the smoke is generated by lighting a small piece of paper, for example,

it is best done over a small tray of watelr so that it can be readily

extinguished. I t would be unfortunate to create hazard w h i l s t mai~ltaining f i r e detectors.

Testing of

heat

is

detectors cannot of course be tested and the only course of action to be

considered

is

satisfa~tory appearance, absence of paint etc.). Should a decorator have

painted over a detector it w i l l probably need to be replaced.

Where restorable heat detectors have been installed it is suggested that one be t e s t e d every s i x months.

The principal maintenance of smoke detectors is regular vacuuming to ensure that entry ports do not become clogged. Heavy smoking or other a c t i v i t i e s in a home can also lead, over the years, to internal depositions

that can cause problems, Ion chamber detectors will become more sensitive and

prone t o false alarms. The radioactive source will then need gentle cleaning. In the case of some photo-electric detectors, deposition on l i g h t source

and sensor w i l l unfortunately dec~ease sensitivfty. Sophisticated testing or

examfnation might therefore be required.

In

detectors however, there are automatic feedback controls that adjust for dust

and variations in the light source, which may occur with a g ~ . Many photo- electric detectors generate

an

fails.

DO

ION

It would be

an

( o r melted) and then ingested there would be definite hazard. The level of radioactivity involved however is comparable to t h a t of the luminous dial on

a wristwatch so about the same degree of attention should be paid to it.

Whilst the detector is in its custcmtay location the fear of hazard should be

completely dismissed. However care should be taken to obey t h e AECB

instructions m the label regarding final disposal of the detector.

WHAT

AN

In the event of an alarm, it must be remembered first, that smoke can be

remarkably toxic and give no warning of impending callapse, and second, that f i r e can develop at an incredible rate, particularly where some modern

furnishings and wall (or ceiling] linings are involved. Immediate evacuation

of a l l occupants is therefore probably desirable. Investigation of the

possibility that the alarm is false or that the fire is extremely small should

be unde~taken only with extreme caution.

If

seen, it is almost invariably wisest to call the f i r e department immediately following evacuation.

An appreciable proportion of fire fatalities result from re-entry of

Last, but not least in importance, an adequate escape plan for each home should be made. I t should be practised at least once every twelve

months, more often

if

alarms include important and usef-ul infomation an escape plans

in

instruction literatuxe.

WHAT APPROVALS OF DETECTQRS SWULD I U3DK FOR?

Underwritersv Laboratories of Canada test and l i s t f i r e detectors

if they prove acceptable. The Province of Alberta has a l s o done t e s t i n g

and produces a list of detectors and make alarms acceptable to the

Building Standards Branch of the Department of Manpower and Labour of

Alberta.

Acknowledgement

Acknowledgement

Is