NBSIR 76-1112

Technical Assessment of

Safety for Hair Dryer/Stylers

Yung-Chi Wu

Product Safety Engineering Section

Product Engineering Division

Center for Consumer Product Technology

September 1976

FINAL REPORT

Prepared for Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20016

NBSIR 76-1112

TECHNICAL ASSESSMENT OF

SAFETY FOR HAIR DRYER/STYLERS

Yung-Chi Wu

Product Safety Engineering Section

Product Engineering Division

Center for Consumer Product Technology

September 1976

FINAL REPORT

Prepared for Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20016

U.S. DEPARTMENT OF COMMERCE, Elliot L. Richardson, Secretary

Edward O. Vetter, Under Secretary

Dr. Betsy Ancker-Johnson, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Acting Director
'

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Table of Contents

Introduction 1

Hazard Potentials 2

Technical Evaluation 3

Discussion 6

Concl usions and Recommendations 11

Technical Assessment of Safety

for Hair Dryers/Stylers

Introduction

The hair dryer/styler is a relatively new small electrical

appliance. In the current marketplace the product generally consists

of a plastic case into which a heater, a rotor and a motor are fitted.

The motor may be activated either by alternating current (ac) or by

direct current (dc). The motor generally is connected across part of

the heating element (resistor) circuit. If it is a dc motor, there will

be a solid state rectifier built into the input circuit, which in turn

is connected in parallel to a part of the resistance (heating element)

in order that the electrical potential across the motor will not

exceed that required by the motor which is approximately 30 volts. The

heating element is made of high resistance metal wire or ribbon which

is wound over sticks or strips of high temperature insulating materials,

such as asbestos, mica or ceramics. The heating element is connected

to a selecting switch which controls the ac line power input to the

product. There is a temperature limiting (T.L.) device connected to

the heating circuit and is generally located either at the center or

the edge of the heating net-work. When the hair dryer is overheated

and the T.L. device reaches the pre-set temperature, the device is

activated to turn off the power circuit for the whole unit. In

addition to the T.L. device, some of the units contain a backup device

such as 'fusible link' which will limit the hazardous temperature in

the event that the T.L. device fails.

Under normal operating conditions the motor drives a fan rotor

which blows air over the heated element, and the warm air is discharged

for hair drying or styling. This appears to be a very efficient small

appliance. Because of its handiness, convenience, and efficiency, it

has become a very popular product as indicated by AHAM's estimated

domestic consumption; about 8.5 million units/year in the period of

1973-1975. However, because of this high volume consumption, substandard products may appear in the highly competitive market, and a high injury trend from this product has already been reported; the

CPSC "FACT SHEET," September 1975, shows that "nearly 780,000 hair

dryers. . . may be a substantial product hazard." Furthermore, "The

Commission estimates that last year (1974) nearly 1000 individuals were treated in hospital emergency rooms for injuries associated with hair

dryers." Clearly, there are problems associated with these products.

Based on the analyses of two previous reports, one by Mary Stefl 1

/

and the other by the author 2 /, the potential hazards associated with

hair dryers have been identified, viz., electrical , mechanical and

thermal hazards. The present study is to investigate the technical

problems associated with those identified hazards. With a clear understanding of those technical problems and with the aid of human factors

assessment, corrective measures for addressing these hazards may be

recommended.
- 2 -

Hazard Potentials

There are three general hazards associated with hair dryers; namely,

electrical, mechanical and thermal hazards ]_/ & 2/. These hazards may

occur separately or combined, they may also occur directly or indirectly.

In any event, the control of an individual hazard potential will be an effective measure for safety. Thus, to control leakage current and

dielectric breakdown in accordance with the requirements of UL or ANSI

standards is a good practice for electrical safety, and to eliminate

sharp edges and sharp points by following the guidelines of the NBS

reports 3/ & 4/, will reduce the mechanical hazards.

In regards to thermal hazards, two effects need to be considered;

one is thermal burn and the other is fire hazard and/or an induced

electrical shock or sparks. The control of thermal burn may be

accomplished by limiting the output air temperature measured as surface

temperature on human skin in accordance with the theory of convective

heat transfer. The limit of this surface temperature may be determined

from the author's previous study 5/, 6/ & 7/, which discussed how the

injury relates to temperature, time and depth of human tissue.

Fire hazards and/or the induced electrical shock or sparks could

be caused by excessive heat accumulation inside the casing. The control

of this hazard may be achieved by limiting the heat generation or by

dissipating it at a faster rate. Of the last two control processes,

the former may be accomplished by a sensitive thermal cut off device

and the latter, by increasing the flow rate of air. Based on these

guidelines for controlling hazards, improved safety measures for the

product may be recommended.

It should be noted that there are human factors related injuries,

such as electrocution, fracture and strain/sprain reported in the NEISS

injury data. Because such injuries do not appear to be specifically

product related and there appears to be no feasible technical solution

to these problems, they are not addressed in this study.

Technical Evaluation

In view of the analysis and the literature cited in the preceding

section, it is understood that only the safety limits for human resistance to injury are known. It is necessary, therefore, to provide

methods or tests to screen out hair dryer/stylers having excessive

physical force/energy so that the hazard potential associated with the

product can be controlled or eliminated. Since some of these hazards occur in a normal operation of the product and some are hidden as induced effects which may occur with possible abuse of the product, a series of tests and measurements were conducted so that apparent hazards

could be screened out and the origin of the hidden ones detected.
Material

s

- 3

-

One pair each of fifteen makes of hair dryer/styl ers were purchased randomly from the open market. Seven pairs of these were of

the gun type having round openings for the air outlet. These are commonly referred to as hair dryers, and are generally sold without

attachments other than a nozzle. Another seven pairs were of the so called hair styler type having rectangular openings for the air outlet

and for comb or brush attachments. The remaining pair was of the

dryer/styler type having round openings for the air outlet and for

comb or brush attachments. One of each pair was used for destructive

tests and the other for nondestrictive tests.

Test Setup and Procedure

The following tests were conducted on selected samples of the above

hair dryer/stylers

1. Careful inspection for workmanship

2. UL "leakage current" and "dielectric breakdown" tests

3. UL cycling test

4. UL temperature tests which include: a) outlet air temperature

and b) surface temperature. Both a) and b) were performed

with and without cheesecloth to cover the air intake opening.

5. UL drop test with two additional heights, 4 and 5 feet,

included for comparison.

6. UL cord strain relief pull test.

Procedures for these six tests, except for the first are described

in UL Standard 859, 1975. The first test was aimed at detecting any

obvious defects both inside and outside of the samples, such as:

a) sharp points or edges on the outside surface, b) metallic surfaces

on the outside of heating element enclosure, c) electrically live

metal parts exposed outside the casing, d) poor electrical connections

inside the casing, e) inadequate clearances between the casing and the

heating element or rotor, f) poor rotor mounting, and g) poor location

of the T.L. device.

In addition to these six tests, two other series of measurements

were performed to ascertain the adequacy of thermal safety requirements

for the product. These were:

1. Measurements of heat flux at a distance of 2.5 cm from the air

outlet. A calibrated heat flux meter, made by Medtherm Corp.

,

Huntsville, Ala., was employed and a thermocouple was located

adjacent to the meter. A hair dryer/styler was held 2.5 cm
- 4 -

(1 in.) away from the meter and hot air was blown onto the

meter until the latter reached its steady state. The EMF's of

the meter and of the thermocouple were recorded and were converted to heat flux and temperature, respectively. This

measurement was repeated for three different hair dryer/stylers.

2. Measurements of outlet air temperatures as a function of air

flow rates, and the temperature near the surface of the temperature limiting device. The setup for these measurements

is shown schematically in figure 1.

The orifice flow meter was made in accordance with the

principle described by the ASME Report on Fluid Meters,

1959 8/. A No. 40 (.005 in. diam.) chromel-alumel thermocouple, T.C. 1, was inserted across the pipe at a distance

of 2.5 cm (1 in.) from the air inlet, which was connected

at connection 3 to the outlet of a hair dryer with a round

opening. The orifice disc was interchangeable at the

connection 1, with three different orifice sizes 1/4, 1/2

and 3/4 of the inside diameter, D, of the pipe, which was 4.3 cm. Connection 2 was used to connect the flow meter with

the hair dryer so that measurements of the out flow air

temperature at 2.5 cm away from the outlet could be accom- plished with and without the flow meter connected. Another

thermocouple, T.C. 2, was located at the vicinity of the

T.L. device. One pressure tape was located at 1 D upstream

from the orifice and a second tap at 1/2 D downstream from

the orifice. A U-tube manometer filled with water was connected to the taps to measure the pressure difference.

The power input line of the motor was disconnected from the

heater circuit and joined to an independent ac or dc source

as required, so that the speed of the motor could be controlled.

In the case of rectangular opening hair stylers, connection 3 was modified to a rectangular funnel, across which three

thermocouples were installed, to accommodate the two different

openings. Otherwise the rest of the setup was the same as described above.

The procedure of measurements was as follows:

A. Before the motor was disconnected:

1. Connection 3 was attached and connection 2 was detached, i.e., separated from the flow meter. The

hair dryer was turned on and the steady state temperatures of T.C. 1 and T.C. 2 were recorded.

2. The orifice disc was removed from connection 1. Connection 1, 2 and 3 were attached. The hair dryer

was turned on and the temperature of T.C. 1 and

T.C. 2 was recorded as in (1).
3 .

- 5 -

% The orifice disc of size 1/4 D was installed in

connection 1. The hair dryer was turned on and

temperature recorded as before. The pressure

difference from the two manometer arms was also

recorded.

i

4. The orifice disc of size 1/4 D was replaced with

1/2 D, and then 3/4 D. For each orifice disc, the

temperatures and pressure differences were recorded.

B. After the motor was disconnected:

The motor was joined to an independent power supply.

The voltage was adjusted to the lowest integral value

which would drive the motor and increased in four to

five equal increments to the maximum allowable integral

value for the motor. At each selected voltage level,

steps 2, 3 and 4 of procedure (A) were repeated.

C. Procedures (A) and (B) were repeated for three different

hair dryers and three stylers.

Results and Calculations

The results of the tests and measurements are listed in the

following tables. The first six tables are self explanatory. In tables VII and VIII, the flow rates were calculated in accordance with

the principles given in ASME Report on Fluid Meters, 1959 8/. The

derivation is given in the appendix. Values in tables IX and X are output air temperatures under various conditions. Temperatures measured

adjacent to T.L. device are listed also in table X. In table XI, the

heat absorbed by the heat flux meter was computed in accordance with the

calibrated value given by the manufacturer.

It should be noted that the remarks in table I are the results of

observations, and those values and remarks in tables II to VI are the

results of UL standard tests. The apparent adequacy of these tests

will be discussed in the next section.

The accuracy of flow measurements depends upon the geometry of

the setup and the accuracy of the manometer. In order to ensure the

reliability of the measurements, two more pressure taps (not shown in

figure 1) at 2.5 D and 8 D from the orifice for upstream and downstream

air flow, respecti vely , were installed to measure the pressure drop.

The results were consistent. Since these measurements were intended

for trend rather than accuracy, no effort to refine the measurement

technique was exercised. It was estimated that the total experimental

uncertainty was about + 10% (higher for low values and lower for high

values)

.
- 6 -

The heat flux measurements were aimed at determining the quantity

of heat that would be received by the human body if the air output of

a hair dryer/styler was blown directly onto it. The 2.5 cm distance

used in these measurements was based on Stefl's study on human behavior ]_/.

Because the heat flux and the surface temperature depend on the flow

characteristics, the uncertainty in these measurements is also estimated

to be about + 10%.

Discussion

In evaluating the results of the first six tests, each test will

be discussed here for its apparent adequacy or deficiency in addressing

the commonly identified hazards. The results of the last two laboratory

experiments are primarily related to thermal hazards which appear to be one of the more common problems associated with hair dryer/styler. A

detailed discussion of these results is also included later in this

section.

The first inspection was subjective in nature and only qualitative

evaluations can be given. In general, no sharp edges or points were

found on the surface of the casing of any of the hair dryer/styl ers examined, except on the attachments to the stylers. Some of those combs

and brushes felt somewhat sharp to the hand, especially in a combing

motion. There was no obvious exposure of metallic surface on heating

element enclosure except three minor cases. No obviously defective

electrical connections were found.

Some undesirable characteristics associated with a few hair

dryer/styl ers were noted. One was related to the mounting of the rotor

unit in the casing, where the clearance between the rotor blades and

the casing was too small, or the fit of the rotor shaft in its bearings

was too loose. The other was associated with the T.L. device in which

the contacts for opening and closing the circuit were not functioning

properly. Any one of these undesirable characteristics with a slight

malfunction could lead to a thermal hazard.

There were three actual cases 9/ of overheating which were caused

by the characteristics stated above. In one case the rotor blade was broken, and a small piece of plastic was stuck to the heating element

causing smoke to be emitted. This was caused by the rotor shaft moving

slightly off its axis causing the blade to contact the ridge inside

the casing. In a second case, hair fragments and dandruff or oily dust were accumulated between the shaft and the casing. The motor slowed

down, and enough heat built up to melt the enclosure of the heater

portion of the casing. In a third case the contact of the T.L. device

was neither completely closed nor opened, which allowed enough current

to energize the heating element but not enough to operate the motor.

As a result, heat built up and the plastic at the opening melted.

All of these defects are design problems which can be corrected.

From the standpoint of safety performance, if those products had had a sensitive T.L. device, properly installed, the last two malfunctions

described above could have been avoided.
- 7

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The results of the next five UL 859 tests are shown in tables 1

1 - V I

.

The second, third and sixth tests appear to be adequate. The part of

the fourth test pertaining to surface tempeature, also appears adequate.

The requirements and criteria for these four tests may be used to screen out those substandard products from reaching consumers. The other

part of the fourth test pertaining to the outlet air temperature, however, involves human thermal resistance and the acceptance criteria

appear to be questionable. This is discussed in the heat flux measurement section of this report.

There are two problems involved with the drop test; the recommended

drop height and the subsequent acceptance criteria. The criteria call

for electrical operational acceptability but does not include requirements

for thermal acceptability. It can be seen from the results of the drop

tests that in some cases the rotor shafts were shifted, blades were

broken, and heating elements were dislocated. Any of these could present

a hazard if the product is to be used repeatedly. The test drop height

of three feet does not appear to cover common occurrence of usage. A

consumer more often uses the product while standing !_/, and the height of

the product at that point would be about five feet. For a more realistic

test the drop height might be increased to five feet, even though the

test results show that at three feet, three out of five samples passed

the test, and at five feet only two passed.

Flow Rate and Air Temperature Measurements

The results of these measurements are listed in tables V 1 1 - X

.

Table VII shows the flow rates at various restrictions with the motor

operated at normal voltage. These data are plotted in figure 2. The

flow restriction, 6, is the ratio of the opening diameter of the orifice

to that of the pipe, and the pipe diameter is approximately equal to

that of the opening of the hair dryer/styl er, so that at 6

= 1, the

extrapolated values for Q, the flow rate, may be taken to be the flow

rate of the hair dryer/styler without restriction. It may be noted from

figure 2 that the measurements for the sample gun type hair dryers yield

practically an identical higher flow rate, whereas those for the hair

stylers are varied and are generally lower. In the latter case, the

lower flow rate appears to be caused by a less efficient rotor working

against the restriction of the opening. Further restriction by attachments such as comb or brush, would reduce the flow rate further and

possibly increase heat as shown in table X.

Table VIII shows the values for flow rates at various power inputs

to the motor. The speed of the motors generally used in these products

is a function of torque that the motor encounters as well as the voltage

input to the motor. Therefore, the voltage on the motor can be varied

to give the equivalent effect of increasing the torque loading. In practical use, the accumulation of hair fragments and dandruff or dust

can increase the applied torque and slow down the speed of the motor,

thereby reducing the flow rate of air. The last factor will influence

the output air temperature which is demonstrated by these values shown

in tablex IX and X and in figure 4.
- 8 -

In table IX, the thermal cut-off temperatures for samples D2 and S2

at high flow restriction (6 = 0.265) are lower than those at a lower

restriction. This illustrates the fact that the T.L. device is sensitive

to heat rather than temperature. At higher flow restriction the air

flow rate is lower as shown in figures 2 and 3. This also means that

the volume of air remains in contact with the heater longer, which in

turn heats up the T.L. device faster. If the heat capacity of the T.L.

device is comparatively low, less heat will be required to activate

the device to cut off the circuit as these two samples showed. On the

other hand, if the heat capacity is high, the heat required for the

device to cut off the circuit will be greater. Also, the output

temperature increases greatly as indicated by sample D1 in the same

table.

It should also be noted that in table X, T<j columns are for the

temperatures adjacent to the T.L. device at the indicated output air

temperature, Ta

. The values for T^ are lower than the output temperatures and change much more slowly. The two temperatures vary by a factor from 2 to 4. These observations further indicate that the T.L.

device is sensitive to heat rather than to the temperature of the

surrounding media. In other words the heat capacity of the T.L.

device is too high. This point may be appreciated further by the

observations of the time lag required for the T.L. device to operate.

In some cases, this took as long as 5 minutes. During this time the

heat build up inside can become highly destructive. Two of the samples

used in this experiment were damaged during the test. (Note that 1000

watts of heat generated for 5 minutes without dissipation is equivalent

to 300,000 joules or about 300 Btu inside the casing.)

These observations indicate what appears to be a basic problem.

The corrective measure is really dependent upon engineering design.

However, if the problem of energy consumption and the efficiency of

performance is immaterial, and only the safety performance is to be

considered, then a sensitive temperature limiting device, properly

installed and calibrated, would eliminate this particular problem.

Heat Flux Measurements

The results of these measurements are shown in table XI, where two

types of values are tabulated. One is the value for heat flux which

would be received by the human body if hair dryer/styler at a distance

of 2.5 cm blows air directly onto it, the other is for temperature

adjacent to the heat flux meter which would be the temperature of hot

air imparted to the human body. The values for heat flux are steady

state values which were reached within about one half of a minute from

the start.

It is of interest to note that in table XI the surface temperature

for sample S4 is higher than that of the other two samples by about 30 C,

and yet the heat flux value for the same sample is lower. This appears

to be inconsistent, but it is not uncommon for convective heat transfer
- 9

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in turbulent flow. In turbulent flow convective heat transfer depends

on many factors such as the shear stress, the velocity and the properties of the fluid and the geometry of the flow system. Any variation

of these parameters will affect the temperature distribution and the

heat flux. The complexity of this problem is beyond the scope of this

study. However, since the three samples tested had significant

differences in the size and shape of output openings, the size and

location of heating elements, and the geometry of protective grids in

the outlet air stream, the variations in heat flux and temperature

values shown in table XI are not unreasonable.

Since safety is the concern of this investigation, the physiological

effects to hair and skin by heat and temperature should be considered

first. According to the Wool Handbook 1_0/, the physical and chemical

properties of human hair are very similar to animal hairs which have

been studied extensively by the wool industry. In these studies it is

shown that if wool is subjected to a wet heat about 100 C, it will lose

most of its resistance to deformation and become more plastic. It will

no longer return to its original length. Moreover, it may even become

brown and brittle with a reduction in its breaking strength. The

utility of the hair dryer/styler is for drying/styl ing hair, but it is

obviously undesirable in the drying process to have hair damaged.

Therefore, the temperature of hot air blown onto moist hair should

preferably not exceed 100 C.

Secondly, if the hot air is blown onto skin directly, the surface

temperature of skin is drastically modified. According to the theory

put forth by Buettner 11/,

Tso= T + 2H t/irX (1)

where T

c = surface temperature of human skin after

receiving heat from hot air, C.

T

r = original surface temperature of human

skin, C. _2

H = heat flux, watt cm .

2 -i -4 -2

x = kpc, thermal inertia, joule s cm C , where k is thermal conductivity; p, density; and

c, specific heat, of human tissue.

t = time, s.

Substituting 35 C for T , and 0.0228 J

2 /(cm

4

s C

2

) for x into

equation (1), it yields 0

T

s = 35 + 7.48 H t ( 2 )
10 -

It is seen that the surface temperature, T , is a function of the heat

flux the skin receives and the time of exposure. However, because of

the human tolerance limit to thermal energy, the freedom of each of

these variables is restricted. Moreover, because the thermal energy is

limited to within the physiological tolerance, either of the two sets

of variables, i.e., temperature-time and heat flux- time may determine

the criteria for thermal safety. Moritz and Henriques 6/ have determined

the temperature time relationship for skin injury, and Stoll 1J has

measured the heat flux-time limitation for safety. Both of these experiments were performed on human subjects. Their results are reproduced

here for reference and are shown in figures 5 and 6, respecti vely.

The results shown in table XI for hair dryer/styl ers may be

compared with either figure 5 or 6. It is apparent that either the

surface temperature or the heat flux is too high, if hot air from a hair dryer/styler is blown directly onto human skin, such as forehead

or neck. Note that of the surface temperatures in table XI, the first

two are approaching and the third one is over, the injurious limit; even

the values for heat flux fall into a rather narrow range of tolerance

time, 15-20 seconds beyond which blisters would form. Of course, the

use of hair dryer/styler is to dry/style hair and hair may act as an insulator to the skin. Also, the appliance is usually kept in motion

during use, in which case no skin surface temperature would reach the

steady state values observed in table XI. However, in some situations,

where the attached comb or brush becomes entangled in the hair, or the

appliance is used for drying/styl ing a child's hair by an adult, the

duration of exposure may extend well beyond the time limit required by

figure 5 or 6, and thermal injury may result. Therefore, for the sake

of safety, the limits for the surface temperature and the heat flux

should be chosen in accordance with figures 5 and 6, respecti vely.

Conclusions and Recommendations

In view of the test results and the performance characteristics of

the hair dryer/styler, the apparent causes for the three general hazards

associated with the product may be evaluated technically as follows:

1. Electrical Hazards. If a product has met the requirements for

the leakage current and dielectric breakdown tests the product

should be reasonably safe. However, there is an induced

hazard to be considered. In the event of reasonable and foreseeable abuse, such as when the hair dryer/styler is dropped

to the floor, there may be no apparent failure on the outside,

and yet the inside wiring or heating element may be dislocated

in such a way as to create an electrical hazard. It is

recommended, therefore, that the hair dryer/styler should be

subjected to the drop test first, then to a limited cycling

test (UL859-26. 28) and then to the leakage current and dielectric breakdown tests. This test sequence should be effective

in reducing those electrical and thermal hazards that are mechanically induced.
- 11 -

2. Mechanical Hazards. Mechanical injuries associated with hair

dryer/stylers were approximately 30% of those reported by

NEISS in 1974-1975, and yet there was no in depth investigation

report ( I DIR) for any of these cases. From the performance

point of view, in normal use or even reasonably foreseeable

abuse, it is difficult to visualize how these injuries would

occur, since the results of a visual inspection show no apparent sharp points or edges other than those on the attached

comb or brush. Even for those, a high force would be required

to lacerate human tissue. Perhaps, in combing or brushing

thick and tangled hair, such force would be applied and the

momentum of this applied force sometimes could cause the

attachment to impact onto other parts of the body resulting

in a laceration or puncture. If this is the case, any unduly

sharp points or edges on all surfaces should be dulled.

3. Thermal Hazards. To control the thermal hazard potential is an inherent difficulty for this small appliance. The heat

capacity is low, for the mass is generally small; and yet a huge energy (as high as 1500 watts or approximately 2 horse

power) is supplied to some models. It is easy to conceive

how such excessive energy (heat) could become destructive.

Obviously, thermal control is an essential requirement in

order to minimize this hazard. The higher the thermal energy

generated, the more difficult the control process becomes.

Since the hair drying/styl ing process depends upon the convective heat transferred to the hair, the efficiency of the

process depends more on the velocity of air flow than on heat.

Therefore, the energy input beyond a certain point appears to

be unnecessary. The human factors study reported by Stefl 1

/

has demonstrated that the average time for the hair drying/

styling process is 7 minutes with no apparent trend related to

the power input to those hair dryer/stylers used in the study

(from 200 to 1100 watts). On the other hand, the high powered

hair dryer/styler is not necessarily a high risk product if the

control mechanism is adequate.

There are three control mechanisms which should be considered:

A. Temperature limiting control - this control should limit

the heat accumulated inside the casing as well as the

output air temperature. Inside the unit the heat accum- ulation should be prevented from reaching a destructive

point, such as flashing, melting or flaming of those

material with low thermal resistance. The output air

temperature should be limited to avoid thermal damage to

hair and/or to skin tissue. Since the power input is

usually large, the heat accumulation is rather rapid if

the heat dissipating process is slowed down. Therefore,

the temperature limiting device should be calibrated in

accordance with the maximum allowable output air temperature. Based on the results shown in table X, the devices
12

investigated were not set for the output temperature. In some cases this temperature reached about 200 C before the

power was cut off. Furthermore, even for inside temperatures many of the devices did not respond fast enough. In some cases, when the motor was slowed down the heat

accumulated for 3 to 5 minutes and reached a destructive

point, causing the casing to soften before the device was activated. This means that the response time of the T.L.

device was too slow. Therefore, the temperature limiting

mechanism or device should be calibrated to limit the air

output temperature and also should be sensitive to the heat

accumulated inside.

B. Air flow rate for thermal control. The out-flow air

temperature of a hair dryer/styler is inversely proportional to the air flow rate, if the power input remains

constant. This also is true for the inside temperature in

accordance with the theory of convective heat transfer

process. Thus, in the interest of safety, an increased

air flow rate would increase the efficiency of the drying

process and reduce the thermal hazards. This is merely a suggestion since this may involve a design problem.

However, if the criteria recommended in A above are to be

satisfied, the suggestion here should be considered in

order to accommodate the high power input.

C. Thermal insulation. Excessive heat may also be controlled

by a process of confinement. Thermal insulation could be

used to limit the direction and rate of heat flow, and

prevent some low thermal resistance materials such as the

plastic casing from being damaged. Again, this is a design problem.

For thermal safety, the first mechanism is to cut off the heat

generation before it becomes excessive; a second one is to

control excessive heat by a process of dissipation and a third

is heat control by confinement. These three mechanisms may be

employed separately or in combination to ensure a safer product
Table 1 . Visual 1 nspect i on

Sample §

Sharp

Points

Sharp

Edges

Metallic'

Surface

SI No No No

S2 No No No

S3 No No No

S4 No No No

S5 No No Yes

S6 No No Yes

S7 No No Yes

DS No No No

D1 No No No

D2 No No No

D3 No No No

D4 No No No

D5 No No No

D6 No No No

D7 No No No

*Metallic surfaces on the outside of heating

element enclosure.
Table II. Leakage Current and

Dielectric Breakdown

Tests

Sample If Leakage Current Dielectric Breakdown

SI Passed Passed

S2 Passed Passed

S3 Passed Passed

S4 Passed Passed

S5 Passed Passed

S6 Passed Passed

S7 Passed Passed

DS Passed Passed

D1 Passed Passed

D2 Passed Passed

D3 Passed Passed

D4 Passed Passed

D5 Passed Passed

D6 Passed Passed

D7 Passed Passed
O Do

CU r— 3o e

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to a) CU -M X) XJ O) t •1

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aj to to to E to to E CU 1

E tO 03 CU -E 1

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to CD 2 E to o

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3

r- 4- t— U 4- • >> c E O +-> • o

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to to CU CU

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CU

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C/) <T3 ro

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CO 43 CL 4-3 4-J E jO to

fd to CU

O •i— o 03 Q_ CU i— CD i- -E 4-> CO CL

4-3 i— rO o to

rO -X 4-> -a e o

~o E ro ro a; E

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CL _l 4—> E *4

CL rO

O E o • X3 CL •i

4-3 CU • CU

X 0) to co to 3: O) _X 03

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CO

C 4-3 to

O -Q 03 e 03 4-3 XJ

•1

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O

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tj

r o e r— o ro -r- 4-J >3 E -E 4-> •> to to • o 03 (J X) ro e X)

SO CD CU r— cu CU 3 CU O CL 4-3 CL 4-3 4-> o >) t- to ro ro i

r— E rz e CU 4— CU CU

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to

at

Lf) o o 03

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CJ (XI

O co

O o co

>> co r— LT3 co '0- CO

c_> to <sO 1 to CO 1— to

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4fc

O) r— CM 00

Q- C/3 OOO E

CO oo

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r-'* o

+-> ajto

CUE

03 • u to

•r- +J 43 tO

ro aj E +-> o

43 CU 3 CJ . CO E CO = ro 43 e CO

<U 3 CU -E X) 43

1— E

CU E

CU • 4- -E o +3 • O

r— CU co to E o O E CL 4- o E

•r- 3 XJ

4-J CL a.) (_> CO

O) CU 3

to rz

4-J CU XJ E E E CU ro O 3 4- (

co • ; CU

4-J r— CO <o CL

4-3 E

CU CO ro i— O to -a E

ro E E

t— CU CU -E rz I 4-3 43O

CD 03

LO E CU co •*- r—

_J 4-3 0 r— ED 03 SZ i

3

JC 3

4-J 03 #3 •i- CU >3 3 E E

CU CU o

CJ E X) E 3 E

03 43 rO X) ro E E E O CU X3

(J CL CU u E 43

rO CU

O

43 dJ E = i

1

•!

d) CO to X) 03

CU CU E X3 E E CU a> O 43 3 4- -M E <U CU

CU E E

CL CL O) E rz

CU CU E 43 x>

CU E (U 3 *r- 43

to to to •i

43 (O r—

to 2

CU to 43 = CU r— 1 CU o CL

to E E

CU 43 (O _E E oo l— O 4t * U -K
Table IV. Air and Surface Temperatures*

Outside Surface Temperature

Sample # Air Temperature, °C of Heater Enclosure, °C

SI 66 81

S5 45 72

S7 75 93

D1 86 -

D2 58 123

D5 67 -

D7 54 _

*Air temperatures were measured 2.5 cm away from opening.

Surface temperatures were measured at the middle circumference, about halfway along the length axis, of the outside

surfaces of heater enclosure. In some of the products,

there was a thin sheet of asbestos inserted between the

heater and the enclosure as thermal insulation. No surface

temperature was measured for those products.
CD

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cn . 0 e 4- 4-> •r- 4-

4->

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cu JC 03 03 cd 2 0 C u

cu 1 E U CD +-> 03 4-

03 cu T3 •>- 4->

O

•r- -Q <D E

CL 03 CL =3 4-3 CU ,

CL

O T3 r— JO O E •1— ,

4-> • cn E cn 0 "O E E O 03 •1

*1— cn +-> E cn 0 O O cn 0 OC 4-> CD E E 2 O O r

I— CC E 4-3 4-> O O 4-3 0> 4- E •1

• -o • JO E -r- E T3 -O

03 CD 03 O) CU

03 E O) E 2 4-3 CD •<— cn O E -0 JO CU

4-> 0) 4-> CU cn JO • 03 CD 03 E cu O E • cu cn

4-3

O 4-3 =5 E 4-3 *0 *r- E 1— E 4-> O 4- 03 O

03 03 • 1— 03 r— E

CL

U • CL 4-3 4-> JO JO

03 -i— e 03 SOU cn CT>

4-3 aj CU <u E E

QJ

CCD E CL 4->

O -r- E 03 O E cu cn E 4-3 03 1— e 03 1— E cu O 03 O Q.JO 0 a> t—

JO cc E

4-> CD

03 E

CL

O “O “O “O "O

O *a T3 T3 T3 •U -O -O

O "O <D cd aj CU CD CD CD <d CD CU CU CU E CO 00 00 cn CO CO CO co CO cn cn co Q 03 00 00 CO cn CO CO CO 00 CO cn CO cn

CU 03 03 CO 03 03 03 ra ra 03 03 03 E CL Q_ Q_ CL CL CL Cl Cl Q_ CL Cl CL CO

O)

CU •O T3 T3 TD T3 -O X3 “O X3 X3 -O t3 CD CU cu CD CD CU 03 03 (D 03 CD CD 03 03 cn cn CO CO cn cn CD CO cn CO CO CD

cn cn CO CO cn cn cn CO cn CO CO CD

03 03 03 ra CO 03 03 03 ra ra 03 03 03

CU CL Q_ Cl Cl CL a. Q. CL CL CL CL Cl

CD CD cn cn cn CD CL Q. CL Cl Cl CL O O O O O O

CL E E E E E E O T3 -O T3 "O TD -a EO 1 CO ' CXI CO •

OsJ CO CO CO CO co

4-3

CLOeo

LO

CO

CD 00 CO >vj- LO LO

CO CO

ID

CO 00=3- oCO

LO

CO 00

>=3- oCD

03CLE

03 CO

CO LD

cn 00

<D r-- cr> 00

Q 3

drops

Passed

Passed
Table VI . Cord Pul 1 Test

(Strain Relief)

Sample ft Passed Fai led Comments

S3 X Cord moved inside under load.

S7 X

D4 X

D5 X

D7 X Cord pulled through strain

relief and out of dryer.
Table VII. Flow Rates at Various Restrictions

Air Flow Rate, Q X 10^, m'Vs

Sampl e 6 = 0.265 3 = 0.5 3 = 0.765 0=1.1

D1 1.98 7.08 11.9 16.7

D2 1.70 6.80 11.9 16.7

D3 2.27 7.36 10.8 16.7

SI 1.13 3.96 6.23 8.9

S2 0.85 2.83 5.10 7.1

S3 1.13 3.96 8.78 11.9

*No restriction, values were extrapolated.
Table VIII. Flow Rates at Various Motor Voltages

Air Flow Rates, Q X 10

3

, m

3 /s

^ Voltage

Sampl

3 6 V 12V 18V 24V 30V

D1 .765 3.90 7.19 10.6 13.3 15.6

.5 2.52 4.98 6.80 8.27 9.91

.265 1.22 1.73 2.10 2.61

D2 .765 5.52 9.43 12.5 15.8

.5 2.78 5.44 7.48 8.83

.265 0.76 1.44 1 .92 2.35
Table IX. Output Temperatures at Various Restrictions

with Constant Voltage to Motor

t°C

Sample 3 = 0.765 8 = 0.50 8 = 0.265

DS 60 92 (94)*

D1 78 124 (200)

D2 67 122 (100)

D3 56 (124) (133)

SI 70 (102) -

S2 72 90 (80)

S3 100 113

*The value in

at which the

the parenthesis was the

T.L. device was activated

temperature

and the

power was cut off.
Table X. Highest Temperatures, T and T,*, at Various

a u Motor Voltages and Flow Restrictions

Sample D1 D2 SI S2

Vo,t 6 T °C VC T

a A: Td

.*c T

a ,°c Td ,°C T

a> °C Td>°C

6 1.0

0.765

0.5

(120)**

(96)

(210)

149 74

(167) 61

12 1.0 115 93 40

0.765 118 65 78 50 108 48

0.5 (195) 148

18 1.0 88 74 36

0.765 97 54 62 41 85 43

0.5 171 121

24 1.0 78 67 35

0.765 85 49 56 38 76 41

0.5 147 102

30 1.0 72

0.765 77

0.5 135

As I s+ 1.0 64 58 58

0.765 78 67 56

0.5 124 122 40

*T is the output air temperature measured 2.5 cm away from the opening ; and T, is the

temperature adjacent to the T.L. device inside a hair dryer/st.yl er.

Q

**The value in the parenthesis was the temperature at which T.L. device was activated and

the power was cut off.

+The "As Is" values are those measured before the dc motor was separated from the circuit.
Table XI. Heat Flux from Hair Dryer/Styler

Onto Blunt Surface

Sample Heat Flux Surface Temperature

2 watt/cm (cal/cm^/S) «C

D4 0.74 (.18) 65

SI 0.61 (.15) 64

S4 0.57 (.14) 95
References

]_/ Stefl , Mary, Interim Report, CPSC Project No. 132, National

Bureau of Standards, Washington, D.C. 20234, Feb. 3, 1976.

2/ Wu, V.C., Interim Report, CPSC Project No. 132, National

Bureau of STandards, Washington, D.C. 20234, Jan. 23, 1976.

3/ McGuire, B. , Sorrells, J.R., and Moore, J.D., NBSIR73-123,

National Bureau of Standards, Washington, D.C. 20234, 1973.

4J Sorrells, J.R. and Berger, R.E., NBSIR73-262, National Bureau

of Standards, Washington, D.C. 20234, 1973.

5/ Wu, Y.C., J. Materials, ASTM, 1,573, 1972.

6/ Moritz, A.C., and Henriques, F.C., Jr., Am. J. Path. 23, 695,

1947.

1J Stoll, A.M., and Chianta, M.A., Aerospace Med. 40, 1232, 1969.

8/ Fluid Meters, Their Theory and Application, ASME Report,

5th ed., 1959.

9/ These samples were provided by CPSC for examination.

10/ Wool Handbook, Von Bergen, ed. Interscience Publishers,

N.Y., 1963.

11/ Buettner, K. , J. Appl . Physiol. 1, 69, 1951.

USCOMM-NBS-DC
ORIFICE

FLOW

METER

Figure

1.

Schematic

diagram

for

temperature

and

air

flow

measurements.
15

10

5

0

T

I

FLOW RESTRICTION, B

Figure 2. Flow rates versus flow restrictions.
I

0 12 18

VOLTS

24 30

Figure 3. Flow rates at various power inputs and at different

flow restrictions: 6 -j = 0.765; 62

= 0.5 and 63 = 0.265.
200

150

100

50

T

()

(A)

01 0 P = 1.0

A P = 0.765

P = 0.5

D2 • P = 0.765

A P = 0.5

(X) THERMAL

CUTOFF

12 18

VOLTS

24 30

Figure 4. Air temperatures at various motor voltages for

different flow restrictfons, 6.
+ ’ >1

01 U V

a) •— a> i o ‘X O X. f1

I— 1/1 io

a> D l-rx: x: ro

to O CL) •r~ c x;

03 4-> «U X X oxo r-- =3 X- O X3 x; cu

i/i aj E

<D r~ -r- X JD 4->

iinc:x

=3 X3

to aj X X

a; o > x>

<l> o

x x x a>

"* CL X $ O -X

Ctrl) C O O O) jO

•«~ X -r4-> r- tO

ro O to r- CT-roj ai e XXXa; aj n a

E E

4-> 4-> to

I I CL) (U qj r

X X C71 =3 3 -r- -M x> x:

ro ro X X CD d<Dr

a ax

E E

ai cu to

cuxo

CO
ABSORBED ENERGY RATE

(cal/cm^'S)

Figure 6. Human skin tolerance time to absorb thermal energy jj

(Log-log scale and 1 cal/cm^* S = 4.186 watt/cm2.)
Append i

Theory of Orifice Flow Meter

A fluid flows through a pipe with diameter, [),and an orifice with

•3 an opening diameter d, then the volume of the fluid flow, Q in m /s ,

will be

Q = v.A = v a A a

(

1 )

where v's are the velocity of fluid in m/s. A and a are the areas of

the pipe and the orifice opening, respectively.

then v = v —

A a A

From the law of energy conservation

1/2 Mv

2 = Mgh

where M = the mass of the fluid

g = gravitational acceleration

h = differential head of the fluid

thus V

a

2

' V

A

2 = 2g (h^-h A ) a A

so that v

g = J

2g(h

a -hA

) • 1 /

1-

( a /A)'

From the Bernoulli's theory:

2

p + 1 /2 p v = cons t

where p = pressure

p = dens i ty

combining eqs . ( 4 ) and (6), and let y

( 2 )

(3)

(*)

(5)

( 6 )

g p , it yields

h A

~

*Pa ~ p a)/

(7)

Putting eq . (7) into (5), the velocity of the fluid in terms of pressure

and diameters of the orifice opening and the pipe becomes
2g (p a

- pJ/y i Ti B ( 8 )

where g = d/D = v a/A

and the flow rate, Q. = a J

2g (P^ - P a )/y

T 1 / / 1 - (T ( 9 )

The actual flow rate through a head meter is practically always

less than the indicated theoretical flow rate, hence a correction factor,

the so-calied "discharge coefficient," C, must be introduced which may

be defined as:

actual rate of flow

C = theoretical rate of flow

so that

Qactua

= a «//' - T

2g (p a

- pJ/y

do)

(ii)

The factor C/y 1 - B' is replaced by K, the flow coefficient, which

has been tabluated in the ASME report 8_/. It should be noted that the

units used in the report are those fph units, To avoid a lengthy conversion, the computation in this report was first in fph units; after

the corrections, SI units were employed.
NBS-1 14A (REV. 7-73)

I uTsTdept. of comm. BIBLIOGRAPHIC DATA

SHEET

1. PURLK A ITON OR REPORT NO.

NBSIR76-1 1 12

2. Gov't Accession

No.

3. Recipient’s Accession No.

4. TITLE AND SUBTITLE

Technical Assessment of Safety

for Hair Dryer/Stylers

5. Publication Date September 1976

6. Performing Organization ( odr 446.03

7. AUTHOR(S)

Yunq-Chi Wu

8. Performing Organ. Report N<

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. Project /Task /Work Unit No. 4461541

11. C ontract /Grant No.

12. Sponsoring Organization Name and Complete Address (Street, City, State, ZIP)

Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20016

13. Type of Report & Perioil Covered

Final Report

14. Sponsoring Agency Code

15. SUPPLEMENTARY NOTES

16. ABSTRACT (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

The hand held dryer/styler is an efficient and popular small appliance, but

a high injury trend associated with such products has been reported. Hazards

commonly associated with the product are electrical, mechanical and thermal.

In this study, the relation between these hazards and human tolerance limits were

analyzed and experiments to investigate the probable cause and potential of such

hazards were conducted. Randomly selected samples of the product were subjected

to relevant safety performance tests of the UL Standard 859. In addition, air

flow rate, heat flux and surface temperature were measured on several samples in

order to determine their thermal hazard potential. Based on the results of

these investigations, corrective measures are suggested.

17. KEY WORDS (six to twelve entries; alphabetical order; capitalize only the first letter of the first key word unless a proper name; separated by semicolons)

Air flow rate; electrical, mechanical and thermal hazards; hair dryer/stylers;

heat flux; safety; temperature; technical analysis.

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