AlllOl 72bEn AlllDt 17flS7fl

NBSIR 76-1097

Illustrative Generic Standard

for the Control of Thermal

Burn Hazards in Household

Appliances

Robert G. Hendrickson

Elizabeth M. Robertson

Rudolph V. Kelly

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

August 1976

Final

! Prepared for Consumer Product Safety Commission

5401 Westbard Avenue

R^thesda, Maryland 20207

-QC

100

.U56

i 76-1097

' 1976

National Bureau of Standards NBSIR 76-1097 "1 Admin. BIdg.

OCT 2 5 1982

ILLUSTRATIVE GENERIC STANDARD ^ FOR THE CONTROL OF THERMAL ' ^

BURN HAZARDS IN HOUSEHOLD Uctoo

APPLIANCES . Li^L^

Robert G. Hendrickson

Elizabeth M. Robertson

Rudolph V. Kelly

Institute for Applied Technology

National Bureau of Standards

Washington, D. C. 20234

August 1976

Final

Prepared for Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, iVIaryland 20207

"•tAU 0» '

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
1
1

TABLE OF CONTENTS

Page

I. INTRODUCTION 1

II. BACKGROUND 3

III. STATEMENT OF PROBLEM 5

IV. APPROACH 6

Summary 6

V. ILLUSTRATIVE STANDARD 8

A. Scope 8

B. Exclusions 8

C. Maximum Temperature Levels 8

D. Test Accessibility Probes 9

E. Test Method 10

1. Determination of Accessible Contact Surfaces 10

2. Test Conditions 10

3. Test Measure 11

Discussion 11

APPENDIX A. TECHNICAL APPROACH A-1

A. Clustering and Fault-Tree Analysis A-2

NEISS Data Source A-2

Empirical Data A4

Fault-Tree Analysis A5

B. Heat-Flow Factors and Equations A-10

Thermal Inertia and Diffusivity A- 11

Basic Concepts A- 11

ii
TABLE OF CCWTENTS (continued)

Page

C. Determination of Surface Temperature Levels A-17>

Current Surface Temperature Limits A- 13

Revised Temperature Levels A- 19

D. Countermeasures A- 22

E. Test Method A- 2 2

APPENDIX B. CONSUMER PRODUCTS ASSOCIATED WITH THERMAL BURNS:

NEISS CLASSIFICATION B-1

REFERENCES ' ' '

"

'

Figures:

Figure A-1: Fault -Tree Diagram for Electric Ranges A6

Figure A- 2: Fault-Tree Diagram for Electric Irons A7

Figure A-3: Fault-Tree Diagram for Electric Coffee Makers A-9

Figure A- 4: Fault -Tree Diagram for Electric Broilers A- 10

Figure A- 5: Surface Temperatures by Thermal Inertia for Selected

Contact Temperatures A- 14

Figure A- 6: Physiological Response Curve A- 15

Figure A- 7: Plot of Current Tenperature Limits A- 17

Tables : , , . ; . ,

.

Table A-1: Basic Product-Type Cluster A-3

Table A-2: Temperature -Contact Time Relationships A- 13

Table A-3: Maximum Acceptable Temperature Limits A- 16

Table A-4: Revised Maximum Surface Temperature Limits A-20

"able A- 5: Revised Maximum Acceptable Temperature Limits A- 23

iii
I. INTRODUCTION

The objective of this work was to develop an illustrative example

of a generic standard for a large number of household, consumer producttypes having a common, explicit bum hazard. The concept of generic

regulation implies that for a large number of product -types it is possible

to define a countermeasure for the perceived hazard and to provide a

satisfactory test method for compliance. In this report, the process of

analysis, modeling, deriving the countermeasures , and specifying appropriate

performance levels for hot surfaces, are presented.

The body of the report is in four parts, the Background (II), Statement

of the Problem (III), Approach (IV) and the Illustrative Standard (V) . The

Background suggests the underlying interest in thermal bums as a serious

national problem; the Statement of the Problem defines the scope of the

perceived hazard addressed by the investigation of this study; the Approach

summarizes the evaluations which are given in detail in the Appendix A; and

Part V is a statement of the illustrative standard for nonfunctional hot

surfaces

.

The principal work on which the illustrative standard is based is

presented in Appendix A, which contains descriptions of the technical

rationale for product -hazard evaluation, measuring heat flow, and formulating

the basic table of maximum surface -temperature levels. Appendix B lists the

products in the NEISS data base associated with thermal bums.

There is no suggestion in this work that the illustrative standard is

complete, nor that it has been developed or refined by review or consensus.

Specific items not fully developed or which remain uncertain in

the report or in the illustrative standard are: (1) the specifications

for the articulated access probes (adult and child) , (2) the specification
for the articulated measuring probe, (3) the procedures for using the

measuring probes (articulated and non- articulated) , (4) a precise determination

of values of thermal inertia, and (5) the details needed to specify the

test conditions more clearly. Also needed are additional research on the

physiological response curves for adults and children, and on possible

operational design changes in the thermesthesiometer to obtain greater

flexibility of use. .^. --^ rJ

All conclusions should be viewed as tentative, and the revised maximum

temperature limits should not be taken as final or authoritative.

The data and information developed in this report are intended only to

demonstrate the generic method and to provide a reasonable basis for the

illustrative standard. It is not intended that any value in this report

should be construed as a position by the National Bureau of Standards

or the Consumer Product Safety Commission. The intent is to show by

example that the concept of a generic standard can contribute efficiency

and simplicity to an otherwise lengthy and complex process.
II. EACKGROUND

The NEISS* data system indicates a high incidence of thermal bums among

the consumer population in 1975. A thermal bum is one in which the source

o£ the bum is heat as opposed to other sources, such as chemicals. These

bums range over approximately 240 product categories and produce severity

of injury great enough to result in 4,900 visits to the 119 hospital

emergency rooms reporting under NEISS. Of these 4,900 bum accidents,

about 45^ involved customary household appliances.

Of the 240 product categories, only about 19 possess appropriate characteristics as subjects for the illustrative standard. These 19, though

less than 101 of the total number of categories, were responsible for

or are involved in about 1,350 emergency room visits during 1975; which,

if projected on a national basis, was about one bum accident per 53,000

households. Although there are no supporting data, the incidence of

"nuisance" bums must be many times this figure.

A study of product -hazard combinations of these 19 product- types

shows that bum accidents result from contact with accessible hot surface

elements of the product. Despite the diversity of product -types,

the hazard, and its manifestation, are common to all of the 19 producttypes, where personal carelessness is not a factor in the accident

sequence

.

Current voluntary, industrial standards should not be judged or

evaluated in the manner of generic standards, however if they are taken

as a group, it is apparent that the primary emphasis is on the protection

^National Electronic Injury Surveillance System: a data collection system

based on accident-injury reports from approximately 119 hospital emergency

rooms

.

-3-
of components of appliances from excessive heat rather than on the

protection of the user against bums. The few standards which contain

surface temperature limits could afford better protection of consumers

from bum accidents. The inadequacies of present standards are in the

inclusion of only a few surface materials used in appliances, in the

consideration of contact times, and in the consideration of physiological

response factors.

The concept of generic safety regulation is, in principle, only an

extension of the product -specific idea applied to many products having a

common hazard to be controlled. The extension to many products is based on the

ability to provide appropriate countermeasures and a test method which

are unambiguous and clearly applicable to all products covered. Reference 1

describes the elements in standards development and examines the relationship

between the product -specific standard and the generic standard.

The recent development of a heat conduction model (Reference 2) and

an instrument (thermesthesiometer) which measures physiological response

(Reference 3) provide a theoretical and practical basis for generating

criteria for the control of thermal bums associated with common household

appliances. ' -

'
III. STATEMENT OF PROBLEM

The cumulative evidence of the NEISS system, the discussion of bum

accidents in the literature, the limited experimental data developed in the

course of this effort, and the fault- tree analysis of product types lead

to the inevitable conclusion that "nuisance" bums are a prevalent hazard

of household appliances, and these bum-accidents may be agents in more severe

accident situations.

It is also perceived that the correction of thermal hazards in household

appliances is within the state of the art, and both theoretical and practicable

methods are available to establish criteria and procedures for improved

standards.
IV. APPROACH

This section summarizes the development of the data and information used

to specify the countermeasures for the perceived thermal bum hazard and

the test methodology for product compliance. The content and conclusions

stated in the following Summary are supported by the technical detail given in

Appendix A. '

The technical approach to the development of an illustrative generic

standard is based on a progression of analyses which begins with existing

statistical evidence, clusters product types, evaluates product -hazard

combinations, describes the theoretical and experimental models and

applications, and ends with a determination of countermeasures and

requirements for compliance. The sole objective of these efforts is to

provide the basis for the content of the illustrative standard. All

conclusions and data are to be considered as tentative and for illustrative

purposes only.

Summary

In order to study the involvement of certain household consumer

products with thermal bum accidents, it was first necessary to: (1)

group the products according to certain characteristics pertaining to

the perceived hazard, (2) establish the basis for selecting appropriate

surface and product element temperature levels, by material and contacttime, and (3) mold these results into a specific statement of countermeasure and test methodology.

Using available statistical evidence and certain criteria of selection,

19 product -types from a total of 240 product categories (listed in Appendix B)

were selected as subjects. These 19 possessed a common hazard for which

-6-
countermeasures (design, insulation, or surrogate materials) could be

identified and a single test method specified to obtain compliance.

Tlie physical and technical details of the product -hazard combinations

were studied in several ways. A theoretical heat flow model contributed

general knowledge of heat-flow phenomena; fault-tree analysis provided a

method for analyzing causes and effects; a measuring instrument called

the thermosthesiometer was used to provide empirical data on hot surfaces;

and physical properties of surface materials were studied with regard to

the role of thermal inertial properties and composite surfaces in heatflow problems. The heat flow model served as a basis (1) for the calculation

of contact ten^eratures , (2) for certain conclusions about thin insulating

surfaces, and (3) for providing an explanation of heat flow phenomena

over short periods of time. The thermesthesiometer provided the capability

for measuring contact temperatures for any surface of interest and for

estimating the thermal inertia for such surfaces in lieu of complex

theoretical calculations. The fault- tree analysis was instrumental in

identifying safe or unsafe features of product- type characteristics and

in deriving the choices for reducing or eliminating the hazard.

These investigations provided the basis for developing a table of

acceptable surface temperature levels, which are based on a selected

time of contact and the thermal inertia of the hot surface or product

elements. This table is given in Appendix A and in the illustrative

standard (Part V)

.
V. ILLUSTRATIVE STANDARD

A. Scope

1. This standard shall apply to all household consumer products

containing a heat source which can have^ as a result of anticipated use^ an

accessibleJ nonfunctional hot surface. A hot surface is defined as a sui'face

whose temperature exceeds an equivalent human contact temperature of 50°C

for a three-second contact time. A nonfunctional surface is a surface

whose location^ position^ or function is not integral to the heat or

radiative output of the product.

B. Exclusions

1. Functional hot surfaces except those surfaces which act as

heat or light shields, collimators, or focusing structures.

2. Surfaces that cannot be contacted after installation.

3. Surfaces that cannot be contacted by the articulated probes

described in Section D, below.

4. Vent openings and surfaces immediately adjacent to such openings.

5. Certain portions of a self-cleaning oven as prescribed in

UL858j Section 54.2.0., September ZO, 1975.

C. Maximum Temperature Levels

1. All products included under Section A and not excluded by

Section B shall comply with the following maximum surface-temperature levels.
Maximum Aoaeptabte Temperature Limits

Surface Temperatures (T ) , °C

Painted metal SO

Porcelain enamel 56

Glass 55

T^l n '^'ti t^ft BO

Aluminum 60

Stainless steel 50

Carbon steel (1%) 50

Zinc 50

Copper 50

Chrome 50

Handles and Knobs °C

50

55

75

50

^The data in this table are for illustrative purposes only and should

not be taken as final or authoritative.

' 1/

A. Surface Materxal—

Bare or painted metalGlass?^/

Plastic 7 , Decorative /indicator metal strivs—

— Surfaces include open comers, decorator strips, portions of the product

which may be used as a work area. There is no exclusion by height from floor

or type of installation.

2/— These temperatures apply for those elements of the product in which a heat

conduction path is provided by direct contact with a heat source. 3/ . . — Thin metal strips on knobs are excellent heat sinks; if these are not

insulated from heat flow they become a severe hot spot.

D. Test Accessibility Probes

1, An articulated test probe will be used to determine accessibility

by an adult hand to remote portions of the product. The probe will approximate

the maximum reach of an adult hand in straight or bent attitudes.

2. An articulated test probe will be used to determine accessibility

by a child's hand to remote portions of the product. The probe will approximate

the maximum reach of a child's hand in straight or bent attitudes.
E. Test Method

. ^ 1 . Determ-ination of aooessible contact surfaces

(a) The subject product will he examined for all accessible

nonfunctional or included surfaces. These surfaces are designated as

test surfaces.

(b) The articulated probes will be used to determine accessibility

to remote surfaces.

2. Test conditions

(a) The product will be activated in an operating mode which

reflects its normal performance result.

(b) Each test surface will be measured for its surface

temperature by a thermocouple probe and recorded.

(o) This test will be repeated for each designed mode of

operation of the product and for all of its normal cycles, range of settings,

and performance results. .

(d) A thermesthesiometer may be used instead of a thermocouple

to obtain a direct measurement of contact temfperature if the test conditions

aj'e satisfied. ...

Z. Test measure

(a) Each recorded surface temperature will be converted to an

equivalent human contact temperature (T^) based on a three-second contact

time and the thermal inertia of the test surface. A four-second contact

time is permitted for handles or knobs unless the test surface has a thermal

inertia greater than 0.07.

(b) If a thermesthesiometer is usedy step 2 (a) is omitted.

(c) All recorded T^ values are compared against the maximum

temperature levels, given in Section C, for compliance to the standard.

-10-
Discussion

Some of the minor bum problems could be corrected by the adoption

and observance of good manufacturing practices. These practices include

using appropriate materials of low thermal inertia, insulation, or

coatings on surfaces; using good design procedures which will reduce

heat flow to particular areas of the product; an(| using good quality

control procedures to test product performance.

A number of additional features, which could be promulgated as good

manufacturing practice or guidelines and would serve as effective

countermeasures , are warning signal displays of dangerous temperature

levels, interlock arrangements, cut-off devices adjusted to improper use,

etc. These require research and development, but they represent ways

and means of reducing bum accidents and should be considered as items

of interest for product safety.

The illustrative standard which appears above is not intended to be

complete, but rather to suggest the tone and content for the control of hot

surfaces associated with some household appliances. This attempt has

not had the benefit of the normal process of examination and consensus

usually associated with the development of a voluntary industry standard.

There are a number of technical matters that need additional study before

data and conclusions can be taken as authoritative.

-11-
(
Appendix A.

Technical Approach

A-1
A. Clustering and Fault-TreC: Analysis

In order to determine how many product -tyjies will be covered by the

illustrative standard, and in order to determine appropriate countermeasurcs

to the hazard, the product-types are (1) clustered according to certain

attributes, and (2) analyzed to determine the precise nature of the producthazard factors. The first process, clustering, is intended to group together

these product-types which have similar hazard characteristics. The

second process, fault -tree analysis, is used to confirm or reject the

initial assignment to the cluster by examining, in critical detail, each

product -hazard factor for the causes of the hazard and the choices for

corrective action. A product-type may be dropped from the cluster if

the character of the hazard is significantly different from the rest of the

product -types or if the countermeasure is peculiar to the product

type and not common to the group. By studying the hazard as it relates to

the product -types and evaluating the information gained from the fault

tree diagrams, a final determination can be made for each producttype about its inclusion under the standard. The details of this development are given in the following sections.

NEISS Data Source

The NEISS data base lists approximately 240 product categories associated

with thermal -bum injuries (see Appendix B) . Many of these injuries are

associated more with behavioral patterns than with product use, and

many of the product categories are not appropriate candidates for inclusion

under our illustrative standard because of their special characteristics.

A-2
Of these 240 product categories, we eliminate those whose special

characteristics make them inappropriate for inclusion under our illustrative

standard. Categories such as torches, charcoal grills, welding equipment,

etc.; products which are not typically associated with household appliance

use such as firearms, vehicles, foods, fireworks, etc., are also excluded.

This screening results in reducing the list of 240 to 19 product categories

of interest for our hypothetical regulation. This list is given in

Table A-1: '

'

,

Table A-1: Basic Product-Type Cluster

Product Frequency'' Product Frequency''

Ranges

Irons

Ovens

Stoves

Broilers, Electric

Cookware, Metal

Cooking Stoves

Space Heaters

Gas Room Heaters

Gas Space Heaters

261

258

195

180

68

46

40

40

31

30

Space Heaters, Electric 30

Fry Pans 28

Heating Pads, Electric 25

Hot Plates, Electric 24

Steam Iron 20

Pressure Cookers 20

Toasters 19

Coffee Makers 19

Table Stoves 18

*1975 data, based on 119 hospitals reporting under the NEISS.

These 19 product categories are involved in about 451 of all bums recorded

by the NEISS for 1975. Since the NEISS system reports only emergency room

cases for those hospitals reporting under the NEISS structure, the total

frequency is not necessarily indicative of the assumed widespread hazard

of "nuisance" bums which are not reported or are reported under different

circumstances, such as at bum treatment centers.
ElEltniC RANGES
FIGURE A-2 FAULT-TREE DIAGRAM FOR ELECTRIC IRONS

A7
A8
A9
important use o£ fault-tree analysis is the development of what is called

the "minimum- cut set." The minirum-cut set is the set of basic events

which can produce the failure or accident, but which cannot be reduced and

still cause the hazard or failure. Minimum-cut sets can be used to study

a variety of safety problems. One of the most important of these is the

identification of factors or conditions common to all or some of the paths

in the fault-tree diagram. If appropriate countermeasures can be specified

for certain conditions in the minimum-cut set, then the correction or reduction

of the failure or accident can be obtained in an efficient manner. This

process amounts to an optimization, in that if countermeasures are applied

to a minimum number of basic events, then a maximum reduction of the hazard

is realized. ,

, Although the minimum- cut set is not shown in the fault -tree diagrams,

the concept was used to identify the countermeasures. Based on the analysis,

it was concluded that the illustrative standard will be generic in scope

because the hazard is shown to be common to the product -types in the cluster,

and a single test method can be specified for all the product- types in the

cluster.

B. Heat -Flow Factors and Equations

It is not appropriate for the purposes of this report to digress into

an extensive review of heat conductivity, but certain aspects of heat-flow

phenomena are relevant as background to the content of the illustrative

standard. '

The material which follows on heat -flow phenomena is based on the work

contained in References 2 and 3, and has been paraphrased for the purposes

A-10
of this report. The need for including selected portions of theory is to

provide a foundation for selectin,g effective countermeasures and to understand the physical limitations and consequences of choices for this standard.

Thermal Inertia and Diffusivity

Thermal inertia (/x) is defined as the square root of the product

of thermal conductivity (k) , specific heat (c) , and density (p)

/a = Ac^ . (1)

Diffusivity (a) is defined as

a = ^ . (2)

cp

^

In general terms, thermal inertia is a way of describing the capability

of a material to move heat from itself to another material. Diffusivity

is a measure of how quickly heat moves from one point to another. Both

T and a are used in the discussions which follow.

Basic Concepts

The basic ideas of heat -flow theory that are relevant to the

illustrative standard are:

(1) the relative temperature distribution is a function of thermal

inertia,

(2) if the time of contact is large, the temperature distribution

behaves as if the interface were not present,

(3) the time required for temperature to reach a particular value is

proportional to the square of the distance and inversely proportional to

the thermal diffusivity.

A-11
(4) given an interval of contact time, the thickness o£ the inaterial

influences whether the maximum rate of dissipation occurs during that

interval or occurs at a time greater than that interval.

The significance of this is that under certain conditions of contact between

human tissue and a hot surface, the influence of a protecting layer is

meaningful only when the contact time is reasonably short.

A basic steady-state equation obtained from the theory, which

relates surface temperature (T^) to skin-contact temperature (T^) and

the themial inertia of the material (/a), is:

T =

-

. (3) s c — — ^ /X

where T is the assumed temperature of the skin. A device called the

P thermesthesiometer has been constructed to measure surface temperatures

as they are perceived by human tissue (T^) . This work is described in

Reference 3. T may be taken as 33°C, so equation (3) becomes

p J = J ^ 0.035 (T^ - 33) .

/X

Equation (4) shows that as the thermal inertia becomes large the contact

temperature approaches the surface temperature.

Figure A- 5 shows the relation between T^ and T^ for a range of values

jf R = 28.6 /x, where R is the ratio of the thermal inertia of the material to

the thermal inertia of skin. The thermal inertia of skin is 0.035, whose

inverse is 28.6. Also shown are various materials according to their value

of R. Figure A-6 shows the physiological response, contact-time data.

A contact time greater than 2 seconds is considered to be a steady- state

A-12
condition for which equation (3) is valid. From Figure A-6 we construct

Table A- 2, which gives the pain response temperature for various contact

times; included also are the corresponding temperatures for the lower

limit of reversible bum injury (shown as Lower Reversible in Table A-2).

Table A-2: Temperature -Contact Time Relationships

Contact Time (seconds) Threshold of Pain (°C) Lower Reversible (°C)

0.5 70 70

1 62 65

2 54 61

4 48* 58

8 46* 56

16 44* 54

*Extrapolated estimates.

The data of Figures A- 5 and A-6, and Table A-2 provide a basis for

evaluating surface tenperature standards as they appear in some current

voluntary standards; this evaluation follows.

C. Determination of Surface Temperature Levels

Current Surface Tenperature Limits

The following Table, A- 3, of maximum acceptable temperature limits is

extracted from a current, voluntary standard for kitchen ranges.

A-13
SUIlFACt lHWI'LilAI UHi;S BY I Mf filVl/Vl IMl K 1 lA I OH Sl'Li:Cn;i) r.UNI AC I l EMPEIiATllllES

FIGURE A-5 SURFACE TEMPERATURE, CELSIUS

A-14
A-15
Table A- 3. Maximum Acceptable Temperature Limits*

op

Surfaces'^

(1) Bare or painted metal 67 152

(2) Porcelain enamel 71 160

(3) Glass 78 172

r41 Plastic 83 182

Handles and knobs^

(1) Bare or painted metal 55 131

(2) Glass , 65 i 149

(3) Plastic 75 167

temperature limits are increased 17°C (31°F) for areas that will be more than 3 feet above floor level

as installed. A cabinet -supported, counter-mounted,

or wall -mounted appliance is to be installed in

accordance with the manufacturer's instructions to

determine which areas are 3 feet above floor level.

Includes plastic with a metal plating not more than

0.005- inch thick; and metal with a plastic or vinyl

covering not less than 0.005-inch thick.

For a self-cleaning or a continuous -cleaning oven, these

temperature limits apply only during the time an oven

door can be opened. At cleaning temperatures v^en the

door is locked, the temperature limits for handles and

knobs are the same as given for surfaces.

^Effective September 30, 1975

Our interest in this table concerns the protection it provides against

severe bums, the diversity of surface materials of interest, and the

extent to which it adequately deals with composite surfaces.

In order to understand the temperature limits for surfaces, handles,

and knobs in a more concrete way we plot the temperatures of the table on

A-16
the surfaoe-temperature versus R graph. This plot is given in Figure A- 7.

The limits for surfaces fall on or to the left of the 55°C threshold line.

At this temperature a safe contact time is two seconds or less. The point

for plastics .is safer, giving about a three-second contact time. The plots

for handles and knobs lie consistently close to the 48°C threshold line,

which provides protection for a contact time of four seconds or less.

A concern is whether it is adequate to try to cover all metals with

one temperature limit, i.e., 67°C. From Figure A-5 it can be seen that

if products are made from alloys or materials with high values of R, the

contact ten^erature will be very close to the surface temperature. At

67°C surface temperature, no more than a one- second contact time can be

tolerated, and this is getting close to non- reversible tissue damage if

longer contact -times occur. A last point concerns the problem of composite surfaces. On a

theoretical basis, composite surfaces are difficult to model because the

assumptions needed to solve the heat conduction equation are difficult to

reproduce experimentally. Consequently, theoretical solutions may be

unsatisfactory to establish useful, operational criteria. Data on

composite surface heat- flow phenomena may have to be developed

experimentally to provide industry with guidelines. The table cited

above provides composite -surface data for only plastic coating and

porcelain enamel. Painted metal and bare metal have the same temperature

limit.

It should be noted that the temperature- limit table. A- 3, was

intended to apply to kitchen ranges and was not necessarily intended as

a generic standard for temperature limits for all product-types of a

similar nature.

A-17
PLOT OF CURRENT TEMPERATURE LIMITS

100 200 300 400 500 GOO 700

FIGURE A 7 SURFACE TEMPERATURE, CELSIUS

A-18
Ranges and other similar products do, nevertheless, create a potential for

severe bums, not so much from hot surfaces as from hot spots, which in

some standards are excluded from the standard temperature limits or not

addressed at all. The results of our analysis of the heat- flow problem

and the points raised on current standards provide a basis for a revised

table of temperature limits.

Revised Temperature Limits

If a reasonable contact time can be ascertained, then the temperature

limit standards follow directly. The selection of a reasonable contact

time, however, is based on a difficult accommodation of providing safety

to the user and feasibility to the manufacturer. Determining the level of

safety and achieving that level involves a number of issues, some of

which may be more a matter of opinion or interpretation than a matter of

fact. Different assumptions about safety requirements lead to different

standards for contact times and surface temperature limits. For the

purposes of illustrating the generic method a conservative decision was

made to use 50 °C as the contact temperature and three seconds as the

maximum contact time.

The selections of a contact time of three seconds and a contact

temperature of 50 °C are founded on the following points:

(1) A contact time greater than three seconds would probably occur in

situations other than in the normal use of the product; therefore, if the

contact time is greater than three seconds, then it is likely that the

situation is abnormal, and protection should be provided;

A- 19
(2) Hot spots are often more hazardous than hot surfaces, and a threesecond contact time is probably the maximum allowable contact time if the

thermal inertia of the spot surface is greater than 0.07;

(3) A contact time greater than three seconds and a contact temperature

greater than 50°-55°C produces pain and may cause injury.

(4) A combination of a four-second contact time and a contact

temperature of 55 °C may be a contributing factor in non-bum kitchen

accidents because of sudden, involuntary reaction to the pain;

(5) If a contact time greater than three seconds is experienced by a

user, but the contact temperature is not greater than 50°C, then the injury

severity is held below the lower reversible damage curve. This conclusion

is based on the admittedly uncertain reliability of the response curves of

Figure A- 6. Until these data are improved, however, the conclusion is not

unreasonable.

Therefore, until the physiological response data can be improved, the

three -second contact time and the 50 °C contact temperature have been chosen

as conservative limits for the illustrative standard.

Taking 50 °C as a compromise contact temperature the maximum acceptable

surface temperature- limit table would become:

Table A-4: Revised Maximum Surface Temperature Limits

Surface Material Maximum Surface Temp. (°C)

(1) Bare or painted metal 50

(2) Porcelain enamel 55

(3) Glass 55

(4) Plastic 82

•', A- 20
The values for the first three surface types differ markedly from

those in current use. For handles and knobs, the plots shown in Figure A7

lie in the region of a four-second contact time. For normal use of the range

this contact time is reasonable, but the hazard may be associated with an

accessory to the handle or knob rather than the handle itself. An

accessory would be a rivet, a support, a bolt, or a structure attaching

the handle to a surface of the appliance. These accessories are usually

made of metal for structural integrity; if this is so, then their heat

conduction properties should conform to safe limits. Our limited,

empirical data on hot spots show, however, that the contact temperature

ranged from 10^ to 145% above the average contact temperature of nearby

surfaces, although not all measured temperatures were considered

hazardous. All of these hot spots were readily accessible to contact

and were located in the "business" part of the appliance. The temperature

limits of current standards do not cover these hot spots directly because

they are not always located on surfaces nor are they classified as

handles or knobs. A typical hot spot on a range is a small metal disc

or strip fastened into the face of a knob. This strip is usually in a

direct line of heat flow from the interior of the range by virtue of

being in contact with or contiguous to a knob stem, which is usually

made of highly conductive material. Other hot spots, hot comers, or

local hot areas on large surfaces occur because the configuration of the

appliance concentrates the heat in particular areas.

In order to correct these hazards it is suggested that the maximum

acceptable temperature limits be amended to apply to hot spots and hot

strips; be based on contact times no greater than three seconds for

A-21
surfaces and four seconds for handles and knobs, as qualified by thermal

inertia; and include a wide range of surface materials. Table A- 5 gives the

revised temperature limits as developed for the illustrative standard.

D. Countermeasures

The perceived hazard of thermal bums may be effectively reduced by

the following actions: (1) tightening the maximum acceptable temperature

limits to correspond to a three-second contact time for surfaces and a foursecond contact time for handles and knobs; (2) using a probe for access

determinations which more closely approximates the dimensions of an

adult's or child's hand; (3) controlling the temperature level of surfaces

of appliances likely to be used as work areas; and (4) including in the

surface elements of the maximum-acceptable- temperature- limits table the

variety of materials likely to be used in the manufacture of appliances

whose thermal inertial values are in excess of 0.07. (This value corresponds

to R=2 in Figures A- 5 and A- 7.)

Depending on the product- type and the manifestation of the hazard,

the ways in which temperature levels are controlled will be through use

of either design changes, new or surrogate materials, coating surfaces

with materials having low thermal inertial properties, or insulation.

Which one is used will depend on a cost-benefit evaluation and the

attitude toward compliance.

E. Test Method

The test procedure for the countermeasures includes a table of

maximum-acceptable-temperature-limits (MATL) for actions (1), (3) and

. A-22
(4) , general specifications for an access probe for action (2) , and a

method for obtaining contact temperatures of human response from surface

temperature measurements. Tliese are discussed separately:

(1) Based on the foregoing analysis, the table of maximum- temperature

limits is revised as follows:

Table A- 5. Revised Maximum Acceptable Temperature Limits*

A. Surface Material— Surface Temperatures (T )

, °C

Painted metal 50

Porcelain enamel 55

Glass 55

Plastics 80

Aluminum * 60

Stainless steel 50

Carbon steel (1^) 50

Zinc 50

Copper 50

Chrome 50

B. Handles and Knobs °C

Bare or painted metal— 50

Glass 2/ ' 55

Plastic ^/ 75

Decorative/indicator metal strips— 50

*The data in this table are for illustrative purposes only and should not be

taken as final or authoritative.

— Surfaces include open comers, decorator strips, portions of the product

which may be used as a work area. There is no exclusion by height from

floor or type of installation.

21— These temperatures apply for those elements of the product in which a heat conduction path is provided by direct contact with a heat source.

3/— Thin metal strips on knobs are excellent heat sinks; if these are not

insulated from heat flow they become a severe hot spot.

A-23
(2) The access probe prescribed in certain current voluntary standards

is not appropriate for simulating the human finger. Its dimensions do

not approximate those of the finger nor is it designed for articulation.

The articulated probe described in UL 1026 possesses most of the necessary

features for good hand or finger simulation. Its dimensions approximate the

adult hand; however, a probe representing juvenile hand characteristics

is necessary.

(3) Compliance with the temperature levels given above requires

measuring product -element temperatures (T^) , according to test procedures,

and converting these temperatures to equivalent contact temperatures for

humans (T )

.

A test procedure prescribes the orientation or use of the product,

important human factors at play in the product's use, and includes in

the measurements to be made the appropriate combinations of variables

and conditions. It is important to include the anticipated uses of the

appliance, such as using its top surfaces as a work area, in the test

scenarios . ,

.

(4) There are two methods to obtain measurements of T^, one based on a

measuring instrument called the thermesthesiometer, the other is the conversion

of a measured surface -temperature to its equivalent T^ value.

The thermesthesiometer is a thermocouple probe designed to simulate

the heat conduction properties of human skin. It measures the temperature

of a hot surface as the skin would measure it, as a function of the length

of time of contact. These data are used to make a theoretical determination

of skin damage or the severity of the injury.

A- 24
Although the thermesthesiometer has been able to verify theoretical

models of heat conduction in which human tissue is a subject, has provided

a device which bypasses the need to estimate thermal inert ial values for

composite services, and has dramatized the importance of contact times in bum

accidents, it is not recommended for extended use in its present configuration

in a quality control, production line context.

An alternative procedure is to measure test surface temperatures in

the usual manner by employing a standard thermocouple probe, regular or

articulated, according to the test procedures of the illustrative standard

or their equivalent, then convert this temperature to an equivalent

human-response temperature. This method requires knowing the thermal

inertia of the surface, and this can be obtained by using the thermesthesiometer

under a strict experimental regimen. It would combine measuring by a

standard thermocouple and by the thermesthesiometer and computing /x

from equation (3) . The value of T^ would have to be stabilized for each

experiment, for large variations in either T^ or T^, or both, would

produce corresponding variations in /x. The final estimate, in theory,

of /a would be given as an interval estimate. From experiments made

over many surface types a table of thermal inertial values could be

developed which when entered with a measured T^ would yield the proper

value of T^. If a thermesthesiometer is to be used directly, in place of

the method given above, then it should be designed to meet the requirements

of the measuring probes as implied in the illustrative standard.

A-25

Appendix B.

Consumer Products Associated with

Thermal Bums: NEISS Classification

B-1
Tlie following list of 240 product catci;orics twc based on the NIHSS

for 1975.

Ranges, Not Otherwise Specified . Irons, Not Otherwise vSpecified

Motor Vehicles, Except Two-Wheeled Vehicles

Gasoline

Sun Lamps

Matches

Cigarettes, Cigars and Pipes

Ovens, Not Otherwise Specified

Stove/Range, With Oven, Gas

Licensed Two-Wheeled Vehicles, Motorcycles, Etc.

Home Structures, Not Otherwise Specified

Heating Systems, Not Otherwise Specified

Foods

Fireworks

Cigarette/Pipe/Cigar Lighters c i. Charcoal

Cookware, Not Otherwise Specified

Radiators, Home

Lighter Fluid

Welding Equipment, Not Otherwise Specified

Furnaces, Not Otherwise Specified

Day Wear

Home Elect. Wiring, Outlets, Fuses, Fuseboxes

Cookware, Metal

Floor Furnaces, Not Otherwise Specified

Torches, Soldering, Cutting, Welding, Unpowered

Gas Furnaces - Excluding Wall, Room, Unit, Duct, Floor Heater

Stove/Range, With Oven, Electric

Irons With Dry Heat

Wires/Cords, Not Otherwise Specified

Portable Gasoline Cooking Stoves/Grills

Space Heaters, Not Otherwise Specified

Outdoor Grills, Not Specified

Light Bulbs

Radiators, Not Otherwise Specified

Heaters, Room, Gas, Floor Type

Clothing

Space Heaters, Gas, Not Otherwise Specified

Ovens, Separate From Ranges, Gas

Electric Fry Pans and Skillets

Fire Arms

Grills, Charcoal, Not Otherwise Specified

Range and Oven Accessories - E.G. Racks, Broiler Pans

Electric Heating Pads

Water Heaters, Not Otherwise Specified

Electric Hot Plates

Lawn Mowers, Not Otherwise Specified

Nightwear

Extension Cords

B-2
Motor Scooters, Minibikes, Etc., (2 or 3 Wheels)

Gas Water Heaters

Pressure Cookers and Canners

Steam Irons

Toasters

Coffeemakers/Teapots, Not Otherwise Specified

Table Stoves, Open Flame

Wax Candles/Paraffin

Electric Space Heaters, Not Otherwise Specified

Metal Pieces, Not Otherwise Specified

Power Mowers, Not Specified

Charcoal Igniters, Chemical

Gasoline, Kerosene and Propane Lanterns and Lamps

Hot Water Pipes

Fireplaces, Not Otherwise Specified

Space Heaters, Electric, Portable

Portable Grills, Charcoal

Appliance and Lamp Cords, Not Attached

Heaters, Wall, Gas

Tableware, Including Insulated Designs

Electric Broilers and Grills

Plastic Products, Not Otherwise Specified

Welding Equipment, Electric

Paper Wrapping Products, Paper Objects

Pressurized Containers

Hair Curlers, Electric, Without Steam

Lamp/Light Fixtures, Lanterns, Not Otherwise Specified

Propane (LP) and Butane Gas Tanks and Fittings

Pipes, Not Otherwise Specified

Ductwork for Heat ing/Cooling Systems, Registers

Steam Pipes

Bathtowels/Cloths , Beach Towels, Dishtowels/Cloths

Mattresses, Not Otherwise Specified

Incinerators, Not Otherwise Specified

Electric Com Popper

Fireplaces, Factory Built, Wood Burning

Beds, Not Otherwise Specified

Hair Dryers

Emergency Flares, Signal Flares

Bicycle and Bicycle Equipment

Bedding, Mattresses, Box Springs, Matt. Covers, Pad

Kerosene

Heat Lamps

Outer Wear

Drapes, Curtains, Inc. Plastic and Shower Curtains

Electric Comb

Gas (LP) Heating Stoves

B-3
Wood Stoves

Radios, All Models

Coal Stoves

Patio Lights /Torches - Fuel Burning

Farm Tractor

Fuel Storage Tanks, Gas Cans

Batteries , Not Otherwise Specified

Soldering Guns and Irons

Mobile Homes and Related Equipment

Roofs and Roofing Materials

Heating Equipment, Portable, Not Otherwise Specified

Germicidal Lamps

Containers, Metal - Cans

Boats, Motors and Accessories for Recreational Use

Rope and String

Go-Carts, All-Terrain Vehicles, Etc., (4 or More Wheels)

Power Ntower, Rotary, Gasoline

Hair Curlers, Hair Pins, Hair Clips, Etc.

Waste Containers

Gas Pipes, Fittings and Distribution Systems

Clothes Dryers, Not Otherwise Specified

Vacuum Cleaners

TV, Not Otherwise Specified

Microwave Ovens Separate from Ranges

Electric Deep Fryers

Floor Furnaces, Gas

Kerosene Space Heaters , Attached

Ovens, Separate from Ranges, Electric

Air Conditioners, Not Otherwise Specified , Flatware, Except Cutlery ; Dishwashers

Fireplace Equipment

Other Kitchen Gadgets, Mix/Measuring Spoons/Cup

Ironers - an le

Alcohol, Not Otherwise Specified

Catalytic Heater (LP or Gasoline)

Hair Curlers, Electric, With Steam

Paint and Varnish Thinners

Paints, Varnish, Shellac, Rust Preventive, Etc.

Home First-Aid, Health Equip., Thermometers, Q-Tips, Etc.

Solvent Based Cleaning and Sanitizing Compounds

Exercise Equipment

Gasoline or Other Fuel -Powered Toys, Model Cars

Grills, Electric, Stationary, Built-in

Hair Brush/Combination - Not Powered

Hand Mowers

Alcohol, Beverage

Stationary Grills, Gas

Stationary Grills, Charcoal

Portable Gas Heating Equipment - LP .

B-4
Football, Activity and Related Equipment

Containers, Plastic, Including Bottles, Bowls, Etc.

Camping Equipment Including Tents, Cots, and Sleeping Bags

Portable Gasoline Heating Equipment

Turpentine

Lubricants, Machine Oils, Engine Oil

Adhesives and Adhesive Products Including Glues

Respiratory Protection Devices

Other Construction Materials

Chain Saws '

, Snow Throwers, Blowers

Garden Tractors

Grills, Gas, Not Otherwise Specified

Liniments, Rubbing Compounds, Including Camphor, Etc.

Bricks, Concrete Blocks

Couches, Sofas, Davenport, Divan, Studio Couches

Electric Table Lamps 5 Floor Lamps

Step Stools

Beds, Springs, Frames, Not Mattresses/Box Springs

Blankets, Not Otherwise Specified

Fireplaces, Built-in

Rugs, Carpets, Not Otherwise Specified

Electric Kettles

Electric Coffeemakers and Teapots

Pillows

Plastic Parts or Pieces, Unknown Product Origin

Other Heating Systems, Including Heat Pumps

Chairs, Not Upholstered or Not Otherwise Specified

Floor Furnaces, Oil

Automotive Tools and Accessories

Cookware, Non-Mdtal Including Glass, Pottery and Ceramic

Cutlery, Unpowered

Sinks

Batteries, Wet Cell

Boilers

Electric Immersion Water Heaters

Bleaches and Dyes, Not Intended for Cosmetic Use

Space Heaters, Electric, Stationary

Waxes, Floor

Faucet Water Heaters

Potholders, Oven Mits, Hot Pads

Coffeemakers and Teapots, Unpowered

Garbage Disposers

Knives, Not Otherwise Specified

Candle Holders, Candlesticks

Sound Recording and Reproducing Equipment, E.G. Tape Recorders

Other Chemicals

Shoe Polish

Refrigerators, Not Otherwise Specified

B-5
Automotive Chemicals

Straw, Drinking

Coal Furnaces

Electric Fences

Bathtub and Shower Enclosures of Non-Glass Materials

Farm Materials Handling Equipment, Not Otherwise Specified

Cutting and Chopping Devices

Resealable Closures

Air Compressors, Separate

Sheets and Pillow Cases

Kerosene Heating Stoves, Not Attached

Cardboard Boxes , Cartons and Other Cardboard Products

Aluminum Foil Wrapping Products

Glass Bottles and Jars, Not Otherwise Specified

Separate Electric Motors

Coffee Grinders, Not Otherwise Specified

Stove, Combination Heating/Cooking

Oil Furnaces

Test Equipment, Voltage Testers

Glass Bathtub and Shower Enclosures

Heaters and Duct Heaters , Gas Unit , Suspended

Gas Ranges Without Ovens

Swimming Pools and Associated Equipment, Not Including Above Ground

Gas, Air and Spring Operated Guns

Slides

Electric Ranges Without Ovens

Blankets/Sheets, Electric

Bars and Bar Stools

Portable Grills, Kerosene

Glass, Unknown Origin

Portable Alcohol Heating Equipment

Electric Griddles

Power Drills

Billiards, Tables, Balls, Etc.

Fire Extinguishers

Charcoal Lighters , Not Otherwise Specified

Guns, Not Otherwise Specified

Thermometer, Cooking

Tennis, Badminton and Squash, Activity and Equipment

Toy Cars and Trucks, Non-Fly Planes, Boats -Not Models

Electric Waffle Irons

Toy Guns and Other Toy Weapons with Projectiles

Furniture, Not Otherwise Specified

Other Ntodels and Their Construction Materials

Non-Heating Toy Home Equipment Including Stoves, Irons, Etc.

Caps and Cap Toys

Fuel for Model Cars, Airplanes, Etc.

Woodbuming Kits

Molding Compounds (E.G., Clay, Play-Dough, Etc.)

B-6
Toys, Not Otherwise Specified

Plumbing Pipes

Blankets, Except Electric and Baby Blankets

Ice Cream Makers, Not Otherwise Specified

Floor Furnaces, Electric

Power Tillers and Cultivators

Outdoor Lighting Equipment, Electric

Electric Dryers Without Washing Machines Attached

Washing Machines with Wringers

Power Mowers, Rotary, Electric

Hatchets, Axes

Gas Lamps

Drinking Glasses, Glass

Bottle Warmers

Sterilizers

Night Lamps

Crib Mattress, Playpen Pad, Crib Bumper Pad, Etc.

Washing Machines, Not Otherwise Specified

Trays Including Folding TV Trays

Gas Incinerators

Electric Water Heaters

Massage Devices

Saunas Including Facial Saunas

Clothes Hangers of All Types

Sewing Machines

Electric Fans, Portable

Industrial Equipment

Wire, Not Electric, Hanging, Construction and Barbed Wire

Ash Trays

Elevators and Other Lifts

Outside Structures, Exterior Walls, Patios, Etc.

Insecticides, Fungicides and Rodenticides

Comb, Not Otherwise Specified

Hair Waving Preparations and Straighteners

Medical Therapeutic Equipment

Other Seasonal Decorations

Hair Curlers, Hair Pins, Etc., Not Otherwise Specified

Trains

Adult Games and Novelty Items

Christmas Tree Lights

Lumber, Boards, Paneling Pieces, Not Part of Structure

B-7
References

1. Hendrickson, R. , Guidelines for the Determination and Development of

Generic Safety Standards , NBSIR 76-1096, National Bureau of Standards,

Washington, D.C., 1976, pp. A1-A8.

2. Wu, Y. C., "On the Control of Thermal Impact for Thermal Safety," AIM

Paper No. 75-713, AIAA 10th Thermosphysics Conference, Denver, May 1975

3. Marzetta, L. A., "Engineering and Construction Manual for an Instrument

to Make Bum Hazard Measurements in Consumer Products," NBS Technical

Note 816, National Bureau of Standards, Washington, D.C., February 1974

4. "Product Safety: Caveat Vendor," Materials Research and Standards , asTM Vol. 12, No. 8, August 1972, pp. 12-17 et seq.

5. Hammer, W. , Handbook of System and Product Safety , Prentice -Hall,

Englewood Cliffs, N. J., 1972, pp. 225-250.
. Oi- I (H E V . 7- 7 J)

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET

1. PUIU.K A riON OR Ki'POR T NO. NBSIR 76-10972. Gov 'r Ac i I- s lull No.

3. K( < i|ii( iii 's Arc (-sioii Nil.

4. TITI.!-: AND SU |VI IT1.1{

Illustrative Generic Standard for the Control of Thermal

Bum Hazards in Household Appliances

5. I'ublication Daic

June 30, 1976

6. Performing Organization ( ode 3053

7. AU rHOR(S)

Robert G. Hendrickson, Elizabeth M. Robertston, Rudolph V. Kelly

8. Performing Organ. Kepori No.

9. PERFORMINC; ORGANIZATION NAMF AND ADDRHSS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, D.C. 20234

10. i^roiect/Task/Work Dim No. 441.1432, 441.2432

11. Contract /Grant No.

12. Spon.soring Org.iii Izat ion Name and C omplctc Address (Street, City, State, ZIP)

Consumer Product Safety Commission

Washington, D.C. 20207

13. Type of Report & Period

< -overi d Final

118

15. SUPPI.FMFNTARY NOTIiS

16. ABSTRAC'T (A 200-word or less factual summary of most significant information. If document includes a significant

bibliography or literature survey, mention it here.)

This document reports on the development of an illustrative generic standard for

controlling hot surfaces associated with certain categories of consumer products. The

development includes evaluations of accident data, fault -tree diagrams, theoretical

heat-flow phenomena, current standards, and application of the thermosthesiometer as a test instrument.

This work is a conpanion to the Guidelines for the Development of Generic Safety

Standards, and as such, it applies the methods and techniques provided in the Guidelines.

Tne principal tool of analysis is the fault -tree method. This method brings to safety

problems a versatile and insightful way of depicting events, conditions, and cruses associated with hazards and accidents.

The intent is to demonstrate the feasibility of the generic approach to controlling

safety aspects of consumer products. Although the illustrative standard is based on a " study of actual data, the conclusions are not to be construed as final or authoritative.

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

)

Burn hazards, household appliances; fault-tree diagrams;

generic safety standards; household appliances; operations research;

thermal burn hazards

18. AVAILABILITY

^

Unlimited

QZ! ^ °' Official Distribution. Do Not Release to NTIS

' Order From Sup. of Doc, U.S. Government Printing Office Washington, D.C. 20402, SD Cat. No. C13

[ 2^ Order F rom National Technical Information Service (NTIS)

Springfield, Virginia 22151

19. SECURITY CLASS

(THIS REPORT)

UNCL ASSIFIED

20. SECURITY CLASS

(THIS PAGE)

UNCLASSIFIED

21. NO. OF PAGES

47

22. Price

$4.00

USCOMM-DC 29042-P74


1