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NBSIR 79-1716

Possible Fire and Electric

Shock Hazards from Hot

Lamps in Miniature Christmas

Tree Light Strings and

Decorations

P. Michael Fulcomer

Product Safety Technology Division

Center for Consumer Product Technology

March 1979

Report to: —'&C — -Consumer Product Safety Commission

100 5401 Westbard Avenue

. U56

Bethesda, Maryland 20016

79-1716

NBSIR 79-1716

POSSIBLE FIRE AND ELECTRIC

SHOCK HAZARDS FROM HOT

LAMPS IN MINIATURE CHRISTMAS

TREE LIGHT STRINGS AND

DECORATIONS

SEP 1 7 197$>

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P. Michael Fulcomer

Product Safety Technology Division

Center for Consumer Product Technology

March 1979

Report to: Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda, Maryland 20016

U.S. DEPARTMENT OF COMMERCE, Juanita M. Kreps, Secretary

Jordan J. Baruch, Assistant Secretary for Science and Technology

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

POSSIBLE FIRE AND ELECTRIC SHOCK HAZARDS FROM

HOT LAMPS IN MINIATURE CHRISTMAS TREE LIGHT

STRINGS AND DECORATIONS

INTRODUCTION

Upon cursory examination, miniature or midget Christmas tree light

strings appear to be innocuous. The total power consumption of these

series-connected sets is considerably lower than that of parallel-connected

light strings, and the miniature lamps seem to operate at lower

temperatures. However, there are several technical subtleties in series

type light string construction and operation which, under certain

conditions of use or predictable misuse, can lead to extremely high lamp

bulb surface temperatures. These high temperatures, in turn, can cause subsequent ignition (fire) and/or electric shock hazards . The

technical subtleties and conditions of use or misuse are discussed in this

report, along with a description of laboratory testing which documents the

potential fire and shock hazards.

The work upon which this report is based was funded by the Consumer

Product Safety Commission (CPSC) as part of their Regulatory Development

program. The CPSC had determined that voluntary standards for series

constructed miniature Christmas tree lights and lighted decorations did not

adequately address existing hazards. Furthermore, some light strings on the market did not meet any recognized safety standard. The Commission

therefore acted to have an offeror develop a safety standard for subsequent

promulgation as a mandatory rule. The author of this report served as technical advisor to the CPSC throughout the standards development process.

PRODUCT DESCRIPTION

The lamps of a miniature Christmas tree light string or decoration are electrically connected in series. The ends of the series circuit are terminated in an attachment plug, by means of which 120 V ac is applied

across the string when the plug is inserted into a household receptacle.

Sometimes two or more series strings are electrically connected in parallel

to a common attachment plug, but the total package is still referred to as

a "light string" or "light set." The total current drawn by a series

string flows through each lamp in the string and is determined by the total

Fire and electric shock hazards can also result from short circuits

(such as caused by staples inserted through the wire) or from poor light

string construction (bare wires exposed, etc). Overcurrent protective

devices and improved methods of light string construction provide

relatively easy and effective solutions to these problems. This report

will mainly concern those hazards resulting from high lamp bulb surface

temperature - a more difficult problem to address.
- 2 -

resistance of all the lamp filaments in the string. The voltage developed

across each lamp is a function of the number of lamps in the series string.

The more lamps there are in series, the less will be the voltage developed

across each lamp.

If a lamp is removed from a series string, none of the remaining lamps

in that string can operate. Lamp burn-out would produce the same effect

were it not for the fact that a shunting device is generally built into

each lamp bulb envelope. The shunt is electrically isolated unless the

lamp filament should open: the shunting device is then activated by the

voltage surge, thereby bypassing the filament and allowing the remaining

lamps in the string to continue operating.

Physically, a miniature Christmas tree light string consists of a number of lampholders, or sockets, connected by wires in series to an attachment plug. Typically, each lamp of a miniature Christmas tree light

set is fitted into a lamp base, with the two lamp leads protruding through

the base and extending upward along opposite sides. When the lamp base is

inserted into a lampholder; the protruding lamp leads make contact with

terminals contained within the lampholder (see figure 1).

A burned out lamp can be replaced by removing the lamp-lamp base

assembly from the lampholder and either (1) replacing it with a similar

lamp-lamp base assembly, or (2) separating the lamp bulb from the lamp

base, inserting a new lamp bulb into the original base, then replacing the

entire assembly into the lampholder. The latter method is likely to be

used if the replacement lamp is supplied without a base or if the supplied

base does not fit into the light string being repaired.

NORMAL OPERATING CONDITIONS

Normally operating miniature Christmas tree lamps dissipate anywhere

from 1/^ to 1 1/3 watts. Hot spot surface temperatures range between 35°C

and 190 C depending upon dissipation and filament placement within the

glass bulb. If the filament is centered, normal surface temperatures

rarely exceed 90 C. Some lighted decorations use miniature lamps which

dissipate up to 2 watts and produce surface temperatures up to 270 C if the

filament is touching the glass envelope. As a rule, the lamps in these

decorations are rigidly held in place.

Temperatures even as high as 270°C are not sufficient to cause

ignition of materials. However, temperatures over 110°C can cause melting

of some lampholder materials, and may subsequently expose electrically live

parts if certain other conditions are present. These conditions include

the contact of a hot lamp with another part of the same or different light

string, or the placing of a decorative attachment over the hot lamp such

that the generated heat cannot readily escape. This subject will be

discussed in more detail later.
- 3

-

VARIATION FROM NORMAL OPERATING CONDITIONS

Under certain conditions, miniature Christmas tree lamp power

dissipation can increase dramatically. Subsequent lamp hot spot surface

temperature can then rise to levels sufficient for the lamp to cause

ignition of a Christmas tree or other materials, such as decorative cotton,

tissue paper, newspaper, etc., with which it may come into contact. There

are two basic sets of circumstances which can cause such excessive power

dissipation by the miniature lamps: one can be initiated by not promptly

replacing burned out lamps; the other can be initiated by placing lamps

into a light string for which they were not designed.

Variation Caused by Not Promptly Replacing

Burned Out Lamps - Cascade Failure

As noted previously, a shunt contained within the miniature Christmas

tree lamp is activated when the lamp filament opens, allowing current to

continue flowing through the string and the remaining lamps to remain

lighted. However, the current through the string increases slightly when

the shunt is activated since the shunt resistance is lower than the

filament resistance. If two or three lamps burn out before any are

replaced, the current increase can become significant. Furthermore, as the

remaining lamps pass the increased current, this tends to reduce their

life. As more lamps burn out, further increases in current result, with

the time between current increases and further burnouts becoming

increasingly shorter. During this time the remaining lamps can dissipate

considerable power before they too eventually open.

One defense against the type of cascade failure just described is to

include an overcurrent protective device in the light string series

circuit: this opens the circuit after the current increases above a specified value. However, most inexpensive devices of this nature rely on

heat generated by high current flow to open a fusing device. For the

device to open fast enough to prevent damage from high hot spot lamp

surface temperatures, a current increase from normal to twice normal is

generally required. Unfortunately, certain lamps can develop hazardous

surface temperatures at currents considerably less than twice their normal

current. Even if light string design were such that an overcurrent

protective device could prevent high lamp surface temperatures in a cascade

failure situation, additional means of protection should be considered for

handling those situations where the protective device either fails to

operate or can easily be defeated by the consumer.

Variation Caused by Placing Lamps Into a Light

Set for Which They Were Not Designed

High lamp dissipation can also result if a consumer should

deliberately or inadvertently insert into a light string one or more lamps

which are not designed for that string. As discussed earlier, light

strings with relatively few lamps in series necessarily drop more voltage

across each lamp in the string than do light strings with a larger number

of lamps in series. A series string of 50 lamps requires lamps ^signed
for approximately 2.4 volts (50 times 2.4 volts equals the 120 V ac household voltage), whereas a series string of 10 lamps requires lamps

designed for 12 volts. To complicate matters further, some 50 light sets use 2.4 volt lamps, while others use 12 volt lamps. (In the latter case

the 50 light set is comprised of five parallel strings, with 10 lamps in

series in each string.) A 35 light set may contain either 3.5 volt lamps

(35 lights in series) or 7 volt lamps (two parallel strings of 17 and 18

lamps in series, respectively).

In the above situations, there is a problem in assuring that the

proper replacement lamps are used in a given string - particularly if the

consumer owns more than one type of light set. Lamps are physically

interchangeable from one string to another. Replacement lamps (either

those which accompany the light string or which may be purchased

separately) are often not furnished with bases: the consumer simply

removes the burned out lamp from its base and inserts a fresh lamp in the

same base. This concept has practical advantages as noted below, but

allows any lamp to be put into any light string, regardless of design.

Furthermore, lamp and lamp base combinations can sometimes be interchanged

as a unit between light sets since (1) many lampholders are sufficiently

similar in design that they will accept lamp-lamp base assemblies of

different manufacture, and (2) different light strings from the same manufacturer typically all use the same style of lampholder.

Small lamp size combined with appearance considerations have, to date,

prevented any meaningful method of identifying the lamp itself. Coding of

the lamp base is possible, but would require that lamps be cemented into

their bases. This would complicate the replacement lamp market and could

make it difficult for' the consumer to find a lamp to fit his string unless

all manufacturers agreed on a standard lampholder configuration. The

latter is unlikely because of the considerable monetary loss to those

manufacturers who would have to change their lampholder and lamp base dies.

(As discussed above, present replacement lamps are typically sold without

bases.) Difficulty in locating a replacement lamp is itself a safety

hazard, since cascade failure can result if burned-out lamps are not

replaced.

Light string current is dependent on the total filament resistance in

the string: a lower total resistance results in a higher current flow. To

keep lamp brightness (which is a function of lamp power dissipation)

roughly equivalent between strings with different numbers of lights, the

lower voltage lamps are designed to operate at higher current than are the

higher voltage lamps. Since power dissipation is the product of voltage

and current, the greater current drawn by lower voltage lamps tends to

bring their power dissipation, and hence their light output, up to the same general range as the higher voltage, lower current lamps

The balance is not perfect. The lamps in a string with only 10 lights

in series burn perceptibly brighter (and will generally dissipate two to

three times more power) than the lamps in strings with 50 lights in series.
- 5

-

The actual current drawn by any series string depends on the

manufacturer's design and on variations in filament resistance from one

lamp to another. Strings with only 10 lamps in series (high voltage lamps)

usually draw from 85 to 100 mA, whereas strings with 17 to 50 lamps in

series (lower voltage lamps) draw 125 to 150 mA or more. Some light

strings manufactured in Italy draw over 200 mA.

If a few high voltage lamps are placed in a string designed for lower

voltage lamps, the high voltage lamps will be subjected to the higher

current that is characteristic of strings using low voltage lamps, and

hence will operate much brighter and hotter than normal. An overcurrent

protective device is of no help in this situation because the higher

filament resistance of the substituted lamps actually lowers string current

slightly. Large reductions in current do not occur because the added

resistance of the high voltage lamp or lamps is only a small portion of the

total series circuit resistance.

BASIC HAZARDS

Fire and electric shock are two basic hazards that can result from

overheated miniature Christmas tree lamps. Fire can result if the lamp

surface temperature becomes high enough to ignite materials which may come

into contact with the lamp. Electric shock can result if the lamp surface

temperature becomes hot enough to melt insulating material with which it

may come into contact, and if electrically live parts are subsequently

exposed and touched by a consumer.

ELECTRIC SHOCK HAZARD

( 7 ) An electric shock hazard can develop from insulation melting

caused by high lamp surface temperatures. This hazard can occur even with

normally operating light strings because the normal operating hot spot

A shock hazard results even if exposed conducting parts happen to be

in the middle of the series circuit. Currents of 80 to 150 mA are common

for series-constructed light strings. Total resistance of the series

string can therefore be calculated by Ohm's Law to be between 800 and 1500

ohms (120 V ac/150 mA = 800 ohms and 120 V ac/80 mA = 1,500 ohms). A value

of 1500 ohms is commonly used to represent electrical resistance of the

human body between perspiring or damp hands. This information and further

calculations using Ohm's Law show that currents in the range of 40 to 52 mA

can flow through the body of a person even though contact with exposed

conducting parts is made at a string location which results in minimum

current flow. Current flow of 5 mA through the human body commonly causes

involuntary muscle contraction. Currents in excess of 18 mA normally

contract the chest muscles, so that breathing is stopped during the shock.

Currents in excess of 50 mA can cause ventricular fibrillation and

subsequent death if the fibrillation is not stopped.
- 6 -

surface temperature of certain lamps, particularly if a lamp filament is

off center, is higher than the melting temperature of some plastic

insulating materials.

As stated earlier, hot spot surface temperatures for normally

operating miniature Christmas tree lamps range between 35°C and 190°C,

depending upon lamp dissipation and filament placement relative to the bulb

envelope. Temperatures over 175°C are unusual, however. The melting

temperature for one of the more popular lampholder materials, polyethylene,

is between 110 and 140°C W, depending on the density of the material.

Another popular lampholder material, polypropylene, has a melting

temperature which ranges between 160 and 175 C 2/

making it safer than

polyethylene. The melting temperature of a third popular lampholder

material, polyvinyl chloride (PVC), depends upon the amount of plasticizers

and other materials added to give characteristics desired of the finished

product. (Pure PVC is rigid, hence, plasticizers are added to increase

flexibility.) The melting temperature of PVC can, therefore, vary between

110 and 180 C, but the type most often used for lampholders in miniature

Christmas tree light strings has a melting temperature between 160 and

175°C - similar to polypropylene.

Under conditions of high ambient temperature, a lampholder may show

some slight deformation before the actual melting temperature is reached.

This is due to stress relaxation in the plastic at its mold release

temperature. A lampholder is normally manufactured by heating the plastic

material above its melting temperature and then molding it to the desired

shape. Following this, the plastic is cooled to a temperature below its

melting temperature and released from the mold. Should the lampholder

again be subjected to the mold release temperature, stresses which may have

built up in the molding process could cause some deformation. Because of

processing economics, the temperature of the plastic when released from the

mold is as close as possible to the melting temperature, the two

temperatures perhaps separated by as little as 10°C.

Exposure of Conducting Parts Due to Contact

Between, a Hot Lamp and Another Lampholder

A hot miniature Christmas tree lamp may not always come in contact

with another portion of the same or another light string or decoration, but

the likelihood of such contact may be quite high. Some lighted decorations

are designed with such close light spacing that contact between one lamp

and another lampholder is almost inevitable. Contact may also occur

because a portion of a light string becomes dislodged from its intended

decorative position and falls against either another portion of the same string or against a separate string. Furthermore, consumers are wont to

exercise considerable ingenuity in designing new and spectacular effects

with light strings and other decorations.

In 1977, a major manufacturer packaged miniature Christmas light

strings with the lamps bunched together inside a plastic cup. When

energized for display purposes, the closely spaced lights shining out the

top of the cup presented an unusual effect. No hazard resulted because the
- 7 -

plastic insulating materials had a high melting temperature. However, a consumer might be inspired by this type of display and decide to try a similar decorating scheme with his own light string, perhaps one with less

safe plastic insulating materials.

In the laboratory, a normally operating 35 light, two-way string

(i.e., two series circuits in parallel, one of 17 lamps and the other of

18) was inserted into a large ceramic coffee mug to achieve a decorative

effect. The visual appearance was striking; unfortunately, not all of the

lamps emerged above the top of the mug. Some lamps became entangled in the

wires below when the lights were inserted into the mug. After

approximately 30 minutes, the normal operating temperature on a few of

these entangled lamps, combined with lack of adequate air flow, caused the

insulation material of surrounding lampholders to melt. One lampholder

melted sufficiently for its lamp to fall out of the holder and open that

portion of the string. However, the other portion of the string continued

operating.

After two hours, the set was unplugged and removed from the coffee

mug. The light string was found to have numerous exposed conducting parts

(see figure 2) and would have presented a serious shock hazard had it not

been unplugged before removal from the mug.

Exposure of Conducting Parts in Non-Contact Situations

Contact between a hot lamp and light string insulating material is not

the only prerequisite for development of an electric shock hazard. Figure

3 shows lampholder melting which occurred when a close-fitting decorative

attachment x was placed over the lamp assembly (lamp, lamp base and

lampholder); figure 4 shows the decorative attachment used. The lamp

assembly was positioned vertically with the lamp pointing down. Air

movement through the attachment was restricted, as might happen if cotton,

artificial snow or other decorative material were used on the Christmas

tree.

(4)

A decorative attachment is a device designed to fit over either the

lamp bulb or the complete lamp assembly of a light string and provide a lighted display of some sort. Decorative attachments are sometimes

supplied as part of a light set, but can also be purchased separately.

They are removable (so that burned out lamps can be replaced) and can,

therefore, be moved from one light string to another to enhance an overall

decorative effect.
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(5) The melting occurred under simulated conditions with a 12 volt

lamp operated in a light string designed for lower voltage lamps. The

current through the lamp, and hence the voltage across the lamp, were much

higher than normal for that lamp, but not for the light string into which

it was inserted. In this test, the 12 volt lamp passed 161 mA and

dissipated 4.5 watts for 10 minutes. The current was then increased by 6

mA to simulate burn out of another lamp somewhere in the series string:

for the next 11 minutes the lamp passed 167 mA and dissipated 5.5 watts.

At the end of that period the lamp filament ceased operation. The melting

temperature of the lampholder material used for the test ranges between 110

and 130°C.

Four other lamp assemblies were tested under similar conditions and

with the same lampholder material. Although considerable melting was observed (see figure 5), no electrical conducting parts were exposed. The

evidence suggests, however, that a potential problem exists for certain

types of lampholder materials. In contrast to these test results, tests of

lamp assemblies in which the lampholder was molded from higher melting

point materials show no such extreme deformations (see figure 6).

Melting similar to that shown in figures 3 and 5 may also result if

the lamp filament is located close to the base of the bulb. Heat from such

a filament location can be sufficient to cause deformation of low melting

point lampholder materials, even without the presence of a close fitting

decorative attachment. Typical lamp surface temperatures at locations next

to the lamp base are plotted in figure.? as a function of lamp dissipation

and filament distance from the base ^

.

A slide wire rheostat is connected in series with the lamp under test

and the 120 V ac supply (see figure 8). The rheostat simulates the total

resistance of other lamps in the string, thereby eliminating testing

problems caused by random failures or poor contacts in these other lamps.

The simulation current is determined by inserting the lamp to be tested in an actual light string, then measuring the current flow in that string.

The slide wire rheostat of the simulated circuit is then adjusted to

provide the predetermined current flow through the lamp under test.

Decrease of the rheostat resistance from the established value simulates

the increase in current which results when one or more lamps burn out.

This new value of current can also be determined beforehand by trial with

an actual light string, if desired.

( 6 ) The data for this figure was obtained in a manner similar to that

described under Detailed Test Descriptions and Test Results in the section

on Fire Hazards.
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-

As discussed earlier, consumer actions (or inactions), such as failure

to replace burned out lamps or replacement with lamps of incorrect voltage

rating, can aggravate a situation and cause lamp power dissipation to rise

considerably above normal. Lamps which usually dissipate 1 watt may thus

dissipate 4 or 5 watts or more. As seen from figure 7, lamp dissipation of

4 1/2 watts produces temperatures at the base of the lamp in the range of

130 to 205 C. The upper range of these temperatures is high enough to

cause melting sufficient to expose the conducting parts of a lamp's own lampholder if low temperature (110-120 C) plastic insulating material has

been used.

FIRE HAZARD

Series constructed miniature Christmas tree light strings and

decorations present a potential fire hazard if lamp surface temperatures

become high enough to ignite materials which may come into contact with the

lamps. Even though there is difficulty in pinpointing the exact cause of

fires related to decorative Christmas material (because the evidence is

usually damaged or destroyed by the fire) , enough evidence has been

gathered to show that miniature Christmas light strings and decorations

have been the cause of at least some fires 3/. To reduce the risk of fire

from these strings, detailed information on the parameters affecting

ignition must be obtained. From this information, one or more hazard

intervention mechanisms can be introduced into the design, manufacture

and/or use of the strings.

The following section gives a brief description of the laboratory test

methodology used to determine if and when ignition can occur from a hot

miniature Christmas lamp. This is followed by a section which gives more

detailed information on the actual tests and their results.

Test Methodology

Most of the data in the literature concerning ignition temperatures is

based on experiments with heat sources much larger in size than miniature

Christmas tree lamps. In many cases the surface temperature of a miniature

lamp is found to exceed the published ignition temperatures of materials

often found in the vicinity of light strings (e.g., dry pine needles,

wrapping paper, tissue paper, decorative cotton, newspaper, etc.).

Additional testing was therefore conducted to determine whether a hot

miniature lamp could indeed ignite typical materials and, if so, at what

temperatures.

Whether any specific material will ignite upon contact with a hot

miniature Christmas tree lamp depends upon: (1) oxygen availability, (2)

the surface temperature of the lamp, (3) the duration of contact, (4) the

heat flow from the lamp to the material and (5) the material sample size.
- 10 -

For a given set of material characteristics and oxygen availability, the

overriding factor is surface temperature of the lamp. Unless this

temperature is above some minimum value, there will be no ignition despite

extended contact times or increased heat flows. At surface temperatures

higher than the minimum required, the contact time necessary to cause

ignition for a given heat flow will, of course, decrease.

Heat flow is a function of lamp power dissipation and bulb geometry.

Since its value normally increases linearly with surface temperature, and

since bulb geometry is similar for almost all lamps, a determination of

actual heat flow is necessary only if the simulation of a miniature

Christmas lamp is required for ignition testing.

Testing for ignition cannot readily be accomplished by utilizing an operating light string. A number of events can intercede, either before

the surface temperature of a particular lamp becomes sufficiently high to cause ignition of materials with which it may be in contact, or before

sufficient contact time has elapsed. These events include: (1) the lamp

filament may open, (2) the lamp shunt may be activated by the abnormal heat

and voltage drop in the lamp and thereby bypass the filament, (3) the

filaments and/or shunts of other lamps in the series string may open

thereby affecting the operating time of the lamp in question, or (4)

lampholder material may melt and cause loss of electrical contact in one or more lamps in the series string, thereby preventing current flow through

the lamp under test.

Because of the above listed factors, a test of 100, or even 1,000,

light strings might not result in a single ignition. However, 40 to 50

million series-connected miniature Christmas light strings and decorations

are sold each year. If conditions were such that only one set in 10,000 were capable of causing a fire, there would still be the possibility for

4,000 to 5,000 fires each year just among the new sets sold. If only 1% of

these possibilities resulted in an actual fire, there would be 40 to 50

fires each year due to miniature Christmas lights.

The previously described difficulty of testing for ignition with an operating Christmas tree light string dictated that initial tests be made

utilizing a simulation. This was accomplished by using a soldering iron

covered with a borasilicate glass sleeve to simulate a hot miniature

Christmas tree lamp.

To achieve the simulation, temperatures on the surface of typical

miniature Christmas tree lamps were first determined as a function of lamp

dissipation and filament placement relative to the lamp envelope. The

soldering iron was then plugged into a variable alternating current supply

and the supply adjusted to provide the indicated temperatures on the glass

sleeve. Slug calorimeter measurements were performed at different surface

temperatures on both the soldering iron simulator and various hot lamps to

verify that heat flow from each was essentially the same (although small

differences can be compensated for by changing exposure time). Ignition

tests were then performed using the simulator. These tests consisted of
- 11

bringing various material samples into contact with the simulator at

increasing simulator surface temperatures, recording the duration of

contact, and noting whether or not either glowing or flaming ignition

resulted.

As a further check on the validity of the simulator measurements,

ignition tests were later attempted on operating hot miniature Christmas

tree lamps. To circumvent some of the difficulties in using a complete

string, the lamps were tested individually. The problems of premature lamp

filament burn out or shunt activation remained, however. Lamp current and

voltage were set by means of a slide wire rheostat in series with the lamp

and the 120 V ac supply. The current and voltage were set to values which

had been predetermined from actual operating conditions. Subsequent

glowing ignitions caused by some of the hot miniature Christmas tree lamps

tested were recorded on video tape.

Lamp filament life was not sufficiently long for surface temperature

measurements to be made during the ignition testing utilizing actual

miniature Christmas lamps, but power dissipation just prior to ignition was

recorded. From the previously determined data relating typical lamp

surface temperatures to lamp power dissipation and filament placement, the

approximate lamp surface temperature at glowing ignition could be

determined and compared to the surface temperatures at which glowing and

flaming ignition occurred for similar materials and times of exposure when

using the soldering iron simulator. Comparison of the data shows that

there is substantial agreement. A discussion of the tests and their

results follows.

Detailed Test Descriptions and Test Results

(a) Determination of typical lamp surface temperatures as a function of lamp power dissipation and filament placement

The initial laboratory tests were designed to determine typical lamp

hot spot surface temperature as a function of both lamp power dissipation

and lamp filament placement relative to the glass envelope. Measurements

were made on lamps of various types and from various manufacturers.

Filament placement was determined visually with the lamp operating at less

than normal power. Each lamp tested was subjected to increasing wattage

dissipation by means of the test set up shown in figure 8. Maximum

temperature on the bulb surface was determined by hand-shifting the

location of a chromel-alumel thermocouple connected to a digital

thermometer which had an accuracy of better than 1°C. The thermocouple

bead was approximately 0.8 mm in diameter. At each new wattage setting the

surface temperature was allowed to stabilize for at least two minutes

before a reading was taken. Longer times were allowed whenever possible,

but if too much time was utilized in lower power measurements, then either

the lamp filament would open or the shunt would activate prematurely - either condition precluding further measurements on that particular lamp.
- 12 -

The data are plotted in figure 9. Data were taken for many more lamps

than are shown in the figure, but only the extremes of filament placement

are plotted. The solid lines indicate lamps in which the filament was touching the glass envelope; the dotted lines indicate lamps in which the

filament was centered. Curves representing all other filament placements

fall within the general envelope defined by the curves shown. Also, for

simplicity, not every curve representing lamps with the two extremes of

filament placement has been .plotted . The curves shown here are those which

typify the observed spread

Because of the relatively short temperature stabilization times

utilized in obtaining the data plotted in figure 9, the recorded

temperatures may be somewhat lower than they might have been had longer

stabilization times been possible. To check this out, single point

measurements (i.e., increasing the lamp wattage immediately to a specific

value and measuring surface temperature only at that wattage) were

performed on several lamps. This procedure permitted much longer

temperature stabilization times, but did not allow a complete curve for any one lamp to be plotted. The data points obtained in this manner could,

however, be compared (for lamps of similar type) with those used in

plotting the curves of figure 9. In every case, the single point data fell

within the range of the corresponding complete curves.

The lamp current measured just before the lamp filament ceased to

operate is indicated at the high end of each curve plotted in figure 9.

This current value is an indication of how likely a lamp might dissipate

the maximum wattage shown on the curve, either in the string for which it was designed or in another string into which it might be placed. For

example, one of the curves for an E-12 lamp indicates that a current of 155

mA was passing through the lamp at its maximum recorded dissipation of A.

watts. The corresponding hot spot surface temperature was 465

U

C. Since

normal current for certain types of presently available light strings is

around 150 mA, and since some light strings manufactured in Italy normally

draw over 200 mA, there is a high probability that the E-12 lamp might

experience 155 mA if inserted into the wrong light string, and hence

dissipate 4.9 watts.

The lamp current indicated at the high end of each curve is also

useful for determining the likelihood that an overcurrent protective device

would avert the high lamp surface temperature which results at that

current. In the foregoing example, the value of 155 mA is only 1.7 times

(7) Filament shape and orientation may also have some effect on the curves. Unfortunately, any such effect could not be determined from the

data collected because practically all lamps currently manufactured have

similar filament shape and orientation. One notable exception is a lamp

with a longer, vertically mounted filament. Curves plotted from the

limited data available for lamps with this filament type show that lamp

surface temperature tends to be lower for a given wattage dissipation and

filament placement. Further investigation is necessary.
- 13 -

higher than the normal current for the 10 light series string in which the

E-12 lamp is designed to operate. Since most overcurrent protective

devices are not designed to work reliably over short time intervals until

the overcurrent approaches twice the normal value, the presence of such a device in the string is not likely to avert a high lamp surface

temperature. Furthermore, if the E-12 lamp were inserted into a string

which normally draws 150 mA, an overcurrent protective device in that

string would be useless in preventing high surface temperatures on the

lamps.

(b) Simulation of a miniature Christmas tree lamp

After typical lamp bulb surface temperatures were documented, the next

step involved development of a simulated miniature Christmas tree lamp

which would not have unpredictable filament burn-out problems. This was

achieved by means of a soldering iron (specifically an Ungar type 6100 iron

with a type 6200 45 watt heat cartridge) that was fitted with a glass

sleeve (by placing a 10 mm ID, 12 mm OD borosilicate test tube over the 10 mm OD body of the heat cartridge). A hot spot on the heat cartridge was

used in the simulations.

The specific soldering iron used in the simulation is noted to enable

exact duplication of the test if desired. However, any soldering iron of

similar or smaller dimensions that can generate temperatures up to 550°C on

its surface could be used for the simulation with the results being

essentially the same.

The soldering iron was supplied from two Variacs (variable voltage

alternating current supplies) connected in series. An ac line regulator,

adjusted to 120 V ac, furnished the input to the first Variac. The output

of the first Variac was set to 140 V ac and supplied the input to the

second Variac. The output of this Variac supplied the soldering iron and was adjusted to obtain the desired surface temperature at the hot spot on

the glass sleeve surrounding the heat cartridge. Output from the second

Variac ranged between 110 V ac for surface temperatures in the range 390-

420 C and 155 V ac for surface temperatures in the range 530-550 C.

Smaller diameter soldering irons were investigated as simulators, but

surface temperature above 450°C could not be achieved with any of the

smaller diameter irons available.

(c) Ignition testing using simulator

Once the simulator was devised, a series of ignition attempts were

conducted. Each ignition attempt consisted of bringing an edge of the

material under test into contact with the previously determined hot spot on

the glass covered heat cartridge. At least ten attempts were rnad^ for each
- 14 -

material at each temperature listed in table 1. If two significantly

different results were obtained for a material/temperature combination,

both results are listed with the corresponding percentage of attempts to

which each applies shown in parentheses; absence of such percentages

indicates uniform results in all trials of the given type. Prior to and

following each series of ignition attempts, the hot spot surface

temperature was measured by means of the same chromel-alumel thermocouple

and digital thermometer as used in the lamp surface temperature

measurements.

The method of applying the material under test to the heat source was

adapted to the type of material and the temperature of the simulator hot

spot. In some instances, the material and temperature were such that a single layer of the sample did not provide a sufficient concentration of

flammable material to result in ignition before the material was consumed

and contact with the hot spot terminated. To counteract this, the material

was folded and applied to the heat source in layers. Layered application

tends to reduce oxygen availability while the sample is in contact with the

heat source, however. Under these conditions, the flaming type of ignition

often did not take place until the sample was removed from the heat source. Layered application was used occasionally for newspaper, tissue paper,

wrapping paper and cheesecloth, but not for the cotton dress or the

cardboard

The results in table 1 show untreated absorbent cotton to be the most

flammable of the materials tested, with tissue paper, cheesecloth and the

cotton dress from a doll shaped Christmas light string decorative

attachment almost as flammable. Dry pine ngedles were not available for

testing. Temperatures of approximately 450 C caused glowing ignition after

10 to 30 seconds for absorbent cotton, newspaper, tissue paper,

cheesecloth, newspaper, and the decorative attachment doll dress. Flaming

ignition was observed for the absorbent cotton at slightly longer exposure

times at the same temperature. According to the curves of figure 9, a hot

spot lamp surface temperature of 450 C can result from lamp power

dissipations of 4.5 to 6.5 watts depending upon filament location.

(d) Ignition testing using miniature Christmas tree lamps

Following the ignition testing utilizing a simulated miniature

Christmas tree lamp, an effort was made to achieve ignition from actual

miniature Christmas tree lamps. The lamp for each trial was inserted into

the test circuit of figure 8. An edge of the material under test was

placed in contact with the lamp surface hot spot (previously determined by

thermocouple measurement at normal lamp power dissipation). Resistance of

the slide wire rheostat was decreased until the power dissipated by the

lamp approached the 6 watt range. Voltage across the lamp and current

through the lamp were then recorded. A glowing ignition was observed in a majority of those instances in which the lamp filament continued to operate

for longer than 2 to 3 minutes. The longer operating time allowed the lamp
- 15 -

hot spot surface temperature to reach a maximum value. The glowing

ignition observations were recorded on video tape.

Approximately 50 lamps of the 12 volt variety from two different

manufacturers, were tested. These lamps were selected for testing because

previous screening had shown that the filament of at least some samples

would continue operating beyond two minutes at 6 watts dissipation. Seven

lamps produced glowing ignition in either the cotton doll dress or sections

of untreated absorbent cotton. For these seven lamps, table 2 provides

information concerning lamp type, filament placement, time to ignition and

lamp power dissipation at ignition. Since the other lamps ceased to

operate in less time than that required by the slowest of the seven to cause ignition, no definitive information about their ignition potential

could be gained.

The tested lamps were of the type commonly used in steady /burning or synchronous flasher light sets. They are not "ballast” lamps - a type

which might be expected to produce even more alarming results.

The absence of flaming ignition in any of the tests utilizing actual

miniature Christmas lamps does not indicate that flaming ignition is

impossible. Rather, it shows only that the maximum surface temperatures

attained on these particular lamps were not sufficiently high to cause it.

Although lamp life was too short for surface temperatures to be measured

during the ignition testing, the effective temperature can be deduced from

the lamp filament location and from the maximum lamp power dissipation (see

table 2) recorded at the time the filament ceased to operate. This

information and the use of figure 9 (which relates lamp surface temperature

to lamp dissipation and filament placement), yields the maximum range of

temperatures to which the surface of the lamp could rise, given sufficient

operating time. The actual temperatures attained may have been somewhat

lower. The maximum surface temperature range as determined for the seven lamps which caused glowing ignition (see table 2) is between 425 and 475°C.

As indicated by the ignition tests with the soldering iron simulator (see

table 1), these temperatures are in a range for producing glowing ignition

on the decorative attachment doll dress, but are not sufficiently high for

flaming ignition.

( o \

' The type of lamp known as a "ballast" lamp is used in light sets where

lamps twinkle on and off independently of one another. These sets differ

from synchronous flasher sets in which the on-and-off is controlled by a single flasher lamp which opens or closes the entire series string at the

same instant. When a light set contains lamps with independent flasher

controls, at least some of the lamps in the series string must remain on at

all times to prevent random sharp rises in current which could burn out all

the lamps at once. The lamps which remain on continuously, called

"ballast" lamps, must have relatively strong filaments in order to absorb

many current surges throughout their life. Prescreening of "ballast" type

lamps showed that some may operate for 20 minutes or longer when

dissipating 6 watts.
16 -

The contact times before ignition are somewhat longer for the tested

lamps (15-90 sec; from table 2) than they are for the soldering iron

simulator (4-20 sec; from table 1). This results, in part, from the

necessity to counteract short lamp life at higher power levels by

initiating contact with a lamp before its surface temperature has reached a maximum value.

When checked again at the completion of the ignition tests, some

tested lamps, surprisingly, still contained an operable filament: the lamp

shunts had apparently been temporarily activated by the elevated voltage

and interior bulb temperature levels. Activation of the shunt would bypass

the lamp filament and give the same observable effect as an open filament.

Once the lamp had cooled after testing, subsequent trial revealed the shunt

to be inactive as the lamps again lighted.

A typical lamp shunt simply consists of three or four turns of #40

aluminum wire wound around the pair of filament support pins. The aluminum

oxide coating the wire acts as an insulator, hence the current normally

flows through the filament. However, if sufficient voltage is developed

across the filament, the oxide coating is broken and the shunt is

activated, thereby shorting the filament. High ambient temperature may

also contribute to breaking through the oxide coating. Further

investigation of this phenomenon is justified since little is known about

the mechanics and predictability of shunt operation at higher power levels.

If shunts in other lamps tested had not acted to bypass the filament, and

hence reduce surface temperature, when they did, those lamps might also

have caused glowing, or even possibly flaming, ignitions.

(e) Measurement of heat flow

Heat flow was measured from both the soldering iron simulator and the

miniature Christmas tree lamps to determine comparability: an exact match

is not necessary since exposure time can be varied to compensate for small

differences. The units of heat flow (or heat flux) are calories per second

per centimeter squared.

The measurements were performed with a slug calorimeter. Two 0.64 cm

(1/4 inch) diameter rods of 99.99% pure copper were machined at one end, one to fit against the soldering iron simulator and the other to fit

against the lamp bulbs. A thermocouple was inserted into a small hole

drilled into the back of each copper slug with thermal contact maintained

by a silicon heat sink compound. Each copper slug was supported in a small

section of high temperature resistant elastomer. The elastomer, in turn, was held by a clamp which was secured to a ring stand. The clamp,

elastomer, and appropriate copper slug assembly could be rotated and moved

down the ring stand to contact the soldering iron simulator or the lamp

bulb.
17 -

Thermocouple readings were begun at the moment of contact between the

copper slug and either the soldering iron or lamp. The readings were

continued until a plot of temperature vs. time became non-linear. The heat

flux could then be calculated by means of the following equation:

MCp dT

y = A

f

• dt

2

where Q = heat flux in calories/sec/cm

M = mass of copper slug in grams

Cp = specific heat of copper, which is .0922 cal/gm/°C

2

= face area of the copper slug in cm

^

= initial linear slope of the temperature-time curve in °C/sec

Initial data for the lamp were taken using a shiny faced slug. Since

the shiny face tends not to register radiant heat flow, additional

measurements were performed using a slug darkened on the face end with

India ink.

Heat flux is plotted as a function of surface temperature in figure 10

for both the soldering iron simulator and the miniature Christmas tree

lamps. Since the lamps exhibit limited operating life at power

dissipations necessary to produce the higher surface temperatures, several

lamps were used to obtain data in this range.

The difference between the heat flux generated by the soldering iron

simulator and that generated by the miniature lamps for a given surface

temperature is not significant. Figure 10 shows an approximate 1.3 to 1 ratio in favor of the simulator, which can easily be compensated for by

increased contact time with the lamps.

RECOMMENDED HAZARD INTERVENTION MECHANISMS

The fire hazard from overheated miniature Christmas tree light strings

and decorations could be reduced considerably, and perhaps eliminated

entirely, by requiring that all miniature Christmas tree lamps meet a maximum wattage/time specification. If lamps are unable to operate at some specified maximum power dissipation for more than a few minutes, lamp

surface temperatures sufficiently high to ignite materials cannot be

realized. The maximum wattage could be specified as a function of both (1)

lamp diameter at the filament location and (2) overall lamp surface area. Larger filament-to-lamp envelope distances and larger lamp bulb surface

areas permit a higher wattage to be dissipated for a given, length of time

before lamp surface temperatures reach the critical range

y

.

(9) Certain types of lamps with long vertically mounted filaments r?ny also

be able to dissipate more power for a given surface temperature rise, but

further investigation of this is required.
18 -

For the sizes of miniature Christmas tree lamps now available, maximum

power should be limited to 5 or 5 1/2 watts over a period of approximately

2 minutes. Most lower voltage lamps already conform to this kind of

limitation.

It should be noted that if an upper limit on the allowable power

dissipation of the lamp is set, the corresponding lifetime of the lamp must

also be taken into consideration. If the former --is set too low the

designed operating lifetime may be too short ‘ thereby giving rise to

premature failure and the concomitant hazards of cascade failure or,.

Other factors being equal,

power of the voltage at which it

equation form as

lamp life varies inversely as the twelfth

is operated. This can be expressed in

12

h-r. . L. ( 1 )

where = lamp life when operated at voltage

L

d

= lamp life at design voltage V

D

The above equation can be rearranged to give

12

V

IL

- —

D

" V,

* L

A

(2)

Normal design life can be calculated from equation (2) if lamp life at

voltage is known.

2

Power dissipated by the lamp is equal to V /R where V is the voltage

across the lamp and R is the filament resistance. Placing a limit on the

maximum allowable lamp power dissipation (exceeding this limit for a specified time should cause the lamp to burn out) is therefore equivalent

to placing a limit on the maximum voltage that can appear across the lamp.

The voltage can be represented by and the specified time by of

equation (2). As seen from the equation, the lower the limiting value of

(or the lower the maximum allowable power dissipation), the shorter will

be the lamp life, L , under normal or design operating conditions.
- 19 -

improper bulb replacement cited earlier. Lamps designed to meet a 5 to 5

1/2 watt limit, however, will operate at their normal dissipation for .at

least 2000 hours - sufficient to last the life of the light string 1

The electric shock hazard from overheated miniature Christmas tree

light strings could be eliminated by requiring insulating material with a higher melting temperature to be used for lampholders. A maximum

wattage/time specification imposed to reduce fire hazard will also, of

course, reduce the electric shock hazard. However, the surface temperature

of even normally operating lamps can be sufficiently high to cause the

melting of some insulating materials. Unless lamp filament placement is more rigidly controlled or lamp envelope size increased, some additional

requirement should be imposed to prevent melting and exposure of

electrically live parts.

( 11 ) Lamp life can be computed via the equations presented in note 10. For

lamps operated at more than + 10% of rated design voltage, these equations

may give values which vary widely from the actual results. Our concern is

with lamps whose operating voltage V

A

is much greater than their design

voltage. Under these conditions, equation (1) may give values for L^ which

are larger than is actually the case. The reason for this is that

operation at higher than normal voltage causes higher than normal lamp

surface temperatures. At surface temperatures above 100°C, the glass used

to manufacture most lamp envelopes begins to outgas. Water vapor caused by

the outgassing is disassociated by the hot filament into hydrogen and

oxygen. The oxygen reacts with the hot tungsten to form tungsten oxide and

thereby reduces filament life, L. , from that predicted by equation (1).

This equation can therefore be assumed to calculate maximum lamp life at

the elevated voltage V.. By this same reasoning, if lamp life at voltage

is actually measured, then equation (2) should give minimum expected

lamp life at normal operating voltages.

Measurements on 12 volt lamps which are dissipating 5.5 watts indicate

a voltage drop across the lamp of approximately 33 volts (V^ = 33 volts).

If operating time at this wattage is 60 seconds (L^ = 60 sec), then the

minimum operating time (Lp) at the normal design voltage (V^ = 12 volts) can be calculated from equation (2) to be 3117 hours. Four weeks of light

string operation per year at 10 hours per day results in 280 hours of

operation per year. At this rate, the lamp would last for 11 Christmas

seasons under normal operating conditions.
- 20 -

The requirement can be either design or performance oriented.

Performance oriented requirements are preferred, where possible, because

they provide a manufacturer with greater freedom of approach to the

problem. On occasion, however, design restrictive requirements may be

necessary because of the unpredictability of the events which may befall

the product to be regulated. This unpredictability of circumstances

precludes specification of a performance test or tests for all situations.

Miniature Christmas tree lights fall into this category.

The best approach for miniature Christmas tree light strings might be

to incorporate both a performance test and a minimum design requirement. A

performance criterion could be applied to fail lamps if there is melting

sufficient to expose electrically live parts when a lamp which is

dissipating maximum power rests against another lampholder from the same

string. The design requirement could specify that lampholder material not

melt to expose electrically live parts when subjected to some specified

ambient temperature. The performance test would provide protection in the

more common hazardous situations, whereas the design requirement would

specify a minimum level of safety for situations that are not easily

predictable. For instance, it is difficult to predict the worst situation

that can result from incorrect lamp replacement because new lamps and new

light strings are marketed each year. It is also difficult to predict the

various acts that a consumer might perform in decorating with a light

string that can cause high temperatures to build up around the lampholders.

AFTERWORD

The foregoing recommended hazard intervention mechanisms, developed in

the course of these investigations, have been included in a Proposed

Mandatory Safety Standard for Miniature Christmas Tree Lights and Similar

Miniature Decorative Lights published by the CPSC in the Federal Register

of May 3, 1978. Since that time, Underwriters' Laboratories, Inc., has

proposed a revision to its voluntary Standard for Safety UL588 on Electric

Christmas Tree and Decorative Lighting Outfits, effective October 1, 1979,

which includes all requirements suggested by NBS and CPSC for the mandatory

standard.
REFERENCES

V Modern Plastics Encyclopedia 1976-1977 (McGraw-Hill Inc., New York,

NY) p. 471.

2/ Modern Plastics Encyclopedia 1976-1977 (McGraw-Hill Inc., New York,

NY) p. 474.

3/ Smith, L. , and Harwood, B. , Incidents Related to Miniature Christmas

Tree Lights - Christmas Season 1977, Consumer Product Safety

Commission (U.S.), Special Report (Aug. 1978). 9p

SHUNT

O-

«*

_l >- “ m

co

co

oj

srscn

separable

parts

-

lamp

bulb,

lamp

base

and

lampholder.
Figure 2. Melted Lampholders and Exposed Conducting Parts

(A, B, C, D and E)

Exposure was caused by contact between hot

normally operating miniature Christmas tree

lamps and low melting temperature plastic

lampholder material. The situation was aggravated by lack of adequate air flow

around contact area.
Figure 3. Melted Lampholder and Exposed Conducting Part (A).

Exposure resulted from heat trapped by a close-fitting

decorative attachment that was placed over the lamp

assembly. The lampholder is constructed from lower

melting temperature plastic.

Figure 4. Decorative Attachment Used in Lamp

Assembly Heating Tests.

A lamp assembly is shown partially

inserted into the attachment.
Figure 5. Lampholders Constructed from Lower Melting

Temperature Plastic After Exposure to Heat

Trapped by a Close-Fitting Decorative

Attachment.

Figure 6. Lampholders Constructed from Higher Melting

Temperature Plastic After Exposure to Heat

Trapped by a Close-Fitting Decorative

Attachment
400

350

300

250

200

150

100

50

0

LETTER

WAS 0B —DESIGN

DISTANC

IS INDIC

SYMBOLS l[ TAINED; AC(

VOLTAGE FO

THAT FIL/

ATED IN mu

1ENTIFY STR

iQMPANYING

R THE LAMP

IMENT IS LOI FOR EACH

INGS FROM V NUMBER S\

VHICH LAMP

r MB0L INDIC ATES

:ated FROM

LAMP TESTEE

THE LAMP E

).

ASE

o s/

\ -S'* o

StsKs?/ y'O

s/y

1 2 3 4 5 6 7

LAMP DISSIPATION (WATTS)

Figure 7. Bulb Surface Temperature Next to Lamp

Base as a Function of lamp Dissipation

and Filament Location.
VOLTMETER

CO «/5 UJ

3Eec ts

UJ

—

eg es 5 £ E

ra

S~cn

fl3 Q ‘

(J

+-> ' raE

<L) • JE T3 U C

<) t-H

CO

CU

S-13

cr>

ividual

Lamp

Assemblies.
500

450

400

350

300

250

200

150

100

50

0

LAMP DISSIPATION (WATTS)

Figure 9. Hot Spot Lamp Surface Temperature

as a Function of Lamp Dissipation

and Filament Placement.
0 100 200 300 400 500

SURFACE TEMPERATURE OF HEAT SOURCE (°C)

Figure 10. Comparison of Heat Flux Produced by

Soldering Iron Simulator and Miniature

Christmas Tree Lamps
TABLE 1

Ignition Test Results for Various Materials Obtained with

a Simulated Miniature Lamp Heat Source

Simulation achieved by means of a borosilicate glass enclosed soldering iron.

Type of ignition and time to ignition are listed in the table. If significantly different

results were obtained for a material/temperature combination, all results are listed with

the corresponding percentage of attempts to which each applies shown in parenthesis.

Material Hot Spot Temperature on Surface of Glass in °C

390-420 440-460 470-490 500-520 530-550

Absorbent

cotton

(untreated

Glow

45-90 sec Flame

30-45 sec

FI ame 10-15 sec Flame

6-10 sec Flame

1-3 sec

Cotton dress

on decorative

attachment

Glow

45-90 sec Glow

7-20 sec Glow4-10 sec Glow (100%)

2-6 sec Flame (30%)

6-15 sec

FI ame

1-5 sec

Tissue paper Glow

75-120 sec Glow15-30 sec Flame10-20 sec Flame

5-10 sec Flame

1-3 sec

Chri stmas

wrappi ng

paper (nonfoil type)

No ignition

after

2 minutes

No ignition

after

2 minutes

No ignition

after

2 minutes

Flame (50%)

5-15 sec

No ignition

(50%)

Flame

1-6 sec

Newspaper

1 !

Glow

30-120 sec Glow

10-20 sec Glow5-10 sec Glow3-8 sec Flame

1-5 sec

Cheesecloth Glow

90-180 sec Glow (100%)

20-40 sec Flame (25%)

30-60 sec

Flame

15-25 sec Flame

5-10 sec

Flame

2-5 sec

Cardboard

(0.5 mm thick)

i

Glow

30-60 sec Glow

15-30 sec Glow

10-20 sec Glow (100%)

5-10 sec Flame (75%)

40-60 sec

Glow (100%)

3-8 sec Flame (80%)

10-15 sec

jf"'

|
TABLE 2

Data on Glowing Ignitions Caused by Actual Miniature

Christmas Tree Lamp as Heat Source

Material igni ted: cotton dress on a doll shaped decorative attachment

#1 #2 #3 #4 #5 #6 #7

Lamp Type

1 H- 1

2

H-l 2 E-l 2 E-l 2 H-l 2 E-l 2 E-l 2

Lamp Color red blue yel low yellow green yel 1 ow green

Fi 1 ame Loca

nt

tion

center center midway

between

center

and edge

center center center center

s

just gni

tion Lamp

Current

175 mA 170 mA 164 mA 178 mA 164 mA 168 mA 168 mA

o

•I- O

+-> 4-J -O sLamp

Voltage

35 Vac 36 Vac 38 Vac 37 Vac 37 Vac 39 Vac 39 Vac

C O

O -r- o S_

CL.

Lamp

Power

6.11 W 6.11 U 6.23 W 6.58 U 6.07 W 6.55 W 6.55 W

Time Lamp at

High Power

Before Ignition

Attempt

20 sec 35 sec 115 sec 60 sec 20 sec 10 sec 15 sec

Time to Ignition

After First

Contact

50 sec 40 sec 15 sec 75 sec 90 sec ; 80 sec 90 sec

Surface Temp, of 425- 425- 425- 450- 425-

450°C

450- 450-

Lamp as Determined

^ from Fig. 9^

450°C 450 C 450°C 475°C 475°C 475°C

J

^Letter symbols identify strings from which lamp was obtained; accompanying number symbol

indicates design voltage for the lamp.

2 Obtained by using "Lamp Power just prior to ignition" and the "Filament Location"

information listed above for each lamp.
NBS-U4A (REV. 9-78)

U.S. DEPT. OF COMM. BIBLIOGRAPHIC DATA

SHEET

1. PUBLICATION OR REPORT NO. NBSIR79-1 716l. Gov’t Accession No. Z. Reclprent’ s Accession No.

4. TITLE AND SUBTITLE

Possible Fire and Electric Shock Hazards from Hot Lamps

in Miniature Christmas Tree Light Strings and Decorations

5. Publication Date March 1979 . Performing Organization Code

7. AUTHOR(S) 8. Performing Organ. Report No. P. Michael Ful comer

9. PERFORMING ORGANIZATION NAME AND ADDRESS

NATIONAL BUREAU OF STANDARDS

DEPARTMENT OF COMMERCE

WASHINGTON, DC 20234

Unit No.

11. Contract/Grant No.

12. SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street, city, state, ZIP)

Consumer Product Safety Commission

5401 Westbard Avenue

Bethesda , MD 20016

13. Type of Report & Period Covered

15. SUPPLEMENTARY NOTES

|_'

; Document describes a computer program; SF-185, FIPS Software Summary, is attached.

16. ABSTRACT (A 200-word or teas factual summary of most sight ficant information. If document includes a si gni ficant bibliography or

literature survey, mention it here.) Failure to replace burned out lamps in series-constructed miniature Christmas

light strings, or replacement with lamps of incorrect voltage rating, can lead

to very high power dissipation by some or all of the lamps in the series string.

Hot spot surface temperatures as high as 470°C were measured for lamps subjected

to simulation of the above conditions. Additional testing showed that contact

with surface temperatures above 390 C can cause glowing ignition, within two

minutes, of cellulose material (e.g., tissue paper, decorative cotton) often

found in the vicinity of Christmas light strings, and can cause flaming ignition

in some samples of absorbent, untreated cotton. Furthermore, the hot spot

surface temperature of normally operating lamps (no excessive power dissipation)

can, particularly if the filament is off center, be higher than the melting

temperature of some commonly used plastic insulating materials such as polyethylene,

Contact between a hot lamp and insulating material can thus cause a shock hazard

due to exposure of current carrying parts. Light string design and performance

requirements that would prevent these hazards are discussed in the report.

This report is based on work funded by the Consumer Product Safety Commission as part of its program for development of a mandatory standard for miniature liqht

sets

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)

Electric shock; fire; flaming ignition; glowing ignition; hazard; ignition temperature; lamp surface temperature; melting temperature; miniature Christmas lights;

plastic insulating material ; polyethylene; series-constructed

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