NUREG/CR-4570

SAND86-0299

RP

DESCRIPTION AND TESTING OF AN APPARATUS FOR ELECTRICALLY

INITIATING FIRES THROUGH SIMULATION OF A FAULTY CONNECTION

Barry L. Spletzer

Frank Horine*

Date Published: June 1986

Sandia National Laboratories

Albuquerque. NM .87185

operated by

Sandia Corporation

for the

U.S. Department of Energy

Prepared for

Division of Engineering Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington. D.C. 20555

Under Memorandum of Understanding DOE 40-550-75

NRC FIN No. A1O01

*K-Tech Corp.

ABSTRACT

An apparatus has been developed that allows the simulation of

a faulty. connection in an electrical circuit by placing a

small resistance heater at the screw of a terminal strip. The

apparatus and associated control system are described in detail. Details of a typical fire produced with the apparatusý:

are presented, along with results of electrical fire initiation attempts with both IEEE-383 qualified and., unqualified nuclear power plant cable.

Repeated use of the apparatus has shown that a self-sustaining

fire can be reliably initiated in unqualified cable with power levels to the apparatus not exceeding 200 W. Such fires may be initiated in approximately 10 min. Large-scale fires have

been initiated with the apparatus, indicating that propagation to any desired fire size is possible. similar efforts

with IEEE-383 qualified cable have begun, but the results are

not yet conclusive.

iii/iv

CONTENTS

Page

Abstract .1i

List of Figures vi

Executive Summary 1

1. Introduction 3

2. Background 3

3. Apparatus Description 6

4. Experimental Results 15

5. Conclusions 24

References 26

V
LIST OF FIGURES

Figure Page

1 Typical Circuit Before and After Inti-oduction

of Faulty Contact 5

2 Cross-Sectional View of Typical Electrically

Initiated Fire Apparatus 8

3 Electrically Initiated Fire Apparatus Before

Test 9

4 Drawing of Heating Element 11

5 Schematic of Power Supply for Electrically

Initiated Fire Tests 12

6 Heater Voltage and Current During Electrical

Fire Initiation 13

7 Heater Power During Electrical Fire Initiation 14

8 Heating Element Resistance During Electrical

Fire Initiation 15

9 Conductor and Terminal Temperatures During

Electrically Initiated Fire 16

10 Additional Wires Positioned Near Initial Bundle to Allow for Fire Propagation 18

11 Early Degradation of Wire Insulation (time:

10:29 min: power: 75 W) 18

12 First Smoke Generation and Blistering of Insulation (time: 12:35 min: power: 100 W) 19

13 Fire Initiation (time: 15:33 min: power: 200 W) 19

14 Fire Propagating Up Initial Bundle (time:

16:07 min: power: 120 W) 20

15 Fire Reaching Secondary Bundle (time:

18:21 min: power: 120 W) 20

16 Fire Consuming Secondary Bundle (time:

19:51 min: power: 120 W) 21

17 Peak of Fire Intensity (time: 20:45 min; power: 0 W) 21

18 Fire Self-Extinguished After Consuming All

Available Fuel (time: 26:59 min: power: 0 W) 22

vi
LIST OF FIGURES (Continued)

Figure Page

19 Electrical Initiation Apparatus Following Fire 22

.20 Setup for Large-Scale Electrically Initiated

Fire 23

21 Large-Scale Electrically Initiated Fire in

Progress 23

22 Large-Scale Fire Setup Following Consumption

of Available Fuel 24

vii/viii

EXECUTIVE SUMMARY

A credible source of reactor plant fires is that of elevated

temperatures caused by heat generated in electrical circuits

that are faulty in some aspect. Previous work in electrically

initiated fires has concentrated on heat generation through

large current overloads. An apparatus has been developed to

simulate heat generation at a faulty terminal strip connection. allowing initiation of a fire without overload

currents. The purpose of this work is to provide a method

for initiating larger fires in a reliable and reproducible

manner for purposes of testing. In developing the apparatus.

emphasis has been placed on realism within the parameters of

circuit design. The apparatus serves to produce a source

fire for testing the manner in which small electrical fires

can threaten electrical cabinets and cabling in nuclear power

plants.

The heat-generating portion of the apparatus is a metal oxide

washer placed under the head of a terminal strip screw. Controllable power levels of up to 300 W are achievable with this

configuration. Several self-sustaining fires have been initiated with this apparatus with peak intensities of up to

800 kW. Power input to the apparatus needed to initiate such

fires is about 200 W for less than 2 min after an initial

1O-min preheat at 50 W.

The initiation of fires with such low wattages and with a

series resistance is of interest since power levels of

greater than 200 W are available to a series resistance in

any 120-Vac circuit that normally draws more than 7 A.

Further. the introduction of a series resistance will not

cause an overcurrent device to deenergize the circuit to

prevent the initiation of the fire.

Self-sustaining fires have been repeatedly initiated in an

unqualified cable (not qualified to IEEE-383 standards) with

polyethylene/polyvinylchloride insulation. Similar efforts

in IEEE-383 qualified cable with cross-linked polyethylene

insulation have begun but are not yet conclusive.

1/2

1. INTRODUCTION

Based on plant operating experience over the last 20 yr. it

has been observed that nuclear power plants are likely to

have three to four significant fires over their operating

lifetimes (i.e.. on the order of one fire every 10 yr or a

frequency of O.1/reactor-yr) [1]. Previous probabilistic

risk assessments (PRAs) have shown that fires are a significant contributor to the overall core-melt probability,

contributing anywhere from 7% to 50% of the total (considering contributions from internal, seismic., flood, fire. and

other events). Because of the relatively high core-melt

contribution, fires need to be examined in detail. Approximately 50% of the fires observed have been of electrical

origin [1]. Therefore. a credible source for these fires is

that of electrical initiation. Previous work in electrically

initiated fires has concentrated on passing large currents

(typically more than 90 A in #12 AWG wire) through cables to

generate sufficient heat within the conductor to cause the

surrounding insulation to ignite [2]. Many circuits are protected by overcurrent devices such that currents of 10 to

15 A will cause the circuit to be deenergized.

This report describes the development of an apparatus to

produce electrically initiated fires in electrical cables

without activating circuit-protective devices that are

sensitive to overlotd currents. The technique of passing

rated currents through appropriate conductors is not considered a credible source since circuits are designed to accommodate the rated current, and heat-generation rates at such

currents are extremely low (e.g.. 1.4 W/ft in #12 AWG wire at

30 A).

This effort was accomplished in support of the Fire Protection Research Program being conducted by the Adverse Environment Safety Assessment Division in the area of investigation

of credible fire sources for control room fires.

2. BACKGROUND

One possible source for an electrically initiated fire in a

current-limited circuit is the existence of a faulty connection. Such a connection would introduce a resistance in

series with the normal load and thus serve as a 1-imited power

source, producing heat at the connection. Conduction of this

heat into the surrounding wires and terminal strip could

ignite the adjacent wire insulation and terminal strip under

appropriate conditions. A technique has been developed by

which a faulty connection at a terminal strip screw may be

simulated and its power level reliably controlled to allow

the effects of various heat-generation rates at faulty

connections to be studied.

The maximum amount of power that may be generated at a

faulty connection in an actual circuit can be determined

3
by maximizing the power available to a resistance placed in

series with the existing load in a constant-voltage circuit.

It should be noted that the addition of any resistance in

series with the load will reduce the total current of the

circuit, thus preventing the condition from being sensed by

an overcurrent trip device (e.g., a fuse or circuit breaker).

Figure 1 shows a typical circuit with a load of R1 before

and after the introduction of a faulty connection with a

resistance Rh. The total applied voltage in both cases is

V; the initial power of the circuit is Pi. The current

through the circuit with the faulty connection is If. the

voltage across the load is V1 . and the voltage across the

faulty connection is Vh. Power dissipated in the faulty

connection is Ph: power to the load with the faulty connection in place is P1 . The Ohm's law relation for the

faulty circuit is

V

f (R 1 + Rh) (i)

The power generated in the faulty connection is

2V2Rh 2

P 1 2Rh = • R (2)

(R 1+ R h)2.

Taking the derivative of the power generated at the faulty

connection with respect to its resistance.

dPh V 2 2Rh

dR h = R1 + Rh)(1 - R1 -h- ) (3,)

Maximizing the faulty connection power by setting the derivative equal to zero.

___ 2__ 2 Rh

o (h - R+Rh! (4)

2R

O0. 1- (R h + R (5)

R = R1 (6)

Therefore, maximum available power is obtained when the resistance of the faulty connection is equal to the load resistance. The faulty connection comprises one-half the total

series resistance of the circuit; by doubling the total circuit resistance, total circuit power is halved. One-half:.of

the reduced circuit power is 'dissipated in the connection.

Therefore

.4
I

v

T" RI

Without Faulty Connection

II

>R1 :V1

With Faulty Connection

Figure 1. Typical Circuit Before and After Introduction of

Faulty Contact

5
- 0.5 V2 0.5V 2 (7) h - Rh + R 1 - 2R 1

and since

=V2

.Pi 2 (8)

it follows that

Ph = 0.25 P. (9)

The maximum power available is then one-fourth the initial

circuit power, and this power may be dissipated at a bad

contact without tripping any overcurrent protective devices;

in fact, the total circuit current is one-half the normal

value.

The above discussion indicates that a faulty connection can

dissipate a theoretical maximum of one-fourth the original

circuit power. A device to simulate a faulty connection is

considered realistic if it can produce the desired power

level in a compact space. Further, the initiation of a fire

by such a device can be considered credible if the power to

the device does not exceed one-fourth the designed circuit

power. A 120-Vac circuit drawing up to 15 A consumes as much

as 1800 W, which allows a maximum of 450 W to be dissipated

in a correctly matched faulty connection.

3. APPARATUS DESCRIPTION

Initial attempts to develop an apparatus to electrically ignite cables concentrated on producing a measured amount of

heat at the end of a wire. The two techniques most widely

attempted were (1) heating the wire end by direct attachment

to a high-current metal heater and (2) flame application to

the wire end with a refractory insulation connecting the bare

portion of the heated wire to the insulated portion. Both

techniques lacked the ability to provide the correct amount

of heat to the wire and had a low degree of geometric similarity with an actual faulty connection. Further, no selfsustaining fire was initiated with either technique.

To provide heat transfer characteristics similar to those

found in actual circuits, the optimum apparatus should be as

similar in geometry as possible to an actual connection. The

final, and successful, effort in developing a source for electrical fire initiation centered on developing a resistance

heating element capable of withstanding the high temperatures

to be encountered, yet small enough to fit under the head of

a terminal strip screw. The use of such an element allows the

cables being ignited to carry the current necessary to supply

6
the heater. The geometry of the space around a :terminal screw

is such that any heating element of homogeneous composition

must have a very small aspect ratio (length of current path divided by area perpendicular to current'flow).. The low aspect ratio, coupled with the relatively low allowable heater

current to be carried through the conductors to be ignited,

requires a high-resistivity heater material to prevent the .required current from causing excessive heating of'the wire.

.-Based on previous work in the initiation of cable fire by high

currents [2]. and on National Electrical Code standards. a

current of 30 A. for the #12 AWGcronductors used, is considered to be an acceptable maximum level that will-'not generate

significant heat.in, the conductors.,.

From the geometry of -a typical terminal strip, a heating ele-

.ment aspect ratio of no more than 10/inch can be accommodated.

A maximum current of 30 A must produce power levels near the

expected 450-W maximum, which requires. a totaltheating element

resistance of about 0.5,ohm with a resulting resistivity of

5 ohm-inches. Several materials were considered for the element but were eliminated because of low resistivity. Major ,,materials considered included carbon (0.02 ohm-inches) and

Nichromem (4,x 10-5 ohm-inches). The material chosen for

the element was a metal oxide used in thermistors: specifically. the thermistor used was-a model FRTI. manufactured by GC

Electronics of Rockford,' Illinois.ý iResistivity of this mate-.

rial ranges from 30 ohm-inchesi to 0..25 o5hm-inches over the

temperature range of, i nterest. A lthlough the current through

the heater was greater by a factor of. 2 than the current in

an actual circuit, the power applied, and thus the heat

available to electrically-initiate a fire, wouldbe readily

available in an actual circuit. A material capable of withstanding the thermal environment, while exhibiting a. resistivity about twice that of -the material described here. would

-,provide volt-ampere characteristics identical to those of a

faulty connection producing maximum heat.

Figure 2"is a cross-sectional diagram of the apparatus developed: *Figure 3 is a photograph of a typical configuration

ready for testing. The apparatus consisted of a phenolic

terminal strip with a brass conductor strip, riveted to the

phenolic, upon which were mounted the heating element and the

necessary electrical connections. The power to the heater

was supplied by establishing a voltage difference between the

Wires attached to the two screws of the conductor strip..

The screw to the left of the Phillips-head screw in Figure 3

was attached directly to one wire, while the Phillips-head

screw was configured as shown in Figure 2. The terminal lug in Figure 2 was attached to the second wire and made electrical contact only with the upper surface of the heating element. A mica washer provided electrical insulation between

the screw and the terminal lug while withstanding the high

7
L

a

- ~ E

0

4.,

0 0

INAI

1,,1

'~'i~ /E-4

0 4)

L.4

C .- 0) E

-C L4

CL fd 0 c 4

'Ul

0 C0

(d 03 F-.L L

+)-

3nC

8
Figure 3. Electrically Initiated Fire Apparatus Before Test

temperatures generated during operation. The insulation was

required here to allow the desired voltage across the heating

element to be established. The lower surface of the heating

element was in contact,,ýwith the terminal strip conductor.

which was connected through the other terminal screw to the

first wire. The heating element had an internal concentric

asbestos insulator that prevented it from contacting the

terminal screw and prevented molten metal from establishing

a current path during high-power tests. This arrangement of

insulation and electrical connections ensured that the applied

electrical current passed axially through the heating element.

The specifics of the heating element are not essential to the

operation of a successful apparatus. The parameter of primary

importance is the generation of enough heat in a small enough

space for a sufficient time. The apparatus described here

presents a set of parameters proven to provide a reliable

source of electrical fire initiation. Approximately 25

self-sustaining fires have been produced with this type of

apparatus.

9
Figure 4 is a dimensioned drawing of the heating element. The

element was constructed by.slicing a commercially .available.

tubular-shaped, metal oxide .thermistor into sections of appropriate length. The element was then fitted with'a washer of

suitable material to provide electrical insulation between

the element inner surface and the terminal screw. Asbestos

gasket material was used to make the inner washer. The metal

oxide thermistor, was chosen because of its relatively high

resistivity and stability at elevated temperatures. However,

the thermistor is intended to exhibit a negative thermal

coefficient of resistivity, which, for the purpose of these

tests, made control of the power to the heating element somewhat difficult. The resistance value of a heating element

can change by a factor of 100 over the temperature range of

interest, typically 700 to 1500OF (210 to 816 0 C).

Most of the attempts at electrical initiation during the

development of this, apparatus were unsuccessful because the

heater lacked the ability to operate for a sufficient time.

The heater was required to operate for approximately 1200 s

(20 min) and may have reached temperatures as high as 1600OF

(871 0 C) during operation. Most failures in heater operation

have been, attributed to the following causes:

* Short-circuiting of the power supply caused by melting

of the conductorstrip and ,flowing of the molten metal

through the heater center. Such failures%were primarily

due to excessive power levels early in a test.

Melting or severe corrosion of the connecting terminal

lug, corrected by using a heavy-gauge lug and reducing

the duration of high power levels.

Heating element cracking. The arrangement of the element

provided.that axial-or radial cracking would not significantly affect the heater. Cracks with a circumferential

component greatly raise the element resistance. Cracks

appeared to be caused by excessive power levels or power

transients. The operating technique developed during the

tests has eliminated these causes. Care must be exercised

during installation to prevent excessive cracking in the

relatively brittle metal oxide element. However, cracking

during installation tends to have no circumferential

component and may not significantly affect element

performance.

A schematic of the heater power supply is shown in Figure 5.

The voltage to the heater was controlled by the manual variable autotransformer. The power transformer was needed to

reduce the voltage and increase the current to the heater to

obtain the desired range of power. The power applied to the

heater was supplied by a variable 60-Hz alternating current

of varying voltage, continuously monitored with the wattmeter

and logged at 1-s intervals during the tests., An approximate

10
Rsbestos Insulator

Heating Element

-0.300

0. 16BB:

I- NN

I 0.070 0.110

Figure 4. Drawing of Heating Element

11
120 VRC

Heater

Variable Power

Rutotransformer Transformer

Figure 5. Schematic of Power Supply for Electrically Initi-

.ated Fire Tests

.reading of heater power level was obtained by monitoring the

input current to the autotransformer. Since transformer

efficiencies are relatively constant over a wide range of

power levels and input voltage of the autotransformer is

constant, the input current was directly proportional to the

heater power. This technique of monitoring power level has

proven useful for remote ignition when the autotransformer

must be situated some distance from the power transformer and

heater [3]. In practice, the remote monitor can indicate the

actual power level to within 5% for levels from 30 to 250 W.

Since the resistance of a heating element varies by a factor

of 100 during a test, the power supply must be sized to be

adequate over the entire temperature range of interest.

Figure 6 is a plot of voltage and current of a heating element during a typical test. Initially. 20 V is required to

begin raising the element temperature. This initial- voltage

corresponds to about 13 W of power to the element. The peak

power requirement of 200 W occurs at about 900 s (15 min)

into the test and requires a current of 28 A. For the tests

conducted, a stepdown transformer with a 120-V primary and a

36-V. 24-A secondary, and a variable autotransformer rated at

7 A. were used.

The power supply performed adequately because of the short:

duration of current above 24 A and the low voltageorequired

at the high Current. The use of a heater with a smaller

12
25

22 Current (

E

- 15 1I

0

> -•

I, End of Pre-heat Ignition

0 200 400 600 820 1000 1222

Time (seconds)

Figure 6. Heater Voltage.and Current During Electrical Fire

Initiation

thermal coefficient of resistivity would simplify the selection of a power supply by providing a more constant voltageto-current ratio.

Control of the power to the heater was complicated by the

heater's negativethermal coefficient, of resistivity This

feature caused the resistance of the heater to decrease with

ýincreasing'temperature. The power supply, driven by a

manually'controlled variable-autotransformer. provided a

controlled voltage to the heater, which caused the power level

of the heater to be inversely proportional to the resistance.,

If the temperatureýof the heater should increase from an

equilibriumpower-temperature state, the power level would

also increase, tending to elevate the temperature further.

The converse would be true for a temperature decrease. Theheater would therefore be difficult to control, especially

when a change in heater temperature was required.

This effect is most pronounced during initial heatup. in which

typically the supply voltage is increased until a power level

of about 50 W is reached, at which time the voltage is decreased at a controlled rate while a 30- to 50-W power level

is maintained. A relatively smooth decrease in heating element voltage can be seen in the first 200 s of the Figure 6

plot. Figure 7 is a plot of the power level corresponding

to the voltage and current plot in Figure 6. Notice the

13
200

150

100

L

3

0

End of Pro-heatt~ Iginition•

0 , t , I I I I I I I I I I I I I I 

0 208 420 620 822 1022 1220

Time (seconds)

Figure 7. Conductor and Terminal Temperatures During

Electrically Initiated Fire

oscillating power levels during the first 200 s of the test.

Figure 8 illustrates the resistance of the heating element

during the test. The rapid drop in resistance during the

first 200 s should be noted.

The oscillations are caused by the heating element's powerlevel instability in a manually controlled voltage mode.

At high temperatures, the cubic dependence of incremental

heat loss by radiation on incremental temperature causes the

element to be relatively stable. Even in the stable hightemperature region, the equilibrium power level is historydependent: that is. increasing the applied voltage momentarily

and returning it to the original level will result in a higher

equilibrium power level. This phenomenon is illustrated by

comparing the heater power to the applied voltage at 400

versus 1000 s (Figures 6 and 7). A power supply capable of

providing a variable, controlled current rather than voltage

control would make the response inherently stable by causing

the power level to be directly proportional to the heating

element resistance. Such a power supply was not deemed

warranted here since sufficient control was achieved by

understanding the element characteristics and closely

monitoring the necessary parameters.

During the bulk of the test, the power level to the heater was

stable enough to allow manual control of the voltage output

14
2:

E

o\

a) U 1

S .5

End of Pre-heat Ign t on

0 , •, , I , , , I , I I , , I I , I , , , I ,

0 200 400 600 800 1000 1200

Time (seconds)

Figure 8. Heating Element Resistance During Electrical Fire

Initiation

of the variable autotransformer. Upon initiation of a test.

the resistance of the heating element was about 100 times the

resistance at testtemperature. This fact required that the

initial voltage applied to the heater be much higher (about

20 V) than the voltage at elevated temperature (about 5 V).

Low-voltage (less than 10 V) application was tried, but the

resulting power levels were'too low to provide the significant. heatup of the element required to cause resistance

reduction and resulting-spontaneous power increase.

The negative coefficient of the heating element also appears

to cause hot spots in the element. Temperature differences

estimated to be greater than 1000 F (38 0 C) have been observed

during testing. If a portion of the element is hotter than

the surroundings. the local resistance is lowered, and a

larger local current density will occur in the area, causing

additional heating and thus amplifying an initially small

perturbation. This-phenomenon has not been investigated but

may be an .oimportant ignition source at high power levels.

Such hot' spots could serve the same purpose in simulated

electrical fire initiation as do electrical arcs in accidental fires.

4. EXPERIMENTAL RESULTS

Tests conducted with seven-strand. #12:AWG. single-conductor

wire:, wire not qualified to IEEE-383 standards, with polyvinylchloride/polye6thylene' insulation, demonstrated that

15
a self-sustaining fire could be produced by providing an

initial power level of 50 W to the heater for 10 min, followed

by gradually increasing the wattage to 150 to 200 W over the

next several minutes. Figure 7 shows the applied power level

for a typical test. The 10-min preheating period at 50 W is

important in producing a self-sustaining fire. Tests without

preheat will produce ignition, but the fire will usually

self-extinguish after several inches of insulation have been

burned. It appears that without the preheat period, the

conductors surrounding the initiation site are at low enough

temperatures to serve as a heat sink, preventing the fire

from propagating. Figure 9 is a plot of temperatures

measured during the test. The conductor temperature was

measured in the wire attached to the heater at a point 3 in.

(8 cm) above the heater. The terminal temperature was that

recorded by a thermocouple placed adjacent to the heating

element. From the plot it can be seen that the temperature

in the conductor was 330OF (166 0C). near its equilibrium

level before'the power was increased beyond 50 W. The

terminal temperature at that time was 700 0 F (371 0 C). In

steady-state temperature tests with this cable at Sandia

National Laboratories, a temperature of 480OF (249 0 C) was

found to be sufficient for cable autoignition [4]. A wire

temperature of 520OF (271 0 C) was observed at ignition. After

ignition, power to the heater was maintained until the heating

element failed. During thb first minute after ignition, the

1400

1200

-. Strip /

m ~/

' 802 /

L 622

L

E

a)

F 222 End of Pre-heat -Ign ition.

0 222 422 602 822 1222 1222

Time (seconds).

Figure.9. Conductor and Terminal Temperatures"

During Electricaliy Initiated Fire

16
power input of the heater contributed to:the development of

the fire. Once the fire had propagated several inches' from

the ignition site., the relatively lw power level of the

heater and the distance of the fire' from the heater made the

contribution of the heater to further. propagation negligible.

The area near the heater had five individual wires positioned

in a vertical bundle (Figure 3). Several% other geometries

with the number of wires ranging from 1 to 30 were also used.

During tests with approximately 10 or more wires placed near

the heater, the fire would not propagate. 1 The failure to

propagate is attributed to the heat-sinking capability of the

large bundle and the limited power levels (.less than' 100 W

during' preheat) of the heater. High-power preheat of large

bundles was attempted-but typically resulted in rapid ignition

such that true preheat :wa`s:not obtained. :;and the 'fire selfextinguished within a few inches of the initiation site.

Experiments with few wires have provenf successful to the

extent that a fire may be initiated and will propagate in a

single vertical wire. The five ."'vertical wires are capable of

sustaining the fire for, the. entire length of the bundle (about

18 in. [46 cm])..

The five-wire bundle should pass within 2 in. (5 cm) of the

heating element. Greater distances reduce the effect of preheating in all but the primary wire and make fire propagation

less likely. The five-wire geometry was chosen to provide a

suffitciently intense fire to ignite the additional fuel. in

the form of more wires. placed' 0 ;to 20 in. (25 to 50 cm) from

the initiation site. Figure 10 shows a typical 'arrangement of

additional'. fuel. Ekperiments, with different arrangements of the additional fuel indicate that the placement is not critical to allowing the fire, to-'propagate. No case was observed

in which the anitial bundle was consumed without alladditional fuel being consumed also.

Figures 11 through 18 show-the initiation and propagation of

an electrf.cally initiated 'f ire.- 'The rapid progress of the

fire after ;initiation is inFigures. "shown 141 and 15, in which

the fire progresses from small flames; on the initial wires

at 16:07 man to flames burning up thee additional wires at

18:21 mii. :,The ,fact that virtualyall available fuel was

consumed (Figure 19) should be* noted.

Propagation of,-such a fire to much' larger geometries can be

accomplished without any changes to the procedure or.- apparatus described. In such, cases, more additional fuel is positioned near the top Of the the test apparatus. 'Numerous

large-scale fire tests of this typ.e have been performed by

Sandia National Laboratories [3]. Figure' 20 illustrates the

setup for a, large-scale electrically initiated fire: .Figure

21 shows the large-scale fire in progress: Figure 22 is a

view of the area after all available fuel was consumed. The

17
Figure 10. Additional Wires Positioned-Near .Initial Bundle to

Allow for Fire Propagation'

Figure 11. Early Degradation of Wire Insulation (time:

10:29 min; power: 75 W)

18
l,

Figure 12. Smoke Generation and Blistering of Insulation

(time: 12:35 min:, power: 100, W)

Figure 13,. Fire Initiation (time: 15:33 min-' power: 200 W)

19
Figure 14. Fire Propagating up Initial Bundle (time:

16:07 min; power: 120 W)

Figure 15. Fire Reaching Secondary Bundle (time:

18:21 min; power: ý 120 W)

20
Figure 16. Fire Consuming, Secondary"Bundle (time:

19:,51 min; power: 120 MW)

Figure 17. Peak of Fire

.power:0N)

Intensity (time: 20:45 min;

21
Figure 18. Fire Self-Extinguished After Consuming All Available Fuel (time: 26:59 min; power: 0 W)

Figure 19. Electrical Initiation Apparatus Following Fire

22
Figure 20. Setup for Large-ScaleElectrically Initiated Fire

Figure 21. Large-Scale Electrically Initiated Fire in Progress

23
Figure 22. Large-Scale Fire Setup Following Consumption

of, Available Fuel

fire shown here reached a peak intensity of 800 kW from the

initial apparatus power input of less than 200 W.

Efforts are currently underway toqproduce similar fires in

IEEE-383. qualified cable. These experiments are not yet

complete. but the results will be reported when sufficient

data have been gathered.

5. ;CONCLUSIONS

The apparatus developed provides a reproducible source for

electrically initiated fires which is realistic in terms of

typical circuit design constraints. This source fire has

been used for testing the manner in which small electrical

fires can threaten electrical cabinets and cabling in nuclear

power plants.

Electrically initiated fires can be repeatedly produced using

heat sources to simulate faulty connections with relatively

low (<200 W) power levels. Such fires have been shown to be

24
self-sustaining and capable.of propagating to proportions

independent of the' 'size of the initiation apparatus.

Although the electrical heaters presently used require lower

voltage and higher amperage than are found in most power plant

circuits, the proper amount of heat is generated at the simulated connection. Use of other suitable materials with higher

resistivity should provide resistance characteristics suitable

for producing sufficient heat from currents typically available in power plant circuits (115 Vac at <15 A)..-

The difficulty of power-level control caused by the inherent

negative thermal coefficient~of resistivity of the metal oxide

heating element can be eliminated through use of materials

with a less-negative coefficient. One such materialis a

silicon thermistor. Further, more realistic simulation of a

faulty connection could be achieved using such a heater in

series iwith the load. This configuration would current-limit

the circuit; such that the resistivity coefficient would not

significantly upset the power levels. However, use of such

a system with a constant total applied voltage might require

that heater power be controlled by varying the size of the

applied load.

Using the apparatus described here, fires are readily initiated and propagated in unqualified cable. No self-sustaining

fire has been electrically initiated in an IEEE-383 qualified

cable: the results are not' entirely conclusive, and further

attempts in this area are being made.

25
REFERENCES

1. W. T. Wheelis. "Nuclear Power Plant Fire Data Base for Use

With a Personal Computer." Sandia National Laboratories.

to be published.

2. D. A. Dube. "Fire Protection Research Program for the U.S.

Nuclear Regulatory Commission. 1975-1981." SAND82-0431.

NUREG/CR-2607. Sandia National Laboratories. April 1983.

3. "An Experimental Investigation of Internally Ignited Fires

in Nuclear Power Plant Control Cabinets - Vol. 1 Cabinet

Effects Test." SAND86-0336. Sandia National Laboratories.

to be published.

4. J. M. Chavez. "Steady-State Environment Cable Damage Testing." Quick-Look Report. Sandia National Laboratories.

July 1984..

26
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D.

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N.

D.

W.

L.

M.

D.

J.

V.

D.

V.

S.

B.

W.

K.

P.

Q. Landenberger L. Garner

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King

Nicolette

P. Nowlen

L. Spletzer (15)

T. Wheelis

D. Bergeron I

W. Dean

5)

28
NRC FORM 33" U.S. NUCLEAR REGULATORY COMMISSION 1. REPORT NUMBER (Assignedby TIDC, odd Vol..No., if any)

(2:84)

NRCM 1102, AA HE

3201,.3202 BIBLIOGRAPHICDATA SHEET NUREG/CR-4 570 SEE INSTRUCTIONS ON THE REVERSE. SAND86-0299

2. TITLE AND SUBTITLE 3: LEAVE BLANK

Description and Testing of an Apparatus-for

Electrically Initiated Fires Through Simulation 4. DATE REPORT COMPLETED

of a Faulty Connection MONTH YEAR

B. AUTHOR IS) APRIL 1986

B. DATE REPORT ISSUED

SBarry L. Spletzer MONTH YEAR

Frank Horine JUNE 1986 7. PERFORMING ORGANIZATION NAME AND MAILING ADDRESS (IcludeZip Code) B. PROJECT/TASK/WORK UNIT NUMBER

Division 6447, Adverse Environment Safety 9PINORGRANTNUMBER

Assessment Division

Sandia National' Laboratories, P.O. Box 5800, 0 FIN NO. A1010

-Albuquerque, NM 87185"

10, SPONSORING ORGANIZATION NAME AND MAILING ADDRESS (Include Zip Code) 1 la. TYPE OF REPORT

U.S. Nuclear Regulatory Commission . echica

Division of Engineering Technology 'Technical

Office of Nuclear Regulatory Research b. PERIOD COVERED (/clusive dates)

Washington, DC .20555

12. SUPPLEMENTARY NOTES

13. P• TR• fM• F(20 words or less)

.An apparatus has been developed that allows the simulation of a faulty,

connection in an electrical circuit by.placing-a .small resistance heater

at the screw of a terminal-strip. The apparatus and associated •control

system are described in detail. Details of a typical fire produced with..

the apparatus are presented, along with results of electrical fire initiation attempts with both qualified and unqualified nuclear power plant,

cable.'

Repeated use of the apparatus has shown that a self-sustaining fire can

be reliably initiated in unqualified cable with.power levels to the

apparatus not.exceeding 200 W. Such fires may be initiated in approximately 10 minutes. Large-scale fires have been initiated with the

apparatus, indicating that propagation to any desired fire size is possible. Fires initiated in qualified cable to date have.been localized-and.

neither self-sustaining nor capable of propagation.

" 14. DOCUMENT ANALYSIS - a. KEYWORDS/DESCRIPTORS

Fire

.Electrical Initiation

Cable Fires

Electrical Fires

b. IDENTIFIERS/OPEN-ENDED TERMS

Faulty Electrical Connection