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RESTRICTED R L REPORT NO. E-3145

REVISED

CABLE THERMO CURVES

/Ii

141

NAVAL RESEARCH LABORATORY

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MISTRICTIED LREOTN.E34

CABLE THERMO CURVES

- c:jjU~ A.T. McClinton, Head, Engineerin~gand Evaluation Section

Probem ,3EO-07 uly14,,1947

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"DIURMYUTION

BuAerAttn: Code TD-4 (6)

ONHAttn; Code N482 (2) .

BuShipsAttn: Code 665(1Attn: Code 660 0.)

NACA, Waý,hingtn (3)

Dir., USNgfL,- ý'

ANEESA .(2)_,

BAGR, Wright Field .. (2)

ATSC, Washington(3

NATC, Patuxent Rfiver

Attn: Library(2.Attn: Navy Secretary()

SIcience and Teelnoi'gy Project

Attn: Mr. 7.H. ~lýeaid (2)

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CONTENTS

Abstract PgIV

Problem Status tv

AUTHORIZATION1

INTRODUCTION1

DEFINITIONS 2-

METHOD,ý OF CONDUCTING IN4VESTIGATION 2

DISCUSSION 0" RESULTS 5VCONCLUSIONS 16

RECOMMENDATIONS 17,

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ABSTRACT

"This report gives the results of an investigation made to determine theshort-time current-carrying capacities of aircraft cables using visual indi-cation of smoke is the limiting factor. The tests w6i4 made on three wiresizes (AN No. 0, AN No. 8, and AN No. 20) in various ambient temperatureson cable from six manufacturers. Curves were obtaiped showing the highestcurrent which can be continuously carried by each size of cable at differentambient temperatures without the occurrence of smolI•. Thermal curves,with smoking and delayed-smoking values, were obtained'for various current-overload applications. Procedure io given for Interpolating smoki anddelayed-smoking curves for wire siles and ambiept temperatures not Ziperi

mentally obtained. Results of this iovestigation thdicate that, for most of thecable used in the Investigation, the maximum conductor temperature and thecontinuous-current values 'pecified In AN-W-1 4k would cause the insulationto smoke.

PROBLEM STATUS

This report concludes the work on this problibm, and, unless the Bureauadvises otherwise, the problem will be closed one month from the mailing

date of this report. '

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R198TRICTED

CABLE, THERMO CURVES [AUTHORIZATION

rher-E-3122-M i resulting from this investigation was requested by ,BuAer letterTED No. NRL F36-2(13), Ser'ial No. 245'704, dated 18 August 1945, Project Direetive I

TEDNo.NRL31'k77, "Cable Thermo Curves."

Much of the electrical equipment in aircraft is used, ~or draws its greatest current,

weight and space c n be saved by the usf snall size cables to carry, for these short.4periods, cret uchierthan those whic 'h could safely be carried continuously. If-overioads are to be nppiied to the cables, it is neýcessary that protective devices be in-stalled to make cert~\irt that the interval of application will riot exceed the safe ....... l.mit

2ý VTh4 characteristics 9ti these protective devices must~be correlated withsahoit-.time ratingsof the cable. i

IU there is not proper coordination between protective equipment and cable ratings,, thecable can easily be overloaded. The most obvious result of this 'oveirioading is the emis,sion of smoke from the cable insulation. This is undesirabl'e in a pl~he owing to the psy-chological effect on the pilot or crcw.

The purpose of this investigition was to determine the continuous and short-time cur-rent-carrying capacities'of 'aircraft cable, using visual evolvement of smoke as the limit-ing factor; and to obtain, thermal curves resulting from overload -applfieatlonh. This

* -irnfrmation is needed for determining the nec essary circuit-breaker ratrigs.

Investigations covering thermal characteristics of certain types of aircraft Icable havebeen made lby thrtf~a manufa~turers (General Electric Company*, Belden Manufa luringCompanyt , and R6ckrbestos Ibroducts Corportationf), but the information obtained 'was notsufficient forcomiputing the 4,esired circuit-breaker calibration curves.

*B. W. Jones and J. A. Scott of General Electric Company. "Shnrit-Timpi Current Ratingsfor Aircraft Wire and Cabl..", AIEE Technical Paper 46-145, May 1946

ft ~ Belden. "Short-Time Overloads on Low Tension Aircraft Cable," Progress Report,

'I t H. S. Moore, Rockbestos Products Corporation, letter to AIEE Subcommittee on AircraftCable, April 28, 1945L

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This, investigation was conducted'onsamiples of cabl e obt ained from the follow - ~~lng manufacturers: Nl

Belden Wknufacturing CompanyIElectric Auto-Lite CompanyGeneral Electric CortipanyGencriM Motors CorporationRockbeistos Products CorporationWhitney Blake Company

The conductor, in all the cable tested con-' BELDEN MANUFACTURING CO.

slated of straiids of tinned copper; Figure AVNI contains other data pertaining to the cable

insulation.Braid -. Coating - Ethylenic polymer

DEFINITIONS saturant Celluloseýacetate base lacq(~r

Certain terms used throughouat this re -. 4 Separator -'None

port are defined as~foilows: Fle oe

a)Maximum Continuous Current - Primary insulation -Vinyll ChlorideThe highest current which can be carriedcontinuously by a specified cable at given,ambient condittons without ~producing visl-'ble. evidence of smoke.

b) Overload'- Any -value greater thanthe maximum continuous current,

c) Smoking Time - The time required

to produce visiber evidence of smoke whenF

an overload is continuously carried'b a

d) Delayed-Smoking Time - The long-est Interval of time an overload can be con-tinuously darriedby a cable without visible G ENERAL MOTORtS CORP.evidence of smoke occurring after the ~zir - aadEecr ivin

rent circuit is opened. Ti OOCVnlt

METHODS OF CONDUCTING Bri 4CtoINVESTIGATION '4 Braid Coating - Lacquer

The inventigation was conducted on 5eatr-onsingle cables in ambient temperatures of Filler -None0, 25, and 45 degre as centigrade In still Rir Primasry Insulation - Vinyliteat approximately sea-level altitude. A~Ncable sizes No. 0, No. 8, and No. 20 wereselected for the lests.

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ELECTRIC AUTO LITE CO. . GENERAL ELECTRIC CO.,DELTABESTON"

braid - Cotton

Brkid Coating - Unknown Braid -1Cotton r

Braid Coating - Unknown

Separator - None

Filler - None F.... " Filler - Asbestos

•:: •" Primary Insulation - VinylitePriiar Inulaion- VnyltePrimary insulation -Polyvinyl Chloride

* WHITNEY BLAKEoCO.

ROCKBESTOS PRODUCTS CORP. I. TRTOBE' "~~~~FIREWA LL" TRTCBE

dCOtton Braid - Cotton

Braid Coating - Celluloqe Nitrate Type Braid Covering - Unknown

-Figu e-ypep -rofte Usedr - None

Bad*Separator - None

Fillr -None*-l Primary Insulation - Whitney Blake PS-11

Prim~ary Insulation - Layers of CelluloseAcetate Butyrate Tape Covered withImpregnated Felted Aslestau

-Figure 1 - 'Xypes of Cable Used for thte Investigation

4 NAVAL RMSEANCI LA BONA ronr RESTRICTED

IA preliminary investigation showed that the most practical method of determiningthetemperature of the conductor at any time during the tests wn,; by calrulatung the increasein resistance as indicated by the change in voltage drop across a section of the cable, thecurrent being held constant by variable carbon-pile resistors paralleled as necessary forthe various overloads.

It was found that thermocouples were 1.nsatisfactory for determining the temperatureof the conductor during overload applications owing to the problems arising in insertingthe junction at the proper position in the coIbductor and because of the thermocouple lag.Thermocouples were used, however, as a check against the voltage drop An determiningwhen the temperature of the cable had stabilized below aty smoking point.

Except when the applied overloads were very high, it was found that a given cablesample cotld ,e used several times without noticeable change in the smoking time, de-lalayed-smoing' time, or temperature rise. When the sample was used more than once,subsequent tests were made only after the temperature of all parts of the cable hadstabilized at room ambient temperature.

* Since the appearance of smoke determnined the length of time the cable could carry aspecified current, it was necessary to have an~accurate method of detecting the smoke.An investigation showed that the best results Were obtained by partially darkening the room,and using a small. desk-type fluorescent lamp Olaced about three inches ,bove the cable.While making measurements for determining maximum continuous currents and delayed-sthoking values, the lamp shade would accumulate the first faint whiffs ot smoke, thus "making detection easy. (During this part of the investigation, the time at which smoke"appeared was not a factor.) A piece of photographer's cloth was hung about ýhree feetbehind the light and a small strip of this cloth was suspended just below the light, abouttwo inches in front of the cable. A photograph of this arrangement is -shown in Figure 2. .

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!4Figure 2 - Photograph of Equipment

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Figure 3 is a schematic diagram of the test circuit. The by-pass circuit consistedof three parallel cables, each of the same size as the cable under investigation, but threeUrnes its~ length. Thi H~ci wnudtrtiiita thg tevib estors.Carbon'piles -were u'sed for the variable resistors, since their change in re-sistance partiallycompensated for the change in resistance of the cable. This made it easier to maintain aconstant current through the cable. Weights of 20 pounds for Size No. 0; 5 pounds for SizeNo. 8; and I pound for Size No. 20 cable were hung over a pulley with a cord connecting theweight to the cable. Thus, any slack caused by elongation due to temperature rise was takenup by the weight, and the tension kept constant. These weights were not sufficient to causcstretching of the cable.

BY-PASS CABLE

S'l• TEST CABLE"

.•PULLEY

VOLTMETER

*WEIGHT

AMMETER SUBMARINE"BATTERY

CARBON PILE tS~R HEOSTAT S

Figure 3 - Schematic Wiring Diagram

.,.DCUSSION OF RESULTS

Figures 4, through 6 show the conductor temperature versus time curves for over-loads applied to the various manufacturers' cables on AN Sizes No. 0, No. 8, and NM.2O ina 25 degree centigrade ambient temperature. Smoking and delayed-smoking values are alsoshown on these graphs. Figure 7 shows thermal curves with smoking-time and delayed-smoking-time values obtained from application of the same current at'different'ambienttemperatures on General Electric Size No. 8.

It will be noticed that the grouping of thermal, smoking-time and delayed-smoking- I,time curves by manufacturers' names is not always consistant for the different sizes ofcable used during the investigation. This is probably due to different graduated scalesused by these companies to determine the relative amounts of primary insulation and...

braid components for the various sizes of cable. For example, it was found that thew7eight ratio of primary insulation and/or braid, to the total weight of the cable might benearly equal on one size for cable fromfour of the manufacturers; whereas, when the same typeof comparison was made using another size, this weight ratio would be about the samefor possibly only two companies; and for a third size, these ratios would be different for•all of the manufacturers.

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Several tests were made to show the effect of increase in insullUton thickness (outsidediameter). The results are shown in Figure 8. No attempt was made to detervalne thethickness cf any material added or removed, but attention is directed to the fact that everymaterial added lecreased the operating temperature of the conductor, and that the type ofouter surface apparently had little effect. This indicates that the heat dissipated from thecable by conduction and convection is very large compared to that dissipated by radiation.

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Figure 8 - Temperature Versus Time Curves ShowingEffect of Change of insulation

Table I shows the temperatures at which the conductors of various cables stabilizedwhen carrying the current indicated.

TABLE 1

25'C Ambient Temperature Temperature ofCurrent Applied Conductor When

Cable Identification (Amperes) Stabilized (VC.)

Belden No. 8 92 91.5General Electric No. 0 305 89.8General Electric No. 8 102.5 - 107.5 95.4 - 104.2General Electric No. 20 19 93.0General Motors No. 0 330 101.4General Motors No. 8 102.5 - 107.5 99.8 - 107.1General Motors No. 20 20 105.2Rockbestos No. 8 92.5 - 95.0 90.4 - 92.7 "Rockbestos No. 20 17 84.5

These currents are the highest that the respective cables could carry without causing theinsulation to smoke while operating in an ambient temperature of 25 degrees centigrade.It should be noted that the conductor teL.peratures listed in Table 1 are lower in nearly allcases than the value given in Army-Navy Aeronautical Specification AN-W-14a, Wiring;Installation of Aircraft. The current ratings given on page 3 of this specification are statedas being based on a maximum conductor temperature of 100 degrees centigrade in an am-blent temperature of 57.2 degrees centigrade.

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RESTRICTED CABLE TISINO CURVESsI

Maximum continuous current without smoking is plotted against cross-sectional areaof the conductor in Figure 9. This curve represents the averago. maximum continuouscurren~t for all the. types of cable investigated, considered collectively for each wire size.n an ambient temperature of 25 df'grees centigrade. In Figure 10 maximum e~ontinuouscurrent Is plotted against ambient temperature. TI

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Th. curves for Sizes No. 0, No. 8, and No. 20 represent average values measured for thesix types of cable investigated; curves for the Intermediate sizes are drawn parallelthrough values at 25 degrees centigrade obtained from Figure 9.

In Figures 11 through 16 smoking time and delayed-smoking tinxe are plotted againstcurrent for the types and siz "I of cable used during this investigSttion.

ANA

Broken Lifts represent Delayed-Sreoking Time TTIME - SECONDS

Figure 11 I Graph for Belden~ Airvine Cable,

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Figure 15 - Graph for Rhietney Blakewl SrtCable

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RESTRICTED CAMLS rVNOo CVRVU i

The curves for AN Sizes No. 0, No. 8, and No. 20 of Figure 17 represent the averagevalues cibtained an cable from the six manufacturers. The shapes of these curves fromapproximately ten thousand seconds to five seconds are based on experimental data. Betowone second the curves are based on calculations df the heat energy required on a no-lossbasis to raise the temperature of the wire size in question to a value of 150 degrees centi-grade, A maximum conductor temperature of 150 degrees centigrade was accepted as themaximum safe tomperature for the shorter time periods (General Electric Company's re-port). The curvN from ten thousand seconds to five seconds is extrapolated from fiveseconds to ote second to join the calculated curve at this point. The curves for otherwire sizes were drawn with the same shape as that for AN Sizes No. 0, No. 8, and No. 20,and asymptotic tip the line representing maximum contlnuous current for the respectivecable sizes as found on Figure 9.

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* ~~Figure 17- Recommended Short-Time Current Ratings in 25*C Ambient Temperature 1for Low-Tension Aircraft Cable Meeing the Requirements of Specification AN-J-C-48 a"- TIE-srID

It should be noted that the continuous-current values given in Figure 10 do not agreewith the values given in Table 1 of Specification AN-W-14a. Agreement of these two setsof values should not be expected since those given in this report, the maximum values thatthe cables can carry without smoking, may impair the cable insulation; whereas, those inSpecification AN-W-14a should neither cause smoking of, nor damage to, the cable insula-tion. A comparison of these values, however, shows that the continuous currents listed inthe specification are higher than those given in this report. This variation Is contrary towhat would be expected.

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BEST AVAILABLE COPY

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A ntkid\ of the tln~a oti nn i•lt time, ai! derayed-smoklng time revealed that there is, 1 , c.Mlhoihli lwfwen (lit, 0A). This is illustrated by the curve of Figure 18.

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SMOKING TIME (SECONDS)

Figure 18-Relationship Between Smoking and Delayed-Smoking Values

Data for this curve was obtained from the numerous curves presented in this report. Itrepresents all cable sizes, currents, ambient temperatures, and manufacturers. TheequatiLon of the curve is

T = 0.46T 1"12d s

wbere Td is the delayed-smoking time and Ts is the smoking time. This relationshipapplies satisfactorily for time periods up to five minutes, and is useful in determiningthe delayed-smoking time from smoking time values, the latter being more easily obtained.

CONCLUSIONS

The following conclusions are based on the results of this investigation:

a) The maximum safe conductor temperature of 100 degrees centigrade given inSpecification AN-W-14a is too high for present aircraft cable.

b) The short-time ratings for AN cable for time intervals between one and tenotiand seconds duration are as shown in Figure 17.

c) The relationship between delayed-smoking time and smoking time for periodsup to five minutes is given by the equation

Td = 0.46T,1

where Td is the delayed smoking time and Ts is the smoking time.

BEST AVAILABLE COPY

RESTRICTED CARLE TNERNO CURVES 17

RECOMMENDATIONS

a) It is recommended that the curves of Figure 17 be used for the short-time rating ofAN cable for time intervals between and ten thousand seconds. It is recommended thatfor periods of time less than one zeconi the maximum current be based on calculations ofthe heat energy required on a no-loss basis to raise the temperature of the wire size inquestion to a temperature of 150 degrees centigrade. 1

i) it is recommended that the maximum safe conductor temperature bf 100 degreescentigrade given in Specification AN-W-14a be removed and that the maximum currentvalues given for single conductors in free air listed therein be replaced by the values inFigure 10 of this report.

PRNC 3223,10 7 4? 1O0

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Naval research laboratory. Report No. E-3145 (Revised)Cable thermo curves, by W. H. Askew. Washington, The Laboratory.

July 194717 p. illus. 27 cm. RESTRICTED

Abstract: Report gives results of investigation to determine theshort-time current-carrying capacities of aircraft cables using visualindication of smoke as the limiting factor. Curves are given showingthe highest current which can be carried by each size cable at differentambient temperatures without the occurrence of smoke. Maximum con-ductor temperature and continuous current values specified in AN-W-14awould cause insulation to smoke.

1. Cables - testing. 2. Cables - insulation for. 3. Aircraft - equip-ment - cables. I. Askew, W. H. 6

Naval research laboratory. Report No. E-3145 (Revised)Cable thermo curves, by W. H. Askew. Washington, The Laboratory.

July 194717 p. illus. 27 cm. RESTRICTED

Abstract: Report gives results of investigation to determine theshort-time current-carrying capacities of aircraft cables using visualindication of smoke as the limiting factor. Curves are given showingthe highest current which can be carried by each size cable at differentambient temperatures without the occurrence of smoke. Maximum con-ductor temperature and continuous current values specified in AN-W-14awould cause insulation to smoke.

1. Cables - testing. 2. Cables - insulation for. Z. Aircraft - equip-ment - cables. I. Askew, W. H. 6

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T"""'I-IED 1r( : (Same) (Bam.)

31ly47 bta%TRACT: U.S./' I E= eeI "G . 6ceO

0 Results are gjien of an investigation made to determine the short time current carryingcapacities of aircraft cables using visual indication of smoke as the limiting factor. Curves0 were obtained showing the highest current which can be continuously carried by each sizeof cable at different ambient temperatures without the occurrence of smoke. Thermalcurves, with smoking and delayed smoking values were obtained for various current overloadapplications. Results indicate'tbat in the majority of cases the maximum conductor tem-perature and the continuous current values listed in specifications would cause the insulationto smoke.

I EO IC501 dd 5 NCV 1)53DII RIBUTION: Couies of this reonrt obtainable frn CAflO (I)IIISON: Electrical Equipment (16) -7 SUBJECT HEADINGS: Cables, Electric (19710)ZlO:Distribution Equipment 5 //d ~ ~ ~ ~~7

ATI SHEET NO.: R-16-5-14C.n..,i Ai D-r,..,,. Of- AIR TECHNICAL INDEX

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