Marshall Tc

7/30/2019 Marshall Tc

1/6

Breakdown Strength of TRXLPE Insulated Cables

after Extended Aging Under Moderate

Test ConditionsHaridoss Sarma and Evangeline Cometa

AT Plastics

Brampton, Ontario

L6W 3G4

Canada

Mark D. Walton and John T. Smith, IIIGeneral Cable

Marshall Technology Center

US Highway 80 East,

Scottsville, TX, 75688

Abstract

Accelerated aging of 15kV cables under moderate test conditions

were carried out for extended periods of time. Two testconditions were used to age two TRXLPE insulated cables: (1)

aging in water- filled conduits as described in AEIC CS5-94 for a

test duration of 600 days, (2) aging in water-filled tanks as perIEEE P1407. It has been shown by these controlled comparative

tests that the breakdown strength of TRXLPE1 is very stable

over extended periods of aging under both conditions. Test cable

failures were recorded during aging of TRXLPE2 insulatedcables per IEEE P1407, although the actual value of the

breakdown strength and its percentage retention over the unaged

reference for some aging time intervals were greater than those

for TRXLPE1 cables. This demonstrates that the value of acbreakdown strength at a given point in any aging scheme is not

necessarily a true predictor of the performance of TRXLPE

insulations. The mechanism of electrical breakdown at elevatedstress under ramp tests can be vastly different from the

breakdown at constant aging stress.

(Key words: accelerated aging, tank tests, TRXLPE, AWTT)

I. INTRODUCTION

It has become common practice to compare the electrical

breakdown strength of aged cables to that of virgin cables,deducing the reduction in the electrical strength as ameasure of the degradation that has occurred under agiven laboratory accelerated aging protocol or in actual

field service. In reality, while the laboratory or field agingproceeds under constant voltage stress, the alternating

current (ac) breakdown strength is measured under a rampvoltage. Hence, there may not be a direct correlation

between ac breakdown strength and degree of aging, i.e., alower value of ac breakdown strength may not necessarily

correspond to higher levels of degradation. Attempts toclarify the relationship between breakdown strength and

actual failure times for XLPE insulated distribution cableshave been studied using accelerated tests in water- filled

tanks and reported earlier [1]. It is also known that EPRinsulated cables have good field service performance, inspite of their lower breakdown strength measured afterlaboratory or field aging [2,3]. Examination of the failure

characteristics under constant aging stress (in addition tothe breakdown strength at a given time during the life of

the cable) is probably more meaningful. Two types of

accelerated aging tests were therefore proposed recently[4]; a fixed-time test procedure and a time-to-failure test

procedure. Both types of tests serve specific and differentneeds in understanding the aging and breakdown behavior

of cable systems. A guideline to record the important test

variables and to analyze and present the data for these twotypes of accelerated aging tests is described in IEEEstandard P1407 [5]. These types of accelerated aging tests

were recently reported for a TRXLPE (tree retardantcross-linked polyethylene) insulated distribution cable [6]and were shown to be complementary in assessing theinsulation degradation. This point is further emphasized

in this paper by carrying out additional controlledexperiments on cables insulated with two TRXLPE

compounds and using statistical methods to analyze theirbreakdown data. The results indicate that the value of ac

breakdown strength at a given point in an aging scheme isnot necessarily a true predictor of the life performance of

TRXLPE insulations. The mechanism of electricalbreakdown at elevated stress under ramp or step rise testscan be vastly different from that for the breakdown atconstant aging stress, which is of more relevance for

cables in service.

II. COMPOUNDS AND CABLE EXTRUSION

Two insulation compounds TRXLPE1 and TRXLPE2were used in the present study. These were of extra cleancable grades. Commercially available conventionalconductor shield compound (CS) based on furnace carbon

black and supersmooth conductor shield compound (SS)based on acetylene carbon black were used in combination

with TRXLPE1. The comparative test cable withTRXLPE2 was manufactured using SS shield only. The

same strippable insulation shield (IS) was used for all testcables.

Test cables were rated 15 kV with 1/0 AWG-19w

Aluminum conductor and 4.4mm (175-mil) nominalinsulation thickness. A true-triple dry-cure manufacturing

line was used for cable extrusion. The concentric neutralconsisted of six 14 AWG copper wires. Production testson these cables were carried out and found to meet theAEIC CS5-94 specification.

0-7803-7285-9/01/$17.00 (C) 2001 IEEE 813


2/6

III. CABLE AGING

The objective of the cable aging program was to comparethe performance of the TRXLPE insulations under one ormore fixed-time accelerated tests, wherein a statistically

significant number of test samples were used for eachpredetermined time interval to measure the response onthe ac breakdown strength. The other objective was toexamine the option of obtaining a combination of

predetermined fixed-time interval, and time-to-failuretests both under one aging condition, sufficiently

moderate and long enough to yield failure data at constantstress and breakdown strength under ramp voltage after

aging. This approach differs from truncating time-to-

failure tests to a predetermined failure event (which were

carried out in the past) in order to compare the residualbreakdown strength of the fully aged (failing) cables ofthe sample population [7].

The first fixed-time accelerated aging protocol used wasthe AEIC CS5-94 accelerated water treeing test (AWTT),wherein cables are aged in a PVC conduit filled withwater. The second fixed-time accelerated aging protocol

used was aging in water-filled tanks as described in IEEEP1407. The details of the test cable construction along

with other information on test parameters are given inTable1.

TABLE 1Accelerated Aging Test Conditions

Parameters AEIC CS5-94 AWTT in Conduit Fixed-time test in Water-Filled Tanks

Test Cables manufactured

using true-triple dry-curemanufacturing line

CS/TRXLPE1/strippable

SS/TRXLPE1/strippableSS/TRXLPE2/strippable

CS/TRXLPE1/strippable

SS/TRXLPE1/strippableSS/TRXLPE2/strippable

Preconditioning Cyclic aging - 14 cycles at 130oC

conductor temperature90

oC conductor temperature for 72 hours

Test Length:in water

above water

6.71m

5.48 m

7.29m

2.06m

Test intervals (Preset time) 120, 180, 360 days (+ 600 days forCS/TRXLPE1 and SS/TRXLPE2)

120, 180, 360, 600 days

No.Samples per interval 3 5

Test Voltage 3Vg (26 kV) 3Vg (26 kV)

Test Temperatures:Conductor in waterIS Surface temperature

Water TemperatureAmbient Temperature

60oC

45oC

Ambient23

oC

75oC

53oC

50oC

23oC

Water Tap water De-ionized water

Heat cycles during aging 8 hours on 16 hours off -5 days/week 8 hours on 16 hours off - 7 days/week

Temperature Gradient:During heat cycleOff heat cycle

15oC

No gradient25

oC

No gradient

It is necessary to note that the accelerating factor due to

temperature is greater in the case of tank tests. Inaddition, because of the controlled tank watertemperature, the moisture uptake is more (from theconductor side during the heating cycle (8 hours) and

from the tank during cooling (16 hours)) in this case. Thetanks were rectangular and of a size sufficient to hold ten

test samples (5 for each test interval). The experiments

were planned in such a way that at any given time the tankwas full with ten samples to achieve thermal balance. Thewater conductivity was maintained during the agingperiod by replenishing the volume of the water lost

through evaporation with de-ionized water. Duplicatecables of the same type were also used in the event of

0-7803-7285-9/01/$17.00 (C) 2001 IEEE 814


3/6

premature failure during aging. The AWTT is simpler in

the sense that individual conduits are used for each testlength. The aging time is the actual number of calendardays for which the voltage was on; it is not integrated overheat cycles.

Several published papers have proven that water trees aredeveloped under both test conditions. Thereforeexamination of water trees and other relevant water tree

diagnostic information (size of water trees, growth as afunction of time etc.) will not be described in this paper

which primarily focuses on the ac breakdown strengthafter aging for each preset time. The AEIC specification

(time steps of 5-min duration and voltage increments of1.6 kV/mm (40 V/mil)) was followed to measure the

breakdown strength after each aging period. Limiteddiagnostic work was carried out on cable failures thatoccurred during aging. This will be presented in aseparate paper.

Statistical methods were used to analyze the experimentaldata on ac breakdown strength and failure times. The

Weibull statistical average, , was obtained from the twoparameter Weibull distribution function given by the

equation F(x) = 1- exp [-(x/)] where x is the randomvariable such as aging time or breakdown voltage and isthe slope or shape parameter of the fitted line. Beta is an

important parameter in that it provides a measure of thespread or range of the data - the smaller the beta, the

larger the spread.

The statistical analysis was carried out using WinSMITHWeibull 2.0Y, a Windows-based Weibull computer

program. The software provides different methods ofestimating and . For most engineering applications,the median rank regression method of curve fitting is

reasonably accurate and is most commonly used. The ac

breakdown results shown in figures 1, 2 and 3 are the values from the rank regression method with 95% upperand 95% lower Fishers matrix confidence bounds.

Another method, the Monte Carlo simulation, featured inthe program provides more accurate estimation of the

confidence intervals for small sample sizes but thecalculations are more complex and time consuming

compared to the instantaneous output when Fishersmatrix is used.

Data sets were also compared to determine if a significantdifference exists between them. This was done followinga graphical approach of comparing their likelihood

contour plots at a given confidence level. The point insidethis contour represents the joint probability of the

maximum likelihood estimate (MLE) of and . Anabsence of overlap between the likelihood contours

indicates a significant difference in the data sets analyzed.A reduced bias adjustment (RBA) factor can be

incorporated in this analysis to improve the accuracy inthe estimation of and the confidence bounds [8,9]. The

contour plots in figures 4a, 4b and 4c were generated

using the newer version of the software that included theRBA factor in the analysis.

IV EXPERIMENTAL RESULTS AND DISCUSSION

A.Breakdown Strength of TRXLPE1 Insulated Cable:

Fig 1 shows the plot of the ac breakdown strength for

CS/TRXLPE1 construction as a function of aging time.AEIC AWTT was extended to 600 days in this case.

Although the AWTT and the Tank Testing used differentpreconditioning methods, the Weibull values forbreakdown strength of the unaged cables are statisticallyequivalent. It can also be seen from the figure that after

120 days of aging under either test conditions the acbreakdown strength of TRXLPE1 decreased from the

value for the unaged, stabilizing thereafter. Theasymptotic value for 600 days in either case was 17.9

kV/mm (455 V/mil). The current results from tank testsare in agreement with those obtained on cables insulatedwith the same TRXLPE and extruded on a dual tandemline and presented in ref [6]. The results for the cable

construction SS/TRXLPE1 are discussed below.

It is interesting to note that the accelerating factors related

to the test temperature and cable/water volume ratio didnot influence the breakdown strength of the aged

TRXLPE1. A parallel aging study (which will be dealtwith in a separate paper) on TRXLPE1 cables extruded in

a dual tandem line and aged for 1300 days in tanks under

4 Vg (greater acceleration on voltage) and 60oC conductor

temperature (equivalent to AWTT temperature) had also

resulted in an average breakdown strength of 25.3 kV/mm(642 V/mil). Within the statistical confidence bounds, the

trend in the decrease of the breakdown strength ofTRXLPE1 seems to be independent of the accelerating

parameters used for aging. A greater portion of thisdecrease seems to occur as soon as the insulation sees

some water. An almost self-stabilization of thebreakdown strength happens beyond this initial reduction.This could be related to the nature of the chemicalinteraction of water with the insulation.

B.Breakdown Strength of TRXLPE2 Insulated Cable:

In the case of SS/TRXLPE2 cables (Fig 2), the cyclic-

aged cables (as the reference for AWTT) showed a valueof 53.1kV/mm (1349 V/mil) for the breakdown strength.

This is significantly different from the value of 38 kV/mm

(966 V/mil) for the cable preconditioned at 90o

C and usedas reference for the tank testing. In both cases, however,

there was a decrease in the strength with subsequent wetaging for 120 days. This decrease was smaller than that

observed for TRXLPE1 with the actual breakdown valuebeing greater than for TRXLPE1. On extended aging of

SS/TRXLPE2 under AWTT, the breakdown strengthfurther decreased. In the case of tank testing, in addition

to a similar decrease, there were three failures at the 5.84

0-7803-7285-9/01/$17.00 (C) 2001 IEEE 815


4/6

Fig.1. AC Breakdown strength of TRXLPE1 cables with conventional

conductor shield

kV/mm (150 V/mil) aging stress, occurring at 478, 488and 568 days. The two remaining samples yielded an

average value for the breakdown strength of 19.3 kV/mm

(490 V/mil). The statistical analysis of the three failuredata yielded a value of 531 days for the mean time to

failure (Weibull ). Thus in the case of TRXLPE2 cables,a test originally started as a preset time test has turned intoa time-to-failure test. Based on the comparative valuesobtained from AWTT and knowing the relative severity ofaging conditions (temperature and cable/water volume

ratio) in each case, cable failures might be expected if theAWTT is extended beyond 600 days

C. Comparison between TRXLPE Insulations and

Conductor shields:

Comparison of the performance of SS/TRXLPE2 againstSS/TRXLPE1 could be obtained from the tank test data

presented in Fig 3. The breakdown strength of tank agedSS/TRXLPE1 is lower than that for SS/TRXLPE2 for all

aging times but unlike in TRXLPE2 group, there were no

cable failures during aging. This was true even in the caseof CS/TRXLPE1 for which the actual breakdown strength

was even lower. Detailed statistical analysis using RBAfactor has also confirmed that this particular tank test

protocol can distinguish between conventional andsupersmooth shields for a plant-made, dry-cured cableinsulated with TRXLPE1, showing the benefit of SS inincreasing the retention of breakdown strength of

TRXLPE1. The asymptotic value for the breakdownstrength after 600 days of aging was 21.7 kV/mm (551V/mil) in the case of SS/TRXLPE1 and 17.9 kV/mm (455V/mil) for CS/TRXLPE1. Thus the tank test conditions

used in this study enabled us to examine the two aspectsof cable aging simultaneously, retention of breakdown

strength and failures at constant stress.

The shortcomings of the AWTT procedure in this regardare evident from the results of this study. The maximum

likelihood RBA contour plots for the ac breakdownstrength data obtained on cable constructionsCS/TRXLPE1 and SS/TRXLPE1 after aging under

Fig.2. AC Breakdown strength of TRXLPE2 cables with supersmooth

conductor shield

Duration of Aging

ACBreakdownStrength

,kV/mm

10

20

30

40

50

60

Unaged 120 days 180 days 360 days 600 days

SS/TR-XLPE1

SS/TR-XLPE2

3 out of 5 samples failedbefore 600 days

Fig.3. AC Breakdown strength of TRXLPE1 and TRXLPE2 cables with

supersmooth conductor shield (tank test @ 3Vg, 75oC)

AWTT procedure, are given in Fig. 4a, 4b, 4c. It is clear

from the contour overlaps that the AWTT protocol cannot

distinguish between CS and SS materials for a plant-made, dry-cured cable insulated with TRXLPE1.

However, AWTT did reveal differences between theinsulation cores TRXLPE1 and TRXLPE2. Therefore,

based on the AWTT results and assuming the breakdownstrength is a true reflection of aging, it is tempting to say

that TRXLPE1 is inferior to TRXLPE2 and there is noperformance benefit in using SS shield in the place of

0

5

10

15

20

25

20 25 30 35 40 45

B

e

t

a

S

h

a

p

e

P

a

r

a

m

e

t

e

r

AC Breakdown Strength (kV/mm)

CS/TRXLPE1

SS/TRXLPE1

Fig.4a. Maximum likelihood (90%)/RBA contour plots for TRXLPE1

cable after 120 days AWTT

Duration of Aging

ACBreakdownStrength(kV/mm)

0

10

20

30

40

50

60

70

80


AEIC AWTT

Tank test3Vg, 75C

Duration of Aging

ACBreakdownStrength

(kV/mm)

0

10

20

30

40

50

60

70

80


Tank Test3Vg, 75C

AEIC AWTT

3 out of 5 samples failedprematurely before 600 days

0-7803-7285-9/01/$17.00 (C) 2001 IEEE 816


5/6

0

2

4

6

8

10

12

14

14 16 18 20 22 24 26 28 30 32

B

e

t

a

S

h

ap

e

P

a

r

a

m

e

t

e

r


CS/TRXLPE1

SS/TRXLPE1

Fig 4b. Maximum likelihood (90%)/RBA contour plots for TRXLPE1


conventional shield (CS). The results from tank testinglead to quite an opposite conclusion. Under the conditions

used, the estimated lifetime of TRXLPE1 cables was>>600 days and that of TRXLPE2 cables 531days. These

in turn were significantly greater than the lifetime ofXLPE cables determined by controlled comparative testsunder identical conditions. In addition, for TRXLPE1cable, the retained breakdown strength for SS shield was

greater than that for conventional shield. In other words,the AWTT (or the breakdown strength from

0

5

10

15

20

25

14 16 18 20 22 24 26

B

e

t

a

S

h

a

p

e

P

a

r

a

m

e

t

e

r


CS/TRXLPE1

SS/TRXLPE1

Fig.4c. Maximum likelihood (90%)/RBA contour plots for TRXLPE1


AWTT) alone will not be able to support a definitive

conclusion about cable performance. As the technology

of cable compounds evolve toward increasing the benefitsfor the end-use, complementary test tools must be used tofully understand how the technology functions and to

derive the maximum benefit from them.

From the studies on aging of distribution cables insulatedwith conventional XLPE, both in the laboratory and in-

service conditions, it is recognized that the degree ofaging is assessed in terms of the amount by which the ac

breakdown strength falls below that of a virgin cable.

Problems arise if this assessment approach is extended toTRXLPE insulations, which are different from XLPE,both chemically and dielectrically. Increases in the highfield conductivity due to the polar nature of additives in

TRXLPEs and their chemical interaction with water caninfluence the space charge limited field differently.Hence the reduction in the ac breakdown strength afterwet aging may not necessarily be solely due to moisture-

induced degradation. This may also explain whyconsistently similar values for the breakdown strength of

TRXLPE1 insulated cables were obtained after agingunder these two similar protocols, whereas TRXLPE2s

performance was significantly different under these sameconditions. Further, comparing the equivalent systems

SS/TRXLPE1 and SS/TRXLPE2 aged in tanks, thebreakdown strength of the former is consistently below 30kV/mm (750 V/mil) for all test times up to 600 days andyet no failures during aging was registered. On the

contrary, failures were registered in the case of

SS/TRXLPE2 after 360 days of aging at which thebreakdown strength falls just below 30 kV/mm. Whetherthis is the threshold limit for TRXLPE2 failures to occur

at constant stress remains to be proven. The aging andfailure mechanisms for these two TRXLPEs seem to be

very different. TRXLPE1 behaves more like some EPRinsulations, which have excellent field performance in

spite of relatively low breakdown strength.

V. CONCLUSIONS

1. Cables insulated with TRXLPE1 were aged for

extended periods of time under two moderate testconditions. The breakdown strengths of these aged cables

decreased initially, then stabilized over 600 days of

testing. There were no failures recorded during agingunder the two tests.2. The conditions used for fixed-time aging in tanks (3

Vg, 75oC conductor in water with water controlled at

50oC) is advantageous in extracting time-to-failure data

while still being used to derive information on breakdownstrength. The statistical time-to-failure for TRXLPE2

under these conditions was 531 days and for TRXLPE1>>600 days. In spite of its higher breakdown strength,

three aged TRXLPE2 cables failed prematurely before thetotal test duration.

3. The AWTT as the fixed-time aging protocol did notdistinguish between the performance of TRXLPE1 when

conventional and supersmooth conductor shields wereused. On the contrary, fixed-time aging in water filled

tanks was able to clearly differentiate the performance ofTRXLPE1 insulated cables with two different types of

conductor shield compounds.4. Both types of tests (fixed-time-aging and time-to-failure) carried out simultaneously, will give a better ideaon the performance of distribution cables and should

continue to be used.5. Statistical analyses of the individual and comparative

data sets are extremely important. The analysis, along

0-7803-7285-9/01/$17.00 (C) 2001 IEEE 817


6/6

with the details of the aging methods and parameters,

should be carefully specified as per IEEE P1407permitting the comparison between different laboratories.

VI. REFERENCES

[1] M.Walton, Aging of distribution cables in controlledtemperature tank tests, EPRI Report TR-108405-V2,Aug 1997

[2] G.S.Seman, C.Katz and G.S.Eager, Jr., Aging studyof distribution cables at ambient temperatures with

surges, EPRI Report TR-108405-V1, Aug 1997[3] C.Katz and M.Walker, Evaluation of service aged

EPR cables, Proc. 43rd

IWCS, Atlanta, GA, p450-459,Nov 1994

[4] R.Bartnikas, R.J.Densley and R.M. Eichhorn,Accelerated aging tests for polymer cables under wetconditions, IEEE Trans. Power Del. Vol.6, p 929-937,1991

[5] IEEE P1407 - Trial use Guide for Accelerated Tests

for Medium Voltage Extruded Electric Power Cablesusing Water-filled Tanks[6] H.Sarma, Accelerated Aging of TRXLPE Insulated

Cables using Water-Filled Tanks, IEEE Trans.Distribution Conf. P40-45, 1999

[7] H.Sarma, Accelerated Life Tests on a New WaterTree Retardant Insulation for Power Cables, IEEE Trans.

Power Del. Vol12, p551-559, 1997[8] R. B.Abernethy, The New Weibull Handbook, 2

nd

ed.(self published, 1993)

[9] W. Fulton, R.B. Abernethy, Likelihood Adjustment:A Simple Method for Better Forecasting from Small

Samples, 2000 RAMS Conference Proceedings.

Haridoss Sarma received his Ph.D in Physics from I.I.T. Madras, India.

He has conducted research for more than 20 years in the field ofdielectrics and insulation for different applications. He has co-authored

several technical publications on his research work. He is presently the

Research and Technical Manager, Wire and Cable Products, with AT

Plastics, Canada. He is a member of the Electrical Insulation and Power

Engineering societies as well as Insulated Conductors Committee (ICC)

of IEEE.

Evangeline Cometa received her B.SC., degree in Chemical Engineering

from Adamson University, Manila, Phils. She held various positions at

Alcatel Canada Wire from 1982 to 1996 primarily involved in solid and

liquid dielectric studies for MV and EHV applications. She is currently

Development Engineer, Wire and Cable Compounds, at AT Plastics. She

has co-authored several technical publications. She is a licensed

Professional Engineer in Ontario and a member of Power Engineering

and Electrical Insulation Societies of IEEE. She is also a member ofInsulated Conductors Committee (ICC) of IEEE.

Mark D. Walton received his BSEE degree from the University of

Texas at Arlington in 1972 and his MSEE degree from the University ofHouston in 1976. From 1973 to 1979 he was with NASA at the Johnson

Space Center in Houston, TX. From 1979 to date he has held different

positions with ACPC, CPI, B ICC Cables and G eneral Cable Corporation

working out of Scottsville, Tx. He is currently Manager of Customer

Testing Services at General Cable's Marshall Technology Center. He is a

senior member of the IEEE and a voting member of the Insulated

Conductor Committee (ICC) of the IEEE. He is also a member of Eta

Kappa Nu, Tau Beta Pi, and is a registered professional engineer in the

State of Texas. He has authored or co-authored 15 technical publications

and holds one U.S. patent.

John T. Smith, III received his BS and MS degrees in Chemistry from

Prairie View A&M University in 1970 and 1973, respectively. From 1973

to 1979 he was with the Dow Chemical Company, Freeport, Texas as a

Product and Process Development Chemist. From 1979 to 1980 he was

employed with the BASF Wyandotte Chemical Company in Detroit, MI,

also as a Product Development Specialist. In 1980, he joined the Alcoa

Conductor Products Company (ACPC) in Scottsville, Texas as a polymerchemist in their R & D laboratory. Upon ACPC's closure of theScottsville facility in 1983, he joined Alcoa's corporate R & D

laboratories in Alcoa, PA. He joined Conductor Products, Incorporated

(CPI) in 1984 as materials Quality Coordinator, Purchasing Manager,

Compounding plant manager and finally Research Laboratory Manager at

the Scottsville, Texas facility. CPI was subsequently acquired by the

Reynolds Metals Co.Electrical Division, where he continued to be

employed as Manager of Materials Technology at the Reynolds/CPI

Technical Center in Scottsville, Texas. From 1992 until 1996, he served

as Product Development Chemist and Customer Technical Services

Manager at the Indianapolis Technology Center, Indiana for the BICC

Cables Corporation, upon its acquisition of the Reynolds Electrical

Division. Since 1996, he has been employed as Director of the Marshall

Technology Center for BICC and the General Cable Corporation, after its

acquisition of BICC Cables. He is a member of IEEE, the Power

Engineering Society, the Insulated Conductors Committee (ICC) of IEEE

and a Working Group chairman of ICC and the Reliability Society of

IEEE. He has authored or co-authored five (5) referred technical papers.

0-7803-7285-9/01/$17.00 (C) 2001 IEEE 818

Documents

Marshall Tc