DC CONTROL BATTERY SYSTEM PROTECTION & COORDINATION

Robert L. Smith, Jr., Life Fellow, IEEE Volts & vars, hc.

PO Box 359 Northville, NY 12134

Abstract - Reliable control battery systems assure proper functioning of well designed, installed and maintained power systems. Battery system failure jeopardizes a power system by eliminating the DC control power source for AC system circuit breakers and protective devices. Failure to protect DC system components also could result in disastrous consequences for the battery system itself. This paper reviews protection of control battery system components including the battery, battery charger and individual circuits. It discusses optimum battery system protective device coordination. Battery selection and charger selection are not discussed. References [q,[7], and [SI discuss these topics.

DC System Failure DC system failure results in the loss of tripping

power for major portions of a distribution system. Serious bum-downs can occur if AC power system protective equipment fails to operate for fault conditions because of contol power failure. References [l], [2] and [lo] as well as incidents brought to the attention of the author indicate the following basic causes for DC system failure: o Operating errors

AC charger switch open Control bus disconnect open Charger misadjusted

Poor maintenance Discharges into fault 10 to 15 secs. or more Overcharging Low or high ambient temperature

Battery Charger DC distribution system Load

o Batterv deterioration

o Faults (short circuits. ground faults, open circuits)

Most of the basic causes of failure result from neglect. This neglect probably stems from the following myths associated with battery application:

1. The battery is a "fault free" zone. 2. The installation is "maintenance free". 3. No battery overcurrent protection is required.

DC system failures occur in one of three modes: o Control power loss with no externally

apparent damage to any DC system component. o Control power loss with damage to DC

control power system components. o Control power loss with damage to the DC

control power system adjacent equipment. Table 1 relates failure modes to possible causes.

Failure Mode Usual Cause

No apparent external Operating errors Battery deterioration Load faults isolated

DC system component DC system faults isolated

isolated

Table 1 - DC System Failure Modes

Battery Faults While less frequent than feeder faults, battery faults

do occur. References [1],[2], and [lo] detail the disasterous results of some such faults. Table 2 shows causes and effects of battery events which may ultimately result in battery faults.

Bartev Protection Overcument protection is of limited use in detecting

internal battery faults. Figure 1 shows three arrangements of fuses or circuit breakers used for large installations. The disadvantages of these systems arise from the inability of overcurrent devices to detect intemal faults since the only source of short-circuit current is the battery itself (chargers usually are supplied with current-limit protection which means they do not provide enough current to trip a circuit breaker or blow a fuse in the battery circuit). Mid-point, between cell, fusing [fig. I(%)] may provide some degree of protection for internal faults but does not interrupt the flow of short-circuit current within one section of the battery. Using more than one "mid-point" fuse [fig. l(c)] raises the rating question.

0-7803-0937-5/93/$3.00 0 1993 IEEE 175

Event

Foreign objects, Fire, Water Corrosive fumes, Vibration

Impacts, Seismic events Loose connections

Physical damage Cell leakage Short-circuits Ground faults

Intercell open circuit

Over-charging Charger Misadj ustment

High Temperature Operation

Gassing, Terminal corrosion Hieh internal resistance

~~

h a d Cycling

I Effect Cause

Ambient Conditions I Internal grid & plate corrosion

Loose inter-cell connections High internal resistance

~~~ ~~~ ~

Unusual Operation

Table 2 - Battery Fault Causes and Effects

Most battery protection recommendations suggest short-circuit protection. "Standard" handbooks and textbooks as well as references [l] through [9] contain no setting or rating recommendations for battery protection. Short-circuit protection prevents the battery from feeding a DC system short-circuit for more than a few seconds. A practical setting opens the battery circuit between three to six times the battery one minute discharge rate.

Figure 1 - Battery Protection Alternatives

Reference [ 11 suggests a fuse rating no smaller than twice the expected continuous load. The fuse selected also must not be damaged when required to pass the maximum instantaneously applied load. This load may be for tripping several circuit breakers simultaneously or recharging their spring closing mechanisms simultaneously. The fuse also must operate prior to the rate of rise curve of the battery current reaching 0.01 seconds. A fuse less than half the battery short-circuit capability, probably about 150 % of the one minute discharge rate, usually meets

these criteria. The Rate of Rise Appendix of this paper shows one method of calculating rate of rise and adds comments on this subject.

Undervoltage protection may provide better battery protection than any type of overcurrent protection since an internal battery bank fault probably would produce a significantly reduced terminal voltage. The recommended charger undervoltage protection could perform this function. The Hardware Appendix of th is paper shows a representative connection diagram for an AC\DC undervoltage protection system.

DC battery undervoltage protection should be set at some voltage about 70 46 nominal voltage since this is the voltage below which circuit breaker trip coils may not operate. The Hardware Appendix lists some of the relays applicable. A time delay of less than 0.5 seconds should be sufficient to over-ride any circuit transients involved provided the demand imposed by charging mechanisms is considered when initially selecting the battery rating.

Since loss of the battery or a decrease in the battery voltage for any more than a very short period means a complete loss of protection for any installation, the common sense approach would be to trip the main breaker to the plant. This, however, would be impossible without any reliable tripping power. An AC alarm with sufficient authority to rouse someone into action may be the only alternative. Perhaps a loud alarm in a manned control room, silenced only from the battery room, is appropriate.

Ground fault detection on ungrounded control battery circuits not only protects the system from interruption on the first ground fault but i t also detects serious battery events not sensed by overcurrent or undervoltage protection. The

176

Hardware Appendix of this paper shows a representative connection diagram for a DC ground detection system.

Any ground on the DC battery system requires immediate attention and should be annunciated in a manner demanding immediate action.

Batery Charger Faults Most battery charger faults result from either

physical damage or misoperation. Physical damage may result in internal short-circuits, ground faults or component failure. Misoperation may produce the same reults. Physical damage includes damage from impacts, vibration, water, dust, foreign objects, corrosive atmosphere, high or low ambient temperature. Misoperation includes overloads and high or low AC or DC voltage with resulting insulation, diode or SCR failure.

Battery Charger Protection Most automatic battery chargers are protected with

AC and DC circuit breakers, a redundant (backup) DC fuse, a current limit feature and both AC and DC surge protection. The AC and DC circuit breakers and redundant fuse protect the charger for internal short-circuits. The current limit feature protects the charger from feeding a DC system short-circuit with more current than the charger rating. The circuit breakers and fuse are sized for the charger maximum rating and system available short-circuit current. The current limit setting usually is set between 110% to 125% of the charger rating.

Additional specified optional system and charger protection can include DC ground detection, AC power failure, AC or DC high and low voltage, charger failure. These features include contacts which may either alarm or shut down the charger.

Load Protection Circuits supplied by a station service control battery usually include circuit breaker tripping and closing, emergency lighting and small emergency lubricating pumps for generators. Individual tripping circuits in switchgear lineups usually are protected with 35 ampere Class H (NECS) fuses for short-circuit protection. Fifteen ampere fuses usually provide both overload and short-circuit protection for individual circuit breaker stored energy closing circuits. Twenty ampere circuit breakers usually protect emergency lighting circuits. Probably the largest DC motor supplied from a battery system is about 15 hp, requiring 200 ampere branch circuit protection supplemented with appropriate motor thermal overload relay heaters.

DC System Protection Overload and short-circuit protection for the DC

system usually is provided by either circuit breakers or fuses sized to protect the circuit wire or cable. Such protective devices not only protect the circuits involved but also are coordinated with source protective devices and are selective with load protective devices. Figures 2 and 3 show single and dual battery systems with some of the typical protective devices such systems include.

Overload and short-circuit protection operates selectively, isolating the faulted circuit. Care must be exercised choosing feeder protective devices to assure proper overcurrent and short-circuit coordination for the system.

Undervoltage protection (#27) usually is C O M ~ Z Z ~ ~ ~

to an alarm system. For an undervoltage due to a DC system fault, all the instantaneous undervoltage relays, usually set at about 70% of the nominal battery voltage, drop out. These relays usually light an alarm light and start a timer to sound an audible alarm. If the fault is on a DC feeder circuit, the appropriate feeder protective device operates, isolating the fault. All alarm lights except the one for the circuit involved go out when the fault is removed from the system and nominal voltage is restored to the rest of the system leaving only the faulted circuit alarm light lit. The timer sounds the alarm, which usually can be silenced with a push button. Locating this silencing button in the battery room would compel the operator to investigate a possible destructive event in the battery location.

Sample DC Control Battery Systems Three sample battery systems will be discussed:

0 Single battery system 0 Dual battery system 0 Redundant DC control system

Each of these nominally 125V DC systems involves typical lead-calcium battery, 60 cells at 2.1 to 2.3 volts per cell. The batteries and automatic charger were selected considering the system demands in accordance with the criteria outlined in references [6], [7], and [SI. Table 3 compares a few of the advantages and disadvantages of these systems. The single battery system shown in Figure 2 involves one battery, one automatic charger, a distribution panel and distribution circuits. This is the usual system found in industrial plants.

The dual batten svstem shown in Figure 3 involves two batteries, one charger, a distribution panel, and distribution circuits. This system provides a redundant battery but is not a redundant system. The main advantage of this system is that adequate protection for internal battery faults can be provided.

The redundant svstem is not illustrated with a figure

177

* I I I I I B A r r E R Y

C B I I I CMFOER I I I

A -

Figure 2 - Single Battery System

F l

I I

I I

I I I I

I =

ca2

A

because various degrees of redundancy can be provided according to how much expense can be allocated for redundancy. The dual battery system is redundant for battery failure and does have battery protection which removes a faulted battery from the system. It does not provide redundancy for a fault in the battery leads or any of the rest of the distribution system. Duplicate battery chargers and battery leads to a distribution panel with two main breakers would provide redundancy for battery and battery leads but not for the rest of the battery system. A system with half the load supplied fiom one of the two batteries and a tie breaker provides a false sense of security. It does not allow for faults in the panelboard since closing the tie circuit breaker would only trip the other main cicuit breaker and shut down the DC control system due to the panelboard fault condition.

A comdetelv redundant svstem could include duplicate single or dual battery systems, two trip coils on each power circuit breaker and two separate control busses for each power circuit breaker. This would provide continuous tripping power for each circuit breaker but duplicate closing mechanisms are not available on present standard design switchgear

*E% j

equipments. Some redundant systems include tie circuit breakers with manual operation permitted to prevent closing on a system fault. Automatic throwuver is not applicable since it could close on a system fault disabling the control system.

Single Battery Sample System Figure 2 shows a sample battery system with

assumed protective devices. The Data Appendix lists the complete rating for equipment shown. For DC short-circuit conditions, the battery charger contribution is ignored since the current limit feature in modem chargers will limit the contribution to only slightly more than the charger rating. The protective circuit breakers and backup fuses included with the charger will not operate under these conditions.

Figures 4(a), (b) and (c) show time current curves for three battery protective devices installed either at the F2 or the battery disconnect switch location.

The fuse selected and shown in fig. 4(a) is somewhat large for the battery shown if the assumed rate of rise of short-circuit current is as shown. It is not selective with the 40A circuit breaker shown for

178

I I

I I I h

P a l RELAY STAT ION

"L -R1 &ERG- CIRCUITS CIRCUITS L l O m

Figure 3 - Dual Battery System

the trip circuit of the AC circuit breaker. The panelboard main breaker acts only as a disconnect since its instantaneous setting is higher than the available short-circuit current at the panelboard.

Figure 4(b) shows the time-current curves with the battery fuse replaced by an instantaneous only molded case circuit breaker set to trip under short-circuit conditions. This selection means that the system will be selective for overloads but not for short-circuits. The battery receives better protection because it will feed the short-circuit for the minumum time any protective device with a practical setting will allow. The main battery leads, however, are not protected in accordance with the NEC which requires instantaneous only circuit breakers to be applied only when they include an overload element. A low set instantaneous circuit breaker protects these conductors better than the hlgh set instantaneous element in a circuit breaker which includes an overload trip. These important circuits, normally lightly loaded, deserve instantaneous protection to remove faults which could result in severe damage.

Figure 4(c) shows the timecurrent curves with the battery fuse replaced by a molded case circuit breaker

with a high set instantaneous. This circuit breaker is selective with downstream protective devices (except for the panelboard main breaker) but allows the battery to feed the fault for up to two seconds. While this compromises battery protection, it should not harm the battery.

If DC system coordination considers overload selectivity only and considers short-circuits an unavaoidable disaster, an instantaneous only circuit breaker seems the proper choice for a single battery system.

Dual Battery Sample System Figure 3 shows a sample dual battery system with

assumed protective devices. Under DC short-circuit conditions, the battery charger contribution is ignored. The current limit feature in modem chargers will limit the contribution to a DC short- circuit from the charger to only slightly more than the charger rating. The protective circuit breakers and backup fuses included with the charger will not operate under these conditions. The time-current coordination curves are shown in figure 5 for parallel battery conditions. Since one battery can feed a

179

10 100 1- - IN-ES AT 1- CT

10 100 1m

IN-

AT 12UV Dc

1.w- - - -

0 J -

10 1111 1oou cuIT(CyT IN Y R U C S

hT I t W CC

Figure 4 - Coordination Curves for Alternate Single Battery Systems

short-circuit in the other battery, an instantaneous only circuit breaker will provide better battery protection without sacrificing overload selectivity.

The coordination curves shown in Figure 5 are drawn from the perspective of the panelboard and main switchgear. The instantaneous setting of the battery circuit breakers is shown on this curve at twice nominal pickup setting. When considering the battery protection, the instantaneous setting is about half the short-circuit contribution from one battery to assure positive tripping. Normally, coordination of multi-source systems cannot be adequately visualized by coordination curves. Overcurrent protective devices should be coordinated considering operating time at the multiples of pickup current through a device compared to downsteam device operating time.

Ultimate Backup System Protection Tragedy resulting from battery system failure can be prevented by installing DC undervoltage devices on each AC source circuit breaker. Such devices are available for both medium and voltage power circuit

such devices since this would shut down the entire plant. Such a shut down could be far less expensive than the destruction of a large drive motor, for Dual Battery System switchgear equipment lineup or important transformer. If the battery fails on a system including large synchronous motors and the field

contactor opens, the loss of tripping power can be an unfortunately disastrous event. It only takes one event of this nature to convince any engineer of the advantages of such backup for a battery system.

breakers. Many engineers are reluctant to specify *o 100 I000 l o o w

Figure 5 - Coordination Curves

180

system

Single battery

Advantages

Least expensive

Dual battery Good battery fault protection

Redundant single battery Fast control power restoration

Redundant dual battery with double power circuit

breaker trip coils

Continuous tripping power

Good battery protection

Disadvantages

Questionable internal battery fault protection

Control power loss results from most DC system faults

More expensive than single battery system

Control power loss results from most DC system faults

More expensive than single or dual battery systems

Questionable internal battery fault protection

Control power restoration depends on operator skill

Most expensive system

Closing power not continuous

Table 3 - DC Control Power System Comparisons

References

[ 11 Richard L. Nailen, "Battery Protection - Where Do We Stand," Trans. IEEE Ind. Appl. Vo1.27, No.4, pp.658-667, Jul./Aug.l991.

[2] Henry E. Lhar, Jr., "Application of DC Control Power for Switchgear," IEEE Pulp & Paper ConferenceRecord, pp. 106- 113, May, 1981.

[3] IEEE 446-1987, "Recommended Practices for Emergency and Standby Power Systems for Industrial and Commercial Applications."

[4] IEEUANSI C2-1990 National Electrical Safety Code, Section 14.

[5] NFPA 70-1993 National Electrical Code, Article 480. [6) E.C. Korbeck, Jr. & J.W. Blankley, "Selection, Use, and

Care of Stationary Batteries for Paper Mill Service," Trans. IEEE Ind. & Gen'l Appl., Vol.IGA-7, Nod, pp. 742-749 NovlDec 1971.

[7] B. Bridger, Jr., "Control and Auxiliary Power Systems for Industrial and Commercial Switchgear Installations," Trans. IEEE Ind. Appl., V0l.W-20, No. 3, pp.667471, May/June 1984.

[8] M.W. Migliaro, "Considerations for Selecting and Sizing Batteries," Trans. IEEE Ind. Appl., Vo1.U-23, No. I , ladFeb. 1987.

[9] M.W. Migliaro, "Maintaining 'Maintenance-Fme' Batteries," IEEE I&CPS Tech. Conf. Record, pp. 69-73, May, 1989

[lo] Kasper, R.J., "ProvidingReliable Switchgear Control Power," Plant Engineering, Jan. 10, 1980 pp.67-70.

[ l l ] "GE Protection & Control - Prcducts Catalog", GEZ-7223A (1/92), GE Meter & Control, 205 Great Valley Parkway, Malvern, PA 19355.

[ 121 "Drive Systems Industrial Control Components", GEP-345C, GE Drive Systems, Salem, VA 24153

(89CH2738-3).

181

HARDWARE APPENDIX

(See References [ll] and [12] for other details)

Rating

24V DC

Battery and Charger Undervoltage Protection

Dropout

18-24V

Dropout Auxiliary Coil 1) (:;? I Calibration

125V DC

125V DC

11 125VDC I 100-140VDC I 120V60HZ 11 100-140V DC 208V 60HZ

100-140V DC 240V 60HZ 250V DC 200-280V

Drawout protective relay including 60hz aux coil and 0.5 sec. time delay drop-out (#27,#27x)

48V DC

125V DC

40-54V DC 120V 60HZ

54-86V DC 120V 60HZ

Rating I I Time Adj.

Rating I Dropout Contacts Glib.

Instantaneous Undervoltage Relay (#27) [Molded Case]

24V DC

48V DC

19-27V 1-N0,l-NC Back

38-54V 1-N0,l-NC Back

I/ 48V DC I 38-54V /I 125V DC 100-14OV

12% DC

25OV DC

100-140V 1-N0,l-NC Back

200-280V 1-N0,l-NC Back

Inst. W Relay (#27) [Drawout Type]

125V DC

48V DC

100-140V l-N0,2-NC Back

38-54V 1-N0,l-NC Front

11 120V60HZ I 0.25-2secs 11

~

125V DC

250V DC

11 208V 60HZ I 0.25-2 secs 11

~

100-140V 1-N0,l-NC Front

200-280V 1-N0,l-NC Front

I[ 230V60HZ I 0.25-2 secs 11

Time Delay Dropout Auxiliary Relay (#27) [Non-drawout]

Instantaneous dropout auxiliary relay (#27X) [Non-drawout]

182

HARDWARE APPENDIX (Cont.)

Representative Connections

Rating Max. R to Grd (#MG) to operate

250V DC 30 ohms

125V DC 15 ohms

48V DC 5 ohms

24V DC 1.25 ohms

* A

I I t I t 27

Contacts (2 coils)

1-NO/coil

1-NO/coil

1-NO/coil

1-NO/coil

AC

I

A

DC

DC Ground Detector Relays

Switchgear Type Drawout Relay

Representative Connections

4 - 4 mi LS CONNECTED

183

CALCULATION APPENDIX

SHORT-CIRCUIT CALCULATIONS FIGURE 2

Battery R, = 120' = 0.1652 0 724A

0 41061 Conductors R, = 2 x 16fr x 2 = 0.0148 61 l@wsc

Circuit Breaker R , = 2 x O.OOO6 0 = 0.0012 61

RI = R, + R, + R, = 0.1812 0

120v 0.1812

Short-circuit at Panelboard = - = 662.4

0 64062 Conductors RI = 2 x 60fr x - = 0.0768 0 l o w

R, = R, + R, + R& + R: = 0.2580 0

120v 0.2580

Short-circuit at Switchgear = - = 465A

SHORT-CIRCUIT CALCULATIONS FIGURE 3

Buttery 1 R, = 120' = 0.1652 62 724A

0 4100 Conductors R, = 2 x 6fr x 2 = 0.0049 0 1CJwfr

R, = R, + R, = 0.1701 n

Paralleled Batteries

Rc = 2 x 18frx - 0.4100 = 0.0148 0 loo?!

Rcb = 2 x O.OOO6 0 = 0.0012 0

RI = R, + R, + R, = 0.0998 62

120v Max. Panelboard Short-circuit = - = 1202.4 0.0998

0 64061 R: = 2 x 603 x 2 = 0.0768 61 lowfr

RI = R, + Rc + R, + RI = 0.1766 n

120v 0.1766

Mar. Switchgear Short-circuit = - = 680A

184

RATEOF-RISE APPENDIX

101 Figures 1 and 2 shown below are approximate copies of Figures 2 and 4 presented in the Nailen paper (Reference [l]). In this paper, ' Nailen states that

5 typical battery circuit time constants range from 10 to : 7 5

80 ms. He states that most battery and fuse 5 manufacturers tend to accept 10 ms as most 2 representative. From Figure 2, offered by the C & $ more complex than the simple approach in Figure 1.

implies that the time constant is significantly shorter

so

D Battery Company, it appears that the subject is

(In Figure 2, E = Voltage, I = Current). Figure 2

U

% 2s

K - than 10 ms.

The Industrial Power Systems Data Book, published by the General Electric Company in the 1950's, calculates (section .173, pp. 1-2) a "typical" battery time constant to be 0.32 ms, using the Figure 1 type of curve.

Conversations with Charley Muller (retired Exide representative) and Felix Garfunkel (engineer - Badger Engineers, Boston) informed me that a battery can withstand discharging into a short-circuit at its terminal for almost 10 to 15 seconds without damage. This could be considered confirmed on page 659 of Reference [l] but no data is given.

Molded case circuit breaker instantaneous elements detect and respond to peak current. For AC circuits, they are calibrated in RMS current equivalent to a pure sine wave peak. Fuses and thermal elements detect and respond to the RMS currents in the protected circuit.

E n 0 1 2 3 4

N W E R OF T I N CONSTANTS FO-LOWIffi FWLT IN IT IAT ION

Fig. 1 Rate of Rise of Battery Current

per Equations

- p - - - - - - - - 1 - - - - - - -

-r - - - - - - - - I

O I I I m 0 Q

EWPD T I Y YILLI-

Fig.2 Another Rate of Rise Concept

185

RATEOF-RISE APPENDIX (Cont.)

t I I I sec amPS

0.01 726(1 - E-') 726 x 0.6321 459

0.02 726(1 - E-? 726 x 0.8647 627

0.03 726(1 - C3) 726 x 0.9502 690

0.04 726(1 - €3 726 x 0.9817 718

To calculate the rate of rise of the DC current, use the following: terminals:

For DC faults at the Figure 2 single battery

In this equation, t, is the begining of any time constant period and

From k discussion, it appears that some work should be done to establish standards for control batteries which would allow easier selection criteria for protective equipment than those criteria now extant.

is the end.

i = _V - R

V = Battery Voltage

R = Circuit Resistance

L T = Circuit lime Constant = - R

= 726A R

v = 120v

120 R = - 726

If T = 10 x lo3 seconds = W R

Then:

L = 0.16529 x 10 x A = Max. Current after t = 0

L = Circuit Inductance

If fuses respond to RMS currents, then the currents shown on the time-current curves for the rate-of-rise of battery short-circuit current should be calculated by:

i = Instantaneous Peak Current

E = 2.71828 I,, = j,: iz dt

186

DATA APPENDIX

Figure 2 Equipment Conductors Battery to Charger 12 feet 2 - s/c #6 AWG Charger to Panelboard 4 feet 2 - slc #6 AWG Panelboard to Switchgear 60 feet 2 - s/c #12 AWG

Battery 120 V , DC; 50 ampere-hours (8 hour rate)

75 A (1 minute discharge rate) 724 A (Maximum Short-circuit)

Interrupting Equipment CB 1, CB2 F1 F2 CB3 CB4 CB5, CB6 F3 F4

Ratings selected by charger mfg Ratings selected by charger mfg 100 A Form 101 Fuse 100 A TFJ therm. mccb, 1250 A inst. 40 A TEY therm. mccb, 1600 A inst. 20 A TEY therm. mccb, 700 A inst. 35 A Class H fuse 15 A Class H fuse

Figure 3 Equipment

Same as Figure 2 above except: Two identical batteries same as figure 2, separated by 6 feet of #6 AWG conductor CB3 150 A TFJ therm. mccb, 1500 A inst. CB7, CB8 30 A TEC mccb, 390 A inst. only

Figure 4(a) Curve Identification

A Max. avail. panelboard short-circuit = 662 A B Max. avail. battery short-circuit = 724 A C CB3 identified above for Fig. 2 D F2 identified above for Fig. 2 E CB4 identified above for Fig. 2 F F3 identified above for Fig. 2

Figure 4(b) Curve Identification

Same as Figure 4(a) except: D Same as CB7 or CB8 identified above for Fig. 3

Figure 4(c) Curve Identification

Same as Figure 4(a) except: D 60 A TEY therm. mccb, 1500 A inst.

Figure 5 Curve Identification

A Max. avail. panelboard short-circuit = 1202 A B Max. avail. battery slc at CO". COM. = 1400 A C CB3 identified above for Figure 3 D 30 A TEC mccb, 390 A inst. only E CB4 identified above for Fig. 2 F F3 identified above for Fig. 2

187