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The two halves of the control panel are essentially the same. Each half is mounted in a steel frame which is joined to the other to form a single structure and is shock mounted to the hull. The starboard control panel consists of the generator levers for the No. 1 and No. 3 generators, starting and reversing levers for the starboard motor, and a bus selector and forward battery lever. The port control panel consists of the generator levers for the No. 2 and No. 4 generators, starting and reversing levers for the port motor, and a bus selector and after battery lever.

1. Starts, stops, reverses, and regulates the speed of the main motors for both surface and submerged operation.

2. Provides for series, parallel, or series-parallel connections of the motor armatures.

3. Provides for uniform speed control of the main motors throughout the entire range of propeller speed from about 38 rpm to 192 rpm submerged, and to about 280 rpm on the surface.

4. Provides for operating the main motors from one or both main storage batteries and from any combination of the main generators.

5. Provides for charging one or both storage batteries with main generators, individually or in combination. Main generators not being used for battery charging may be used for propulsion power.

7. Provides for operation ahead on one propeller shaft and astern on the other at any speed within the designated operating range.

8. Provides, by means of shore connections, for charging the main battery from shore or tender.

3A4. Simplified power circuit description. a. The main control cubicle circuit (Figure 3-6) consists essentially of two buses, the motor bus and the battery bus to which the main power units are connected by means of their associated contactors in order to provide the various operating combinations. The motor bus is the one to which the main motors are

The motor bus can be split for operation of the motors on one side independently of the other side (BUS TIE OPEN), closed for parallel operation of both motor groups (BUS TIE CLOSED), connected to the battery bus for battery operation of the main motors (BATTERY BUS), and lastly, for series operation of all motors, the positive side of one motor bus can be cross-connected to the negative side of the other motor bus, so that by proper closing of the motor contactors, all four motors can be placed in series for slow speed operation on the battery bus (SLOW).

Either or both batteries can be connected to the battery bus by closing their respective contactors which in turn are controlled by one operating lever.

b. Excitation and control circuits. As indicated in Figure 3-7, excitation power is furnished from either the forward or after battery

e. Protective circuits. Motor, generator, and battery contactors are provided with overload protection of the trip-free, holding coil type. An overload relay is placed on each side of each armature and each battery. All overload relays associated with each group of contactors are connected in series with a holding coil. The holding coil is an electromagnet which, when de-energized, allows the trip-free mechanism to operate. For the description of this mechanism see Section 3A14. The protective circuits are shown in Figure 3-8.

1. One main propulsion control panel and operating bench with necessary instruments, rheostats, operating levers, etc.

2. One after contactor group comprising:

a. Port and starboard motor reversing switches.

b. Port and starboard motor starting contactors.

c. Bus selector switches.

3. One forward contactor group comprising:

a. Port and starboard main generator contactors.

b. Forward and after battery contactors.

c. Motor bus tie contactors.

All parts are mounted in a number of steel frames which are joined to form a unit. The assembly is supported on rubber shock mounts which are secured to the hull.

a. Two reverser levers. These levers are used to change the direction of rotation of the main motors by reversing the current flow through the armature. One lever is for the 2 starboard motors, and the other is for the 2 port motors. Each lever has 3 positions, AHEAD, OFF, and ASTERN.

b. Two starter levers. Each of the starter levers, 1 for the 2 port and 1 for the 2 starboard motors, has a STOP position and 5 operating positions, SER. 1, SER. 2, SER. 3, PAR. 1, and PAR. 2. The starter lever is used for cutting in a resistance in series with the armature, thus keeping the starting current down to a minimum. As the motor picks up speed, the resistance can be cut out of the circuit when the armature is at running speed and the current reaches a normal value, putting it across the line voltage. The starter levers have 3 series

c. Four generator levers. One lever is provided for each of the 4 main generators. The levers have an OFF position and 2 operating positions, MOTOR BUS and BAT. BUS.

NOTE. On Westinghouse controls the operating positions are GEN. BUS and BAT. BUS.

The function of these levers is to place any desired generators on the battery bus for charging the batteries, or any one or all of the generators on the motor buses for propulsion. An extra mechanical latch on each lever prevents accidental movement from the OFF position.

d. One battery selector lever. This lever has an OFF position and 3 operating positions,

3A7. Mechanical and electrical interlocks. The 10 operating levers have notched steel bars attached to them and mechanically interlocked with each other by slide bars. In addition, an electrical interlock is provided on each starter lever. This interlock consists of a solenoid whose circuit is completed by cam-operated contacts attached to the shaft of the field rheostat handwheel. The resulting interlocking arrangement of the operating levers is as follows:

1. A motor starter lever cannot be moved from the STOP position unless the corresponding motor field rheostat is in at least 75 percent full field position (electrical interlock).

2. The reverser lever cannot be moved unless the corresponding motor starter lever is in the STOP position.

3. The battery lever cannot be moved if the bus selector lever is in the SLOW or BAT. BUS position except that it may be moved between the FWD. BAT. and BOTH BAT. positions at any time.

4. The bus selector lever cannot be moved unless both motor starter levers are in the STOP position, except that the bus selector lever can be moved between the MOTOR BUS-BUS TIE OPEN and MOTOR BUS-BUS TIE CLOSED positions at any time. The motor starting levers cannot be moved when the bus selector lever is in the OFF position.

5. A generator lever cannot be moved from the OFF position to the MOTOR BUS position unless the bus selector is in the OFF position.

CAUTION. If one generator is already in the MOTOR BUS position, any other can be thrown at will. Hence this interlocking arrangement does not prevent the operator from placing a dead generator on a live motor or battery bus and seriously damaging the machine.

6. The auxiliary latch must be lifted before a generator lever can be thrown to the MOTOR BUS or BAT. BUS position. To operate a lever with an auxiliary latch requires the use of both hands, the object being to make the operator realize the importance of the step and cause him to think before he makes a particular selection.

7. The bus selector lever cannot be thrown to the BAT. BUS or SLOW position unless all generator levers are in the OFF or BAT. BUS position, nor can any generator lever be thrown to the MOTOR BUS position if the bus selector is in the BAT. BUS or SLOW position.

3A8. Overload relays. Each battery, motor, and generator is protected against short circuit and overload by means of overload relays connected as shown in Figure 3-8. The overload relay contacts open when the current exceeds a certain value, thus de-energizing the holding coil and permitting the contactors in the overloaded circuit to open.

The relays are provided with adjustable, calibrated tension springs for the purpose of adjusting the current at which the relays open. Since the relays are of the instantaneous acting type, they must be set rather high to prevent tripping due to current peaks which may occur during starting and maneuvering. The battery relay is usually set for 12,000 to 14,000 amperes. The generator and motor relays are usually set for 10,000 to 13,000 amperes. For specific calibrations of the various relays refer to the manufacturer's instruction book.

3A9. Reverse current protection. A reverse current relay (Figures 3-13 and 3-14) is provided for each main generator to protect it and its driving engine when charging batteries. These relays are adjusted to operate at a low reverse current value. In the event of reverse current flow (current flowing from battery to generator) of sufficient value, the relay contacts open, thus deenergizing the holding coil circuit and causing the generator contactors to open. The relays normally are set to operate at 300

CAUTION. These relays do not act in sufficient time to prevent damage to a generator if it is accidentally connected to the battery when
Figure 3-13. Main generator reverse current relay, closed.

Figure 3-14. Schematic diagram of main generator reverse current relay.

3A10. Field discharge resistors. The field discharge resistors connected across each generator and motor shunt field serve to limit the inductive voltage rise across the field during opening of the field switch. The resistors used on Cutler-Hammer and Westinghouse equipment consist of wire wound resistors connected across the field terminals just before the field circuit is opened. General Electric employs "Thyrite" (trade name) units (Figure 3-15) which are composed of a ceramic material, having very high resistance at low voltages and low resistance at high voltages. They are permanently connected across the field terminals. In both types of installation, the energy of the discharging field is dissipated in the resistors in the form of heat, thereby protecting the field coils from the high voltage that results from the sudden opening of an inductive circuit.

Figure 3-15. GE "Thyrite"field discharge resistor.

3A11. Motor and generator field rheostats. The 2 motor rheostats port and starboard, and the 4 main generator rheostats are of similar design. General Electric employs a fixed commutator type, the individual bars of the commutator being connected to taps on the field resistor. The contact brush is rotated through bevel gearing by a handwheel on the front of the panel. Each rheostat has 90 steps of resistance.

Whenever 2 or more main generators are

Figure 3-16. Field rheostat, G.E. commutator type.

3A13. Motor starting resistors. One starting resistor is provided for each motor armature. These resistor units consist of steel straps or cast grids made of an alloy containing mainly nickel, copper, and iron. The capacity of the resistors is sufficient to carry the full motor armature starting current for approximately 1 minute when the motor starting lever is in SER. 1 position, plus 1 minute in the SER. 2 position, and 1 minute in the PAR. 1 position, provided the motors are operated for 1 minute in the SER. 3 connection when moving from SER. 2 to PAR. 1.

The resistors will stand a duty cycle of 2 minutes on, 1 minute off, and 1 minute on with 900 amperes flowing through the resistors, and not exceed 390 degree C during this cycle. These

Figure 3-18. G.E. main motor starting resistors.

3A14. Contactors. All contactors that may be required to operate under load are provided with arc chutes and magnetic blowout coils for circuit interrupting duty. All contactors, with the exception of the motor bus-bus tie contacts and the contactors which short out the starting resistance have, incorporated in their operating mechanism, a trip-free feature that allows the contactor to open independently of camshaft position. After such opening, due to overload, reverse current, and so forth, the camshaft must be returned to the OFF position to reset the trip mechanism before the contactor can again be closed.

3A15. Ground detector equipment. The ground detector equipment provided on the main control panel consists essentially of the following parts:

1. A double-scale voltmeter with a range from 0 volts to 500 volts on both sides of the center.

2. A rotary selector switch for selecting the particular circuit to be tested on motors or generators.

3. A battery selector switch for selecting the particular battery or polarity to be tested, either positive or negative alone, or both.

4. A resistor and push-button switch connected in parallel with the voltmeter. The purpose of this circuit is to lower the effective resistance of the voltmeter circuit to one-tenth of its regular value. It is provided primarily to increase the accuracy of measurement of low insulation resistance that is encountered on main power cables when they are carrying full load current continuously.

Instructions for the operation of this equipment are given in Section 3C4.

When moving the operating levers from one position to another, the operation should be firm, fast, and positive. Slow or hesitant operation will draw out sustained arcs between the arc tips, thereby causing excessive burning. However, this does not mean that they should be slammed from one position to another as this will cause rapid wear of the parts. One exception to this rule should be noted: When moving the motor-starting lever from a SERIES position to a PARALLEL position, a momentary positive stop is made in the STOP position before moving into the PARALLEL position. This allows time for the series contactor arc to collapse before the parallel contactors are closed. This stop is also necessary when returning from the PARALLEL position to the SERIES position.

CAUTION. If this precaution is not observed the supply may be effectively short circuited and the resulting fire will certainly damage the control equipment. This casualty has occurred several times in submarines on patrol, putting the control cubicle out of commission.

In operating the starting lever in a sequence of positions, the motor ammeters indicate a sudden high current as each position is reached, but, as the speed of the motor increases, this current decreases to a more or less steady armature current, indicated by a steady position of the motor ammeter pointer. The most successful operation of the motor control is obtained by waiting for a steady motor ammeter indication while in one lever position before moving the lever to the following position.

The normal operating position of the starter levers for SLOW operation (that is, with the bus selector in the SLOW position) is in the SER. 3 position. If the starter levers are moved to the PAR. 1 and PAR. 2 positions, thereby increasing the propeller speed and motor load, the series selector contactor will be overloaded.

CAUTION. The holding coil control switch should be opened slowly to allow for collapse of the induced voltage in the coils. Sudden opening of the switch may cause an induced voltage that will break down the circuit insulation so that repair or replacement will be necessary.

When using the same number of generators on each side, the bus selector lever should be in the MOTOR BUS TIE OPEN position, thereby disconnecting the port and starboard motor buses. For one-generator or three-generator operation, it should be in the MOTOR BUS TIE CLOSED position in order to supply equally both port and starboard motors, and for balancing the generator loads. For two-generator or four-generator operation, when the generators are divided equally between port and starboard, the bus selector lever may be in either the CLOSED or OPEN position. The OPEN position is preferred because it tends to prevent an electrical fire from spreading from one side of the cubicle to the other and permits independent control of the motors and generators on each side. Further, opening of contactors under overload on one side will not affect the operation of the other side.

In parallel operation of main generators, with the generator field rheostats mechanically clutched together for common operation, a mechanical interlock on each clutch handle prevents the turning of any rheostat far enough to disconnect its field. On older type GE controls this is accomplished by a switch on the clutch that bypasses the field contactors. The clutch cannot be engaged for combined operation if the field contactors are open.

It is possible for the bus selector lever to be moved from the BUS TIE OPEN to the BUS TIE CLOSED position at any time, thereby possibly throwing generator bus voltage across nonrotating motors or generators. Before moving the BUS selector lever from the BUS TIE OPEN to BUS TIE CLOSED position, make the following checks:

2. Check to see that if the generator levers in one side (port or starboard) are not in the MOTOR BUS position that the starter lever on that side (port or starboard) is in the STOP position.

For one- or two-generator operation always operate motors in series. For three- or four-generator operation always operate motors in parallel.

3B2. Optimum operating conditions. Detailed instructions for starting and operating the propulsion system in various combinations are given in the manufacturer's instruction book for the vessel. Due to the slight differences among the interlock systems for the several classes of submarines, it is not possible to give a single set of operating instructions that will cover all systems correctly. In order to get the best in performance and reliability out of the propulsion system, there are a few fundamental points which must be observed. They apply equally to all types of electric drive submarines, although the exact values of current, voltage, speed, and so forth, must be obtained from the manufacturer's instruction book.

3B3. Adjustment of generator field current. In general, the best engine operation requires that the generator be run more slowly as the power output becomes less. The power is equal to the product of the voltage and the current produced by the generator. Since the voltage depends on the speed and field current, if the field current remains constant and the speed is reduced as the load is lessened, the generator voltage will decrease in proportion to the load. This condition requires that the armature current remain constant for all loads. Since the major part of the losses is due to the armature current and since the ventilation becomes less as the speed is reduced, the net result is overheating of the generator. Therefore, the best results are obtained when the generator is run at the maximum speed compatible with good engine performance and also at maximum field

3B4. Adjustment of motor field current. While submerged, the motor field strength is the only control over the speed of the vessel (aside from the connection of the armatures in various combinations of series and parallel). On the other hand, when operating from the generators, the fields of the motors are adjusted to obtain the desired output from the engines. When starting and maneuvering, the field should always be kept at full strength to increase the available torque and reduce current peaks. For steady operations, the motor field current should be adjusted to give the desired load on the generators being used. It varies, depending on the number of engines used for propulsion. For example, with one engine on propulsion with the two motors on each side in series, it is necessary to weaken the motor field to about 80 percent of normal to load the generator to its full rated load. With two engines, one on each side, and the two motors on each side in series, the motor fields must be weakened still more, (to approximately 62 1/2 percent of normal) to fully load the two engines. For three generators on propulsion, with all four motors connected in parallel, the motor fields must be increased to approximately 110 percent of normal to obtain a full load on the engines. For four generators, full load should be obtained with normal full field on the motors. However, under conditions of foul hull and so forth, full load may be reached at a propeller speed lower than designated, in which case more than the normal field will be required. This condition is unfavorable to the motors as it has the effect of reducing the series field, causing poor parallel operation. Therefore, should full load be reached at a propeller speed less than that designated, operation should be restricted and when used, the voltage of the system should be made as high as possible by increasing the generator field current.

3B5. Maneuvering. Maneuvering should always be performed on an even number of generators divided between the two sides. This makes it possible to control the two shafts entirely independently of each other and also to control the speed by generator field control which makes much smoother operation possible.

3B6. Reversing. A quick reversal can be made either from the generators or the battery. For a generator reversal, it is desirable to have an even number of generators so that if an overload relay is tripped, power to only one screw would be affected. Reversal from the battery may be used when only one engine is in use at the time or for other reasons. On some vessels a reversal on the battery can be made faster than on the generators, but on others, which require the generators to be taken off the motor bus first, it takes approximately the same time. Maximum braking effect will be obtained when the current is held to a maximum (approximately 150 to 200 percent) and the transitions when no power is put out, are made as fast as possible. The recommended procedure for reversal on the engines is as follows:

All steps should be made smoothly with the current maintained as close to maximum as possible. If too much power is used in reversing, the propeller will cavitate and no increase in braking effort will be obtained.

3B7. Adding a generator to a live bus. Whenever a generator is added to a live bus, its voltage should be adjusted to slightly above that of the bus. This will prevent reverse current from flowing to the generator. When adding a generator to the motor bus, its speed and load should be equalized with the others on the bus

3B8. Propulsion from auxiliary generator. On a few vessels it is possible to put the 300-kw auxiliary generator on propulsion entirely free of the battery. However, on most vessels this is not possible. The auxiliary generator can, however, be used for propulsion without raising the battery voltage to the undesirable values which result in excessive evaporation. This is done as follows:

1. Start the auxiliary generator and connect to the battery. Adjust its voltage to 260 to 270 volts.

2. Start the motors from the battery and run in series. Adjust the motor field rheostat to give the desired shaft speed, being careful not to exceed the current rating of the auxiliary generator.

3. Make final adjustments of voltage with the auxiliary generator field rheostat, and of current with the motor field rheostat.

The above procedure is based on the assumption that the battery is fully charged. If the battery is being charged, the voltage must necessarily be determined by the state of the battery charge. It should be noted that the auxiliary generator must carry all auxiliary power as well as propulsion, and the current, therefore, should be read from the auxiliary generator ammeter rather than from the ammeters on the control cubicle.

3B9. Battery charging with a propulsion generator. Any propulsion generator not being used for propulsion may be placed on battery charge. It is necessary only to adjust its voltage slightly above battery voltage and throw its lever to BAT. BUS. The charging current is adjusted with the generator field rheostat. One battery may be charged independently of the other by placing the battery selector lever in the position for the battery to be charged.

CAUTION. Make certain that the auxiliary power bus tie does not parallel the two batteries when they are not paralleled in the propulsion control cubicle. The auxiliary power bus tie should never be closed except when all

3B10. Operation.In Figures 3-20 to 3-27 are shown, for General Electric single unit propulsion control equipment, the movements and final position of the operating levers for various operating conditions noted in the titles. These

3B12. Safety precautions. The following are the more important safety precautions to be observed in handling this equipment:

1. Do not enter the main control cubicle when the buses are energized.

2. Do not leave the motor and generator field circuits energized when the machines are not in service except for the purpose of keeping the machines from 5 degree to 10 degree F warmer than the surrounding air in order to prevent condensation of moisture on the windings. Even then the field current should never be more than is necessary for that purpose.

3. Do not operate the motors and generators with field currents in excess of the recommended values.

4. Never place a generator on a live bus

5. If the lubricating oil pressure on the motors and reduction gears fails, stop the motors immediately and ascertain the cause of the failure.

6. Never release the control lever latches unless preparatory to moving to a new position. Make certain that a lever engages the slot in its new position.

7. Do not operate the machinery with the safety devices or interlocks disconnected.

8. Do not advance the motor starting levers to the next position until the armature current has dropped to a reasonable value.

9. Make frequent inspections to insure that no tools or loose objects are inside the control cubicle or in such a position that they can fall into it or any part of the operating gear.

10. Whenever possible, deenergize the control cubicle and make frequent inspections for loose nuts and bolts and other connections.

11. Never operate the motors or generators at greater than rated armature current.

3C2. Contactors. It is essential that the

Figure 3-29. Carbon impressions of contact surfaces.

Contacts should be examined to see that foreign material has not accumulated on the silver surfaces. Surfaces should be kept aligned so that a carbon paper impression will show that at least 60 percent of the contact area, well-distributed over the entire surface, is making contact. Use only a fine file for dressing the surfaces. These switches function only as circuit-selector switches and are never required to make or break their contacts with current flowing. The wear of the silver contact surfaces is therefore negligible and the contacts should require replacement only after long periods of service.

Since these switches are of double throw design, it is necessary that both the upper and lower contact gaps and wipes be equal. If they are not equal, and all switch units operated by a common camshaft show the same irregularity in the gap and wipe dimensions of the upper and lower contacts, the difficulty is probably caused by a loose cam or by slippage of the moving contact assembly on the vertical supports. Contacts should not be shimmed to correct for irregular gaps or wipes.

3C4. Ground detection.A ground detector system is installed in the propulsion control cubicle to test for grounds on a number of circuits. It consists essentially of a zero-center ground detector voltmeter with selector switches so arranged that the voltmeter can first be connected from ground to the positive side of a circuit and then from ground to the negative side of the circuit. If the voltmeter reads zero in both cases the circuit is not grounded. If one side of the circuit has a dead, or very low resistance ground, the voltmeter will read zero when connected from ground to the grounded side, and will read full circuit voltage (voltage

a. Battery grounds. 1. To detect battery grounds, turn the machinery ground detector selector switch to OFF. A study of Figure 3-32 shows that the battery selector switch can be used to connect the ground detector voltmeter to each battery in each of the following ways:

a) From ground to positive battery terminal.

b) From ground to negative battery terminal.

2. When connected according to c) above, the voltmeter will deflect in one direction for a ground or grounds on the positive cable leg or positive half of the battery; and in the opposite direction for grounds on the other half of the system. There will be no deflection of the voltmeter if the battery is grounded at the center; or if the battery and cables are symmetrically grounded on both sides of the center; or even if the system is unsymmetrically grounded on both sides of the center if certain relations exist between the positions of the grounds and their resistances. Consequently, when the voltmeter is connected to the battery in this way, a voltmeter deflection definitely establishes the

3. A conclusive test for the presence or absence of grounds is made by noting V_p, the voltmeter reading when connected to the positive battery terminal, and V_n, the voltmeter reading when connected to the negative battery terminal. These are the ground detector voltmeter readings when it is connected according to a) and b) above. If both V_p and V_n are equal to zero, there are no grounds. If either is different from zero, or if both are different from zero, there is at least one ground on the battery or on cables or equipment connected to the battery.

a. On the newer submarines, grounds on the battery alone, when disconnected from its cables by opening the battery disconnect switches (main and auxiliary power and emergency lights) in the battery tank, can be detected and measured by using the voltmeter mounted on the individual cell voltmeter panel (see Section 5A9).

b. Test on machines in operation. To detect grounds on machines in operation, turn the ground detector battery selector switch to OFF, or in some installations to TEST LIVE CIRCUIT. Turn the machinery selector switch to connect the ground detector voltmeter from ground, first to one side of the machine and then to the other side. If the ground detector reads zero in both cases, there are no grounds. If either reading is different from zero, or if both are, there is a ground on the machine or in the circuit to which it is connected. It may be noted in Figure 3-32 that the ground detector voltmeter can be connected to either the positive or the negative side of the armatures, but to only one side of the field circuits. A zero reading on one side of a field circuit is not sufficient to show that no part of the circuit is grounded, but further test on the machine when it is idle will check this point.

c. Test on machines when idle. To make a test on machines that are idle, first turn the machinery selector switch to OFF, then turn the battery ground detector selector switch to TEST DEAD CIRCUIT. This connects two high resistances in series across one battery and

CAUTION. Always turn all ground detector selector switches to OFF when testing has been completed. Furthermore, never attempt to use more than one ground detector system at a time. There are several installed but they should be used one at a time.

3C5. Use of ground detector voltmeter to measure insulation resistance. A megger is not suitable for measuring the resistance of battery grounds because a battery, unlike a generator, cannot be deenergized. The ground detector voltmeter system, however, can be used not only to detect grounds but also to measure the insulation resistance to ground from batteries or from energized equipment or circuits. The insulation resistance to ground is found by using the equation:

R = R_v ((E/(V_p - V_n) - 1)

In this equation, E represents the voltage across either a motor, generator, or battery as the case may be; V_p and V_n represent the reading of the ground detector voltmeter when connected to the positive and negative terminals of the motor, generator, or battery; R_v is always the resistance of the ground detector voltmeter.

Consider, for example, a submarine with a 50,000-ohm ground detector voltmeter on the submarine control cubicle. With the battery connected to the control cubicle, the battery voltage E was observed to be 243 volts, and the readings of the ground detector voltmeter were from each leg to ground V_p = 10, and V_n = 192. The insulation resistance to ground from the complete circuit including the battery and

R = 50,000 ((243/202) - 1) = 10,000 ohms

This was a low value of insulation resistance and required further investigation. The battery was isolated from the cables by opening the battery disconnect switches, and additional measurements were made on the battery alone by using the ground detector voltmeter on the individual cell voltmeter panel. The ground detector voltmeter installed on this panel had a resistance of 30,000 ohms. The battery voltage E was 243 volts, while V_p and V_n were 9.2 and 10.6 volts respectively. The insulation resistance to ground from the battery alone was, therefore:

R = 30,000 ((243/19.8) - 1) = 338,000 ohms

The cause of the low resistance observed at the control cubicle was, therefore, not the battery, but the battery cables. Tests made on these with a megger showed that one leg had a low resistance and this was responsible for the low resistance found for the complete circuit. Similarly, by use of the above formula, resistance to ground of any generator or motor may be measured while it is operating. It must

The accuracy obtained in measuring insulation resistance with a ground detector voltmeter depends upon the accuracy and resistance of the voltmeter used and the value of the insulation resistance being measured. Insulation resistances that are either very large or very small as compared to the voltmeter resistance are determined only approximately. Insulation resistances not too greatly different from the voltmeter resistance can be measured with considerable accuracy.

Very accurate measurements of insulation resistance can be made when needed by deenergizing the circuit and using an insulation resistance tester. The chief advantages of the ground detector voltmeter are that it can be used to measure insulation resistance at any time a circuit is in use, and that it furnishes a means of making a continuous check on insulation resistance.