30

2 Ι Ο 2 0 y υ ζ

1 0

t o o 1 0 0 0 1 0 0 0 0 F R E Q U E N C Y - C Y C L E S PER S E C O N D

1 0 0 0 0 0

Figure 3. Impedance versus frequency of driven-rod ground system

terminals of the two buried wires with the voltage Vr across the resistor. The surge generator was discharged a t about 500 volts.

As the results show, the impulse imped­ance is somewhat smaller than the d-c leak­age resistance. No at tempt was made to record the voltages during the initial one or two microseconds.

Tests have also been made on a buried wire using high-frequency steady-state cur­rents of low magnitude and it may be of interest to note the reduction in impedance of the buried wire as the frequency is in­creased, as illustrated in figure 3 of this dis­cussion. In all of the grounds tested a reduction in impedance with increase in frequency took place. For certain types of grounds, however, the reduction was greater than in others.

E. D. Sunde: In his discussion, Mr. Hagen­guth questions the small difference shown in figure 6 between the maximum value of the surge impedance for unit step current and for a current of 2.5-mierosecond wave front. The surge impedance for the latter current (figure 6, curve 2) is derived from the surge impedance for unit step current (figure 6, curve 1), using the superposition theorem expressed in equation 17 of the paper. I t would thus seem to follow that the two im­pedances shown in figure 6 are mutually consistent, regardless of the assumptions on which the surge impedance for unit step current is based. The voltage in response to a unit step current is, of course, consider­ably larger than for a current of 2.5-micro-second wave front.

That the surge impedance for small values of time does not depend to any considerable extent on the wave front may also be con­cluded by inspection of the surge impedance for unit step current, as shown in figure 2 of the paper. For a wire at a depth of 0.3 meter, there is a rather small variation in the surge impedance in the interval between 0.01 and 0.1 microsecond, so that it may without serious error be regarded as con­stant, and somewhat larger than 50 ohms, for times less than 0.1 microsecond. On the latter assumption the surge impedance for times less than 0.1 microsecond would be the same regardless of the wave shape of the current, and somewhat larger than 50 ohms. Since the maximum surge imped­ance is obtained for small values of time, it

would appear that the latter does not de­pend to any considerable extent on the wave front, when the latter is less than 2.5 micro­seconds, as implied by Mr. Hagenguth.

The Accuracy of W a t t - H o u r Meters on Intermittent Loads

Discussion and authors* closure of paper 4 0 - 4 0 by M . A . Faucett, C. A , Keener, and M . S. H e l m , presented at the A I E E winter conven­tion, N e w York, Ν . Y . , January 2 2 - 2 6 , 1 9 4 0 , and published in A I E E T R A N S A C T I O N S , 1 9 4 0 (August section) pages 4 3 3 - 4 1 .

Wm, M. Young (Ohio University, Athens): This paper indicates very interesting re­sults on a very major problem, a problem which vitally affects the revenue received by utilities and the charges made to consum­ers that have loads which are not steady in character. The problem is extremely im­portant and naturally creates wide interest.

Because of the large number of meters studied, general statements as to behavior can hardly be made, however, particularly with reference to the more modern meters, might I inquire if the authors have studied effects of change in ambient temperature on an instrument under intermittent load? Certainly it would seem tha t the heating effects caused by currents of short duration which produce a 200 or 300 per cent over­load if checked by an indicating watt­meter might cause local heating which would aversely affect the results.

A major problem still exists in determin­ing the effect of a complex wave of either current or voltage or both simultaneously. This problem has been partially attacked by another paper presented a t this same time ["Watt-Hour Meter Performance With Power Rectifiers," C. T. Weller, Η. E. Trekell, and F. O. Stebbins, A I E E TRANSACTIONS, volume 59, 1940 (August section) pages 749-57]. I t would seem, however, tha t it is extremely important to have definite data on the behaviorism of modern meters under these highly com­plex conditions. Have any results been obtained for a period less than one-half cycle or for periods less than the decelera­tion time of the instrument?

With present meters if the load cycle changes, figure 5 is apparently of consider­able importance. I t indicates tha t each type of intermittent load of short duration

should be separately studied by the utility and the meter very carefully adjusted if the intermittent load constitutes a major portion of the customer's load. In order that charges be fair and revenue just, the compensator apparently must be adjusted to the center point of its range between forward creep and reverse creep.

Has the effect of the back lash in gear trains been studied? Certainly we are dealing with a type of gear which must be produced very cheaply and one in which the torque for all angles of mesh of the gears is not constant. Other companies in other fields than electrical engineering have had considerable difficulty with variation in torque because of variation in the contact angle between gear teeth as rotation pro­ceeded. This variation in torque may be of considerable importance, particularly for cycles of rather short duration because of the static friction effect.

I t is to be assumed tha t the time of cir­cuit closure was governed and controlled although this is not definitely stated as far as I could discover. There is much more inter­esting work to be done on this problem and I certainly hope tha t we will have more good subsequent reports of progress in the future.

Η. E. Trekell (General Electric Company West Lynn, Mass.): This paper by Messrs. Faucett, Keener, and Helm has proved very interesting, first, because of the novel approach to the problem of determining the response of watt-hour meters to loads of short duration, and second, because of the magnitude of the errors found. In addition to finding the magnitude of errors, the method of analysis should also serve to point the way to improvement in case it is war­ranted.

I t has not been generally believed that errors as large as those found were present in watt-hour meters of recent or modern construction. For this reason tests have just been completed on the type 7-30 single-phase meter now being manufactured by the General Electric Company. In the first of these tests, the aid of Professor Η. E. Edgerton of the Massachusetts Inst i tute of Technology was enlisted. By the use of the high-speed stroboscopic-light camera, pictures were taken of the meter disk during acceleration and deceleration periods. Fig­ure 1 of this discussion shows the response found a t 100 per cent load and corresponds to figure 1 by the authors.

During acceleration the meter reached

ω<η 3 0

«5 ! 10! Α

C .— D

0.1 0.4 0 . 5

Figure 1 .

0 . 2 0 . 3 T I M E - S E C O N D S

Disk motion at 1 0 0 per, cent load unity power factor

A-B—Acceleration p e r i o d — t o w i t h i n o n e p e r cent of steady-state speed

B-C—Steady-state p e r i o d C - D — D e c e l e r a t i o n p e r i o d — t o w i t h i n o n e

per cent of steady-state s p e e d

1940, VOL. 5 9 Discussions 1 1 8 1

within one per cent of the ultimate speed in about ten cycles or 0.17 seconds. Also, the time required for deceleration to within one per cent of the speed at the start of the deceleration period was found to be very close to ten cycles. The loss in displacement during acceleration and the gain during deceleration were found to be nearly equal to the steady-state displacement during two cycles. The difference between the loss in displacement during acceleration and the gain during deceleration was so small that greater accuracy in reading the displace­ment was needed to assign finite values to the error found. The results did show, however, tha t for the conditions of figure 2 of the authors' paper, the error was less than one-fifth of that shown. The displace­ment was read to 0.1 degree by using the teeth on the edge of the disk. In the tests the current was started and stopped at the zero of the steady-state wave and a load of constant resistance was used.

I t was found that the damping and inertia constants of the meter disk were the pre­dominant factors in determining the re­sponse as the speed-time curve was nearly a pure exponential function with an ex­ponent of — (D/J)t where D is the damping torque per unit speed and / is the polar moment of inertia. If Τ be the steady-state torque applied or released, it may be shown that the loss in displacement during accelera­tion is equal to TJ/Di2 for a true exponential function and that the gain during decelera­tion is Τ JID22 where D\ and D2 are the respec­tive damping constants. The value for D2

should be less than that for D\ and by the* amount of the current-flux damping torque which is small in modern meters. This un­doubtedly explains why the error is positive and small and also why the error is larger in older meters.

For loads of very short duration such as are shown by the authors in figure 3, it is thought that the above concept must be modified to include the reactance of the meter disk. I t is thought that the disk react­ance may keep the steady-state driving torque from being established instantane­ously causing a delay of a small fraction of a cycle which effect would be difficult to evaluate or detect by means of the stro-boscopic-light pictures. A second test which may be considered more straight­forward was employed in finding the re­sponse for these loads of very short duration. This consisted of measuring the intermittent load by a reflecting dynamometer watt­meter which was sufficiently ballistic that the light beam did not oscillate more than one per cent of the scale reading. The registration of the watt-hour meter was then compared with the wattmeter in conven­tional manner. The results are shown in table I for loads of one to eight-cycle dura-

Table I

Error in Watt-Hour Meter Registration

Current Start- Current Start-T ime Time ing Near £ e r o ing Near Peak Load Load of Steady- of Steady-

on off State Wave State Wave (Cycles) (Cycles) (Per Cent) (Per Cent)

8 2 2 + 0 . 6 - 0 . 1 4 2 6 + 0 . 7 . . , - 0 . 7 1 2 9 , . + 0 . 5 . . .' - 1 . 9

tion at 100 per cent load and unity power factor.

These results show errors much less than those reported by the authors. Negative results for the condition where the load begins and ends near the peak of the steady-state wave indicate tha t the reactance of the disk is also a factor in determining the error. However, regardless of the conditions causing the errors with intermittent loads, it is evident tha t the present-day meter performs with remarkable accuracy which should be entirely satisfactory for most applications.

Hamilton Tread way (Illinois Commerce Commission, Springfield): The subject of watt-hour meter accuracy is one of con­siderable interest and importance to engi­neers engaged in the regulatory field. In the past the standards of meter accuracy formulated by the state public-utility com­missions have been predicated on the basis of constant load. Most of the rules were adopted when the principal use of electricity by the residential and commercial cus­tomers was for lighting and small motors. With the increase in the use of electric-welding equipment, flashing electric-light signs, X-ray machines, and other rapidly fluctuating loads the problems raised by the operation of intermittent loads on the dis­tribution systems of the electric utilities have become of increasing importance. The authors' paper in dealing with one of the problems raised by these changes in the character of the use of electricity is particu­larly timely.

Many of the existing rules of the state commissions permit errors in meter regis­tration up to 4 per cent. The authors ' conclusion that the maximum error on very rapidly fluctuating loads may reach 20 per cent would therefore prove of considerable importance in the case of a customer whose load is continually transitory in nature and is metered separately. A meter serving such a load separately, though meeting the present standards of accuracy under exist­ing methods of testing in the field, may actually be considerably in error. In such cases in the future it may be necessary to give some thought to the character of the load served by the meter when tests are conducted.

The authors do not indicate whether the tests were made under conditions of con­stant voltage. In the field we know tha t fluctuating loads and voltages go hand in hand. Since the potential flux of the meter will vary directly with the voltage the effect of the fluctuating voltage would seem to be of some importance. Except where the distribution system has been specially designed for a heavy fluctuating load the instantaneous drop in voltage when such loads come on the line is usually quite large. Information on the effect of these fluctuations in voltage on the accuracy of meters serving intermittent loads should prove of value.

A. B. Craig (Boston Edison Company, Boston, Mass.): During the fall of 1927, an investigation of the performance of watt-hour meters on intermittent loads was made b y C. H. Ingalls, then meter engineer for the Boston Edison Company,

M O T O R - D R I V E N — J R O T A T I N G S W I T C H ν Ό Ν " 5 0 P E R C E N T I * O F F " 5 0 P E R C E N T

S E R I E S M E T E R N O . 2

M O T O R - D R I V E N R O T A T ­I N G S W I T C H F O R U S E W I T H F O U R M E T E R S

N V O N " 2 5 P E R C E N T " O F F " 7 5 P E R C E N T

Figure 2

The results of Mr. Ingalls' investigations have not been published. Our comments are based upon the data obtained in this original investigation.

The fundamental approach to this prob­lem by the authors and by Mr. Ingalls are radically different. The authors have analysed individual impulses whereas, in our test methods, the average of a large number of impulses was used. Figure 2 of this discussion shows the test methods employed by Mr. Ingalls.

A constant load registers through the master meter and, by means of a motor-driven rotating switch this load is shifted from one series meter to another. For intermittent tests with loads' " o n " one-half the time and "off" one-half the time, a

0 4 0 8 0 120 160 2 0 0 P E R C E N T O F R A T E D C U R R E N T

- P O W E R F A C T O R 1.0

Figure 3

A—Disk constant 0 . 2 5 β — D i s k constant 0 . 3 C — D i s k constant 0 . 6

one-half segment commutator is used with two meters. For tests with intermittent loads "on" one-quarter of the time and "off" three-quarters of the time, a one-quarter segment commutator is used with four meters. The difference between the registration of the master meter and the sum of the series meters is of course the error in registration due to intermittent loads.

This method of test has the advantage that as many impulses as are necessary or desirable can be obtained. In some of the tests conducted by our company, over 100,000 impulses were used. Special regis­ters were placed on the watt-hour meters so that the actual number of disk revolu­tions could be determined to one-tenth of a revolution. In all tests, loads were applied for a sufficient time so that no, meter had

1182 Discussions A I E E TRANSACTIONS

Figure 4

1 0 0 p e r cent l o a d , unity p o w e r factor, disk constant 0 . 2 5

D — " O n " 5 0 p e r cent ; " o f f " 5 0 pe r

cent

£ — " O n " 2 5 pe r cent ; " o f f " 7 5 p e r

cent

I.0 2 3 4 5 6 θ Ι Ο 2 0 3 0 4 0 6 0 8 0 Ι Ο Ο C Y C L E S P E R " O N P E R I O D * O F I M P U L S E

fewer than 100 revolutions of the disk. The maximum error in reading is thus one-tenth of one per cent.

We believe that this method is superior to that described by the authors, in that it is a simple, practical test method and one which can be made more nearly to simulate actual field operating conditions. Although we are in agreement with the authors as to the types of errors involved, we differ widely as to the magnitude of these errors.

From the data presented in figure 3 of this discussion, we draw the following conclusions:

1. That the error in registration is positive and is proportional to the load.

2. That for meters of the same rating the error is inversely proportional to the square of the meter test constant.

The data in figure 4 of this discussion are in close agreement with the conclusion of the authors, tha t the maximum error occurs when the load duration is from two to four cycles. However, in figure 3 of their paper the authors show errors from four to seven times greater than measured by us. Although they state that there may be errors as great as 20 per cent, our data leads us to the conclusion that , when the meter is subjected to full-load impulses, the maximum error will not exceed i y 2 per cent, and that for smaller loads and modern meters a much smaller error may be ex­pected.

We are offering no discussion on various other conclusions drawn by the authors inas­much as they are of a very general nature and our own conclusions can be considered as well within their statements.

L. A. Burckmyer, Jr. (Cornell University, Ithaca, Ν. Y.) : This paper states that, as the duration of the "on" period of a load cycle is diminished, the watt-hour-meter error increases to a maximum and then ap­proaches zero as the load duration ap­proaches zero.

This statement is supported by (a) a brief verbal argument, (b) curves from direct tests, and (c) curves derived from measurement of acceleration and decelera­tion rates.

The argument reads "The registration error due to intermittent load must ap­proach zero as the acceleration portion of

the load cycle approaches zero or 100 per cent of the load cycle. At some other value of the duration of load the error must be a maximum." There is no question but that the error due to intermittent load must approach zero as the acceleration portion of the load cycle approaches zero, that is, as the load approaches a condition of not being intermittent. But the conditions stated for the other extreme are not so acceptable. Incidentally, the other extreme is not even correctly defined in the passage quoted, for it is obviously impossible for the acceleration portion of the load cycle to approach 100 per cent of the load cycle.

The meaning of the curves of figure 3 however, appears to be entirely clear. The registration error is represented as rapidly declining to zero as the duration of load is reduced from about three cycles to zero. This result, contrary to the implications of the passage quoted above, could not well have been anticipated. Let us consider a meter in which the driving torque is directly proportional to load power, the retarding torque due to the permanent magnet is directly proportional to disk speed, and the friction and potential-flux torques are ex­actly equal to the torque produced by the light-load compensator at all speeds.. We will recognize, however, the existence of a further retarding torque proportional to the product of current and speed. The qualitative performance of such a meter agrees closely with all the characteristics observed in practice and with those de­scribed in this paper but for one excep­tion: The percentage error on intermittent load loads reaches a maximum as the dura­tion of load approaches zero. (See ap­pendix of this discussion.) The value of the percentage error is finite, and deter­mined by the relation between the re­tarding torque due to load current and that due to the permanent magnet. With torque values that make performance a t low power factors and at overload acceptable, this error is small.

While this analysis disposes of the quoted argument presented by the authors, it of course cannot refute their test results. I t must be borne in mind, however, tha t the region under dispute involves load durations of less than one cycle. There is a question of the reliability of the authors' incompletely described method of direct test under such conditions, and undoubtedly the determination and graphical application

of speed-time curves for such brief load duration is subject to error. In the curves of figure 1, 0.9 of the steady-state slope is reached in about 0.2 second, so even a t the end of what is called the "constant-velocity" period the slope is about one per cent less than that which would occur under continuous load. This makes it difficult to locate accurately the asymptotes to the curves. I t seems tha t a direct test with a ballistic wattmeter would be indi­cated.

I t is admitted that there may be little practical importance in the question raised, and that the pulsating character of most of the torque components may make the dura­tion of load less important than the instan­taneous value of voltage at the moment load is applied. Further, the nature of the transient associated with some types of intermittent load might invalidate any laboratory or analytical predictions, even those involving load durations of a number of cycles.

In regard to light-load adjustment, the authors repeatedly stress the need of making the setting with extreme "care". Since it is stated, however, that meters adjusted within the limits of commercial accuracy may show 20 per cent error under inter­mittent load, and that this error can be reduced by readjustment of the light-load compensator, it appears tha t the proper setting of this control is not a question of care but of technique. Detailed instruc­tions for securing optimum adjustment would, have formed a valuable contribu­tion. In their absence it must be inferred that only by direct experimental study of performance under intermittent load can this be accomplished. Such procedure is, of course, impracticable in most cases.

APPENDTX

If a constant load is applied continuously, a speed (N) is attained, a t which the driving torque (Γ) corresponding to load power is just equal to the retarding torque (Tm) due to the permanent magnet plus that (Tc) due to current.

During acceleration, at any speed n, these retarding torques are reduced to ( r w - f -Tc)n/N, or Tn/N, so the torque available for acceleration is T—Tn/N or (T/N) (N-n). Therefore

dn dt 4:TT2IN

(N-n) = C(N-n) (1)

where I is the moment of inertia of the disk and C is a constant for a given load condition. The solution of equation 1 for an initial condition of η = 0 at t = 0 gives

n = N(l-e-ct) ( 2 )

The number of disk revolutions com­pleted in a time h after power is applied is

ndt = Nh-~ (1-6-€ίη (3)

During deceleration with zero load cur­rent, the decelerating torque becomes Tmn/N, so

dn dt

- = — C'n (4)

1940. VOL. 5 9 Discussions 1183

where

C = C- (5)

The solution of equation 4 for an initial condition of η — Ν when t — 0 gives

n = Ne (6)

and the number of disk revolutions com­pleted between the time t2 and standstill is

ndt-N

c -C'h (7)

To determine the number of total revolu­tions, we must find the value of h which corresponds to a disk speed equal to that at the end of the acceleration period. From equation 6 and equation 2,

i 2 = - ^ l o g ( l - e - c ' )

and substituting this value into equation 7, the number of revolutions during decelera­tion, following application of power for a time t\ is

( 8 )

so the total number of revolutions, from equations 3 and 8, is

I t should be Ntit so the per cent error due to intermittent load is ιο°(ΗΉ Cti

(9)

As h approaches 0, the quantity in brackets approaches the value C, or the meter over-registers by 100 TjTm per cent.

M. A. Faucett, C. A. Keener, and M. S. Helm: We are pleased to note that sev­eral of those who discussed this paper have presented experimental data. Their re­sults agree with ours in many particulars and we wish to thank them for their con­tributions. In the interest of brevity, we necessarily omitted many details which might have been of interest and the dis­cussions have been valuable additions.

Many factors which might be of impor­tance, in particular applications of meters, were omitted in this investigation. To have included such factors as voltage variation, temperature changes, and wave form, would have made the interpretation of results difficult. Such an increase in the number of variables would lead to so complex a situa­tion that the effect of any one would be nearly impossible to evaluate. The in­fluence of such factors as were omitted was minimized by keeping conditions involving them as nearly constant as was feasible. In answer to Mr. Young's question regard­ing the temperature effects of heavy loads of short duration, it might be pointed out that the thermal rating of meters is for continuous loads of such magnitudes and no errors of importance should be expected from this cause.

I t is difficult to reconcile the third para­graph and appendix of Mr. Burckmeyer's

discussion. The mathematical analysis of the performance of a watt-hour meter under even the simplest circumstances re­quires more than the assumption of simple exponential acceleration and deceleration. The component torques are not all simple functions of time or flux and differ greatly in their natures. Conclusions drawn from an analysis based on incomplete premises may be misleading. When equations fail to explain an observed natural phe­nomena, it is usually Nature that is correct.

In the interpretation of the curves of figure 3, only such points as represent in­tegral cycles should be considered, although a continuous curve was used to indicate the trend of such points. Fractional cycles introduce harmonics or unidirectional com­ponents in the current waves. This com­plication was purposely avoided in this investigation. An investigation, by the authors, of the influence of these factors has been in progress for some time as a part of the study of the magnetic conditions in meters. I t is recognized tha t intermittent loads, in general, need not be of integral cycles duration and tha t the behavior of meters on the general types of loads would probably deviate somewhat from that indi­cated for integral cycles.

In a like manner, under practical condi­tions, no guarantee can be made tha t the time of circuit closure will repeatedly occur at the same point on the cycle. A notable exception to this is the electronic-controlled welder. The curves of figures 1 and 3 should be considered as the average of a large number of possible cases. The ex­perimental test curves were obtained with circuit conditions approximating the elec­tric welder.

I t is difficult to present quantitative re­sults for meter studies without specifying all of the conditions involved, including the make, the type, or the age of the meter. For this reason, it was the intention of the authors that the study be considered more in a qualitative than a quantitative sense. For the presentation of curves, however, quantitative results are necessary. The cases chosen as illustrative are neither the best nor the worst, but are between those extremes. The quantitative results ob­tained on any meter were dependent upon the adjustments of tha t meter, especially the friction compensation.

The 20 per cent error mentioned in the paper was the greatest encountered in the investigation. This value should not be considered as the maximum possible error or that which might be expected in any particular meter. In all cases, the meters were adjusted within the limits of com­mercial accuracy. The meters used in the tests had no gear trains. However, it would seem probable that the gear train would not offer appreciably more opposition to disk rotation with an intermittent load than under normal operation. In a meter in good condition, this component torque is practically negligible.

The magnitude of registration errors reported by Mr. Trekell compares closely with that found by the authors in tests on that type of meter. The conclusions he has drawn check those included in the paper regarding the use of the modern meters on loads of intermittent character.

In the preliminary tests made by the authors, a switching arrangement similar

to tha t reported by Mr. Craig was used. I t was, however, put aside in favor of the other method which seemed to offer more material for analytical study. The errors observed by the two methods compared favorably.

The test data submitted in the discussions bear out the authors ' observation that maximum errors occur with load durations of from two to four cycles. Since these data were obtained by widely different methods, the check is of particular interest.

W a t t - H o u r - M e t e r Perform­ance W i t h Power Rectifiers

Discussion and authors' closure of paper 4 0 - 4 2 by C. T. Wel ler , H . E. Trekell, and F. O . Stebbins, presented at the A I E E winter con­vention, N e w York, Ν . V., January 2 2 - 2 6 , 1 9 4 0 , and published in A I E E T R A N S A C ­T I O N S , 1 9 4 0 (August section) pages 4 4 9 - 5 7 .

Walter C. Wagner (Philadelphia Electric Company, Philadelphia, Pa . ) : The authors are to be sincerely congratulated upon the effective work which they have done and upon their method of presentation. All re­search work of a reliable character tha t can be done on this subject will serve to make more clear the present atmosphere of un­certainty that faces the operating companies when measuring loads of this character.

Some three years ago, when I approached some of the electrical manufacturing com­panies, plans were outlined for making a joint investigation, combining the work on the test floor with tests in the field in order to include all elements of the problem in­volved in metering six-phase rectifiers. At that time, in 1937, the General Electric Company representatives stated that , at the first opportunity, when a power recti­fier was available on their test floor, they would arrange to have the necessary inte­grating and indicating meters installed, and tests made following the procedure proposed to their engineers. In 1938, upon their advice and invitation, we, in conference with their representatives, completed the details of the tests and co-operated in mak­ing the tests on a six-phase power rectifier on the test floor a t Schenectady. During these years and later, tests were made jointly with others, and also independently, on six-phase rectifiers, in line with the original program. The data obtained from these other sources are in the course of ex­amination, but the information so far is not uniformly in a satisfactorily close agree­ment with the data presented in this paper. I t will probably be necessary to make addi­tional tests to determine what factor or factors give rise to these differences.

Watt-hour meters operating on the induc­tion principle are susceptible to inaccuracies due to departures from sine wave and changes in voltage, power-factor, frequency, and temperature to varying degrees, de­pending upon the design of the watt-hour meter. No reference has been made to the possible effect upon the metering current and potential transformer performance and particularly when operating under service conditions. Note also that the actual periodic field calibration of service meters is

1184 Discussions A I E E TRANSACTIONS