NOTES:

1. To be leveled with means provided. Portable instruments rated at 0.1 percent accuracy shall be provided with a means for determining and obtaining a suitable degree of levelness. 2. Applies to single-range instruments whose range is 15 volts or higher or multirange instruments with a range ratio not exceeding 5 to 1.

3. All a-c voltmeters are calibrated to allow for 0.026 ohm lead resistance. Where a lead resistance of 0.026 ohm influences accuracy to a degree equal to 2 of the accuracy rating of the instrument, leads shall be supplied with the instrument and shall be used to obtain rated accuracy.

4. Multirange combinations are limited in range coverage in certain design types, and it should be pointed out that to obtain wide coverage in a single instrument, special arrangements may be required.

5. It is not required that these instruments be operable on direct current.

6. This maximum value shall not be exceeded at any time within six hours.

7. This requirement is limited for multiple-range instru ments to those having a ratio of highest full scale range to lowest full scale range of 4 to 1.

8. In the case of instruments having nonlinear scales, these values shall apply only in those portions of the scale where the divisions are equal to or greater than two-thirds the width they would be if the scale were evenly divided.

9. To be taken at 80 percent of full scale value. 10. No separate tabulation is shown for currently available instruments of rated accuracy class 0.75 or for other intermediate classes occasionally used. In the interest of standardization the classes have been limited to those shown.

11. This allows for the purchase of instruments not desig. nated as magnetically shielded, as a concession to present manufacture or particular characteristics. Such instruments are not limited in external field influence.

thought here was that the work already accomplished would be quite useful to industry and it seemed much in order to make it immediately available, particularly in view of the fact that a document on portable instruments would probably require several years in the making. It also seemed in order to ask the sectional committee for specific authorization to complete such a standard on portable instruments because such coverage was a bit vague in the original assignment. Everett S. Lee (General Electric Company), chairman of the committee, immediately put the question, and the sectional committee approved.

Portable Instrument Standard
Considered

Shortly thereafter the first meeting was called to consider specifically the portable instrument standard. Throughout the previous discussions on switchboard and panel instruments most everyone had the portable instruments in mind and there was much discussion at the committee luncheons as to how we might approach the subject. With this background the approach to the portable instrument standard was fairly straightforward and it was generally agreed that the endeavor should be to tabulate in compact form the specific requirements in somewhat the same fashion as had previously been used for the panel and switchboard instruments.

However, it was obvious that there was no standard for outside dimensions and the instrument casings actually varied from one manufacturer to another in shape, size, material, and appearance. The common denominator, however, appeared to be the accuracy class and scale length. Tabulations, therefore, were started for the several types of instruments against accuracy class.

As to types, there are a-c voltmeters and ammeters, d-c voltmeters and ammeters, and wattmeters. It was decided to omit the more complex and less used types such as power factor meters, frequency meters, and the like, and concentrate on the more common kinds.

The accuracy classes were selected from those offered by the various manufacturers and without too much regard to such prior limited listings as had been made. Some compromise was needed to secure common classes between manufacturers but such compromise was generally forthcoming.

Study Results

The result of this study was a series of four tabulations, one of which is shown on the facing page. This plate is printed here to indicate the possibility of tabulating the requirements whereby a very great deal of information can be placed on a single sheet to the end that a much better standard can be made available for those using such information in their daily work.

Some details may be of interest. It was found that some manufac turers were producing portable instruments to certain values of rated accuracy where, however, that accuracy would not be maintained when connected to a circuit for a long period of time. In many instances the demand for multiple range instruments had resulted in limitations of power dissipation to the point where working errors sometimes exceeded the accuracy rating. Representatives of the utilities and of laboratories using the instruments were quite critical of the manufacturers and at one time it looked as though a stalemate would result. Finally a new influence was proposed, "Sustained Operation Influence," stated to be the change caused by energizing the instrument over extended periods of time over and above the indications of the instrument when first connected to the circuit. After much discussion, values were assigned for this sustained operation influence. It is believed that this is the first time that recognition has been given to working errors of portable instru

ments.

Platform Influence

In a somewhat similar fashion the matter of the effect of placing the instrument on an iron table came to the fore. Certain types of instru

ments appeared to be affected by adjacent iron and, accordingly, "Magnetic Platform Influence" was listed, defined, and limiting values stated.

The matter of positioning of portable instruments always has been a question in the past. Portable instruments are designed for the high accuracy which can be obtained with the instrument horizontal, or, more importantly, with the axis of the moving system vertical. In this position minor changes in the balance of the moving element are eliminated. But there never had been a clear statement in this regard. Users have persisted in blocking up precision instruments at an angle to facilitate reading in some particular laboratory test to the evidently unre alized detriment to overall accuracy. Making a 14 percent accuracy instrument more readily seen at the expense of a possible 1 percent error is frequently attempted and the instrument maker blamed for the error.

"Position Influence"

"Position Influence" was, therefore, defined. As might be expected, this was a graded value whereby the highest accuracy instruments must be leveled as exactly as possible "with the means provided"; usually such instruments are provided with a small bubble level. But instruments having a 1 percent rated accuracy can be tipped as much as 5 degrees from the horizontal without exceeding the established criterion.

Other items tabulated have been used before in one form or another but, again, specific values had not been given and were utlimately assigned by dint of much discussion and the inevitable give and take and compromise in a working committee.

New Document

When the work was better than half complete, J. W. McNair, American Standards Association staff, reported that the printing of the original panel and switchboard standard was rapidly being exhausted and he recommended that instead of bringing out the portable instrument standard (Continued on page 58)

M

ANY switchboard and portable instruments are purchased each year by the Public Service Electric and Gas Company of New Jersey for new installations and replacements. Approximately 11,000 electrical switchboard instruments are constantly in use in our power plants and substations. Day and night they help to provide a constant dependable source of power to our customers and supply information for the efficient operation of the generation, transmission, and distribution facilities. In addition, approximately 3,000 portable instruments are distributed throughout the various departments of our Company and are carried by techni cians on their tours to make special tests, to check the accuracy of or to calibrate installed instruments, and to test various components of the system in an effort to anticipate and prevent incipient power failures.

The constancy and adequacy of the service we provide our customers depends greatly upon the continued ac

curacy, in operation, of the station instruments. In order that we may be able to rely on their results, they are calibrated periodically against high quality portable instruments.

In our Laboratory we are equipped with primary electrical standards and precision master instruments. These instruments are used to make scheduled tests on all portable instruments, basic acceptance tests on new types of instruments, and detailed tests on new instruments to insure their meeting purchase specifications. Thus we are able to maintain a high degree of accuracy on all instruments from the customer's premises back to the generators.

Our instrument and test engineers have made a detailed study of electrical instruments particularly from the viewpoint of operation and test. However, policy and good practice dictate that the American Standard for Electrical Indicating Instruments be consulted in detail for all acceptance and detailed tests on new indi

Bell Telephone Laboratories

cating instruments. Under no condi tion is memory or the understanding of any individual or group permitted to substitute for the written word of the standard.

The 1938 edition of the standard is presented in a prose form and vari ous non-consecutive paragraphs or sections many times refer to the same general point or test; because of this, much time has been spent unproduc tively in leafing through this edition and studying many of the paragraphs in selection of specific tests and their method of application to a particular instrument. The new 1951 edition. for the first time, tabulates all the detailed requirements for each type and class of switchboard, panel, and portable instrument. Also listed in the tabulation are specific references to the written definitions and instructions that pertain to the individ ual requirements. The written matter is presented in a direct and straightforward style with a noticeable effort to eliminate ambiguity. The presen tation of the specification in this ready-reference form will eliminate uncertainties as to a possible over sight of some important characteris tic as well as what may or may not be expected of each type and class of instrument.

How important this can be is illus trated by an experience of one of the Armed Services, which recently came to our attention. Operating in one of our northern states during the winter, the Service engineers were disturbed to find an abnormally large error apparently in their electrical instruments. Attempts to adjust the instruments were of no avail. In an urgent call to the manufacturer they complained of a 4% error whereas the instrument was listed in the 1% class. Investigation by the manufac (Continued on page 54)

E

by H. C. Koenig

VERY effort is being made to

write a standard for recording instruments that will compare favorably with the American Standard for Electrical Indicating Instruments. Two subgroups, whose membership is well diversified and includes men with considerable experience in the recording instrument field, are active under subcommittee 2 of Sectional Committee C39. One, on Classifications and Requirements, is working under the chairmanship of D. A. Young, Westinghouse Electric Corporation. The other, on Definitions, has E. E. Scott, Armed Services Electro Standards Agency, as chair

man.

The immediate objective of these committees is a revision of AIEE Standard No. 40. The detailed requirement sheets of the American Standard for Indicating Instruments are being used as a guide in presenting the material in the new edition.

It is planned to move slowly in this revision. No attempt is being made to cover the entire field of recording instruments at this time. The immediate objective, on the other hand, is to lay the groundwork with specifications and tests covering most of the recording instruments but for the present to deal specifically only with the direct-acting type and to cover only the ordinary run of instruments -ammeters, voltmeters, wattmeters, etc. Once this objective is accomplished, it will be possible to move on to broader fields.

The history of standards for electrical recording instruments started in 1933 when the American Institute of Electrical Engineers issued a Report identified as No. 40. At that time the chief purpose was to define the terms and conditions which characterize the rating and behavior of the recording instruments. Some years later the Instruments and Measurements Committee of the AIEE under

took a revision of No. 40; in July 1947 this revision was eventually approved as AIEE Standard No. 40.

Although the revised standard was an improvement over the original report, in the writer's opinion it left much to be desired. For years the writer, with Dr H. B. Brooks and Dr F. B. Silsbee of the National Bureau of Standards, and others, tried to "put some teeth" into the standard, to include some definite performance requirements. These efforts met with little, if any, success. The standard, in the main, still consists of a list of definitions and terms. It appeared to be hopeless to expect anything more than this for some years to come.

However, something has happened during the past year or so which makes it appear that the matter is not quite as hopeless as the writer had felt it was. This is approval of American Standard for Electrical Indicating Instruments, issued by the American Standards Association first as C39.1-1949, and now available in the revised and enlarged 1951 edition. This standard did for indicating instruments just what should have

been done for the recording instruments in the revised AIEE Standard No. 40. In this 1951 edition, for the first time, the indicating instrument standard contains detailed-requirement sheets covering the performance of switchboard, panel, and portable instruments. With the development of this standard, the thought occurred to those of us who are concerned with recording instruments, that if it can be done for indicating instruments, why not for recording instruments? The writer promptly recommended to the Instruments and Measurements Committee of the AIEE that, in light of what had been accomplished in the indicating instrument field, we again revise Standard No. 40. This recommendation was made even though Standard No. 40 had only been issued a few years ago. This revision is being done by Subcommittee 2 of ASA Sectional Committee C39, as indicated above. It is hoped that this revision will be published before the end of 1951. It is sure to be welcomed by every user and manufacturer of recording instruments.