While it is true that most engineers tend to be a conservative lot, most of your standards and information in textbooks is based upon a combination of experience, mathematical and observed experimentation, theory, fact, compromise and consensus. Standards development meetings, and developing consensus between domestic and global standards, can get quite exciting with debate, disagreement and even the occasional subterfuge. There is nothing more exciting than watching competing machine and component manufacturers along with scientists, laboratory representatives, and the users of the machines and components, all with their own agenda, trying hard to agree, while not sharing competitive information. However, it is also important to understand (with only a few exceptions), that virtually all of the engineers involved in the standards that we use are dedicated and interested in the good of the industry, with the few maverick self-interested engineers tending not to last long. In other cases information is shared on new development that helps the industry move forward.
The reality is that standards development can be quite exciting and informative. One such case is the most referenced Institute of Electrical and Electronics Engineers, Inc. (IEEE) standards in the maintenance industry: IEEE Std 43-2000: "IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery" (IEEE 43). The original IEEE 43 standard was issued in 1974 and remained virtually unchanged up through 2000. It primarily represented about a Century's worth of experience with data identical to publications published in 1913, and earlier.
A lot of changes occurred in the 1970s and later, in relation to the electrical insulation industry, including the use of polymers and advances in both epoxy and polymer sciences for electrical insulation systems. In the meantime, much of the data in use for the 1974 standard could be traced back to the experiences from oil and/or tar and paper insulation systems.
Every so often, IEEE standards must be reaffirmed. The reaffirmation process involves the review of an existing standard to determine if it is still useful, valid, or requires updating. It involves the creation of a standards committee working group who is tasked to review, advise and either modify or reject the standard. If the standard is accepted or modified by the group, it is put out through the IEEE standards authority for general announcement and voting by interested engineers, with a minimum number of votes being required before it is released.
IEEE Std 43-1974 was revised by the 2000 working group over the course of several years and re-released with some of the original core document attached, but with far more explanation and detail. The standard has since been re-affirmed with the stipulation that specific issues are reviewed, a process which is ongoing at the time of the drafting of this article. IEEE 43 is the responsibility of the IEEE Materials Subcommittee of the IEEE Dielectrics and Electrical Insulation Society (DEIS), the IEEE society that oversees a majority of the electrical testing standards for electrical insulation systems and machines that we use consistently in our industry.
What is IEEE Std 43?
The scope of IEEE 43 is described as follows:
"This document describes a recommended procedure for measuring insulation resistance of armature and field windings in rotating machines rated 1 hp, 750 W or greater. It applies to synchronous machines, induction machines, dc machines, and synchronous condensers. It does not apply to fractional-horsepower machines...The document also describes typical insulation resistance characteristics of rotating machine windings and how these characteristics indicate winding condition. It recommends minimum acceptable values of insulation resistance for ac and dc rotating machine windings."1
IEEE 43 provides us with some very useful information within a scope of all machines over one horsepower in size, including the definitions of insulation resistance and polarization index. The purpose is outlined as follows 1:
•Define insulation resistance and polarization index of the winding of a rotating machine;
•Review the factors that affect or change insulation resistance characteristics;
•Recommend uniform test conditions;
•Recommend uniform methods for measuring insulation resistance with precautions to avoid erroneous results;
•Provide a basis for interpreting insulation resistance tests to estimate winding suitability for service or for an overvoltage test. In particular, this standard describes typical insulation problems detected by the insulation resistance tests;
•Present recommended minimum acceptable insulation resistance values and polarization indices for various types of rotating machines.
Starting in the 2000 edition, changes were made to the minimum acceptable value from the traditional 1 MegOhm plus 1 MegOhm per kilo-Volt rating of the machine, or 1.5 MegOhms for a 460 Volt machine and 5 MegOhms for a 4,160 Volt machine. The new values were presented as shown in Table 1 (note: Table 1 wording represents the present recommended change to IEEE 43 and not the existing standard).3
The standard describes the specific conditions that will affect insulation resistance, including temperature, test voltage, and moisture. While the moisture requirements suggest that the machine should be tested above the dew point; that is the limit to the guidance other than noting that humidity can cause dramatic changes to test results. Recommended test voltage values are presented based upon machine voltage, and temperature adjustments are recommended to 40°C to a standard chart and formula.
Prior to IEEE 43-2000, there were few limits related to Polarization Index (PI). The change was made such that insulation resistance values over 5,000 MegOhms, adjusted to 40°C, should not be tested for PI. Very little explanation was forthcoming until the present draft changes to the IEEE 43, or IEEE Std P43-2000 (Revision of IEEE Std 43-2000), Section 12.2.2 Applicability of polarization index when IR is greater than 5000 MegOhms, "When the insulation resistance reading obtained after the voltage has been applied for 1 min (IR1) is higher than 5,000 MegOhms, based on the magnitude of applied direct voltage, the total measured current (IT) can be in the submicroampere range. At this level of required test instrument sensitivity, small changes in the supply voltage, ambient humidity, test conditions, and other non-related components can greatly affect the total current measured during the 1- 10 minute interval required for a PI. Because of these phenomena, when the IR1 is higher than 5,000 MegOhms, the PI may or may not be an indication of the insulation condition and is therefore not recommended as an assessment tool."3
Tests previously referenced, including dielectric absorption and limits are not mentioned in the present or upcoming edition of the IEEE 43. However, such types of tests are now referenced in Annex A, including a more in-depth discussion of the meaning and interpretation of PI and newer insulation systems.
Present Conditions Surrounding IEEE 43
The designation of 'P' in front of a standard number, such as IEEE Std P43-2000, indicates that the standard is presently under development. In these conditions, IEEE provides a statement on the cover of all 'P' documents that the standard is presently under development and may have changes made such that use of the standard is at the risk of the user.
IEEE is also a professional organization in which a significant amount of information is publicly available. For the IEEE Power Engineering Society (PES) Materials Subcommittee, which falls under the responsibility of DEIS, information on the present ongoing work related to IEEE P43 (and other insulation standards) can be found at http://ewh.ieee.org/cmte/pes/materials/workinggroups/p43/p43.htm 2. Anyone may go and view ongoing work, comment, and/or volunteer, although only IEEE members are able to vote on the IEEE standards.
The primary issue that was raised with the Affirmed IEEE 43 was that it related to all machines above 1 horsepower for new, repaired, and existing machines up to and including large generators. Motor manufacturers identified that the change to the minimum insulation values might affect specific National Electrical Manufacturers Association (NEMA) portions of the NEMA MG-1 standard (Motors and Generators), and in particular, how it related to sealed insulation systems. The discussions are ongoing, with most of the discussion hinging around the fact that none of the NEMA members had raised the issue that the original 1 MegOhm plus 1 MegOhm per kilo-Volt was an issue, especially in the wet-winding test: (Reference Section 12.3.1 of the IEEE P43) "Sealed Winding Conformance Test - Final test specifications for new machines often require the exposed areas of the stator winding to be completely soaked with an aqueous solution of known surface tension. A measurement of insulation resistance is used to establish that the insulation system is sealed against ingress by moisture. The detailed test procedure and an acceptance criteria are provided in IEEE 4, NEMA MG-1." This is post-scripted with the following note: "NEMA MG1 gives a minimum insulation resistance for a wetted winding of kV+1 MegOhm (awaiting feedback from NEMA MG1 Technical Committee Chair on rationale for this minimum value)."3
Once this issue was identified, additional potential problems were identified within the standard such as some of the diagrams not fully, or properly, describing the true conditions of an insulation system. The argument was made that while the original purpose of the diagrams was to generically demonstrate conditions, they were never meant to be used for actual calculations of insulation condition. However, numerous cases were identified where technical arguments were made using these diagrams, resulting in erroneous conclusions. One such issue includes the equivalent circuit diagram identified in the document as Figure 1. The decision must be made whether to leave it as is, remove it, or qualify it further in the description. The argument and decision has been in process for close to 18 months at the time of drafting of this article with lively discussion at each meeting.
IEEE standards development can be an exciting process, especially once it is understood that most standards are created only through the participation of professionals that bring many differing viewpoints, experiences and agendas to the table. The meetings can certainly become quite lively as everyone contributes their input to the ongoing debate. It is not easy to develop consensus among the cast of characters, and everyone who participates in the process should be commended for spending their time in an effort to move our industry forward.
The result is demonstrated in the IEEE 43 standard, which is heavily used in the reliability and maintenance community for the testing and evaluation of rotating machines. Following major changes in the 2000 version of the standard after 26 years, and over a century of experience, it has been discovered that additional modifications are required in order to meet modern requirements and changes to the chemistry of modern insulation systems. This includes developing a consensus between standards bodies and within the IEEE P43 committee, itself.
1.IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery, IEEE Standard 43-2000.
2.IEEE Materials Subcommittee IEEE P43 Working Group, Available: http://ewh.ieee.org/cmte/pes/materials/workingroups/p43/p43.htm
3.Draft IEEE Recommended Practice for Testing Insulation Resistance of Rotating Machinery, IEEE Standard P43-2000
Howard W Penrose, Ph.D., CMRP is the President of SUCCESS by DESIGN® a reliability services and publishing firm, Editor-in-Chief of the IEEE DEIS Web (http://www.ieee.org/go/deis/), Founding Executive Director of the Institute of Electrical Motor Diagnostics, and President of AllAmericanHybrid.com. Dr. Penrose specializes in commercial/industrial rotating machinery, hybrid/electric vehicle system, and electrical system reliability, maintenance, manufacturing, design, diagnostics, training, and forensic analysis. He is the 2009 Bronze Medal Axiom Business Book Award Winner (Operations Management Category) for his book "Physical Asset Management for the Executive" and the author of "Electrical Motor Diagnostics: 2nd Edition." SUCCESS by DESIGN may be found at http://www.motordoc.com .