Revision: 0 PAGE 1 X-Rays and Vacuum Interrupters INTRODUCTION: Vacuum Interrupters made by Eaton carry a label that warns the user about the possibility of X-radiation. This warning needs some explanation so the reader can understand that under normal conditions, X-radiation is not detectable above normal background levels, and therefore is of no concern. And further, that only in rare situations when such emissions can occur is there a need for concern and protection. The label presently reads as follows: CAUTION This Device may emit X-rays if voltage higher than rated maximum is applied across the open contacts, or if the contacts are spaced less than rated stroke. In such cases personnel must be protected with appropriate shielding. This is a brief warning statement, small enough to fit on the label of a vacuum interrupter, intended to alert the user about a possibility of X-radiation. However, in reality, X-radiation is only produced under special conditions, and then, the only personnel that need be concerned are those involved in the testing of a vacuum interrupter. The discussion that follows is intended to put the issue of X- ray emission in perspective for both test personnel and anyone else that may be in the vicinity of vacuum interrupters or switching devices. 1. X-rays may be produced by a vacuum interrupter, but only under special, abnormal conditions X-rays that are measurable above the natural background level may possibly be produced by a vacuum interrupter only under the following special conditions: a) a voltage is applied to an interrupter whose contacts are open, and b) the applied voltage is typically much higher than the normal operating voltage of the interrupter, and c) the applied voltage is typically above about 50 to 60 kv, rms., (Although x- rays have been observed at voltages as low as 15 to 20 kv rms, this is a very rare occurrence.), and d) some amount of current driven by the high voltage is flowing through the open interrupter. (It is the current that produces the x-radiation, so the application of a high voltage from a source with severe current limiting may produce a much lower level of x-radiation than normally expected at that same voltage using a supply capable of a larger current output.)
Revision: 0 PAGE 2 2. A vacuum interrupter under normal conditions does not produce X- rays: X-rays that are measurable above the natural background level are not produced by a vacuum interrupter under the following conditions: a) sitting in a box b) sitting on the self c) in a de-energized switch d) in an energized switch when the interrupter contacts are closed e) in an energized switch at the normal operating voltage f) in a breaker switching either load currents or fault currents g) in a breaker undergoing impulse voltage withstand testing 3. X-rays are only an issue of concern for test personnel. AC or DC high voltage withstand testing is the only condition under which an interrupter with its contacts open can have a sustained voltage applied that is high enough in magnitude to possibly produce measurable x-rays. Therefore, the only people that need be concerned about x-ray exposure are: - high voltage test personnel at the vacuum interrupter factory, - high voltage test personnel at the switchgear factory, or - maintenance personnel in the field. Moreover, the level of x-rays produced increases as the magnitude of the applied voltage is increased. Therefore, the potential for x-ray emission is greatest during tests in a factory, since these tests are performed at 100% of the rated power frequency withstand voltage of the device. In addition, high voltage tests are performed frequently in a manufacturing setting and often by the same few people, thus providing the possibility of more accumulated exposure over time if in fact the person is exposed to any x-rays at all. Tests performed in the field are normally only performed at 75% of rating, and therefore, the level of x-ray production is automatically lower. Moreover, maintenance tests are performed very infrequently which greatly reduces the opportunity for an accumulation of exposure by one individual. Consequently, high voltage test personnel at a vacuum interrupter factory and at a switchgear factory are the people that need be most concerned with the possibility of x-ray exposure. 4. Distance avoids excessive exposure to x-rays during high voltage tests. A respectful, safe separation distance between test personnel and energized parts is always required during high voltage withstand testing, and, when testing vacuum interrupters, such a respectful, safe separation distance between test personnel and the vacuum interrupter under test also avoids the possibility of excessive exposure to x-rays. X-ray production is very directional from an
Revision: 0 PAGE 3 emitting source, and, moreover, the intensity of x-rays decreases as the separation distance increases. Therefore, the most expedient means of insuring a safe environment for test personnel is to maintain a minimum distance between test personnel and the vacuum interrupters under test. TABLE 1 at the end of this report which lists minimum recommended distances distance between test personnel and the vacuum interrupters under test provides a convenient guide for the test personnel to follow. 5. Minimum recommended distances are based on ANSI standards The Vacuum Interrupters manufactured by Eaton comply with the requirements of ANSI C37.85-2002 entitled Alternating-Current High- Power Vacuum Interrupters - Safety Requirements for x-radiation Limits. Interrupters, tested as specified in Section 5 (of C37.85-2002), shall be in compliance with this standard if x-radiation emitted does not exceed the following: (1) 0.5 milliroentgen per hour at the maximum operating voltage shown in column 2 of Table 1 of C37.85-2002. (2) 15.0 milliroentgen per hour at the low frequency dielectric withstand test voltage shown in column 3 of Table 1 of C37.85-2002. That is, at a distance of 1 meter from the interrupter, the observed x-radiation emitted complies with items (1) and (2) above. As reported in some excerpts from Appendix A of C37.85-2002, As a result of evaluating the results of the aforementioned tests, the manufacturers (of vacuum interrupters) concluded that neither the general public or users will be subjected to harmful X-radiation due to normal application and operation of 15.5 kilovolt-rated vacuum interrupter devices when applied within their assigned ratings and when the voltage applied across the open contacts of the interrupter is 15.5 kilovolts or less. The manufacturers also concluded that at the permissible user powerfrequency withstand test voltage of 37.5 kilovolts (the field test voltage is 75% of the rated withstand voltage), radiation levels are negligible for vacuum interrupters rated 15.5 kilovolts. Normal electrical safety precautions require the user to be at a distance from the interrupters that provide sufficient protection.... A minimum distance of 2 to 3 meters can normally be expected to be used for reasons of electrical safety. Since this test is performed only during maintenance, which normally occurs only once in a 1-year to 3-year period, (2000) 1 minute tests per year exceeds by a factor of 10 or more the number of interrupters a user such as a utility would be expected to test in a year.
Revision: 0 PAGE 4 Concerning the 50 kilovolt 1-minute withstand test voltage applied by the switchgear manufacturer during the factory test, the Vacuum Interrupter manufacturers believe that the higher level of possible emitted x-radiation could be injurious to personnel. Therefore, the manufacturers recommended that they take appropriate precautions to protect personnel and meet local codes by using monitoring devices and safety measures as required. Since the 1978 reaffirmation of this standard, manufacturers have conducted tests on interrupters rated through 38 kilovolts, as reflected in this (1985) revision. This testing and concurrent field experience has demonstrated that these higher voltage ratings meet the requirements of this standard. 6. Minimum recommended distances are further based on x-ray measurements. From many tests on 15 and 27 kv vacuum circuit breakers with separation distance of 1 to 3 meters, it has been observed that x-ray levels are almost indistinguishable from the background level at voltages up to 40 kv, AC rms. and just barely above background but still less that 0.5 milliroentgen per hour at voltages up the 60 kv, rms. So, as a general recommendation, x-ray levels from the testing of vacuum interrupters will be well within safe limits for test personnel that are located at a distances of 2 to 3 meters from the interrupter under test at test voltages up to 60 kv, rms. An x-ray survey was performed for a Cutler-Hammer 38 kv vacuum circuit breaker to determine the potential radiation dose that could be received by personnel testing these higher voltage rated circuit breakers. - Based on the survey results the radiation levels did not exceed 0.5 mr/hr at a distance of 3 meters from the device when the applied voltage was 60 kv AC rms. (the end user s test voltage) with all three phases of the device energized at the same time. - Under normal testing conditions performed by the circuit breaker manufacturer, the test voltage does not exceed 80 kv AC rms. and the maximum instantaneous radiation level did not exceed 3.5 mr/hr from a distance of 3 meters from the device (the ANSI limit is 15 mr/hr). ANSI Standard C37.85 (1989) was used as a guide for radiation exposure limits and test procedures. The background level where these tests were performed was about 0.2 mr/hr.
Revision: 0 PAGE 5 The effects of extra shielding were also evaluated in these tests - With a 3 mm thick steel shield and a separation of 3 meters, the x-ray levels at 60 kv AC rms are indistinguishable from the background level. At 80 kv AC rms the x-ray levels are only about 0.8 mr/hr, which is a negligible dose for testing personnel and only about 1/20th of the 15 mr/hr limit set in ANSI C37.85 (1989). - With a 1.5 mm thick portable lead shield and a separation of 3 meters, the x-ray levels at 60 kv AC rms are indistinguishable from the background level. At 80 kv AC rms the x-ray levels are only about 0.4 mr/hr, which is a negligible dose for testing personnel and only about 1/40th of the 15 mr/hr limit set in ANSI C37.85 (1989). 7. An x-ray absorbing barrier provides an extra measure of protection A barrier of steel or lead provides some extra protection for high voltage test personnel to avoid excessive x-ray exposure. Barrier Material Barrier Thickness x-ray level at 3 meters reduced to: Steel 3 mm less than 1/4 of the unshielded case Lead 1.5 mm less than 1/8 of the unshielded case For fixed test installations, either steel or lead can be provided as portable screens or fixed walls. For field tests, if the circuit breaker mechanism enclosure is located between the test personnel and the vacuum interrupter(s) under test, then the steel enclosure provides an extra barrier to reduce x-ray levels. 8. Vacuum Switchgear manufacturers should provide separation, monitoring and training (and shielding at higher test voltages or restricted spaces) for x-ray exposure precautions. For a switchgear manufacturer, good practices to insure that x-ray levels are within safe limits for test personnel include the following provisions. (1) Maintain at least the minimum recommended separation between the test personnel and the test object. (2) Provide x-ray cumulative dose monitoring of all test personnel. (3) Provide training of all test personnel in the hazards of x-rays and safe test practices. (4) At test voltages of 60 kv and above where significant x-ray levels may be observed, provide shielding between the test personnel and the test object for extra protection. This is also advisable in cases with restricted spaces where the minimum recommended separation is not practical. Establishing a high voltage test area where a minimum clearance between the vacuum interrupter under test and the test personnel is 3 to 4 meters is the first priority. In this manner, switchgear of all voltage ratings can be tested in the
Revision: 0 PAGE 6 same location and no special consideration between one voltage and another need be considered since the largest separation distance is employed. Exposure monitoring is also recommended for all high voltage test personnel or others that are regularly in close proximity to the test by wearing film badges that are checked monthly for accumulated radiation exposure. This practice monitors the actual accumulated exposure experienced by the people involved and provides a warning when the exposure exceeds allowable limits in a 1 month time period. This practice is followed at the Cutler-Hammer Horseheads Vacuum Interrupter plant where separation and shielding are in place. In our experience, no accumulated exposure is ever detected with the monitors worn by our test personnel. Training of high voltage test personnel should be provided to outline the potential hazards of x-rays from vacuum interrupters and to describe the proper precautions to take to keep the risk exposure to minimum levels. Shielding of the test object is an additional provision to consider under some circumstances. An enclosure for the switchgear device or barrier between the test personnel and the switchgear device under test consisting of sheet metal walls and possibly a portable lead screen or two can provide some additional protection from x-ray exposure. Such additional measures are recommended for test voltages of 60 kv and above where significant x-ray levels may be observed. Shielding is also recommended in cases with restricted spaces where the simpler measure of providing the minimum separation is not practical. In this case, adequate shielding can insure that x-ray levels immediately outside the enclosure are always within safe levels. The switchgear manufacture should also consult local and state codes for other requirements that should be considered. R. Kirkland Smith, Ph. D. Manager, Power Test Lab Eaton Corporation Phone: 607 796 3370 200 Westinghouse Circle FAX: 607 796 3364 Horseheads, NY 14845-2277 USA E-mail: KirklandSmith@eaton.com
Revision: 0 PAGE 7 TABLE 1 - Minimum Recommended Distances from a Test Object during High Testing of Vacuum Interrupters Applicable Standard Rated System Rated Power Frequency Withstand Factory Test at 100% of Rated Withstand Field Test at 75% of Rated Withstand Minimum Recommended Distance from Test Object Measured X-ray Emission kv, rms. kv, rms. kv, rms. kv, rms. kv, DC Meters milliroentgen per hour IEC 694 3.6 10 7.5 10.6 2 to 3 <background IEC 694 3.6 10 10 2 to 3 <background ANSI C37.06 4.76 19 14.3 20.2 2 to 3 <background ANSI C37.06 4.76 19 19 2 to 3 <background IEC 694 7.2 20 15 21.2 2 to 3 <background IEC 694 7.2 20 20 2 to 3 <background ANSI C37.06 8.25 36 27 38.2 2 to 3 <background ANSI C37.06 8.25 36 36 2 to 3 <background IEC 694 12 28 21 29.7 2 to 3 <background IEC 694 12 28 28 2 to 3 <background ANSI C37.06 15 36 27 38.2 2 to 3 <background ANSI C37.06 15 36 36 2 to 3 <background IEC 694 17.5 38 28.5 40.3 2 to 3 <background IEC 694 17.5 38 38 2 to 3 <background ANSI C37.06 15.5 50 37.5 53 2 to 3 <background ANSI C37.06 15.5 50 50 2 to 3 <0.5mR/hr IEC 694 24 50 37.5 53 2 to 3 <background IEC 694 24 50 50 2 to 3 <0.5mR/hr ANSI C37.06 25.8 60 45 63.6 2 to 3 <background ANSI C37.06 25.8 60 60 2 to 3 <0.5mR/hr ANSI C37.06 27 60 45 63.6 2 to 3 <background ANSI C37.06 27 60 60 2 to 3 <0.5mR/hr IEC 694 36 70 52.5 74.2 3 to 4 <0.5mR/hr IEC 694 36 70 70 3 to 4 <3.5mR/hr ANSI C37.06 38 80 60 84.8 3 to 4 <0.5mR/hr ANSI C37.06 38 80 80 3 to 4 <3.5mR/hr