Download Vacuum Interrupters and X

ELECTRICAL
VACUUM INTERRUPTER APPLICATION NOTES
Filename: VIAN0008 - X-Rays and Vacuum Interrupters
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 Xray 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 xrays 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.)
ELECTRICAL
VACUUM INTERRUPTER APPLICATION NOTES
Filename: VIAN0008 - X-Rays and Vacuum Interrupters
Revision: 0
2.
PAGE 2
A vacuum interrupter under normal conditions does not produce Xrays:
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
ELECTRICAL
VACUUM INTERRUPTER APPLICATION NOTES
Filename: VIAN0008 - X-Rays and Vacuum Interrupters
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-Voltage 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.”
ELECTRICAL
VACUUM INTERRUPTER APPLICATION NOTES
Filename: VIAN0008 - X-Rays and Vacuum Interrupters
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.
ELECTRICAL
VACUUM INTERRUPTER APPLICATION NOTES
Filename: VIAN0008 - X-Rays and Vacuum Interrupters
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
ELECTRICAL
VACUUM INTERRUPTER APPLICATION NOTES
Filename: VIAN0008 - X-Rays and Vacuum Interrupters
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
200 Westinghouse Circle
Horseheads, NY 14845-2277 USA
R. Kirkland
Smith
Phone:
FAX:
E-mail:
607 – 796 – 3370
607 – 796 – 3364
[email protected]
ELECTRICAL
VACUUM INTERRUPTER APPLICATION NOTES
Filename: VIAN0008 - X-Rays and Vacuum Interrupters
Revision: 0
TABLE 1
Applicable
Standard
PAGE 7
-
Minimum Recommended Distances from a Test Object
during High Voltage Testing of Vacuum Interrupters
Rated
System
Voltage
Rated
Power
Frequency
Withstand
Voltage
Factory
Test
Voltage
at 100%
of Rated
Withstand
Voltage
Field
Test Voltage
at 75% of Rated
Withstand Voltage
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
IEC 694
3.6
3.6
10
10
7.5
10.6
10
2 to 3
2 to 3
<background
<background
ANSI C37.06
ANSI C37.06
4.76
4.76
19
19
14.3
20.2
19
2 to 3
2 to 3
<background
<background
IEC 694
IEC 694
7.2
7.2
20
20
15
21.2
20
2 to 3
2 to 3
<background
<background
ANSI C37.06
ANSI C37.06
8.25
8.25
36
36
27
38.2
36
2 to 3
2 to 3
<background
<background
IEC 694
IEC 694
12
12
28
28
21
29.7
28
2 to 3
2 to 3
<background
<background
ANSI C37.06
ANSI C37.06
15
15
36
36
27
38.2
36
2 to 3
2 to 3
<background
<background
IEC 694
IEC 694
17.5
17.5
38
38
28.5
40.3
38
2 to 3
2 to 3
<background
<background
ANSI C37.06
ANSI C37.06
15.5
15.5
50
50
37.5
53
50
2 to 3
2 to 3
<background
<0.5mR/hr
IEC 694
IEC 694
24
24
50
50
37.5
53
50
2 to 3
2 to 3
<background
<0.5mR/hr
ANSI C37.06
ANSI C37.06
25.8
25.8
60
60
45
63.6
60
2 to 3
2 to 3
<background
<0.5mR/hr
ANSI C37.06
ANSI C37.06
27
27
60
60
45
63.6
60
2 to 3
2 to 3
<background
<0.5mR/hr
IEC 694
IEC 694
36
36
70
70
52.5
74.2
70
3 to 4
3 to 4
<0.5mR/hr
<3.5mR/hr
ANSI C37.06
ANSI C37.06
38
38
80
80
60
84.8
80
3 to 4
3 to 4
<0.5mR/hr
<3.5mR/hr