AMETEK MX CTSL - Manual (Page 20)

User Manual MX-CTSL Compliance Test System
California Instruments Revision J 20
2.4.1 Test Classes
The standard divides products to be tested in to four classes, A, B, C and D. Each class has its own
harmonic current limits. Class A is the default class, meaning if a product does not fall into the categories
for class B, C or D, it is by default class A. The product classes are defined as follows:
Class Description
A All motor driven equipment, most “domestic” appliances and
virtually all 3 phase equipment (<16 A rms per phase)
B All portable electric tools
C All lighting products, including dimmers, with an active input
power above 25 Watts.
D Products having a power range from 75 to 600 Watts AND a
current waveform that has a special wave shape.
Test limits are most stringent for Class-C and Class-D equipment. These classes were established to cover
Table 2-3: EN 61000-3-2 Class Descriptions
the wide range of products that uses “cheap” switching power supplies with a rectifier capacitor input.
These units typically exhibit relatively high odd harmonics. Since this type of supply is so commonly used,
the effect of a large quantity of products like this can add up to significant problems. To limit this effect, the
current harmonic limits for class-D equipment are specified in mA/W instead of an absolute current value
as is done for class A and B. Many consumer and professional devices such as TV’s, VCR’s, stereos,
PC’s, fax machines, printers etc. may fall into the more stringent class D category.
Class-C limits are not specified in absolute values either but rather as a percentage of the fundamental
current. The third harmonic limit is also a function of the power factor so is harder to meet as the power
factor decreases. Neither class C nor D devices have to meet even current harmonics limits.
Since class C and D limits are load dependent, the power level and power factor have to be determined for
each test. In case of fluctuating power levels, it may be necessary to measure the power level during the
test and adjust the limits used to pass or fail the unit under test dynamically.
This dynamic nature of the class C and D limits has resulting in some confusion and a possible problem of
inconsistent test results when using IEC test systems for different vendors. The Standards body working
group for harmonics is in the process of clarifying some of the class and limit issues and these efforts are
likely to result in a revision of classes and limit calculations sometime in the year 2001. Until a new
standard is established, the CTSMXL software maintains a database of test limit values than can be
updated over time if needed without changing the core program.
CTSMXL software makes dynamic classification for Classes A and D. The classification follows the
following principle. If during either a Class A or D test the EUT becomes a Class A device, the EUT will be
considered a Class A device. If the EUT is a Class D device through out the test, then the device is a
Class D device. In other words, Class A is a latching classification. Please note that even though the
classification may change during a test, a Class A (or D) test will still be performed according to Class A (or
D) specification.
2.4.2 Steady State versus Transitory Harmonics
Two types of harmonics testing are called for in the EN 61000-3-2:1998 standard, a quasi steady state and
a transitory one. The transitory harmonics tests allow for equipment that has power demands that vary over
time to temporarily exceed the harmonic current limits by as much as 50 %, as long as such elevated levels
do not occur for more than 10 % of the test period. Since the minimum test time required is 2.5 minutes or
150 seconds, this means the current harmonics can be as high as 150% of the standard limits for no more