AMETEK CTS 3.0 - Manual (Page 25)

User Manual Compliance Test System 3.0
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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 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 a
nd 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 CTS 3.0 software maintains a
database of test limit values than can be updated over time if needed without changing the core
program.
CTS 3.0 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 IEC 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
Table 2-3: IEC 61000-3-2 Class Descriptions