In the past, standards were simpler and required extremely limited testing with ohmmeters and dielectric testers. This is not the case today, where lightning and surge tests are included in most Standards. In addition, many companies are testing finished goods in sophisticated ways; and requiring their subcontractors to perform tests on subassemblies before being incorporated into the finished product. Finally, some technologically-advanced companies are seeing their new product development outstrip the existing Standards, with unanticipated uses that require unique testing.
Any of these scenarios may require the design team or manufacturer to obtain specialized equipment. If subcontractors are required to test by the OEM or internal test specifier they may not have test engineers on hand to define the specification and obtain the needed equipment.
So how does the OEM or subcontractor learn to navigate through these uncharted processes and ensure they are testing correctly?
Sourcing Surge Test Equipment: A Checklist
1.The test specification: Whether delivered from an OEM or internal test specifier, or mandated by an outside agency, a proper test spec is crucial.
A. Does it make sense? Metaphorically, if you are building umbrellas,
a rain test makes sense. If you are making candy, it does not.
B. Is it complete? In order to define a surge test, specific parameters are required.
1. A complete surge test waveform can be defined in each of the following instances:
a. Waveform rise time and duration for voltage and current;
amplitudes; and waveform tolerance. Amplitude must be defined as
either the source voltage or the waveform peak voltage.
b. Waveform rise time and duration for voltage and current, amplitudes
for either waveform and the output impedance desired; and
waveform tolerance. Amplitude must be defined as either the
source voltage or the waveform peak voltage.
c. A circuit diagram with a power source voltage; with component and
source tolerances.
2. The surge test program must include application information:
a. Between which two points on the part is the surge to be applied?
b. How many surges are to be applied to each part?
c. Are there positive or negative polarity requirements?
3. What is the pass/fail criterion?
C. Does it test the parameter in question? It is extremely important to take a close
look at the test specification, including the point where the surge is to be applied.
Does it physically test your part?
D. How will the test impact production? Think about the overall impact the test will
make on your production. How long will the test take to conduct, and will this
time negatively affect the production schedule? Should more than one tester
be considered to keep throughput acceptable?
E. How will your facility handle the testing? Does this area of the plant
have infrastructure to support the test (personnel, power, network connections
etc.)? If the test uses high voltage or high current, can it be designed to keep
test personnel safe, and other personnel clear of the danger?
F. Did you schedule check testing early enough? Make sure the design engineers
know about the test up front, and that qualification tests are scheduled early
enough so any test failures can be evaluated and dealt with in an
economical fashion.
2. The equipment specification
A. Type Testing: This test is applied once in the product’s life to make sure the
design meets the requirement. In the past, these type tests were conducted on
a design sample, and never revisited. However more and more OEMs are
requiring type-style tests to be applied to 100 percent of production.
1. Is the Standard the correct one for the type of equipment being tested?
Reading the name and Scope of the Standard should provide information
about whether it is the correct Standard to test the product you are building.
There are some instances where a test that is described in one Standard is
referenced in another, but if the name of the Standard doesn’t correlate, it’s
certainly worth a phone call to your OEM or internal test specifier to make sure
that the test specification is correct for your product.
2. Does the specification received for the type test agree with the version of the
test in the latest copy of the Standard?The latest version can be found online;
if the Standard reference of the test you are being asked to perform is
different, there could be a good reason but clarification from the OEM or
internal test specifier would be prudent.
3. Is the surge test properly defined according to the requirements specified in
#1. above? A surge test needs either a current or voltage waveform and an
impedance, or both waveforms. Definitions of the waveshape for rise and
duration times, as well as references for waveform tolerance are also required
for a complete definition of the waveform. In the past, waveforms were
defined when the output was open (for voltage waveforms) or shorted (for
current waveforms), but as allowed by IEC 61000-4-5, some Standards and
OEMs are defining waveforms when delivered into a defined load. Clear up
this point early.
4. Does the specification received include the Standard paragraph, including
the requirements for all Tables and Footnotes, and is the specification in
accordance with the Standard? Even if the OEM test is adapted from a
Standard’s Type test, it makes sense to check the entire spec to make sure
something isn’t missed. A footnote can change or possibly waive the test.
a. Things to check: Tester impedance 2, 12 or 42 ohms? Depending on
the test condition, the surge equipment can be made with various
impedances. Most tests will have a 2 ohm, 12 ohm or 42 ohm or 500
ohm impedance.
b. Is the test performed while the equipment is energized? Most
production tests would be conducted without power applied, but type
tests can specify this condition. It’s a good thing to consult the OEM or
internal test specifier if this is not defined.
c. Is the test voltage correct for the Insulation Class? In cases where the
surge test is checking for a dielectric breakdown, output voltage is
usually assigned by a table showing the Insulation Class of the tested
part, and the voltage potential it sees during operation. If so, consult
with the OEM or internal test specifier about which Insulation Class
he/she is testing to, and the voltage level, to double check the proper
test voltage. If this voltage is set too high the test may overstress the
insulation.
d. Is the test voltage correct for the creepage and clearance distances?
Surge tests are sometimes used to judge the effectiveness of air gap
insulation. The clearance distances, and the surge voltage for that
clearance, are given in Tables in the Standard. These should be
double checked.
B. Production Testing – does it test your product?
1. Is this test reasonable in scope? Many production tests consist of a dielectric
withstand and ground continuity test. These tests concern only the primary
circuits of the part. If your part does not see primary voltage, these tests may not
be required.
2. Does the test have a reasonable pass/fail point? If you are asked to perform a
hipot test between mains and ground, it should be in the range of
1500Vac/2100Vdc. If the test is between primary and secondary, it should be in
the range of 2500-3000Vac. If your test spec exceeds these limits by a
considerable margin, it’s time to confirm with the OEM or internal test specifier.
3. Other considerations
1. Is the test accurate enough to find a sample that is prone to failure in the field? Make sure the test makes sense for the product being tested. For example, if your product has mains voltage connections, it’s not appropriate to apply the 500V hipot test for secondary parts. Testing should be appropriate, so potential field failures can be fixed in the manufacturing facility.
2. Is the method of tester control specified? Are you to keep records of all failed parts, or of all tested parts? Is this recordkeeping done manually, or by the test equipment? Is the test equipment to stand alone, or run with the assistance of a computer or PLC? What are the portability requirements; is it to be designed to be easily moved to another location or is it to be fixed in place?
3. Is the testing being done safely? Most importantly, it is crucial to ensure that the equipment is designed to keep operators and uninvolved plant personnel safe.
Jeffrey D. Lind, president of Compliance West USA, has contributed his 33 years of extensive electrical engineering expertise to help further the advancement and expansion of the Compliance West USA brand. Lind’s comprehensive understanding of the industry, ranging from sales and marketing to product development, has helped establish Compliance West as a driving force within the medical and industrial testing industries. Lind received his BS in electronic engineering from Cal Poly San Luis Obispo in 1976.
