Test
Laboratories
The Value of Pre-compliance Testing
F

or some manufacturers, the cost of outfitting an entire full-compliance EMC test facility is not justified. These entities do not have enough sales volume, build unique one-off units, have tight margins, or lack the resources to staff an entire on-site, full-compliance test facility.

These types of manufacturers must rely heavily on the services of out-of-house, third-party test laboratories to properly test their products to confirm compliance before placing their devices on the market. The major problem with this approach, however, is that should the product fail at the test lab, the result will be delayed product shipments and missed revenue opportunities.

Utilizing early-stage in-house pre-compliance testing and sound engineering judgment, along with pre-compliance test data will boost confidence that the product will pass when it is eventually sent out-of-house for full compliance certification work. Learning in-house, using pre-compliance test methods as early as possible (when first prototypes are available), instead of at the end of the product development program (when attempting certification tests at the full-compliance test facility) will inevitably save manufacturers time and money. During final certification testing, there is immense pressure to meet your production release deadlines and being charged big dollars for lab time by the hour adds a whole other layer to that pressure. Having a product fail at this point in the process could be fatal to the project, especially if competitors get their product to market ahead of yours.

Pre-compliance Versus Full-compliance Testing
Pre-compliance testing utilizes ad-hoc test methods, not in strict adherence to published standards. With pre-compliance testing, there are no restrictions placed on how the tests are accomplished. It’s a process free from any constraints dictated by compliance standards. No assessors coming in to spot check the test methods employed. Some of the equipment can be made in-house, purchased on the used market, and may not fully comply with standards. Pre-compliance test methods just need to work well enough and provide good enough data to make an engineering judgement as to whether the product will or will not pass when tested formally. Pre-compliance testing has the advantage that tests can be started and stopped at any time, the equipment under test (EUT) modified with an EMC fix (common-mode choke, low-pass filter, etc.), and the test conducted over again at your leisure.

Pre-compliance testing can prepare the system for the formal conformity assessment by determining worst-case configuration and functional parameters of the EUT before formal certification testing. This saves considerable time by not having to explore these parameters at a third-party facility during expensive full-compliance testing.

On the other side of the same coin to pre-compliance testing is full-compliance testing. Full-compliance testing is testing that is conducted in strict accordance with published standards using expensive equipment and detailed procedures. The test labs performing these types of tests should be accredited to perform them. The labs undergo periodic assessments by experts who are highly skilled and knowledgeable in the performance of these highly technical test methods. Laboratory accreditation ensures that the lab has passed rigorous benchmarks in the performance of testing procedures as they apply to specific standards. The only downside to full-compliance testing is that there is a cost associated with their service as it can be costly to build full-compliance test systems and to staff them with qualified engineers and technicians.

Examples of Pre-compliance Test Methods
  • Measuring common-mode current from cables at your workbench (office or laboratory environment) using current probes and an inexpensive pre-compliance spectrum analyzer. Measured currents less than 5mA indicate the product will pass Class B radiated emissions. Currents less than 15mA will pass Class A radiated emissions. Current probes are effective pre-compliance tools.
  • Measuring conducted emissions at your workbench using an inexpensive line-impedance-stabilization-network (LISN) and inexpensive pre-compliance spectrum analyzer.
  • Using inexpensive antennas and an inexpensive pre-compliance spectrum analyzer at your workbench (office or laboratory environment) to measure radiated emissions in the near-field from the entire product (cables and enclosure).
  • Using an inexpensive ESD simulator to perform ESD immunity testing at your workbench (office of laboratory environment).
  • Near-field E & H probe measurements (more of a troubleshooting method).
Problems with Pre-compliance Test Methods
Depending on the sophistication of your pre-compliance test setup, you might not be able to obtain data that correlates well with full-compliance methods. This means you won’t be able to skip testing at a full-compliance test site because you won’t know with any certainty where your product fails the real compliance test. A solution to this dilemma is to plan for at least one pre-scan run to be conducted at the full-compliance facility to obtain a baseline of where your product has issues. Using this data, you can go back to your office and use your pre-compliance and other troubleshooting methods to fix all areas of concern prior to heading back to the full-compliance lab.
inside view of EMC test facility
Summary
Without much effort and expense, it is possible to perform pre-compliance testing in a manner that saves considerable time and money without commercial or financial risks associated with purchasing a full-compliance test facility. It is not just plugging in the equipment and taking data; one needs to know what the errors are during the pre-compliance measurement process, the latter of which is beyond the scope of this brief article. To find more information on how to properly conduct pre-compliance testing, errors of the pre-compliance measurement process, and EMC troubleshooting please check out the excellent references listed below.
References and Further Reading
  1. Ott, H., Electromagnetic Compatibility Engineering, John Wiley& Sons, 2009.
  2. Andre, P., Wyatt, K., EMI Troubleshooting Cookbook for Product Designers, SciTech Publishing, 2014
  3. Montrose, M.I. & Nakauchi E.M., Testing for EMC Compliance, Approaches and Techniques, IEEE Press/Wiley-Interscience, 2004
  4. Williams, T., EMC for Product Designers Fifth Edition, Newnes, 2017.
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