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Preparing for component EMC testing

A quick reference guide

The test specification

It is best to obtain a copy of the specification you will be testing to at least a month prior to testing. If  the specification cannot be located contact UL for assistance. Spend a few hours reading through the applicable sections. Be sure to also read the spec's "Section 1," which often contains information relevant to all the test methods in the spec.

The wiring harnesses

Determine what wiring harness lengths will be needed for each EMC test. Build these and set them aside. Label them "For EMC Testing Only."

Modes of device operation

Determine how the device will be operated during EMC testing. For immunity testing, select the modes that are most likely to be disrupted by RF and microwave interference as well as any modes that are distinctly different in their functions. For emissions testing, select the device's "noisiest" modes as well as any modes that are distinctly different in their functions.

These determinations might be difficult to make, and when no past EMC test results are available as a basis for the decision, some engineering judgment might be required. When the device only has one mode (e.g., on, operating, etc.), the choice will be obvious. Do not try to test in every device mode unless necessary. All of these decisions should have already been made in a customer-approved EMC test plan.

Load/simulator boxes

Remember that any load/simulator box used to exercise the device will probably be placed outside the chamber with its wires routed to and from the device via the chamber low-pass filter panel. This is especially critical for radiated emission testing.

Monitoring the device

The device's performance must be monitored during immunity testing. For radiated immunity, this must be done from outside the chamber. Examples of this would be voltage levels monitored on a voltmeter or stripchart recorder, visual monitoring of LEDs or movement of solenoids on the device with an in-chamber video camera, or running a diagnostic with an attached PC.

UL supplies meters, stripchart recorders, video cameras, scopes, etc. for this monitoring. If you need to monitor a communications bus or other diagnostic with a PC, you will have to supply this PC.

Your customer-approved EMC test plan should detail what to monitor.

Number of samples

Do not duplicate tests on several samples of the same device model unless contractually required, since this can greatly affect the cost and time of testing. Three of the same is usually enough for validation testing. One or two is typically sufficient for development testing.

Your engineer/technician

If planning to escort the device for initial setup or to monitor the device throughout testing, send someone familiar with the device. Even the most competent tech or engineer, if not normally involved with the project, often shows up unprepared to operate the device or unfamiliar with EMC. This could cause significant testing delays.