Below are the questions asked during the live event, along with their respective answers.
Q: You mention drawings throughout the presentation. Can you give a description or show a drawing?
A: Drawings should show the dimensions of the layout, such as distance from the front edge of the ground plane, distances from the floor, distance from the walls of the enclosure. Should also show how the cables shall be laid out on the table, how they shall be isolated 5 cm from the ground plane, and call out specific terminations of shields and all grounding.
Q: Can one laboratory write the procedure and another laboratory test to the procedure?
A: Yes. Although this is difficult at times because one laboratory may perform a test differently than another, this is definitely possible and I’ve done it in my career.
Q: How long should it take a laboratory to develop a test procedure?
A: Depending on the complexity, once the laboratory has all of the required information about the EUT, 2-4 business weeks should be sufficient.
Q: Why should the height of the test table only be 80 – 90 cm?
A: Most EMC standards use this requirement. The purpose is so that all devices are tested the same (within reason), and so the floor ground plane effects are minimized.
Q: Does the conductive tabletop have to be bonded to the wall of the chamber? Can it be bonded to the floor instead (since same reference ground as wall of chamber but easier to access (due to cones and ferrite tile on wall)?
A: It can be bonded to the wall of the floor. From paragraph 184.108.40.206 of MIL-STD-461G: The metallic ground planes shown in Figures 2 through 5 shall be electrically bonded to the floor or wall of the basic shielded room structure at least once every 1 meter. The metallic bond straps shall be solid and maintain a five-to-one ratio or less in length to width. Metallic ground planes used outside a shielded enclosure shall extend at least 2.5 meters beyond the test setup boundary in each direction.
Q: The closed-loop method is requested by the RS103 test in the standard. In this method, the sensor position in the test setup is significantly affecting the field level due to the EUT presence. Can we include the pre-calibration method in our test procedure for the RS103 test? Is this allowed in any part of the standard?
A: You could include it and justify why you’re proposing to do it that way. As long as it is accepted, it shouldn’t be a problem. Just remember that you may get some push-back from the approver.
Q: In DI-EMC-80201C, one section that has perplexed me is the test instrumentation section regarding defining antenna factors, current probe factors, etc. Often this is not known, as this is info only the test house could provide and we may not know which test lab we will test. Can you describe methods of meeting this criteria if the DID?
A: This is why I recommend having a test lab write your procedure. If you know what equipment they’re going to use, you can include some factors from the manufacturer as part of each method.
Q: How do you document how video quality is assessed during susceptibility – how much clarity loss is acceptable?
A: This depends on the manufacturer of the EUT and the contractual requirements. Let’s say, for example, that you’re testing a display. The contractual requirements state that there shall be no loss of image, flickering, screen tearing, etc, during the performance of susceptibility testing. If that’s the case, and there is some type of issue seen during testing, it could constitute a failure. However, if there is an issue, but the screen is still legible, then it may be up to the customer to approve or consider it a failure. It’s on a case-by-case basis.
Q: Often we try not to regurgitate all the words from 461. How liberal can we be using references to 461 rather than re-writing the 461 procedures in our test plans?
A: When in doubt, it’s best to include as much information from the standard as possible. I don’t like leaving things out and open for interpretation.
Q: How do you bond the earth of a three-phase 400Hz generator used for aircraft, which would be on wheels?
A: This depends on the recommendation of the manufacturer and the configuration of how the generator is used. I’m afraid I cannot give a definitive answer.
Q: As a manufacturer, why are aircraft representative cables a hard requirement? If we are selling an I/O box, ultimately cables should be the customer’s responsibility.
A: The cables are on the customer. However, the cables attached during test need to replicate what is present during installation for a myriad of reasons. They will represent the shielding and impedance of the installation cables. When the test stimulus is applied to the EUT and cables attached, you will perform a test that is representative of what will actually be happening during installation. This is crucial for EMC testing.
Q: Do we need to use a LISN for testing CE102 or just a probe to get the signal given to the EMI receiver?
A: The purpose of the LISN is two-fold.
- The LISN provides the correct, controlled impedance of the power-input lines. The limits of CE102 are based on the impedance of the LISN as well as other factors. Taking the measurement of the emissions on the power-input lines is meaningless without a controlled impedance.
- The LISN provides a calibrated RF measurement port to connect to a receiver via a coaxial cable. So, yes, a LISN is essential to perform an accurate test for CE102.
Q: Can an EMC Lab. which didn’t write the EMC Test Procedure, do the EMC test, or does it have to be done in the same EMC Lab?
A: Yes. Although this is difficult at times because one laboratory may perform a test differently than another. This is definitely possible and I’ve done it in my career.