Re: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where?

Douglas Powell Mon, 12 Oct 2015 13:13:03 -0700

Jerry,

The "stricter" limits are not limits at all. Best practice says to always provide margin.

While you may have a sample that passes with one or two dB of margin. This is usually accepted by most companies as adequate for that sample alone.

First, I would ask for 3 dB of margin for the test setup. I have found that even if you leave all the same test gear and test samples overnight and then retest again, you will always have a variance. Adding 3 dB Is generally enough to cover for this.

I also asked for an additional 3 dB of margin because of manufacturing variance. Small differences in wire routing, component values and RF grounding impedance can make a big difference. Often, a sample taken to an EMC test site is specially built for the purpose. I sometimes call this a white glove sample and it represents an ideal build. The question is can this product be built again in six months, by someone else, and still maintain the same exact results? Another cause of variance is capacitor or ferrite core tolerance in EMI filters. Ferrite cores are know to have up to 20% tolerance from one lot to the next. Adding 3 dB to the margin is generally enough for this concern.

Now, an engineer may argue with you and say passing is passing even with zero margin. And the engineer would be correct. However, try this with 3 or 10 more samples and guaranteeing a 100% passing result is difficult to prove the very least.

It's not news that good design practice allows for some margin and not just for EMC.

All the best,

Doug

Douglas E Powell

doug...@gmail.com

http://www.linkedin.com/in/dougp01

From: Itzenheiser, Jerry (GE Healthcare)

Sent: Monday, October 12, 2015 1:26 PM

To: EMC-PSTC@LISTSERV.IEEE.ORG

Reply To: Itzenheiser, Jerry (GE Healthcare)

Subject: [PSES] Stricter limits than legal (CISPR11, IEC, etc,) Where?

Hello EMCers,

I would like to askâ¦

Is there anyone out there that tests to stricter limits than the legal (CISPR, IEC, etc.) limits? If so, what was the rationale behind selecting the stricter limits? Our engineering teams are curious as to where the stricter recommended limits come from, such as the 6dB margin for emissions testing.

Thanks,

Jerry Itzenheiser Jr

EMC Technician - Waukesha

GE Healthcare

Global Engineering Technologies

EMC Laboratory Waukesha

T + 262-548-2217

M + 262-720-8846

gerald.itzenhei...@med.ge.com

www.ge.com

3000 N. Grandview Blvd.

Mail Code W618

Waukesha, WI 53188

-
----------------------------------------------------------------

This message is from the IEEE Product Safety Engineering Society emc-pstc discussion list. To post a message to the list, send your e-mail to <emc-p...@ieee.org>

All emc-pstc postings are archived and searchable on the web at: http://www.ieee-pses.org/emc-pstc.html

Attachments are not permitted but the IEEE PSES Online Communities site at http://product-compliance.oc.ieee.org/ can be used for graphics (in well-used formats), large files, etc.

Website: http://www.ieee-pses.org/
Instructions: http://www.ieee-pses.org/list.html (including how to unsubscribe)
List rules: http://www.ieee-pses.org/listrules.html

For help, send mail to the list administrators:
Scott Douglas <sdoug...@ieee.org>
Mike Cantwell <mcantw...@ieee.org>

For policy questions, send mail to:
Jim Bacher <j.bac...@ieee.org>
David Heald <dhe...@gmail.com>