EMI Shielding

I am currently designing an EMC panel for a screen room. The panel is a Hoffman EMC rated. It will contain 4 Newport display meters, a reset pushbutton (Siemens 51-HAAS), and an emergency stop pushbutton (Siemens 51-HPAS). All devices will get their dc power and inputs from a PLC panel outside the screen room. Therefore, only one cable will be run to the panel.



Any opening in an EMC panel is bad. An acceptable shielding solution was determined for the incoming cable, and the openings for the 4 display meters. The switches are a challenge.



I have purchased two siemens switches for modification to make them EMC rated with my panel since I was unable to find any that would fit our criteria. They are hazardous rated, but apparently according to “senior engineers” they are still not “good enough”. Apparently there is a ratio they want to keep – L/D > 10 where L is the length between the penetration hole of the panel to the electronics of the switch and D is the diameter of the opening. Based on my investigation this ratio is determined by the shielding effectiveness (SE) you desire. The shielding effectiveness is determined by the frequencies you are trying to shield from. When I pushed back and asked where the ratio came from I was told it was a good “rule of thumb”. I believe I am designing to garbage, but that is what I have to work with. This is the first panel ever designed for our group with the EMC requirements.



Does anyone have any books, resources, or vendor products they can recommend for designing an industrial EMC shielded panel? I have several books on the topic (most by the author Michael Mardiguian- an expert). Majority have nothing on switches, some recommend purchasing EMC rated switches (those that are recommend are miniature and for airplanes). If switches are discussed it is a mere paragraph with few solutions, just – yes switches are bad, and put a metal bushing around them. None of these books discuss using the L/D ratio in design.



Thanks.

I would imagine you would need to consider what the switches were switching as well.

If the switches were being used to switch discrete DC input into the PLC, filtering the I/O lines using a good LC or pi filter would certinally help (as long as the plc I/O can tolerate it). This would be good to minimize conducted RF from the PLC panel into the screen room. If the switches are intended to directly switch AC (which the Estop might?) then it gets more intense.

When working with our design engineers on RF immunity / emission issues, we usually push for filtering / slowing down transitions etc.. first, with shieilding as part of a layered approach.

Some info on EMC for systems can be found here:
http://www.compliance-club.com/keith_armstrong.asp

Although the info is more for EM Compliance issues towards the CE mark than for installations within a screen room - the theory from this and other articles might help.

Thanks for the link

Thanks for the link. I posted my question to the compliance club for general suggestions on products, or reference materials.

They responsed instead with solutions - which was awesome they did that.

Basically, I was encouraged to use a filtered bulkhead in the screen room wall. I will now build a case to my manager that this is the most cost effective approach.

Most articles I find for EMI issues are for Printed Circuit Boards, so the industrial applications are few and far between.