MIL-STD-461D, How do you maintain EMC w/cabinet open?

[RFIGUY]

I was wondering if anyone has heard of anyone addressing the problem of maintaining EMC when the cabinet that contains EMI/RFI sensitive equipment is open for maintenance or testing. All the Regulations (MIL-STD-461D, Regulatory Guide 1.180, EPRI TR-102323, FCC, etc.) talk about the limits that must be maintained in order to maintain EMC, and to get to that limit you must provide shielding (EMI cabinet). But the issue of opening these cabinets to perform maintenance, while the equipment is in use, is not addressed. Has anyone heard if the industry has or is going to address this issue?

 

[Janus101]

It's an interesting point you make, and I've never heard of it being adressed earlier. I work in the oil & gas industry, ie offshore platforms and gas treatment plants, and EMC design is often adapted.

I do not have any suggestion, but here is a little Java applet which I belive adress your concern:

http://webphysics.davidson.edu/Applets/Poisson/Faraday.html

I'll follow this thread with great interest.

 

[busbar]

Short of being able to cut over to a redundant system and isolate or otherwise take the subject equipment off line, it may not be possible to eliminate the EMI with the "normal" enclosure being breached.

I am not familiar with 461D, but does the standard address {or categorically exclude} the issue? Possibly oversimplified, but a solution may be to disable the enclosed equipment prior to access, either by shutdown or bypass. Recognized that it may not be a trivial task.

 

[RFIGUY]

I realize that the only way to meet EMC guidelines are to either deactivate the equipment or apply some redundant shielding to meet the emissions limits set forth by the standards (they are all relatively close to one another) However, none of the codes/standards require this.

I guess I should restate my original question a bit clearer. I was wondering if anyone out there knows why it seems that the EMC community seems to ignore real world situations that require the equipment, within a cabinet, to be tested/maintained while still fully functioning. I work at a nuclear power plant and it is not feasible to just deactivate equipment to perform testing/maintenance (say for example a redundant power supply within a cabinet fails). Not only do standards and regulations “seem” to ignore this; they don’t even cover why it is not an issue. The closest thing I have found that touches on this issue is that the FCC has a disclaimer that manufactures put on their products. “…shall caution the user that changes or modifications not expressly approved by the party responsible for compliance could void the user’s authority to operate the equipment” i.e. the equipment has exceeded FCC limits (FCC Part 15.21) It seems to me that if you have to shield equipment to meet EMC limits, that the issue of opening the cabinet should also be covered.

 

[dbaird]

No answer for you but I would ask is it really an issue? I was once involved in a project that used a rather large DC power supply for electroplating wafers for semi-conductor industry. We took extra precautions to great length in designing the enclosure using all the special hardware devices/seals for door and enclosure openings while selecting correct materials. When third party reviewer came to test compliance for CE mark, the equipment was tested both door closed and open during normal operation. To our amazement the test results showed no significant difference whether door open or closed! I suggest that if it worries you, and you have genuine concern, have it tested. You might be pleasantly surprised.
David Baird
mrbaird@hotmail.com

Sr Controls Engineer

EET degree.

Journeyman Electrician.

 

[IRstuff]

For the most part, it's simply not a significant problem. The EMI environment of 461 deals primarily with radars and radio transmitters. If you have none in your vicinity, there is no problem. Additionally, you must have RF susceptible devices, such as optical or RF receivers. Again, if there are none in your circuitry, there is no problem. Finally, the levels of emitted susceptibility are comparable to having a radar dish about 10 ft away, and most environments are significantly more benign.

If you can operate a transistor radio with no interference near your equipment, then there should be no problem.

TTFN