How do you maintain EMC with cabinetÆs 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 (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 anything on this issue?

 

[BrianG]

What kind of "maintenance" do you need to perform? If you need to be able to change certain equipment modules while the rest remains powered up and operating then this can be difficult to achieve but not impossible. It would usually require a much more complex housing e.g. with inner and outer "skins" or "pidgeon holes" for each module to plug into, which maintains the shielding on the units which are not being removed.

If it's more a question of adjustment, then ideally your design should include suitable adjustment control points, test connectors, etc., brought to a controlled shielded area. This allows adjustment without "opening the box" and the test area is covered with a flap or similar, once the equipment has been adjusted, just to make sure.

 

[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 in standards/regulations.

 

[rumbaldking]

I have puzzled out safety standards on sort of the same lines and finally figured out that they make a big differentiation between operator area and service area. If you can seal the cabinet, the interior is considered the service area. Perhaps there is a similar difference between operating with sealed cabinet and being serviced.

 

[hacksaw]

such matters are addressed in your design objectives as they relate to maintenence.

 

[FOC]

I'm new to this forum but I'm on the EPRI committee which is updating the TR and I was also on the MIL-STD-461E committee, so I hopefully can shed some light on the matter. Unfortunately the problem is one not of the EMC community ignoring the maintenance issue but one of $$$. The spec's are for the final production unit to be sold to the end user and maintenance is not an issue..Not that it is ignored, but it is left up to the end user to work around..not really a good situation, as you have seen but not usually a problem. Once you open the "shielded/clean/sensitive" area you need to be aware that interference may be experienced. This is not usually a problem in the military environment but the Nuclear plant is a different environment. I will bring this topic up at our next meeting...which may be soon..If you would like to discuss further let me know