Control Panel Design: Tips and Tricks

Enclosure ratings, filters, input and output, networking, and safety are among enclosure design considerations.

04/24/2012


All control panel design activity must start with an awareness of the enclosures working environment. Follow the NEMA environmental rating for the area for all components in the panel. For example, in many food plants all external surfaces are coated daily with sanitizing foam producing airborne corrosives, and then rinsed with 80 lb force of hose-directed water. This requires NEMA 4x rated equipment for this area. Using a NEMA 12 enclosure, which provides some protection against water ingress from dripping or light splashing, would not meet the requirements of this area. Don’t mix and match components within an enclosure, in particular for externally mounted devices on the panel.

Where risk of arc flash exists, panels should be externally stickered, alerting workers of such risk. Requirements for working in such panels should include wearing appropriate personal protection equipment for the panel.

When dealing with pneumatic equipment in the panel, always exhaust the air outside the panel. A recommended practice is to put the pneumatic penetrations low in the panel. Moisture in pneumatic air is a common problem, so placing the entry low will insure there are no drips on electronic equipment, should moisture ever become a problem. It is also a good idea to use a low micron prefilter for biologics and some water, coupled with a coalescing filter, to remove the majority of water in the air.

From an equipment protection standpoint, wiring conduit penetrations should never be at the top of the panel. Moisture and water in conduit lines are not that uncommon. Penetrations near the bottom are the safest from an equipment protection standpoint but will be harder to work with for the electricians. Mid-level side-mount penetrations are a common compromise.

While not necessary, systems will be easier to maintain over time with true earth grounds versus floating grounds. This can help mitigate panel equipment damage in the event of inadvertent cross-connects between two different potential levels.

Input/out racks

A good practice is to segregate signal types. TriCore engineers group discrete I/O and analog I/O by voltage type. Always run analog signals in shielded cable. It is a good practice to minimize interference as combining different voltage types in internal panel wiring ducts. (Conflicting voltages should cross at right angles to minimize interference.)

Motor control panels

Motor starters and drives often have networked cable. Generally this should be isolated from high-voltage cable, unless using a high-voltage-rated Ethernet cable. It is a best practice to have a physical barrier within an enclosure for high- and low-voltage sections, although size and cost considerations often come into play. Touch-safe terminals should be used, with cover plates used for high-voltage power distribution.

- David McCarthy is president and chief executive officer, TriCore Inc. Edited by Mark T. Hoske, content manager, CFE Media, Control Engineering.

www.tricore.com

www.nema.org

http://controleng.com/integration



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