Enclose as Needed

To enclose or not to enclose, that is the question. So perhaps the decision isn't a Shakespearean epic, but it could be a tragedy nonetheless if you pay to enclose when you don't have to, or don't enclose and shorten the life of what you're installing... or that of the nearby operator. Greater options for distributed controls bring greater responsibilities for protecting assets where they're lo...

03/01/2006


AT A GLANCE

 

  • Enclosure selection

  • Check the ratings

  • Standard, modified, custom

  • Heat calculations

  • Materials, options

Sidebars:
Mounting considerations


To enclose or not to enclose, that is the question. So perhaps the decision isn't a Shakespearean epic, but it could be a tragedy nonetheless if you pay to enclose when you don't have to, or don't enclose and shorten the life of what you're installing... or that of the nearby operator.

Greater options for distributed controls bring greater responsibilities for protecting assets where they're located. For areas with ignitable dust or fumes, for instance, electronics must be enclosed or of low-enough voltage with a design considered intrinsically safe.

Two common standards are NEMA enclosure ratings from the National Electrical Manufacturers Association and ingress protection (IP) ratings from the International Electrotechnical Commission (IEC).

While wording differs on designations from the two standards bodies, comparisons have been made (table). Each classification denotes amount of protection against what. For instance, NEMA 4 is intended for indoor and outdoor use to protect against dust and hose-directed water. IEC IP54 offers protection against objects under 1.0 mm diameter touching internal parts, ensuring that dust doesn't effect product performance and that water splashing from any direction will not harm.

Yes or no

Lynna Chin, product manager, Rockwell Automation, says the purpose of any enclosure is to house and protect the equipment or controls, and act as a protective barrier between the operator and equipment. Therefore, enclosure selection is highly dependent on the environment of the application and the size of equipment or controls it houses. Check the rating on the equipment housing and compare it to the worst the location could deliver.

Other factors in enclosure selection, Chin says, are ease of:

  • Customization: how easily can the enclosure be modified to accommodate service connections, etc.;

  • Installation: how readily can the enclosure be mounted and how effortlessly can equipment or controls be mounted within; and

  • Maintenance: how accessible are equipment and controls in the enclosure for maintenance and repair without sacrificing security.

Catalog or beyond

Should you select a standard catalog enclosure, adapt a standard catalog enclosure, or specify a custom one?

David Crooks, general manager, Fibox Enclosures, says the standard enclosure is an off-the-shelf catalog item, for example, a Type-1 junction box. 'Standard enclosures are large-volume production items, normally the least expensive choice. Their use will require compromises on the part of the user, but for a one of a kind control panel, a standard enclosure is usually the best choice.'

At the opposite end of the spectrum is the 'custom' enclosure, Crooks says, which is designed and produced exactly as required, with varied material choices, finishing, dimensions, hole patterns, cutouts, and modifications ensuring the delivered product is ready for final product assembly. 'Custom is often the most expensive option, but it can be cost effective if the quantities are large.'

In-between is the 'customized standard' enclosure, a standard enclosure machined with required holes, cutouts, slots, and recesses. Modifications make the enclosure functional, permitting mounting of switches, viewing of indicators, and input/output connections, as the application requires, Crooks says. This is usually the most cost-effective choice for OEMs producing low- to medium-volume products. It requires first selecting the best enclosure option, then designing to fit within the package. 'This can complicate the design process,' he says, 'but the result is usually worth the effort.

2 holes or 40 recesses

Who does the modifications depends on complexity, quantity, and capabilities. Complex cutouts, such as slots, keyed holes, or recesses, require special tooling or a CNC milling machine. Modifications can be 'difficult, especially if quantities are large, hole pattern complex, and tolerance critical. Repeatability, from enclosure to enclosure, requires precision equipment and expertise that comes only from doing a lot of modification work,' Crooks suggests.

Many progressive enclosure manufacturers now offer full customization services, he adds. Because the cost of CNC machinery (specifically designed for enclosure modification) has dropped significantly in the last few years, while capabilities have increased dramatically, Crooks says, holes, slots, cutouts, milled recesses, and other modifications can be done more 'quickly and at reasonable cost for even small runs.'

Materials selection

Common enclosure materials include steel (carbon or stainless), thermoplastic, polycarbonate, and fiber glass. Chin says Rockwell Automation plastic enclosures are a thermo-plastic polyester-blend material that eases customization without requiring special tooling. The material is lightweight, which facilitates installation without compromising strength, she says; cost is lower compared to traditional sheet metal and fiber glass polyester material.

Typically, equipment or controls housed in an enclosure are mounted onto a steel mounting plate where the designer needs to identify the equipment location on the mounting plate and pre-drill the holes to mount the equipment, Chin says. A 'pegboard' plastic mounting plate design allows equipment installation onto the plate without having to create any template and pre-drill any holes. Just place the equipment on the grid design as desired and use self-tapping screws to mount to the plastic mounting plate, Chin advises; the grid design eliminates drilling requirements and equipment installation time.

Dissipate heat

No matter what the material, calculations need to be made to ensure appropriate heat dissipation, says Betty Jackson, senior product manager, Hoffman. Sealed enclosures may require modification to manage heat generated by industrial controls, which are decreasing in size while increasing in capacity. A basic PLC accompanied by related components, for instance, can generate 200 W of heat; when placed in a sealed enclosure 3 ft x 2 ft x 1 ft, she says; that heat load increase the enclosure's internal temperature to 36

For every 18

'Installing closed-loop air conditioners, closed-loop heat exchangers, fans, or vents in enclosures are all means of dissipating component-generated heat,' Jackson says. Once a solution fitting the application is chosen, the 'system designer must determine how the enclosure will be modified to accommodate the thermal management components.' Options include modifying the enclosure in-house, hiring a panel shop or system integrator, or ordering the enclosures modified from the manufacturer. Jackson sees 'an increase in manufacturer-modified enclosures. Manufacturer provided holes and cutouts for thermal management solutions decrease on-site component installation time, provide accurate, high-quality solutions, and are consistent on repeat orders.'

Enclosure options—beyond materials, sizes, shapes, and ratings—vary widely, and include doors, viewing options, compartments, racks, shelving, accessory hardware, handle variations, and locks. Many Web sites list options or allow feature selection online.

Among considerations outlined by Rittal Corp. include corrosion, shock, and vibration protection; security from unwanted access (animal, vegetable, or mineral); shielding from electromagnetic or radio frequency interference; environmental (moisture, winds); and thermal (passive shields or active heating or cooling).

NEMA and IEC enclosure rating comparison

NEMA type

IEC designation

This comparison, based on tests specified in IEC Publication 60529, was listed at www.nema.org/stds/ics1.cfm .
Source: Control Engineering with information from NEMA 250 from NEMA Standards Publication 250-1997 'Enclosures for Electrical Equipment (1000 V maximum),' available from Global Engineering Co. at www.global.ihs.com .

1

IP10

2

IP11

3

IP54

3R

IP14

3S

IP54

4 and 4X

IP56

5

IP52

6 and 6P

IP67

12 and 12K

IP52

13

IP54



Other references
For comparison tables on enclosure materials, EMI/RFI shielding properties, LCD refer-ence/supplies, and IP & NEMA protection classifications from Evatron, click here .

Related reading on enclosure research, trends, and products from Control Engineering includes:



Mounting considerations

Enclosures can be mounted on floors, walls, ceilings, and equipment, and on various stands, rails, and mounting arms.

Brian Evans, managing director of the Evatron Group, an enclosure provider for electronic and electrical components, often gets questions about mounting, and how that influences door access, electronics' functionality and integrity, aesthetics of the system, safety, tampering, or theft.

Lugs and screws—among two traditional mounting methods—are exposed and can be undone. Evatron's 'invisible mounting kit' hides screws, and with additional hard wiring, contacts could trigger alarm systems when data protection and sensitive equipment are threatened.



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