Cabinet Signs

The following article originally appeared in the June 1998 issue of Signs of the Times magazine.

By Bill Dundas

World War II precipitated a technological boom that created new American industries overnight. Two developments in the years immediately following the war gave rise to an entirely new type of sign. The genesis of the plastics industry, coupled with the commercial development of fluorescent lighting, made internally illuminated signs feasible and practical.

Sun Oil Co. (Philadelphia) helped pioneer modem cabinet signs on a large scale, by installing acrylic-faced Sunoco diamond signs at its service stations beginning in 1947. These pole-mounted ID signs were illuminated by two internal, cold-cathode tubes. In the early, post-war period, cold-cathode tubing thrived in general lighting applications. Soon, however, the availability of standard fluorescent tubes in various lengths relegated cold-cathode lighting to a marginal status.

After World War II, Sunoco was one of the earliest quantity users of internally illuminated, plastic, cabinet signs.

By 1950, DuPont Plastics (Arlington, NJ.) was marketing Lucite® acrylic sheet, and Rohm and Haas (Philadelphia) had introduced Plexiglas®. During the ’50s and ’60s, many existing neon signs were replaced by internally illuminated plastic models, and many sign professionals believed that, perhaps, neon signs would not survive this new competition.

Just as neon’s innate qualities guaranteed its survival, the inherent advantages of internally illuminated cabinet signs propelled this signage form into a dominant, sign-industry position. The practically unlimited palette from which designers may choose rates first among these advantages. Although neon is available in various colors, the ability to paint, screenprint or digitally print plastic and flexible faces opens up countless design possibilities.

Second, manufacturers of internally illuminated, cabinet signs don’t require the highly specialized skills possessed by neon tubebenders. Hence, the plastic sign’s introduction delivered the same kind of technological shot-in-the-arm for the 1950s sign industry that computer-aided signmaking equipment would provide three decades later.

Third, internally illuminated, fluorescent signs cost less to maintain than neon signs in outdoor applications. This is true because the entire lighting system is protected from dirt and weather by the sign cabinet and plastic face panel(s). Unlike exposed neon or channel letters with remote transformers, a properly designed cabinet sign encloses all the secondary wiring connections.

Loss of message also relates to overall maintenance cost. Neon signs notoriously display interesting messages when a portion of the tubes is dead or flickering. Even when the partial message isn’t embarrassing, this situation demands immediate service to restore proper identity.

Conversely, an internally illuminated sign’s message is unaffected by daylight lighting problems, and the sign continues to show a complete message at night, even when some of the lamps are burned out. This eliminates the user’s burden of facing an emergency situation every time his sign experiences a lighting problem.

Although some people say outdoor neon signs are equally durable, a moonlight drive down Main St. after a rainstorm illustrates that cabinet signs stand up considerably better to the elements. The average sign user isn’t impressed by the technical reasons why his neon sign burns out after a storm; he just knows that it costs him a lot of money.

Structural design
Some fabricators apparently believe that anything with four sides and light bulbs in the middle constitutes a sign cabinet. But a sign cabinet is more than merely a frame for plastic or flexible sign faces; it’s a structural unit. At a minimum, a properly designed electric sign cabinet must fulfill three essential functions:

  1. To support and retain the face panels that contain the sign’s message
  2. To house and protect the internal lighting system
  3. To withstand various natural forces that impact the sign

Although these requirements seem simple enough, many sign builders continue to miss the mark.

To fulfill each of these functions, a sign cabinet must be engineered like any other structure. This is particularly true for pole-mounted outdoor signs, which must be designed to resist substantial wind pressures. According to Benjamin Jones, PE, the author of “Engineering Sign Structures” . . .  “To correctly engineer the internal structure of a freestanding sign cabinet, you must determine the loads to be resisted, decide on a practical frame layout, select the type and size of component members, and detail all critical connections.” This means that if you’re bolting together something that merely looks like a sign cabinet, you’re playing with fire.

Fig. 4: This drawing shows the basic design of steel-frame pole signs (left), aluminum extrusion pole signs (right) and internal details for rigid and flexible-face signs (center). Sag braces comprising steel or aluminum rods resist the cabinet’s tension load to maintain the sign’s square shape.

A properly designed outdoor sign cabinet should carry a design wind pressure rating (Fig. 3) measured in pounds per square foot (psf). The geography and topography upon which the sign is erected, plus the sign’s mounting height above ground, determine the specific minimum design wind pressure. Jones provides an in-depth discussion of the structural design process for sign cabinets in Chapter 4 of “Engineering Sign Structures.” In general, however, most sign cabinets are supported by steel-truss frames, extruded-aluminum frames or a combination of steel and aluminum (Fig. 4).

Electrical design
A “Catch-22” situation commonly arises when designing an electric sign cabinet. As we have seen, each type of sign requires a specific structural framework. But the sign also needs proper illumination. This combination presents conflicts that some sign builders haven’t successfully resolved.

Because outdoor cabinet signs commonly develop water leaks over time, the vertical orientation of fluorescent lamps should be avoided. Rainwater that enters the top of the cabinet drips onto the lamps’ surfaces and runs down into the lower sockets. This is one of cabinet signs’ most common service problems. Unfortunately, center-pole cabinet signs incorporate vertical steel supports, which usually dictate vertical lamp placement.

Fig. 5: Signs mounted atop poles (left) typically require vertical lamps because of the center support member. Ballasts (shown in red) should be mounted on the lower portion of the cabinet’s sides. Signs mounted between poles (right) may incorporate a horizontal raceway between adjacent lamps to support the ballasts. The ballast secondary circuits (shown as “A” and “B”) may be staggered to decrease the frequency of required maintenance.

Horizontal lamps leave insufficient space to mount ballasts on the cabinet’s sides unless the cabinet frame is at least 12 inches wide. For narrower cabinets, this problem can be overcome by mounting the ballasts on a horizontal, metal raceway placed midway between two of the lamps (Fig. 5). Some pole signs incorporate notched, center-support beams to permit horizontal lamping. This design can create maintenance headaches, however, because it’s difficult to remove and reinstall the lamps without breakage. Vertical lamping’s occasional necessity makes a waterproof cabinet design particularly important.

Ballast placement is another important aspect of sign-cabinet design. Although many mass-produced signs violate the rule, ballasts should not be installed on sign-cabinet bottoms where water collects. Service mechanics often open signs and discover ballasts surrounded by puddles. Neither should ballasts be mounted upside down on top of the cabinet. This is the hottest part of the sign, and the sun’s constant rays only exacerbate this fact. Also, if the ballast mounting holes penetrate the cabinet top, water leaks are likely.

Ideally, you should mount ballasts vertically on the lower portions of the cabinet’s sides or other sturdy supports (Fig. 5). Ballast manufacturers label their products to indicate the proper vertical orientation. Curl the wires on vertically mounted ballasts to form “drip loops” proximate to where they enter the ballast case. These loops prevent rainwater from “wicking” down the wires and into the ballast case.

A common sign-builder error is mounting ballasts with double-nut bolts. If nuts are placed between the ballast case and the inner cabinet surface, the ballast’s heat can’t be transferred efficiently through the cabinet to the outside air. Ballasts should always be mounted flush against the metal cabinet surface to facilitate this “heat-sinking” process. Use stainless steel or brass fasteners to attach ballasts, because ordinary steel bolts rust and become difficult to remove.

Staggering the secondary-lamp circuits greatly reduces your customer’s annual maintenance costs (Fig. 5). Instead of placing all the lamps operated by one ballast in succession, wire the sign so that every other lamp is operated by the same ballast. The customer benefits, because the sign will continue to appear evenly illuminated despite a few bad lamps. Many customers will appreciate that this design allows sign operation for longer periods between service. The sign builder should always remember, however, to indelibly label or color-code the lamp sockets to make service personnel aware of the staggered circuits.

Placement and mounting of lamp sockets similarly impact sign-cabinet design. The spacing between adjacent lamps determines the relative brightness of the sign, depending on the cabinet’s width, the type of face panel(s) and any internal obstructions. Also, the dimension between each lamp’s opposite sockets is critical to maintain proper electrical contact, as well as to facilitate maintenance. All lamp sockets must be attached to firmly supported raceways to avoid electrical and maintenance problems.

Most importantly, sign builders must seal outdoor sign cabinets to prevent rainwater leakage and safeguard the lighting system. Outdoor signs should not be designed with access covers on top of the cabinet. These metal covers often get bent or damaged after years of service. Sign mechanics don’t always reattach all the screws that fasten these covers to the cabinet. Sign builders should never assume that an access cover won’t eventually loosen or blow off an outdoor sign. If a sign is designed for access through the sides or bottom of the cabinet, problems with these metal covers are less likely to allow saturation of the internal components. Additionally, all bolt, screw and rivet holes drilled through the tops of cabinets must be properly sealed to prevent leakage. Also, the bolts that attach sign-hoisting hardware should always be replaced and sealed after installation.

Finally, because an energized cabinet sign is essentially a “hot box,” vents placed near the tops of the cabinet sides allow hot air trapped inside the sign to escape. These vents typically comprise 4- to 6-inch round or square holes protected from rainwater by a louvered cover. Scientific tests conducted on sign ballasts prove that the cooler the ambient operating temperature, the longer the ballast’s life span.

Building quality
The apparent simplicity of a sign cabinet stripped of its plastic or flexible faces is deceiving. This simple structure masks the intelligent, economical design that distinguishes a high-quality sign from a tin box. Attractive, durable signs marry engineering and experience. Because sign-cabinet design demands more than sheer mathematics, manufacturers who successfully combine all essential criteria gain an important competitive advantage. Intelligent design serves customers better and enhances the sign builder’s business. In a larger sense, quality products defend the sign industry from regulatory excess by demonstrating that sign professionals recognize appropriate product standards.


Photo by James Walsh

Wade Swormstedt

Wade Swormstedt

Wade is Executive Director of the Foundation for the Advancement of the Sign Industry. Formerly he was Editor and Publisher of Signs of the Times magazine.

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