It is said that three simple rules govern the packaging of electronics today simplicity, versatility and reliability. But sometimes, conforming to these rules is not as easy as it looks, particularly when one is designing a product that must achieve an IP rating, rigorous screening levels and conformance with the Low Voltage Directive.
Nevertheless, these were the key design criteria behind the new ‘Cubic Desktop’ range of enclosures from SRS Products. Developed to meet the IP55 rating, they offer protection against dust penetration and water ingress when the enclosure is subjected to jets of water from all directions.
EMC screening has been achieved via gaskets that are fitted in-board of the IP seals. This guarantees that the gaskets are protected from any moisture contamination. Screening levels can be increased by reducing the interval between each gasket. Typical screening levels achieved are 40dB at 100MHz up to 500MHz.
The unit also features full earth bonding of the enclosure frame and cladding. This is accomplished through a specially designed corner joint which can give up to a Class 1 earth bonding, with reference to some requirements of the Low Voltage Directive. This ensures that the enclosure is sufficiently earth protected to house an open framed power supply unit (PSU). The inherent earthing at each corner assists in shielding.
The patented earthing component is designed so that it could be used within the moulded corner construction of the enclosure. This makes it possible to dispense with the need for an additional earthing kit, eliminating the inconvenience of assembling an earthing kit into the finished unit and moving the wires to one side to fit an electronics package.
Additionally, any earthing post could be fitted anywhere inside the enclosure without being restricted to the earthing kit wire length or extending a wire to suit.
The corner extrusions (Figure 1) were designed from scratch, as was the glass loaded nylon corner moulding. Diecasting was originally specified for the corner but was found to easily shatter.
A major concern was the design of the beryllium copper earthing piece, which was not easy to make, although the tooling needed to be simple. Radii and varying thicknesses, to provide the necessary reinforcement, had to be introduced, to add to the problem.
RFI and EMC screening is aided by clips. It was found that by applying nickel plating, assembly was made easier as it acted as a lubricant. Figure 2 shows the method of achieving earthing, screening and continuity. Adhesive bonding was used throughout.
Water sealing to meet IP55 was quite complex. At one stage it was found that even with seals water was still finding ways in. Tests indicated that most seals tended to act as a pump and direct water to areas where it was not wanted.
Eventually, seals resembling windscreen wipers were designed beads were added at the edge, and different height ribs incorporated. When the hollow seal is depressed, the ribs all act to ensure no water enters the enclosure. Coloured seals make it easy to check that everything has been installed correctly.
SRS Products Tel: (01279) 635500