By Howard Minnick
Cable glands—sometimes referred to as strain relief or cable connectors—are used in electrical equipment and panel construction to ensure panel integrity is maintained at the point of entry by the electrical cable. Typically, as shown in Figure 1, holes are drilled into the side of the enclosure or panel to allow cables to pass to the inside where they connect to various electrical components. The cable gland is designed to mount to the enclosure while also providing a secure strain relief to the cable and sealing all possible openings between the enclosure, cable and fitting (see Figure 2).
|Figure 1||Figure 2|
Cable glands are available in a wide variety of materials, and selection is based on the application and the environment where they will be installed. The four main types of cable glands with respect to material are: plastic cable glands, brass cable glands, aluminum cable glands and stainless cable glands.
The installer must carefully select the cable gland type that best suits the application. In applications such as the food and beverage industry, for example, where washing down equipment and panels is done on a regular basis, it is normal to use stainless steel cable glands since they provide very good cabinet protection against dust, water and other vapors. Other issues to consider when selecting material include cost, impact resistance, whether shielding is required, and the enclosure or panel material to which it is being mounted.
The entry thread or gland size relates to the threaded portion of the cable gland that feeds through the sidewall of the equipment or panel. Although there are other types, the most popular are the PG and metric threaded cable glands. The cable gland can be threaded into a drilled and tapped hole, such as those in Figure 3, or they can feed through a clearance hole and a lock nut applied to the threaded portion that extends into the equipment. Although a lock nut is not required when used in threaded holes, many customers will use them for an extra level of security. For enclosures that have special requirements due to thickness, or where it is necessary to ensure enough room for a lock nut, extended thread versions are available.
Another very important criteria for the selection of cable glands is the overall diameter of the cable that the cable gland must accept while continuing to provide strain relief and a sealed interface between the outside and the inside of the equipment. Cable glands are typically rated for a minimum and maximum cable diameter, such as 13mm to 18mm. If the cable is smaller then normal, then the customer has the option of selecting a reduced cable entry version of the cable gland. Finally, if multiple cables are to feed through a single cable gland, then it might be possible to use a multi-entry seal and plug of the variety in Figure 4.
As mentioned earlier, one of the important jobs a cable gland must perform is to protect the integrity of the cabinet or panel. In many parts of the world ingress protection is defined by the IEC and given a rating based on the level of protection—such as IP65, IP67 or IP68. In the U.S., the level of protection is defined by NEMA standards such as NEMA 4 or 4X for cable glands that can accept wash down. Care must be taken to select a cable gland with a rating required by the application. The cable glands achieve the different protection ratings through the use of seals and washers between the gland and the equipment and the gland and the cable.
Numerous special application cable glands are available from most suppliers. One popular version is the EMC cable gland that is used with cables that have a braided shield and provide a means of attaching the shield to the gland (see Figure 5). Once the shield is attached to the cable gland, it is then connected to the equipment or panel—thereby providing a connection to ground and eliminating any electromagnetic disturbance.
Cable glands, like most electrical components, are application driven—therefore customers must be careful in the selection process. Consideration must be given to material types, degrees of protection, size of the cable and the associated gland, method of mounting, special approvals such as Ex and a wide range of other criteria. The good news is that many suppliers today offer a complete range and have developed online selection tools that assist the customer in making that decision.
About the author: Howard Minnick is CEO of Automation Systems Interconnect, a manufacture and distributor of electrical and electronic components since 1999.