Selecting cable strain reliefs

Whether for North American or international applications, strain reliefs provide a liquid-tight seal on the power cord entry and, at the same time, provide extra protection to power cords. Strain reliefs extend cord life and reinforce the cord connection to your equipment. The strain reliefs described in this section are intended for use in mechanically securing a power-supply cord to a piece of electrical equipment in accordance with IEC 60335 and other international standards.

Available in a variety of styles, strain reliefs seal the power cord entry and protect power cords to extend cord life and reinforce the connection to your equipment.

Choosing a strain relief

The first step in specifying strain reliefs is to determine which general type will work best. For applications requiring small-to-medium-sized cable, several varieties of strain reliefs are available. Aluminum strain reliefs offer the best performance for larger cable sizes.

Depending on the application, you may wish to specify a strain relief with integral flex relief. The flex relief protects the cord from stress that is applied perpendicular to the axis of its entry into the equipment. The last step is to select the correct size strain relief for the cord with which it will be used.

After installing the cable and the strain relief, the product should be able to pass a pull test. UL calls for the most stringent pull test, which consists of a 13.5-kg weight fixed to the cable for one minute. After the minute is over, a visual inspection is done to determine if the cable has slipped from the strain relief.

Where a strain relief fits in

Some styles of strain reliefs are threaded and can be assembled in two different ways. The mounting threads of the strain relief can be threaded directly into the panel of the equipment, or the strain relief can be eased through a clearance hole in the panel and fitted with a locking nut.

Obviously, each of these options require a different sized clearance hole in the equipment panel. Some manufacturers provide specification charts and drawings for their strain reliefs to help determine the size of the hole that must be made to accommodate the strain relief selected.

Options for mounting threads include NPT (National Pipe Thread), the American standard and PG (Panzer Gewinde), a European standard. PG threading is sometimes referred to as metric threading, however, these threads are not truly metric. There is a metric system of threading that is slowly replacing PG threads, but very few people have actually started using metric threads at this time.

Strain reliefs come in different thread types, including NPT and PG.

At the opposite end of the strain-relief body are Acme threads, which are used to attach the dome nut and flex nut. This style of threading is designed to handle pressure under high torque conditions.

Selecting a thread system

The thread system you choose will depend on the location the equipment will be used. Equipment intended for use in North America should make use of NPT strain reliefs and equipment intended for use in Europe should make use of PG threaded strain reliefs. Some manufacturers also offer PG to NPT adapters.

When ordering strain reliefs, consider the following factors:

* Thread size

* Cable range (outer diameter)

* Dome or flex style

* Metal or nylon type

* Color

* Threaded or knockout

* Seal ring or O-ring

Dome-style strain reliefs

Named because of their domed shape, dome-style strain reliefs are designed to protect the wire entering the equipment. They help to secure the wire connection to the equipment, and provide longer life to the cord by preventing it from rubbing against a rough or sharp edge where the cord passes through the equipment panel.

Dome style strain reliefs are typically offered in Nylon 6/6 (polyamide) and nickel-plated brass, among other materials. These strain reliefs are also available with a multicable insert and can be designed to accommodate up to eight holes.

Accessories such as strain-relief adapters can be useful when equipment needs to be converted from, in this case, PG to NPT.

The insert is used when there are two or more cables entering the equipment. Manufacturers such as Interpower often assist in custom design an insert to meet specific design needs. The inserts are made of durable water-resistant elastomer.

Flex-style strain reliefs

Flex style strain reliefs are named because they offer flexible bend protection to the cord entering the equipment. They are designed to eliminate the possibility of the power cord kinking at the point where the cord enters the equipment. The spiral shaped spring wraps around the cord and adds strength to the cord where it is most vulnerable to damage.

Flex-style strain reliefs offer flexible bend protection to the cord entering the equipment.

90° angled strain reliefs

Angled strain reliefs with a 90 bend can be used with both a flex or dome nut. These parts add strength and support to the cord while adding a 90 angled snap elbow to minimize wire strain in a bending application. The snap elbow opens to 180° for easy insertion of the cable, then closes to 90°, providing added strain relief and a 10.5-kg/m2 liquid tight seal.

Strain reliefs for larger cable

For larger cable, aluminum strain reliefs are available with or without integral flex relief. This type of flex relief features a design similar to that of the old “Chinese finger torture” devices.

As pull on the wire increases, the flex style relief’s grip on the cable increases, reinforcing the strength of the connection to the equipment. These strain reliefs are designed to accommodate larger cables ranging from 15.9 to 49.2 mm.

Using conduits

Some companies also offer conduits for use with nipples or elbows. Highly flexible corrugated conduits are made of Nylon 6/6, are flame retardant, and self-extinguishing.

When selecting conduits one should keep in mind factors such as resistance to crush, heat, UV exposure, and certain chemicals. Wires or cable are threaded through conduit in situations where the wire and cable need extra protection (from elements or physical damage) or where users may need to add additional wires at a later time.

Push-in conduit systems connect flexible nylon conduits to either a straight or 90 push-in fitting. Polyamide nipples feature a ridge that snaps into the grooves of the corrugated conduit to hold the conduit in place.

Conduits can be mounted turnable for non-liquid-tight applications by pushing the conduit into the fitting until it stops. For rigid, liquid-tight applications the conduit should be pushed farther into the fitting until it clicks again. These fittings are available in the nipple version or the 90° angled elbow version strain relief. ■

For more on cable strain reliefs, visit http://electronicproducts-com-develop.go-vip.net/packaging.asp.

Interpower Corporation is the premier supplier of power systems components for electrical and electronic components.