Why Do I Need a Strain Relief?
North American and international strain reliefs from Interpower Corporation provide a liquid-tight seal on the power cord entry and, at the same time, provide extra
protection to power cords (Figure 1). 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.
Where Does a Strain Relief Fit In?
All styles of Interpower 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. Our specification charts and drawings for each style of strain relief found in this section should make it simple to determine the size of the hole that must be made to accommodate the strain relief selected.
NPT and PG Thread Types Compared Back to the top
Interpower strain reliefs are available in two
different types of mounting threads: NPT and PG (Figure 2). NPT (National Pipe Thread) is the American
standard. PG (Panzer Gewinde) is 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.
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.
Most of the items shown in this catalog are available with either NPT or PG threads. 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. For convenience, Interpower also offers a PG to
NPT adapter (Figure 3).
When ordering strain reliefs, consider the following factors:
1) Thread size
2) Cable range (O.D.)
3) Dome or flex style
4) Metal or nylon type
5) Color
6) Threaded or knockout (If knockout, locking or not)
7) Seal ring or O-ring
Dome Style Strain Reliefs Back to the top
Dome style strain reliefs, named because of their domed shape, are designed to protect the wire entering the equipment (Figure 4). 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. Interpower dome style strain reliefs are available in Nylon 6/6 (polyamide) and nickel-plated brass. These dome style strain reliefs are also available with a
multi-cable insert and can be designed to accommodate up
to eight holes. This insert is used when there are two or more cables entering the equipment. We can also help custom design an insert to meet your design needs, but there must be at least 1mm distance between holes. 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 (Figure 5).
90° Angled Strain Reliefs Back to the top
Our 90° angled strain reliefs can be used with both a flex or dome nut. They 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 10.5kg/m2 liquid tight
seal (Figure 6).
Strain Reliefs for Larger Cable
For larger cable, we offer aluminum strain reliefs 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 49.2mm.
When Do I Use Conduit? Back to the top
Interpower Corporation also offers conduit for use with nipples or elbows. This highly flexible corrugated conduit is made of Nylon 6/6, and is flame retardant and
self-extinguishing. It features excellent crush resistance,
resistance to 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.
The push-in conduit system connects flexible nylon conduit to either a straight or 90° push-in fitting. The polyamide nipples feature a ridge that snaps into the grooves of the corrugated conduit to hold the conduit in place. The conduit 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.
Assembling Strain Reliefs
Make strain relief assembly simple with Interpower Corporation’s deep socket wrench. This tool is made of alloy steel, which makes it handy and light weight. The tool is slit open along the full length of the socket to accommodate the installed cable during tightening of the dome nut and some flex nuts. The deep socket wrench is available in many sizes.
Selecting a Strain Relief for Application Back to the top
The first step in specifying Interpower strain reliefs is to
determine which general type will work best. For applications requiring small to medium-sized cable, we offer several varieties of strain reliefs. The aluminum strain
reliefs offer the best performance for
larger cable sizes.
Depending on your 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. See our Cable section.
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 kilogram 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.
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