Strain Reliefs InformationStrain reliefs protect the lifeline of electrical products by absorbing the forces of push and pull that may be exerted on the flexible wires, cables, or tubing.

 

How Strain Reliefs Work

 

Strain reliefs are manufactured in a variety of sizes and shapes, and using various materials.  All strain reliefs serve one or more of three main purposes:

  

  • Strain Reliefs InformationExtra protection against overbending, which extends the life of the cable.  This protection is also useful against cutting or fraying by sharp edges of nearby equipment.
  • Reinforcement of the cord's electrical connection.  These products keep the cord in a stationary position to prevent the conductors from twisting or pulling.

  • Providing a liquid-tight seal on the cord's connection area.  This prevents moisture from entering and possibly damaging the component or machine the cord is connected to.

Strain reliefs can also be classified by style, depending on their flexibility or rigidity.  Dome style products are fitted to the cable at the connection point and prevent the cable from rubbing against a rough or sharp edge where the cable meets the device or electrical panel.  Flex style strain reliefs create a flexible bend at the connection point, protecting the cable against kinking or breakage.

 

Specifications

 

The GlobalSpec SpecSearch database contains information about numerous strain relief specifications, including type, material, and size.

  

 

Type

 

Strain relief types include cable bushings, cable glands, romex connectors, and wire ties. 

 

Cable Bushings

 

Bushings are hollow liners that encircle conductors or cable to provide electrical isolation. 

 

Strain reliefsCable Glands

 

Cable glands are also known as cord grips.  They serve to secure the end of a cable to a piece of equipment or device.  Cable glands include a provision for making an electrical connection from the cable to the device.

    

This video provides an excellent overview of cable gland construction and installation:

  

Video Credit:  Hawke International

 

 

Strain Reliefs InformationRomex Connectors

 

Romex connectors attach to romex cables to seal and support it as the cable enters a junction box or enclosure.

  

Wire Ties

 

As their name implies, wire ties are plastic devices designed to bundle wires, cables, or tubing.

 

Material

 

Strain relief products may be manufactured using different materials, each with its own unique characteristics.

 

  • Aluminum is a light and malleable metal that features excellent conductivity, reflectivity, and oxidation resistance.
  • Brass is an alloy of copper and zinc, and features superior strength, ductility, conductivity, and environmental resistance.  It typically costs more than other materials.
  • Nylon is a synthetic polymer that has similar qualities to brass but is cheaper to buy and manufacture. 
  • Various types of polymer are used as strain relief materials, including polyester, polyolefin, PTFE, and PVC.  While each polymer has different characteristics, most have good chemical resistance, flexibility, and versatility.
  • Steel and stainless steel have excellent corrosion resistance and pressure ratings.

Strain Reliefs InformationSize

 

Buyers of strain reliefs should carefully consider specifications related to the product's size.

 

Diameter

 

A strain relief's diameter refers to the diameter of the cable designed to pass through the product.

 

Mounting Size

 

Mounting size refers to the size of a strain relief's threads, which are used to connect the strain relief to a junction box or other device.  Thread sizes are standardized by the National Pipe Thread (NPT) in North America and metric sizes elsewhere. 

 

NPT sizes are determined by the nominal size of the pipe or tube.  For example, a 1/4" NPT standard describes the thread size on a 1/4" pipe.  Metric sizes all share the common format [M][number] where [number] is the nominal pipe size in millimeters, such that an M6 pipe specifies that the nominal pipe size is 6 mm.     

 

The following tables detail common NPT and metric thread sizes.  In both tables, OD refers to the pipe's outer diameter, and pitch refers to the distance between threads.

 

NPT size

1/4"

3/8"

1/2"

3/4"

1"

1 1/4"

1 1/2"

2"

OD (in.)

0.54

0.69

0.84

1.05

1.32

1.66

1.9

2.38

Pitch (in.)

.056

.056

.071

.071

.087

.087

.087

.087

Threads/inch

18

18

14

14

11.5

11.5

11.5

11.5

 

Metric size

M6

M8

M12

M16

M20

M25

M32

M40

M50

M63

OD (in.)

0.24

0.31

0.47

0.63

0.79

0.98

1.26

1.57

1.97

2.48

Pitch (in.)

.04

.05

.06

.06

.06

.06

.06

.06

.06

.06

 

Table Credits:  Wikipedia | Interpower

 

References

 

Interpower - How to choose a strain relief

 

Image Credits:

 

SAB North America | Fastenal | 1-1111 | Installation Parts Supply | Digi-Key Corp.


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