Why
Do I Need a Strain Relief?
North American and international strain reliefs from Interpower
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. Interpower strain reliefs
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?
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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.
Each of these options requires a different sized clearance
hole in the equipment panel. Our specification charts and
drawings for each style of Interpower strain relief found
in this product section should make it easy to determine the
size of the hole that must be made to accommodate the strain
relief selected.
Ordering and Selecting
a Strain Relief for Application
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Ordering
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
Specifying for Application
The first step in specifying Interpower strain reliefs is
to determine which general type will work best. We have strain
relief designed for applications requiring small to medium
sized cable. The aluminum strain relief types 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 cable or power cord with which it will be used. After
installing the cable (Figure
7) 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 one-minute is over, a visual inspection
is done to determine if the cable has slipped from the strain
relief.
NPT and PG Thread
Types Compared
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Interpower strain reliefs are available in two different types
of mounting threads: NPT and PG. 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 product section 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
2).
Dome Style Strain
Reliefs
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Dome style strain reliefs, named because of their domed shape
(Figure
3), 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 (Figure
4). 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
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Flex style strain reliefs are named because they offer flexible
bend protection to the cord entering the equipment. Interpower
offers straight and 90° angled
flex style strain reliefs. 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
6).
90° Angled Strain
Reliefs
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Our 90° angled strain reliefs (Figure
5) 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/m liquid tight seal.
Strain Reliefs for
Larger Cable
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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 reliefs
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 To Use Conduit
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Interpower 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, and exposure
to UV agents 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 so that it is 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 style strain relief.
Assembling Strain
Reliefs
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Make strain relief assembly simple with Interpower's deep
socket wrench. These tools are made of alloy steel, which
makes them handy and lightweight. The tools are 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 four sizes.
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