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Cable and Harness
Testing Made Easy
High voltage testing
makes it easier to find insulation problems,
stray conductors, and contamination in your cables.
The following are
some common questions about high voltage or hipot testing for cable
assemblies. What is the purpose of high voltage testing? What defects
are found? Is it safe for my operators? What mental images do you have
about testing cables with high voltages? What images do people on the
production floor have when they get failures? The following information
will help you understand how high voltage tests identify insulation
defects and provide a better overall idea of what hipot testing is.
What is "hipot" testing?
Many people are familiar with a continuity test. A continuity test
checks for "good connections." You do a continuity test by seeing if
current will flow from one point to another point. If current flows
easily enough then the points are connected. Many people aren't familiar
with a hipot test. Hipot is short for high potential (high voltage). A
hipot test checks for "good isolation." You do a hipot test by making
sure no current will flow from one point to another point. In some ways
a hipot test is the opposite of a continuity test.
Continuity Test—makes
sure current flows easily from one point to another point.
Hipot Test—make
sure current won't flow from one point to another point, and turn up the
voltage really high just to make sure no current will flow.
In the simple case, a hipot test takes two conductors that should be
isolated and applies a very high voltage between the conductors. The
current that flows is watched carefully. Ideally not much current will
flow. If too much current flows, the points are not well isolated and
they should fail the test.
Why run a high voltage test?
You
use a hipot test to make sure you have good isolation between the parts
of a circuit. Having good isolation helps to guarantee the safety and
quality of electrical circuits. Hipot tests are helpful in finding
nicked or crushed insulation, stray wire strands or braided shielding,
conductive or corrosive contaminants around the conductors, terminal
spacing problems, and tolerance errors in IDC cables. All of these
conditions might cause a device to fail.
What kinds of high voltage tests are there?
There are three common high voltage tests.
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Dielectric
Breakdown Test
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Dielectric
Withstanding Test
-
Insulation
Resistance Test
What is dielectric
breakdown testing?
With dielectric breakdown testing you are trying to answer the question,
"How much voltage can I apply between the wires before the insulation
fails?" You increase the voltage until the current suddenly increases.
You are finding the highest voltage the cable can stand before it fails.
Once the cable fails it is usually damaged or destroyed.
What is dielectric withstand testing (DW)?
In
dielectric withstand testing you are trying to answer the question,
"Will this cable withstand a required voltage for a required time?" You
apply the voltage for the amount of time and watch the current that
flows. Ideally no current flows and the cable is not harmed.
What is insulation resistance testing (IR)?
In
insulation resistance testing, you are trying to answer the question,
"Is the resistance of the insulation high enough?" You apply a voltage
and very carefully measure the current. You then calculate the
insulation resistance using Ohm's Law (R = V/I).
How do these "hipot" tests affect quality?
All
of these tests are tools you can use to better understand how a cable
will perform and to monitor any changes in the cable's performance.
Dielectric breakdown testing is used in product design and qualification
stages. It helps establish the maximum voltage of the design. It can
also be used on a random sample basis to verify that the maximum voltage
is not changing. Dielectric breakdown testing may be required during the
development of assemblies used in critical applications.
Many test specifications require a Dielectric Withstand Test on every
cable produced. The test is usually done at about 75 percent of the
typical breakdown voltage. It is done as a safety net. The test is
sensitive to arcs or corona so it often finds terminal spacing problems,
over-mold problems, tolerance errors in IDC cables, or any problem that
might produce arcs. This test doesn't significantly degrade the cable.
The Insulation Resistance test is typically done on every cable tested.
It is usually done at 300 to 500 Vdc with 100 to 500 Megahoms
resistance. The test is very sensitive to contamination in the assembly
process. Solder flux, oils, mold release agents, and skin oil all can
cause problems. This test excels at identifying insulation that will
conduct in the presence of moisture. Doing this test on every cable
allows you to detect contamination changes in the manufacturing process.
Additional High Voltage Testing Resources:
Guidelines for using Voltage to Detect Insulation Defects
High Voltage arc distance
With all the high
voltage being used, what about my safety?
During a hipot test you may be at some risk. The risk can be reduced by
using a tester designed to be safe and by using that tester according
the manufacturer's instructions.
Products being designed today usually must comply with product safety
regulations. Some of these regulations work to reduce the chance of you
receiving a harmful electrical shock. Modern equipment is more likely to
follow these regulations. When it comes to hipot charge, energy, and
voltage you should select the safest machine that will still test your
cables.
To minimize your risk of injury from electrical shock make sure your
hipot equipment follows these guidelines:
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The total charge
you can receive in a shock should not exceed 45 uC.
-
The total hipot
energy should not exceed 350 mJ.
-
The total current
should not exceed 5 mA peak (3.5 mA rms)
-
The fault current
should not stay on longer than 10 mS.
If the tester doesn't
meet these requirements then make sure it has a safety interlock system
that guarantees you can not contact the cable while it is being hipot
tested.
These guidelines come from the test standard EN61010-1, Safety
requirements for electrical equipment for measurement, control and
laboratory use, April 1993,
CENELEC. Over the last decade many of the safety regulations have
been harmonized (standardized) and EN61010-1 is similar to UL 61010A-1
(formerly UL3101-1).
While you are testing cables there are several
things you can do to reduce the risk even more:
-
Verify the
correct operation of the safety circuits in the equipment every time
you calibrate it.
-
Follow all of the
manufacturer's instructions and safety guidelines.
-
Don't touch the
cable during hipot testing.
-
Allow the hipot
testing to complete before removing the cable.
-
Wear insulating
gloves.
-
If you have any
health condition that can be aggravated by being startled, don't use
the equipment.
-
Don't allow
children to use the equipment.
-
If you have any
electronic implants, don't use the equipment.
Where is the high
voltage applied?
To
understand how hipot testing works you'll need to understand where to
connect the high voltage supply. Hipot testers usually connect one side
of the supply to safety ground (earth ground). The other side of the
supply is connected to the conductor being hipoted. With the supply
connected like this there are two places a given conductor can be
connected: high voltage or ground.
When you have more than two contacts to be hipot tested, connect one
contact to high voltage and connect all other contacts to ground.
Testing a contact in this fashion makes sure it is isolated from all
other contacts.
What happens when you test something more
complicated than just contacts?
A series of contacts that are connected with wires, resistors,
capacitors, diodes, and other components is called a "network" of
connections (or net). To hipot test a net you connect all of the
contacts in the net to high voltage and connect all other contacts in
the device to ground. For example, if you have a wire that connects two
pins, the high voltage will be simultaneously apply to both of those
pins and the entire wire will be raised in voltage. All other wires and
pins will be held at ground. If you have a resistor that connects two
pins, both pins are raised in voltage; the voltage drop across the
resistor is always zero. The entire resistor is raised in voltage. In
short, all pins of a component see the same voltage at all times.
Applying the voltage in this fashion makes sure the body of the
component is isolated from the rest of the device.
Where is the current measured?
During the hipot test the current that flows out of the high
voltage supply is measured.
What causes current to flow through an insulator?
Insulation does not conduct current. If you use enough voltage, even the
best of insulations will allow some current to flow. There are several
reasons current will flow through insulation during a hipot test.
Resistance, capacitance, arcs, electrochemical effects, and corona are
all effects that describe current flow. All of these effects add
together during a hipot test to shape the outcome of the test.
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