Cable,
Harness and Wiring Testing Information
How and why should I test a cable? |
How much time and money could you save every year? |
Measuring small Resistances |
Testing other Components |
How
and
why
should
I
test
a
cable?
|
What can go wrong? |
1. You have built the wrong cable.
2. Some connections that should be made are not.
3. Some connections that should not be made are.
4. Some connections that should be made almost are.
5. Some connections that should not be made almost are.
There
are
many
tests
that
can
be
applied
to
a
cable.
Deciding
what
are
the
most
appropriate
tests
is
not
always
obvious.
This
article
provides
an
overview
of
the
tests
that
can
be
applied.
It
explains
how
they
work
and
the
most
appropriate
application.
Level 1: Full visual and mechanical examination. |
This test requires no equipment other than a good pair of eyes, although a magnifier will help to spot the less obvious faults. Key points to include are:
·
Check
that
correct
parts
are
being
used
(not
just
pattern
but
specified
quality
level).
·
Cable
length.
·
Wire
colour.
·
Wire
gauge.
·
IDC
cables
are
correctly
registered.
·
Crimps
are
correctly
positioned
(ensure
that
insulation
isn't
trapped).
·
Contact
fully
into
housing.
·
Pull
test
on
Crimp.
(Example
can
be
found
supplied
by
Daniels
Manufacturing
Corporation)
·
Contacts
are
free
of
contamination
e.g.
Flux.
·
Loose
or
broken
strands.
It
is
important
that
the
crimp
force
of
tools
is
monitored
as
incorrect
pressure
can
lead
to
loose
crimps
(force
too
low)
or
broken
wire
strands
(force
too
high),
Level 2 Electrical check for continuity/Shorts. |
This test is normally carried out at a low voltage and current, typically 5Volts at a few milliamps. The simplest tester is a battery and a buzzer. This is sometimes called ringing out a cable.
For each connected pin in the cable carry out the following
1.
Ensure
that
it
is
connected
to
all
the
points
shown
on
the
drawing.
2.
Check
against
all
the
other
pins
to
ensure
that
there
are
no
shorts.
This is repeated for every pin.
This
test
will
find
a
high
proportion
of
faults,
but
there
are
some
disadvantages
in
performing
it
manually
·
You
may
miss
a
connection,
possibly
due
to
a
lapse
in
attention,
an
interruption
or
misreading
the
drawing.
·
If
there
is
an
intermittent
fault
the
process
of
holding
on
the
probes
may
make
the
connection.
·
Shorts
can
be
missed
as
the
check
is
ignored
for
some
(or
all)
of
the
pins
on
the
assumption
that
a
short
is
not
possible
in
a
particular
configuration.
·
The
process
is
very
slow.
·
This
process
is
very
expensive.
There are automatic test systems that allow you to carry out the above test in a few seconds. The tests are exhaustive rather than exhausting.
·
All
connections
are
checked.
·
The
cable
is
checked
against
a
golden
sample
or
a
previously
stored
master.
·
The
test
can
be
repeated
to
check
for
intermittent
faults.
·
All
possible
shorts
are
checked
for.
·
The
test
takes
little
more
than
the
time
to
connect
up
the
cable.
·
The
equipment
will
typically
recoup
the
cost
in
less
than
a
year.
See
Appendix
A
For small standalone test areas a tester like the B801 "Sharon" cable tester provides a cost-effective solution. A more comprehensive connection tester is the B857 "Tracy" cable tester . Both Testers can use the same interface jigs and connectors so it is easy to mix the two, or to start with the Sharon and invest in the Tracy at a later date.
B801 "Sharon" cable tester | B857 "Tracy" cable tester |
Level 3 Electrical check for resistance. |
Levels 1 and 2 will satisfy the majority of customers, but there are occasions when further testing is required. This is normally when failure of the cable is very costly or may cause a safety hazard. Measuring the resistance of the cable would be the preferred next step. The resistance of a cable may vary for a number of reasons.
·
Wire
outside
spec.
·
Poor
soldered
joint.
·
Faulty
crimp.
·
Contamination
on
contact.
·
Contact
outside
specification.
The simple way to measure the resistance of a cable is to use an ohmmeter. This may not be the best solution as the resistance of a cable is typically in the region of an ohm or less. This is similar to the uncertainty of the ohmmeter. This uncertainly is caused by variation in the lead resistance and the resistance of the probes. The better solution is to use a Kevin resistance meter or 4-wire tester. See Appendix B for a technical description of how 4 wire testing works. An example is the BA765 "Henry".
You should bear in mind that the resistance of the cable will vary depending on its specification and manufacturer. The resistance will also vary with temperature. A tolerance should be allowed to account for this. Measuring at a constant temperature will make the job easier.
This measurement will add considerably to the confidence in the cable, however manual measurement does take a considerable amount of time. An automatic system such as the BA765 "Henry" cable tester / harness tester will carry out the test in a number of seconds. It is worth considering this system, as there are many advantages over manual measurement.
·
All
connections
are
checked.
·
The
cable
is
checked
against
a
known
spec.
·
The
test
can
be
repeated
to
check
for
intermittent
variation.
·
The
resistance
can
be
stored
in
an
Access
database
for
SPC
·
All
testing
is
documented
automatically
and
printed
evidence
or
bar-codes
can
be
used
·
There
is
no
chance
of
the
wrong
values
being
used
as
the
part
is
tested
against
a
part
number
defined
specification.
·
The
equipment
will
typically
recoup
the
cost
in
less
than
a
year.
See
Appendix
A
Level 4 Electrical test for insulation (Insulation resistance). |
Damage to the insulation or contamination may not be detected by testing at levels 1 to 3. If the insulation system fails it can be very costly. An IR Test measures the electrical insulation resistance of the system. This is sometimes also called a Mega test. Failure of this test may be due to one or more of the following.
·
Damaged
insulation
·
Contamination
by
flux
or
finger
residues
·
Excessive
humidity
This measurement is carried out at a high voltage in order to measure the resistance accurately, however the voltage is not intended to stress the cable. The test would typically be carried out at a voltage in the range 500 to 1000 V although if sensitive components such as capacitors have been fitted for EMC purposes then the test may have to be carried out at a much lower voltage. The tests are usually carried out at DC so that only the resistive current is measured. An AC test would include the capacitance of the cable in the measurement.
The test voltage should be allowed to settle for a short while so that any current due to the cable capacitance being charged can be nulled out. An adjustable dwell time ensures the system has stabilised. Attempting to ignore this will produce unreliable readings.
This measurement will add considerably to the confidence in the cable, however manual measurement does take a considerable amount of time. An automatic system such as the BA765 "Henry" cable tester / harness tester will carry out the test faster and will enable the operator to deal with other tasks during the testing period.
It is worth considering this system, as there are many advantages over manual measurement.
·
All
connections
are
checked.
·
The
cable
is
checked
against
a
known
spec.
·
All
testing
is
documented
automatically
and
printed
evidence
or
bar-codes
can
be
used
·
There
is
no
chance
of
the
wrong
values
being
used
as
the
part
is
tested
against
a
part
number
defined
specification.
·
The
equipment
will
typically
recoup
the
cost
in
less
than
a
year.
See
Appendix
A
·
Automatic
testing
allows
the
operator
to
be
protected
from
high
voltages
using
interlocks.
This
is
an
important
area
of
operator
safety.
Level 5 Electrical test for insulation (Dielectric Strength Test). |
This test is carried out at a high voltage but it differs from the Insulation Resistance test in the following ways.
1. The purpose of this test is to stress the insulation so that the insulation breaks down, it is typically carried out at twice the operating voltage plus 1000 volts.
2. The current measured in the "Insulation Resistance Test" is the average current after the system has stabilised. In a "Dielectric Strength Test" the peak current is measured so that if flashover occurs then this will be recorded as a failure.
3. The test is typically carried out at AC, however if permitted by the specifying authorities the test can be carried out at DC using 1.4 times the voltage.
4.
This
test
can
detect
points
that
are
nearly
touching
such
as
wire
ends,
if
flashover
occurs
this
would
be
logged
as
a
fault.
It
should
be
remembered
that
at
the
typical
test
voltage
of
1500V
DC
flashover
will
only
occur
if
the
points
have
a
separation
of
less
than
0.2mm