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 |
|
Appendix B Measuring small Resistances
|
|
Appendix
B
Measuring small Resistances |
| Choosing Two wire or Four wire (Kelvin) testing. |
A
2-wire
tester
such
as
AVO
or
digital
multimeter
has
one
problem.
When
you
measure
a
resistance
with
a
2-wire
tester
you
also
measure
the
resistance
of
the
test
leads.
This
will
be
in
the
range
0.01
to
1
Ohms
and
is
typically
100
milliohms
for
a
Digital
Multimeter.
If
you
want
to
measure
resistances
of
less
than
10
ohms
you
should
consider
using
a
4-wire
tester.
Resistance
is
usually
measured
by
passing
a
current
through
the
test
piece
and
then
measuring
the
voltage
drop
across
it.
The
current
source
and
the
voltmeter
use
the
same
leads.
This
is
why
a
2-wire
tester
is
inaccurate
for
measuring
low
resistances
(fig
1).
This is why a 2-wire tester is inaccurate for measuring low resistances.
RS
:
-Resistance
of
sample
(e.g.
Cable
or
Harness)
RL
:
-Resistance
of
lead
VM
:
-
Measured
voltage
VM = (RS + RL + RL)I
The
meter
will
read
RS
+
RL
+
RL
instead
of
reading
RS.
The alternative is to use 4 wires. i.e. One pair of wires for the current source and another pair of wires for the voltage measurement (fig 2).
The
current
due
to
the
voltage
reading
Im
will
be
so
small
that
it
is
negligible,
so
VM
=
RS
*
I
.
| Interface considerations. |
It is important that current and voltage wires are connected as close to the sample as possible. If you are testing very small resistances > 50 mOhms, this may mean using specially designed connectors with two separate points making contact with each pin of the sample being tested. This approach can also help when the resistance of connectors becomes significant due to wear.
A
four-wire
test
system
can
appropriate
when
testing
very
large
harnesses
as
the
length
of
wire
from
the
tester
to
the
harness
can
become
significant.
| Thermal EMF. |
When the junction of 2 dissimilar metals is heated (e.g. by the testing current) a voltage can be generated leading to an error in the resistance being measured. It is possible to remove this effect by reversing the polarity of measurement. This phenomenon is unlikely to be encountered during testing of most cables.
Banair
has
been
manufacturing
electronic
test
equipment
for
over
20
years.
4-wire
testing
is
one
of
the
topics
that
we
get
asked
about
again
and
again
so
we
have
produced
this
card
explaining
how
it
works.
Look
out
for
other
cards
explaining
test
methods.