Extech 380560 Data Sheet

Extech Instruments Corporation

• 285 Bear Hill Road • Waltham, MA 02451-1064 USA

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Milliohm Meters

What is a Milliohm?

The units of measure for resistances less than 1 Ohm
are Milli- and Micro-Ohms. One milliohm (1mΩ) is
equal to one thousandth of an ohm (0.001 Ω). One
microohm (1µΩ) is equal to one millionth of an ohm
(0.000001 Ω). For example, 0.5 ohms (one half an ohm)
is equal to 500mΩ.

Why bother measuring resistances so low?

It may seem like too small a resistance to be of any
consequence but many safety standards (UL, CSA,

VDE, etc.) require that such resistances be measured

during the manufacture of consumer products, hand
tools, and many appliances. A good example is the
testing of kitchen appliances.

Many appliances require AC power (115 or 230VAC

signals) and although AC signals are not supposed
to come in contact with the user, accidents and
faults can arise. To overcome these problems,
manufacturers provide ground paths in appliances to
carry high potentials to ground, away from the user
in the event of a product fault. If even the slightest
amount of resistance is present in such paths, injury
can result. Milliohm measurements are also useful in
the development of electronic components such as
capacitors and inductors.

Why are 4-wire connections required when making
milliohm measurements?

Test leads always introduce a small amount of
additional resistance into a measurement. This is
acceptable in non-critical measurements such as
megohm resistances. However, when measuring
milliohms in a critical application, any additional
resistance can render the measurement useless. This
is why 4-wire (Kelvin) connections are imperative.

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