Application Note

2 Fluke Corporation ABCs of multimeter safety
CAT 0
Transient protection
The real issue for multimeter
circuit protection is not just the
maximum steady state volt-
age range, but a combination of
both steady state and transient
overvoltage withstand capabil-
ity. Transient protection is vital.
When transients ride on high-
energy circuits, they tend to be
more dangerous because these
circuits can deliver large currents.
If a transient causes an arc-over,
the high current can sustain
the arc, producing a plasma
breakdown or explosion, which
occurs when the surrounding air
becomes ionized and conduc-
tive. The result is an arc blast, a
disastrous event which causes
more electrical injuries every year
than the better known hazard of
electric shock. (See “Transients–
the hidden danger” on page 4.)
Measurement categories
The most important single con-
cept to understand about the
standards is the Measurement
category. The standard defines
Categories 0 through IV, often
abbreviated as CAT 0, CAT II, etc.
(See Figure 1.) The division of a
power distribution system into
categories is based on the fact
that a dangerous high-energy
transient such as a lightning
strike will be attenuated or
dampened as it travels through
the impedance (ac resistance) of
the system. A higher CAT number
refers to an electrical environ-
ment with higher power available
and higher energy transients.
Thus a multimeter designed to
a CAT III standard is resistant to
much higher energy transients
than
one designed to CAT II
standards.
Within a category, a higher
voltage rating denotes a higher
transient withstand rating, e.g., a
CAT III-1000 V meter has supe-
rior protection compared to a CAT
III-600 V rated meter. The real
misunderstanding occurs if some-
one selects a CAT II-1000 V rated
meter thinking that it is superior
to a CAT III-600 V meter. (See
“When is 600 V
more than 1000 V?” on page 7.)
Figure 1. Location, location, location.
Understanding categories:
Location, location, location
Table 1. Measurement categories. IEC 1010 applies to low-voltage (< 1000 V) test equipment.
Measurement
category In brief Examples
CAT IV Three-phase at
utility connection,
any outdoor
mains conductors
•
Refers to the “origin of installation,” i.e., where low-voltage
connection is made to utility power
• Electricity meters, primary overcurrent protection equipment
• Outside and service entrance, service drop from pole to
building, run between meter and panel
• Overhead line to detached building, underground line to well
pump
CAT III Three-phase
distribution,
including single-
phase commercial
lighting
•
Equipment in fixed installations, such as switchgear and
polyphase motors
• Bus and feeder in industrial plants
• Feeders and short branch circuits, distribution panel devices
• Lighting systems in larger buildings
• Appliance outlets with short connections to service entrance
CAT II Single-phase
receptacle
connected loads
•
Appliance, portable tools, and other household and similar
loads
• Outlet and long branch circuits
– Outlets at more than 10 meters (30 feet) from CAT III source
– Outlets at more that 20 meters (60 feet) from CAT IV source
CAT 0 Electronic
•
Protected electronic equipment
• Equipment connected to (source) circuits in which measures
are taken to limit transient overvoltages to an appropriately
low level
• Any high-voltage, low-energy source derived from a high-
winding resistance transformer, such as the high-voltage
section of a copier