Fluke 360 AC, 789, 87V, 113, 289 Service Guide
...
ABCs of
multimeter safety
Multimeter safety and you
Application Note
Don’t overlook safety—your
life may depend on it
Where safety is a concern, choosing a multimeter is like choosing a motorcycle helmet—if
you have a “ten dollar” head, choose a “ten
dollar” helmet. If you value your head, get a
safe helmet. The hazards of motorcycle riding
are obvious, but what’s the issue with multimeters? As long as you choose a multimeter
with a high enough voltage rating, aren’t you
safe? Voltage is voltage, isn’t it?
Not exactly. Engineers who analyze multimeter safety often discover that failed units
were subjected to a much higher voltage
than the user thought he was measuring.
There are the occasional accidents when the
meter, rated for low voltage (1000 V or less),
was used to measure medium voltage, such
as 4160 V. Just as common, the knock-out
blow had nothing to do with misuse—it was
a momentary high-voltage spike or transient
that hit the multimeter input without warning.
TRUE RMS MULTIMETER
189
TEMPERATURE
mA
A
10A MAX
FUSED
V
COM
A
CAT
400mA
1000V
FUSED
Voltage spikes—an
unavoidable hazard
As distribution systems and
loads become more complex,
the possibilities of transient
overvoltages increase. Motors,
capacitors and power conversion equipment, such as
variable speed drives, can be
prime generators of spikes.
Lightning strikes on outdoor
transmission lines also cause
extremely hazardous highenergy transients. If you’re
taking measurements on electrical systems, these transients
are “invisible” and largely
unavoidable hazards. They
occur regularly on low-voltage
power circuits, and can reach
peak values in the many thousands of volts. In these cases,
you’re dependent for protection
on the safety margin already
built into your meter. The
voltage rating alone will not
tell you how well that meter
was designed to survive high
transient impulses.
Early clues about the safety
hazard posed by spikes came
from applications involving
measurements on the supply
bus of electric commuter railroads. The nominal bus voltage
was only 600 V, but multimeters rated at 1000 V lasted only
a few minutes when taking
measurements while the train
was operating. A close look
revealed that the train stopping and starting generated
10,000 V spikes. These transients had no mercy on early
multimeter input circuits. The
lessons learned through this
investigation led to significant
improvements in multimeter
input protection circuits.
Test tool safety standards
To protect you against
transients, safety must be built
into the test equipment. What
performance specification
should you look for, especially
if you know that you could
be working on high-energy
circuits? The task of defining safety standards for test
equipment is addressed by the
International Electrotechnical
Commission (IEC). This organization develops international
safety standards for electrical
test equipment.
Meters have been used
for years by technicians and
electricians yet the fact is that
meters designed to the IEC 1010
standard offer a significantly
higher level of safety. Let’s see
how this is accomplished.
From the Fluke Digital Library @ www.fluke.com/library
Transient protection
The real issue for multimeter
circuit protection is not just the
maximum steady state voltage range, but a combination of
both steady state and transient
overvoltage withstand capabilit y. Transient protection is vital.
When transients ride on highenergy 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 conductive. 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 concept 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 environment 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 superior protection compared to a CAT
III-600 V rated meter. The real
misunderstanding occurs if someone 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.)
Understanding categories:
Location, location, location
CAT 0
Figure 1. Location, location, location.
Measurement
category In brief Examples
•
CAT IV Three-phase at
utility connection,
any outdoor
mains conductors
CAT III Three-phase
distribution,
including singlephase commercial
lighting
CAT II Single-phase
receptacle
connected loads
CAT 0 Electronic
Table 1. Measurement categories. IEC 1010 applies to low-voltage (< 1000 V) test equipment.
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
•
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
•
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
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
2 Fluke Corporation ABCs of multimeter safety
It’s not just the
voltage level
In Figure 1, a technician working
on office equipment in a CAT 0
location could actually encounter
dc voltages much higher than the
power line ac voltages measured
by the motor electrician in the CAT
III location. Yet transients in CAT 0
electronic circuitry, whatever the
voltage, are clearly a lesser threat,
because the energy available to
an arc is quite limited. This does
not mean that there is no electrical hazard present in CAT 0 or CAT
II equipment. The primary hazard
is electric shock, not transients
and arc blast. Shocks, which will
be discussed later, can be every
bit as lethal as arc blast.
To cite another example, an
overhead line run from a house
to a detached workshed might be
only 120 V or 240 V, but it’s still
technically CAT IV. Why? Any
outdoor conductor is subject to
very high energy lightning-related
transients. Even conductors buried
underground are CAT IV, because
although they will not be directly
struck by lightning, a lightning
strike nearby can induce a transient because of the presence of
high electro-magnetic fields.
When it comes to Overvoltage
Installation Categories, the rules
of real estate apply: it’s location,
location, location...
(For more discussion of Installation
Categories, see page 6, “Applying cat-
egories to your work.”)
Independent testing
Independent testing is the
key to safety compliance
Look for a symbol and listing
number of an independent testing lab such as UL, CSA, TÜV or
other recognized testing organization. Beware of wording such
as “Designed to meet specification ...” Designer’s plans are
never a substitute for an actual
independent test.
How can you tell if you’re
getting a genuine CAT III or CAT
II meter? Unfortunately it’s not
always that easy. It is possible
for a manufacturer to selfcertify that its meter is CAT II or
CAT III without any independent
verification. The IEC develops
and proposes standards, but it
is not responsible for enforcing
the standards.
Look for the symbol and
listing number of an independent testing lab such as UL,
CSA, TÜV or other recognized
approval agency. That symbol
can only be used if the product
successfully completed testing
to the agency’s standard, which
is based on national/international standards. UL 61010-1,
for example, is based on IEC
61010-1. In an imperfect world,
that is the closest you can come
to ensuring that the multimeter
you choose was actually tested
for safety.
What does the
symbol indicate?
A product is marked CE
(Conformité Européenne)
to indicate its conformance to
certain essential requirements
concerning health, safety,
environment and consumer
protection established by
the European Commission
and mandated through the
use of “directives.” There are
directives affecting many
product types, and products
from outside the European
Union can not be imported and
sold there if they do not comply
with applicable directives.
Compliance with the directive
can be achieved by proving
conformance to a relevant
technical standard, such as
IEC 61010-1 for low-voltage
products. Manufacturers are
permitted to self-certify that
they have met the standards,
issue their own Declaration
of Conformity, and mark the
product “CE.” The CE mark is
not, therefore, a guarantee of
independent testing.
Tool Tip
Non-contact voltage detectors are a
quick, inexpensive way to check for
the presence of live voltage on ac
circuits, switches and outlets before
working on them.
1. Verify the voltage detector function is working properly.
2. Make sure the detector is rated for the level of voltage being
measured and is sensitive enough for your application.
3. Make sure that you also wear the appropriate PPE based on
the environment you're in.
3. Make sure you’re grounded (through your hand, to the floor),
to complete the capacitive voltage connection.
4. Make sure the hazardous voltage is not shielded.
Use only a digital multimeter or contact type voltage tester to
test for the absence of voltage.
This meter has a built-in non-contact voltage tester.
ABCs of multimeter safety Fluke Corporation 3
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