Fluke 289 Application Note

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ABCs of

multimeter safety

Multimeter safety and you

Voltage spikes—an

unavoidable hazard

As distribution systems and

loads become more complex,

the possibilities of transient

overvoltages increase. Motors,

capacitors and power con-

version equipment, such as

variable speed drives, can be

prime generators of spikes.

Lightning strikes on outdoor

transmission lines also cause

extremely hazardous high-

energy transients. If you’re

taking measurements on elec-

trical 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 thou-

sands 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 rail-

roads. The nominal bus voltage

was only 600 V, but multime-

ters rated at 1000 V lasted only

a few minutes when taking

measurements while the train

was operating. A close look

revealed that the train stop-

ping and starting generated

10,000 V spikes. These tran-

sients 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 defin-

ing safety standards for test

equipment is addressed by the

International Electrotechnical

Commission (IEC). This organi-

zation 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.

AmA COM V

TEMPERATURE

A

TRUE RMS MULTIMETER

189

400mA

FUSED

10A MAX

FUSED

CAT

1000V

Application Note

From the Fluke Digital Library @ www.fluke.com/library

Don’t overlook safety—your

life may depend on it

Where safety is a concern, choosing a multi-

meter 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 multi-

meters? 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 mul-

timeter 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.

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

ABCs of multimeter safety Fluke Corporation 3

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.

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 electri-

cal 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 tran-

sient 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 is the

key to safety compliance

Look for a symbol and listing

number of an independent test-

ing lab such as UL, CSA, TÜV or

other recognized testing organi-

zation. Beware of wording such

as “Designed to meet specifica-

tion ...” 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 self-

certify 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 indepen-

dent 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/interna-

tional 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.

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.

4 Fluke Corporation ABCs of multimeter safety

Protection against two major electrical hazards

Transients–the hidden

danger

Let’s take a look at a worst-case

scenario in which a technician

is performing measurements

on a live three-phase motor

control circuit, using a meter

without the necessary safety

precautions.

Here’s what could happen:

1. A lightning strike causes a

transient on the power line,

which in turn strikes an arc

between the input terminals

inside the meter. The circuits

and components to prevent

this event have just failed or

were missing. Perhaps it was

not a CAT III rated meter.

The result is a direct short

between the two measure-

ment terminals through the

meter and the test leads.

2. A high-fault current–possibly

several thousands of amps–

flows in the short circuit

just created. This happens

in thousandths of a second.

When the arc forms inside

the meter, a very high-pres-

sure shock wave can cause a

loud bang—very much like a

gunshot or the backfire from

a car. At the same instant,

the tech sees bright blue arc

flashes at the test lead tips–

the fault currents superheat

the probe tips, which start to

burn away, drawing an arc

from the contact point to the

probe.

3. The natural reaction is to

pull back, in order to break

contact with the hot circuit.

But as the tech’s hands are

pulled back, an arc is drawn

from the motor terminal to

each probe. If these two

arcs join to form a single arc,

there is now another direct

phase-to-phase short, this

time directly between the

motor terminals.

4. This arc can have a tempera-

ture approaching 6000 °C

(10000 °F), which is higher

than the temperature of

an oxyacetylene cutting

torch! As the arc grows, fed

by available short circuit

current, it superheats the

surrounding air. Both a

shock blast and a plasma

fireball are created. If the

technician is lucky, the

shock blast pushes him away

and removes him from the

proximity of the arc; though

injured, his life is saved. In

the worst case, the victim

is subjected to fatal burn

injuries from the fierce heat

of the arc or plasma blast.

In addition to using a multi-

meter rated for the appropriate

measurement category, anyone

working on live power circuits

should be protected with flame

resistant clothing, should wear

safety glasses or, better yet, a

safety face shield, and should

use insulated gloves and shoes.

A lightning strike causes a transient

on the power line, creating an arc

between the meter’s input terminal

and resulting in loud noises.

Then, a high current flows in the

closed circuit which is formed.

An arc starts at the probe tips.

If those arcs join, the

resulting high-energy

arc can create a life-

threatening situation

for the user.

When you pull the probes away

as a reaction to the loud noise,

arcs are drawn to the motor

terminals you’re probing.

4

1

1 2

3

Figure 2. A worst-case scenario—potential arc blast sequence.

ABCs of multimeter safety Fluke Corporation 5

Transients aren’t the only source of

possible short circuits and arc blast

hazard. One of the most common

misuses of handheld multimeters

can cause a similar chain of events.

Let’s say a user is making

current measurements on signal

circuits. The procedure is to select

the amps function, insert the leads

in the mA or amps input termi-

nals, open the circuit and take a

series measurement. In a series

circuit, current is always the same.

The input impedance of the amps

circuit must be low enough so that

it doesn’t affect the series circuit’s

current. For instance, the input

impedance on the 10 A terminal

of a Fluke meter is .01 W. Compare

this with the input impedance on

the voltage terminals of 10 MW

(10,000,000 W).

If the test leads are left in the

amps terminals and then acciden-

tally connected across a voltage

source, the low input imped-

ance becomes a short circuit! It

doesn’t matter if the selector dial is

turned to volts; the leads are still

physically connected to a low-

impedance circuit.* That’s why the

amps terminals must be protected

by fuses. Those fuses are the only

thing standing between an incon-

venience–blown fuses–and a

potential disaster.

Use only a multimeter with amps

inputs protected by high-energy

fuses. Never replace a blown fuse

with the wrong fuse. Use only

the high-energy fuses specified

by the manufacturer. These fuses

are rated at a voltage and with a

short circuit interrupting capacity

designed for your safety.

Overload protection

Fuses protect against overcur-

rent. The high input impedance of

the volts/ohms terminals ensures

that an overcurrent condition is

unlikely, so fuses aren’t necessary.

Overvoltage protection, on the other

hand, is required. It is provided

by a protection circuit that clamps

high voltages to an acceptable

level. In addition, a thermal protec-

tion circuit detects an overvoltage

condition, protects the meter until

the condition is removed, and then

automatically returns to normal

operation. The most common ben-

efit is to protect the multi meter from

overloads when it is in ohms mode.

In this way, overload protection

with automatic recovery is provided

for all measurement functions as

long as the leads are in the voltage

input terminals.

AmA COM V

TEMPERATURE

A

V

AmA COM V

TEMPERATURE

A

V

TRUE RMS MULTIMETER

189

400mA

FUSED

10A MAX

FUSED

CAT

1000V

ACOM

Figure 3. Misuse of DMM in Ammeter Mode.

While most people are aware of the

danger from electric shock, few real-

ize how little current and how low a

voltage are required for a fatal shock.

Current flows as low as 30 mA can

be fatal (1 mA=1/1000 A). Let’s look

at the effects of current flow through

a “typical” 68 kilogram (150 pound)

male:

• At about 10 mA, muscular paraly-

sis of the arms occurs, so that he

cannot release his grip.

• At about 30 mA, respiratory paraly-

sis occurs. His breathing stops and

the results are often fatal.

• At about 75 to 250 mA, for exposure

exceeding five seconds, ven-

tricular fibrillation occurs, causing

incoordina tion of the heart muscles;

the heart can no longer function.

Higher currents cause fibrillation at

less than five seconds. The results

are often fatal.

Now let’s calculate the thresh hold for

a “hazardous” voltage. The approxi-

mate body resistance under the skin

from hand to hand across the body

is 1000 W. A voltage of only 30 V

across 1000 W will cause a current

flow of 30 mA. Fortunately, the skin’s

resistance is much higher. It is the

resistance of the skin, especially the

outer layer of dead cells, that protects

the body. Under wet conditions, or if

there is a cut, skin resistance drops

radically. At about 600 V, the resis-

tance of the skin ceases to exist. It is

punctured by the high voltage.

For multimeter manufacturers and

users, the objective is to prevent

accidental contact with live circuits at

all costs.

Look for:

• Meters and test leads with double

insulation.

• Meters with recessed input jacks

and test leads with shrouded input

connectors.

• Test leads with finger guards and a

non-slip surface.

• Meter and test leads made of high-

quality, durable, non-conductive

materials.

Use the right high-energy fuses

Electric shock

* Some multimeters, such as the Fluke 80 Series,

have an Input Alert which gives a warning beep

if the meter is in this configuration.

Arc blast and electric shock

6 Fluke Corporation ABCs of multimeter safety

Multiple categories

There’s one scenario that some-

times confuses people trying to

apply categories to real world

applications. In a single piece of

equipment, there is often more

than one category. For example,

in office equipment, from the

120 V/240 V side of the power

supply back to the receptacle is

CAT II. The electronic circuitry,

on the other hand, is CAT 0. In

building control systems, such

as lighting control panels, or

industrial control equipment

such as programmable con-

trollers, it is common to find

electronic circuits (CAT 0) and

power circuits (CAT III) existing

in close proximity.

What do you do in these

situations? As in all real-world

situations, use common sense.

In this case, that means using

the meter with the higher

category rating. In fact, it’s not

realistic to expect people to be

going through the category-

defining process all the time.

What is realistic, and highly

recommended, is to select a

multi meter rated to the highest

category in which it could pos-

sibly be used. In other words,

err on the side of safety.

Shortcuts to

understanding categories

Here are some quick ways to

apply the concept of categories

to your every day work:

• The general rule-of-thumb

is that the closer you are to

the power source, the higher

the category number, and the

greater the potential danger

from transients.

• It also follows that the greater

the short-circuit current avail-

able at a particular point, the

higher the CAT number.

• Another way of saying the

same thing is the greater

the source impedance, the

lower the CAT number. Source

impedance is simply the total

impedance, including the

impedance of the wiring,

between the point where you

are measuring and the power

source. This impedance is

what dampens transients.

• Finally, if you have any

experience with the applica-

tion of transient voltage surge

suppression (TVSS) devices,

you understand that a TVSS

device installed at a panel

must have higher energy-

handling capacity than one

installed right at the com-

puter. In CAT terminology, the

panelboard TVSS is a CAT III

application, and the computer

is a receptacle-connected

load and therefore, a CAT II

installation.

As you can see, the concept

of categories is not new and

exotic. It is simply an exten-

sion of the same common-sense

concepts that people who work

with electricity professionally

apply every day.

Work safely

Safety is everyone’s responsibility but ulti-

mately it’s in your hands.

No tool by itself can guarantee your safety.

It’s the combination of the right tools and

safe work practices that gives you maximum

protection. Here are a few tips to help you in

your work.

• Work on de-energized circuits whenever

possible. Use proper lock-out/tag-out

procedures. If these procedures are not

in place or not enforced, assume that the

circuit is live.

• On live circuits, use protective gear:

– Use insulated tools.

– Wear safety glasses and arc rated face

shield if required

– Wear insulated gloves; remove watches

or other jewelry.

– Stand on an insulated mat.

– Wear approved clothing, not ordinary

work clothes.

• When making measurements on live

circuits:

– Hook on the ground clip first, then make

contact with the hot lead. Remove the

hot lead first, the ground lead last.

– Hang or rest the meter if possible. Try to

avoid holding it in your hands, to mini-

mize personal exposure to the effects of

transients.

– Use the three-point test method, espe-

cially when checking to see if a circuit

is dead. First, test a known live circuit.

Second, test the target circuit. Third,

test the live circuit again. This verifies

that your meter worked properly before

and after the measurement.

– Use the old electricians’ trick of keeping

one hand in your pocket. This lessens

the chance of a closed circuit across

your chest and through your heart.

Applying categories to your work

Always wear approved personal protective

equipment (PPE), including arc rated clothing,

leather over rubber gloves, safety glasses, and

an arc-rated face shield or hood, both with hard

hat and hearing protection.

7 Fluke Corporation ABCs of multimeter safety

Understanding voltage

withstand ratings

IEC 61010-1 test procedures

take into account three main

criteria: steady-state voltage,

peak impulse transient voltage

and source impedance. These

three criteria together will tell

you a multimeter’s true voltage

withstand value.

When is 600 V more than

1000 V?

Table 2 can help us understand

an instrument’s true voltage

withstand rating:

1. Within a category, a higher

“working voltage” (steady-

state voltage) is associated

with a higher transient,

as would be expected. For

example, a CAT III-600 V

meter is tested with 6000 V

transients while a CAT III-

1000 V meter is tested with

8000 V transients. So far, so

good.

2. What is not as obvious is the

difference between 6000 V

transient for CAT III-600 V

and the 6000 V transient for

CAT II-1000 V. They are not

the same. This is where the

source impedance comes in.

Ohm’s Law (Amps = Volts/

Ohms) tells us that the 2 W

test source for CAT III has six

times the current of the 12 W

test source for CAT II.

The CAT III-600 V meter clearly

offers superior transient pro-

tection compared to the CAT

II-1000 V meter, even though its

so-called “voltage rating” could

be perceived as being lower.

It is the combination of the

steady-state voltage (called the

working voltage), and the cat-

egory that determines the total

voltage withstand rating of the

test instrument, including the

all-important transient voltage

withstand rating.

A note on CAT IV: Test

values and design standards for

How to evaluate a multimeter’s safety rating

Category IV voltage testing are

addressed in IEC IEC 61010-1

standard.

Creepage and clearance

In addition to being tested to

an actual overvoltage tran-

sient value, multimeters are

required by IEC IEC 61010-1 to

have minimum “creepage” and

“clearance” distances between

internal components and circuit

nodes. Creepage measures

distance across a surface.

Clearance measures distances

through the air. The higher the

category and working voltage

level, the greater the internal

spacing requirements.

The bottom line

If you are faced with the task of

replacing your multimeter, do

one simple task before you start

shopping: Analyze the worst-

case scenario of your job and

determine what category your

use or application fits into.

First choose a meter rated

for the highest category you

could be working in. Then,

look for a multimeter with a

voltage rating for that category

matching your needs. While

you’re at it, don’t forget the test

leads. IEC 61010-1 applies to

test leads too: they should be

certified to a category and volt-

age as high or higher than the

meter. When it comes to your

personal protection, don’t let

test leads be the weak link.

Look for category and voltage ratings of test leads and multimeters.

Working Voltage Peak Impulse

Measurement (dc or ac-rms Transient Test Source

Category to ground) (20 repetitions) (W = V/A)

CAT 0 600 V 2500 V 30 Ohm source

CAT 0 1000 V 4000 V 30 Ohm source

CAT II 600 V 4000 V 12 Ohm source

CAT II 1000 V 6000 V 12 Ohm source

CAT III 600 V 6000 V 2 Ohm source

CAT III 1000 V 8000 V 2 Ohm source

CAT IV 600 V 8000 V 2 Ohm source

Table 2: Transient test values for measurement categories. (50 V/150 V/300 V

values not included.)

Fluke Corporation

PO Box 9090, Everett, WA 98206 U.S.A.

Fluke Europe B.V.

PO Box 1186, 5602 BD

Eindhoven, The Netherlands

For more information call:

In the U.S.A. (800) 443-5853 or

Fax (425) 446-5116

In Europe/M-East/Africa +31 (0) 40 2675 200 or

Fax +31 (0) 40 2675 222

In Canada (800)-36-FLUKE or

Fax (905) 890-6866

From other countries +1 (425) 446-5500 or

Fax +1 (425) 446-5116

Web access: http://www.fluke.com

©2006-2014 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 10/2012 1263690J_EN

Modification of this document is not permitted

without written permission from Fluke Corporation.

Fluke. The Most Trusted Tools

in the World.