Fluke 87V Industrial Users Manual

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A common misconception in

choosing a multimeter is that as

long as you choose one with a

high enough voltage rating, you’re

covered. However, the voltage rating

is only part of the story. Engineers

who analyze multimeter safety

often discover that failed units were

subjected to a much higher voltage

than the user thought was being

measured. This can occur when a

meter, rated for low voltage (1000 V

or less), is inadvertently used to

measure medium voltage. Or, it

can result from a momentary high-

voltage spike or transient that hits

the multimeter input without warning.

These transients are “invisible”

and largely unavoidable but they

occur regularly on low-voltage

power circuits, and can reach peak

values in the many thousands of

volts. Your safety depends on the

safety margin built into your meter.

And that safety margin is based on

several specifications beyond just

the voltage rating alone.

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 equipment not

directly connected to mains

• 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

Understanding measurement categories

The International Electrotechnical Commission (IEC)

publishes international standards for all electri-

cal, electronic and related technologies. Multimeters

designed to the IEC ANSI 61010-1 standard offer a

significantly higher level of safety.

CAT 0

IEC measurement categories are based on the fact that a dangerous high-energy transient, such as a

lightning strike, will be dampened as it travels through the impedance (ac resistance) of the system.

The higher the IEC CAT number, the higher the power

available and the higher the voltage transients

The most important thing to understand about the

IEC safety standards is “Measurement Category”. The

IEC 61010-1 standard divides the power distribution

system into four categories:

Multimeter safety

by the numbers

How to evaluate a multimeter’s

safety rating

Within each measurement category, a higher voltage

rating denotes a higher transient-withstand rating.

So a CAT III 1000 V meter offers superior protection

compared to a CAT III 600 V rated meter. However, a

problem can occur if someone selects a CAT II 1000 V

rated meter thinking that it is superior to a CAT III

600 V meter.

To understand the difference you need to under-

stand the 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 tell you a

multimeter’s true voltage-withstand value.

When is 600 V more than 1000 V?

To understand an instrument’s true voltage-withstand

you need to consider that:

• Within a category, a higher “working voltage”

(steady-state voltage) is associated with a higher

transient. 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.

• The 6000 V transient for CAT III 600 V and the

6000 V transient for CAT II 1000 V are not the

same. This is where source impedance comes in.

Ohm’s Law (Amps = Volts/Ohms) tells us that the

2 Ω test source for CAT III has six times the current

of the 12 Ω test source for CAT II. Therefore, the

CAT III 600 V meter clearly offers superior tran-

sient protection compared to the CAT II 1000 V

meter, even though its so-called “voltage rating”

could be perceived as being lower. It is the com-

bination of the steady-state voltage (called the

working voltage), and the measurement category

that determines the total voltage-withstand rating

of the test instrument, including the transient

voltage withstand rating. Refer to Table 2 for more

specifics.

Bottom line

If you are looking to replace your multimeter, a good

place to start is to analyze the worst case scenario of

your job and determine what Measurement Category

that scenario fits into. Then choose a meter rated for

the highest category you could be working in, and

with the highest voltage rating you will need within

that category. And don’t forget the test leads. IEC

61010-1 applies to test leads too so they should be

certified for a category and voltage as high or higher

than the meter.

Measurement

Category

Working Voltage (dc or

ac-rms to ground)

Peak Impulse Transient

(20 repetitions)

Test Source

(Ω = V/A)

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.) ©2014 Fluke Corporation.

Specifications subject to change without notice.

Printed in U.S.A. 4/2014 6002399A_EN

Modification of this document is not permitted

without written permission from Fluke Corporation.

The key to safety compliance

How can you tell if you’re getting a genuine CAT IV, CAT III, or

CAT II meter? It’s not always easy. A manufacturer can self-certify

that its meter is at a certain CAT level without any independent

verification. The IEC develops and proposes standards, but it is

not responsible for enforcing those standards.

So the best way to determine that a meter’s CAT certifica-

tion is genuine is to 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. Beware of wording such as

“Designed to meet specification ...” Designer’s plans are never a

substitute for an actual independent test.

Helpful tips for applying categories to your work

• The general rule-of-thumb is that the closer you are to the

power source, the higher the CAT number, and the greater the

potential danger from transients.

• The greater the short-circuit current available at a particular

point, the higher the CAT number.

• The greater the source impedance, the lower the CAT number.

Source impedance (or total impedance) includes the impedance

of the wiring between the point where you are measuring

and the power source. Source impedance is what dampens

transients.

• A transient voltage surge suppression (TVSS) device installed

at a panel must have higher energy-handling capacity than

one installed right at the computer. In CAT terminology, the

panel board TVSS is a CAT III application, and the com-

puter is a receptacle-connected load and therefore, a CAT II

installation.

• A single piece of equipment may have 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 elec-

tronic circuitry, is CAT 0. In building control systems, such as

lighting control panels, or industrial control equipment such

as programmable controllers, it is common to find electronic

circuits (CAT 0) and power circuits (CAT III) existing in close

proximity. Always select a multimeter rated to the highest

category in which it could possibly be used.

Look for these test lab marks on your

meter. Don’t trust meters that haven’t

been independently tested!