Application Note

ManualsBrandsFluke ManualsMeasuring instruments289

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.

COM

TEMPERATURE

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.

Summary of content (7 pages)

PAGE 1
ABCs of multimeter safety Multimeter safety and you 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.
PAGE 2
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 capability. Transient protection is vital. When transients ride on highenergy circuits, they tend to be more dangerous because these circuits can deliver large currents.
PAGE 3
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.
PAGE 4
Protection against two major electrical hazards Transients–the hidden danger When the arc forms inside the meter, a very high-pressure shock wave can cause a Let’s take a look at a worst-case loud bang—very much like a scenario in which a technician gunshot or the backfire from is performing measurements a car.
PAGE 5
Arc blast and electric shock V Use the right high-energy fuses Electric shock TEMPERATURE A mA A COM V 189 TRUE RMS MULTIMETER V A A mA A 10A MAX FUSED COM COM 400mA FUSED TEMPERATURE V CAT 1000V Figure 3. Misuse of DMM in Ammeter Mode. 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.
PAGE 6
Work safely Safety is everyone’s responsibility but ultimately 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.
PAGE 7
How to evaluate a multimeter’s safety rating 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.