electrical_measurement.ppt

©2003 Fluke Corporation Electrical measurement safety 1
Electrical measurement safety
Understanding hidden
hazards and new safety
standards

Goals
• Awareness of electrical measurement hazards
• Understand international safety specifications for DMMs and
scopes
• Understand the four installation overvoltage categories
• Learn how to minimize and avoid electrical measurement hazards
Outline
• What electrical power can do to a DMM
• Common safety hazards
• Meter safety inspection
• IEC Safety Standards
• Arc blast
• Meter and scope safety check list
Goals of this education program

Handheld test tool safety
Last known earthly
residence of
automotive fuse
used to replace
original fuse
Test leads survived intact
How not to save time...

Test leads destroyed
13.8 kV
arced
over to
test
probes.
Insides were barbecued.
This DMM had a hot date with 13.8 kV.

Probe tips
burned off
250V fuse
didn’t
open in
time
The wrong meter to use on a power circuit.
Poor quality leads and
probes led to injury.

The electrician suffered severe burn
injuries on his hand and arm.
Fingerprints burned
into probes

Typical work environment

Aftermath of an accident

If it melts metal, what does it do to people?

• Arc from transients (lightning, load switching)
Protection: Independent certification
to meet CAT III-1000 V or
CAT IV 600 V
• Voltage contact while in continuity or resistance
Protection: Overload protection in OHMs up to the meter’s volt rating
• Measuring voltage with test leads in current jacks
Protection: High energy fuses rated to the meter’s voltage rating
Use meters / testers without current jacks
• Shock from accidental contact with live components
Protection: Test Leads double insulated, recessed / shrouded,
finger guards, CAT III – 1000 V. Replace when damaged
• Using meter or tester above rated voltage
Protection: Good karma
Common DMM / tester hazards

Three common errors that are avoidable
Common DMM safety hazards
• Measuring voltage while test
leads are in the current jacks:
short-circuit!
Protection: Fluke meters use
high energy fuses.
• Contact with ac or dc power
source while in Ohms mode.
Protection: Use a meter with “Overload Protection”.
Functions are self-protected to the meter’s
rated voltage.
• Using meter above rated voltage, i.e., on medium
voltage circuits.
Protection: Good karma

Check test lead resistance:
Step 1: Insert leads in V/  and COM inputs.
Step 2: Select  , touch probe tips. Good leads are 0.1 - 0.3 .
How do you check a single test lead?
Visually check for:
• CAT III-1000 V/CAT IV-600 V rating
• Double insulation
• Shrouded connectors, finger guards
• Insulation not melted, cut, cracked, etc.
• Connectors not damaged: no insulation pulled away from end
connectors
• Probe tips: not loose or broken off
Safety inspection
Test leads and probes

Safety inspection
Step 1: Plug test lead in V/  input. Select  .
Step 2: Insert probe tip into mA input. Read value.
Step 3: Insert probe tip into A input. Read value.
Is the fuse okay? What would an open fuse read?
Checking meter fuses on most meters

With leads in V/  and COM inputs:
Step 1: Select V and put probes
in a live outlet.
Will you damage the meter if you...
Step 2: Select mV
Step 3: Select 
Step 4: Select A.
Overload protection is only to DMM’s rated voltage.
Overload protection on volts inputs
Safety Inspection

New IEC Safety Standards

International Electrotechnical Commission
• IEC 61010 is the new standard for low voltage
“test, measurement and control equipment”.
• IEC 61010 provides much improved protection
against “overvoltage impulse transients” -
voltage spikes.
• IEC 61010 is the basis for:
• ANSI/ISA-S82.01-94 (US)
• CAN C22.2 No. 1010.1-92 (CAN)
• EN61010-1:1993 (EUR)

IEC 61010 key concepts
• CATEGORIES: CAT I to CAT IV
• The greatest danger from transients is in
the high categories, because they could
trigger an arc blast.
• IMPULSE TESTING: No failure allowed
• Meters must be tested by being hit with a
specified number of transients, with
specified peak voltages.
• INTERNAL SPACING: increased
• Clearance (distance through the air) and
• Creepage (surface distance) are increased.
Protection against overvoltage transients

Overvoltage category
• The level and energy of voltage impulses is
dependent on the location. The closer the location
is to the power source, the higher the available fault
current, the higher the category
• IEC 61010 defines four locations or categories:
CAT IV “Origin of installation”
Utility level and any outside cable run
CAT III Distribution wiring, including “mains” bus, feeders
and branch circuits; permanently installed loads.
CAT II Receptacle outlet circuit; plug-in loads.
CAT I Protected electronic circuits

Category locations

Common sense ways to think of categories
Overvoltage category
• The higher the short circuit fault current available,
the higher the category
• High energy transients are much more dangerous,
because they can trigger an arc blast
• The greater the source impedance, the lower the
category
• Transients are dampened by system impedance as
they travel from the point where they were generated.
• TVSS (transient voltage surge suppression)
devices are sized larger (more joules) at the
panel than at the receptacle outlet.

Determining the true voltage
withstand rating
WITHIN each Category:
• There are designated “working voltages”
(50, 150, 300, 600, 1000 V).
• A higher voltage has a higher transient withstand
• CAT IV example:
CAT IV – 600 V: 8 kV impulse
• CAT III example:
CAT III – 600 V: 6 kV impulse
CAT III – 1000 V: 8 kV impulse
• CAT II example:
CAT II – 600 V: 4 kV impulse
CAT II – 1000 V: 6 kV impulse

When is 600 V more than 1000 V?
• CAT III-600 V or CAT II-1000 V?
• The greater the source impedance,the lower
the Category:
• CAT IV-600 V:
• 8 kV impulse
• CAT III-600 V:
• 6 kV impulse
• 2 ohm test source
• CAT II-1000 V:
• 6 kV impulse
• 12 ohm test source
• A CAT III-600 V 6k V test impulse has 6 times the
current of a CAT II-1000 V 6 kV test impulse!

First the CAT, then the voltage
• Voltage rating by itself can be misleading.
• CAT III-1000 V (8 kV transient) is safer than CAT III-600 V
(6k V transient)
• But CAT III-600 V is safer than CAT II-1000 V
• First know the category you are working in, then choose
the appropriate voltage rating.
• If you ever measure power circuits, you should use
a CAT III-600 V or CAT IV 600 V/CAT III-1000 V meter.
• And CAT IV 600 V/CAT III-1000 V test leads and probes.

CAT III-
600 V
CAT III-
1000 V
CAT IV-600 V
CAT III-1000 V
Look for CAT III or CAT IV markings

Levels of CAT III protection
CAT Transient with Fuse and Clearance Creepage
2  Source overload (air) (surface)
Rating
III-1000 V 8000 V 1000 V 16.0 mm 16.0 mm
IV-600 V
III-600 V 6000 V 1000 V 11.5 mm 14.0 mm
II-1000 V
II-600 V 6000 V 600 V 11.5 mm 11.5 mm

IEC sets standards but does not test or inspect
for compliance.
A manufacturer can claim to “design to” a
standard with no independent verification.
To be UL-Listed, CSA or TUV-Certified , a
manufacturer must employ the listing agency to
TEST the product’s compliance with the standard.
Look for the listing agency’s emblem on the meter.
“Listed” vs. “designed to”

Brand A Brand B Brand C
Markings CAT II – 750 V CAT III D of C to
1000 V Input IEC 1010-1
CAT II – 1000 V Cat III – 1000 V CAT III – 1000 V
Creepage
clearance
3.7 mm 2.5 mm 7.5 mm
Doesn’t Doesn’t Doesn’t
comply comply comply
with 5.7 mm with 16 mm with 16 mm
Transient
tests
Input protection Display Input protection
components window components
opened breakdown opened
under high @ CAT II level
voltage
Tested @
But can the product pass testing...
“Designed to IEC 1010-1”

1 Flashover inside meter
3 Arcing at the terminals 4 Arc blast
2 Fault current in test leads

Misuse of DMM in ammeter mode

Fuse protection on amps inputs

CAT IV-600 V
CAT III-1000 V
What’s the bottom line?
• If you work on power circuits, you
need a CAT III-600 V or CAT IV-600 V/
CAT III 1000 V meter.
• Look for the CAT rating and voltage
rating marked near the input jacks.
• CAT or voltage rating alone can be
misleading
• Look for independent certification.
UL 3111

• If you use a scope on power
circuits, you need a CAT III-600 V
scope and scope probes.
• Look for the CAT rating and voltage
rating marked near the input jacks.
CAT III-600 V

CAT IV-600 V
CAT III-1000 V
Safety must be built-in
• An industrial grade meter devotes
10 % - 15 % of components exclusively
to protection.
• Built-in protection against the most
common safety hazards:
• High voltage transients and danger
of arc-over
• Voltage contact while in continuity
or resistance mode
• High integrity components
• Voltage measurement while test leads are
plugged into amps jacks
• High energy fuses
Overload protection
on all functions
1000V high
energy fuses

Unless a meter was specifically designed to
meet CAT III-600 V or higher, it is not safe to use
on power circuits. Most meters produced before
1997 do not meet the standard.
Older Fluke
70 Series-III
CAT II-600 V
UNDER RATED
New 170 Series
CAT IV-600 V
CAT III-1000 V
Original
Fluke 70 Series
NOT RATED
Newer meters also have
additional features and
capabilities
Larger displays
Back light
1000 Vac capability
Capacitance
Frequency
Magnetic hangers
Temperature
3X dc accuracy
2X ac accuracy
Min / Max Record
Probe holders
Battery door
What about my old meter?

Meter safety checklist
Insist on these safety features:
• Fused current inputs
(high energy fuses).
• Overload protection on the ohms function.
• Test leads that have shrouded connectors and
finger guards.
• Recessed input jacks.
• Meet the latest safety standards
(CAT III-600 V or CAT IV 600 V/CAT III 1000 V)
and are independently certified.

Meter safety checklist
Watch for:
• Cracked or oily case
• Broken input jacks
No meter is safe when improperly used.
• Use meters within their rating.
• Use meters designed for measurements on
power circuits.
• Use replacement fuses approved by the
manufacturer.

Test lead safety checklist
Don’t let test leads be a weak point
• CAT III-1000 V or CAT IV 600 V/
CAT III 1000 V rating
• Double insulation
• Shrouded connectors
• Arc Flash Hazard consideration using
specialized probes and PPE materials
• Finger guards
• Insulation not damaged: not melted, cut,
cracked, stretched
• Connectors: no insulation pulled away
from end connectors
• Probe tips: not loose or broken off (too short)

Safety first
Safe practices include but are not limited to:
• Whenever possible, work on de-energized circuits.
Follow proper lock-out/tag-out procedures.
• Use well maintained tools and appropriate safety gear
• Safety glasses, insulated tools, insulating gloves,
flash suits, insulating mats, etc.
• Don’t work alone.
• Practice safe measurement techniques.
• Always connect the grounded lead first, hot second.
• Disconnect the hot lead first, grounded lead second.
• Use the three-point test method.
• Test known circuit, measure target circuit,
then re-test known circuit.

Oscilloscope safety
Category ratings
• Select a scope and probes
and clamps for the worst
case category
Voltage ratings
• Working voltage
• Transient voltage
Overvoltage Working voltage Peak impulse Test source
category (dc or ac – rms to grnd) transient (Ohm = V/A)
(20 repetitions)
CAT I 600V 2500 V 30 ohm source
CAT I 1000V 4000 V 30 ohm source
CAT II 600V 4000 V 12 ohm source
CAT II 1000V 6000 V 12 ohm source
CAT III 600V 6000 V 2 ohm source
CAT III 1000V 8000 V 2 ohm source

Oscilloscope safety
Line powered bench scopes
• Use a differential or isolation probe
to separate the earth ground
connection of the scope from high
energy circuits that are also
referenced to earth ground.
• Isolate bench scope grounds only
in conjunction with
differential/isolation probes.
Differential
Input
+
–
50Ω BNC
Output

Typical use of a differential probe
Oscilloscope safety
Typical use of a differential probe
DC VOLTAGE
DC VOLTAGE
TO MOTOR

Oscilloscope safety
Battery powered scopes -
• Have inherent ground isolation for superior
common mode noise rejection
• Some come standard with CAT III 600 volt probes
for measurements in high energy circuits

Oscilloscope safety
Probes
• Beware of bench scope probes - they are usually
CAT I - 500 volts
• Don’t use CAT I or II divider probes on CAT III circuits
• Don’t use probes with exposed metal parts
• Don’t use probes without specified ratings
• Read the manual for safe probe connections - they
may vary greatly between instruments

Oscilloscope safety
Safe practices
• De-energize circuits
• Use protective gear
• Do not exceed instrument voltage and category ratings
• Use dc coupling - ac coupling may not reveal
dangerous voltages
• Use 3-point test method
• Test known live circuit
• Test target circuit
• Test known live circuit again
• Avoid holding or touching the scope if possible

CAT III circuits - Make sure
you use Cat III leads AND
scope AND current clamp
Oscilloscope safety
Current clamps
• Clamps have category and working voltage ratings
• Do not exceed them

CAT III-600 V
• If you use a scope on power circuits, you need a
CAT III-600 V scope and scope probes.
• CAT II scopes and probes are mainly for loads
that plug into a receptacle outlet.

Equipment of overvoltage category IV
is for use at the origin of the installation
(utility service).
• Outside and service entrance
• Service drop from pole to building
• Run between meter and panel
• Overhead line to detached building
• Underground line to well pump22
CAT IV

• Premises wiring: “mains”circuits, i.e.,
bus and feeders and distribution panels
• Permanently installed loads: motors,
lighting systems, drives, load centers
• Typically separated from utility service
by at least a single level of transformer
isolation
• Does not include receptacle plug-in
loads, except in the case of heavy
appliance outlets with “short”
connections to service entrance
CAT III

Loads that plug in at receptacle outlet
• Examples of such equipment are
appliances, portable tools and other
household and similar loads
• All outlets at more than 10 m (30 ft)
from Category III
• All outlets at more than 20 m (60 ft)
from Category IV
CAT II

Equipment in which measures are
taken to limit transient overvoltages
to an appropriately low level
Examples are protected electronic
circuits. A copier that has an internal
step-up transformer and 1000 Vdc is still
a CAT I-1000 V machine, because the
current levels are so low
CAT I

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