©2003 fluke corporation electrical measurement safety 1 electrical measurement safety understanding...

©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 destroyed13.8 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 ratingUse 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 inspectionTest 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 ratingWITHIN 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 markingsLook 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 mmIV-600 V

III-600 V 6000 V 1000 V 11.5 mm 14.0 mmII-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 1010-1CAT II – 1000 V Cat III – 1000 V CAT III – 1000 V

Creepageclearance

3.7 mm 2.5 mm 7.5 mmDoesn’t Doesn’t Doesn’tcomply comply comply

with 5.7 mm with 16 mm with 16 mm

Transienttests

Input protection Display Input protectioncomponents window components

opened breakdown openedunder 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


What’s the bottom line?• 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 VCAT III-1000 V

What’s the bottom line?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 dangerof arc-over

• Voltage contact while in continuityor 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-IIICAT II-600 V

UNDER RATED

New 170 SeriesCAT IV-600 VCAT III-1000 V

Original Fluke 70 SeriesNOT 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 checklistInsist 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 checklistWatch 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 checklistDon’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 safetyCategory ratings

• Select a scope and probes and clamps for the worst case category

Voltage ratings

• Working voltage

• Transient voltageOvervoltage Working voltage Peak impulse Test sourcecategory (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 safetyLine 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.

DifferentialInput

+

–

50Ω BNCOutput


Typical use of a differential probe

Oscilloscope safety

Typical use of a differential probe

DC VOLTAGE

DC VOLTAGE

TO MOTOR


Oscilloscope safetyBattery 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

What’s the bottom line?• 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

©2003 fluke corporation electrical measurement safety 1 electrical measurement safety understanding...

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