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Electrical measurement safety

Electrical measurement safety. Understanding hidden hazards and new safety standards. Goals of this education program. Goals Awareness of electrical measurement hazards Understand international safety specifications for DMMs and scopes Understand the four installation overvoltage categories

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Electrical measurement safety

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  1. Electrical measurement safety Understanding hidden hazards and new safety standards

  2. Goals of this education program 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

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

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

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

  6. Handheld test tool safety The electrician suffered severe burn injuries on his hand and arm. Fingerprints burned into probes

  7. Handheld test tool safety Typical work environment

  8. Handheld test tool safety Aftermath of an accident

  9. Handheld test tool safety If it melts metal, what does it do to people?

  10. Common DMM / tester hazards • 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

  11. Common DMM safety hazards Three common errors that are avoidable • 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

  12. Safety inspection Test leads and probes • 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

  13. Safety inspection Checking meter fuses on most meters 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?

  14. Safety Inspection Overload protection on volts inputs 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.

  15. New IEC Safety Standards

  16. 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)

  17. IEC 61010 key concepts Protection against overvoltage transients • 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.

  18. 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

  19. Category locations

  20. Overvoltage category Common sense ways to think of categories • 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.

  21. 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

  22. 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!

  23. 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.

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

  25. 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

  26. 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”

  27. “Designed to IEC 1010-1” But can the product pass testing... 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 Tested @ 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 Creepage clearance Input protection Display Input protection components window components opened breakdown opened under high @ CAT II level voltage Transient tests

  28. 1 Flashover inside meter 2 Fault current in test leads 3 Arcing at the terminals 4 Arc blast

  29. Misuse of DMM in ammeter mode

  30. Fuse protection on amps inputs

  31. 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 CAT IV-600 V CAT III-1000 V

  32. 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

  33. 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 CAT IV-600 V CAT III-1000 V

  34. What about my old meter? • 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. 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 Original Fluke 70 Series NOT RATED New 170 Series CAT IV-600 VCAT III-1000 V Older Fluke 70 Series-III CAT II-600 V UNDER RATED

  35. 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.

  36. 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.

  37. 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)

  38. 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.

  39. 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 Category ratings • Select a scope and probes and clamps for the worst case category Voltage ratings • Working voltage • Transient voltage

  40. + 50Ω Differential Input BNC Output – 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.

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

  42. 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

  43. 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

  44. 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

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

  46. 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. CAT III-600 V

  47. CAT IV • 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

  48. CAT III • 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

  49. CAT II • 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

  50. 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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