Fluke 289 Application Note
2015-09-09
: Fluke Fluke-289-Application-Note-805131 fluke-289-application-note-805131 fluke pdf
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![ABCs of multimeter safety Multimeter safety and youVoltage spikes—an unavoidable hazardAs 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 standardsTo 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.AmA COM VTEMPERATUREATRUE RMS MULTIMETER189400mAFUSED10A MAXFUSEDCAT 1000VApplication NoteFrom the Fluke Digital Library @ www.fluke.com/libraryDon’t overlook safety—your life may depend on itWhere 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.](https://usermanual.wiki/Fluke/Fluke289ApplicationNote805131.216203909-User-Guide-Page-1.png)
![2 Fluke Corporation ABCs of multimeter safetyCAT 0Transient protectionThe real issue for multimeter circuit protection is not just the maximum steady state volt-age range, but a combination of both steady state and transient overvoltage withstand capabil-ity. Transient protection is vital. When transients ride on high-energy circuits, they tend to be more dangerous because these circuits can deliver large currents. If a transient causes an arc-over, the high current can sustain the arc, producing a plasma breakdown or explosion, which occurs when the surrounding air becomes ionized and conduc-tive. The result is an arc blast, a disastrous event which causes more electrical injuries every year than the better known hazard of electric shock. (See “Transients–the hidden danger” on page 4.)Measurement categoriesThe most important single con-cept to understand about the standards is the Measurement category. The standard defines Categories 0 through IV, often abbreviated as CAT 0, CAT II, etc. (See Figure 1.) The division of a power distribution system into categories is based on the fact that a dangerous high-energy transient such as a lightning strike will be attenuated or dampened as it travels through the impedance (ac resistance) of the system. A higher CAT number refers to an electrical environ-ment with higher power available and higher energy transients. Thus a multimeter designed to a CAT III standard is resistant to much higher energy transients than one designed to CAT II standards.Within a category, a higher voltage rating denotes a higher transient withstand rating, e.g., a CAT III-1000 V meter has supe-rior protection compared to a CAT III-600 V rated meter. The real misunderstanding occurs if some-one selects a CAT II-1000 V rated meter thinking that it is superior to a CAT III-600 V meter. (See “When is 600 V more than 1000 V?” on page 7.)Figure 1. Location, location, location.Understanding categories: Location, location, locationTable 1. Measurement categories. IEC 1010 applies to low-voltage (< 1000 V) test equipment.Measurement category In brief ExamplesCAT 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 pumpCAT 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 entranceCAT 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 sourceCAT 0 Electronic • 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](https://usermanual.wiki/Fluke/Fluke289ApplicationNote805131.216203909-User-Guide-Page-2.png)
: Fluke Fluke-289-Application-Note-805131 fluke-289-application-note-805131 fluke pdf
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