Fluke 179 Fluke Multimeters - ABCs of Multimeter Safety Multimeter Safety and - Page 7

How to evaluate a multimeter's safety rating Understanding voltage withstand ratings IEC 1010 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. Measurement Category Working Voltage (dc or ac-rms to ground) Peak Impulse Transient (20 repetitions) Test Source (Ω = V/A) CAT I 600 V 2500 V 30 Ohm source CAT I 1000 V 4000 V 30 Ohm source CAT II 600 V 4000 V 12 Ohm source CAT II 1000 V 6000 V 12 Ohm source CAT III 600 V 6000 V 2 Ohm source CAT III 1000 V 8000 V 2 Ohm source CAT IV 600 V 8000 V 2 Ohm source When is 600 V more than 1000 V? Table 2: Transient test values for measurement categories. (50 V/150 V/300 V values not included.) Table 2 can help us understand an instrument's true voltage withstand rating: 1. Within a category, a higher "working voltage" (steadystate voltage) is associated with a higher transient, as would be expected. For example, a CAT III-600 V meter is tested with 6000 V transients while a CAT III-1000 V meter is tested with 8000 V transients. So far, so good. 2. What is not as obvious is the difference between the 6000 V transient for CAT III600 V and the 6000 V transient for CAT II-1000 V. They are not the same. This is where the source impedance comes in. Ohm's Law (Amps = Volts/Ohms) tells us that the 2 Ω test source for CAT III has six times the current of the 12 Ω test source for CAT II. The CAT III-600 V meter clearly offers superior transient protection compared to the CAT II1000 V meter, even though its so-called "voltage rating" could be perceived as being lower. It is the combination of the steady-state voltage (called the working voltage), and the category that determines the total voltage withstand rating of the test instrument, including the all-important transient voltage withstand rating. A note on CAT IV: Test values and design standards for Category IV voltage testing are addressed in IEC 1010 second edition. Creepage and clearance In addition to being tested to an actual overvoltage transient value, multimeters are required by IEC 1010 to have minimum "creepage" and "clearance" distances between internal components and circuit nodes. Creepage measures distance across a surface. Clearance measures distances through the air. The higher the category and working voltage level, the greater the internal spacing requirements. One of the main differences between the old IEC 348 and IEC 1010 is the increased spacing requirements in the latter. The bottom line If you are faced with the task of replacing your multimeter, do one simple task before you start shopping: analyze the worstcase scenario of your job and determine what category your use or application fits into. First choose a meter rated for the highest category you could be working in. Then, look for a multimeter with a voltage rating for that category matching your needs. While you're at it, don't forget the test leads. IEC 1010 applies to test leads too: they should be certified to a category and voltage as high or higher than the meter. When it comes to your personal protection, don't let test leads be the weak link. Look for category and voltage ratings of test leads and multimeters. Fluke. Keeping your world up and running.™ Fluke Corporation PO Box 9090, Everett, WA USA 98206 Fluke Europe B.V. PO Box 1186, 5602 BD Eindhoven, The Netherlands For more information call: In the U.S.A. (800) 443-5853 or Fax (425) 446-5116 In Europe/M-East/Africa +31 (0) 40 2675 200 or Fax +31 (0) 40 2675 222 In Canada (800)-36-FLUKE or Fax (905) 890-6866 From other countries +1 (425) 446-5500 or Fax +1 (425) 446-5116 Web access: http://www.fluke.com ©2006 Fluke Corporation. All rights reserved. Printed in U.S.A. 5/2006 1263690 A-EN-N Rev H