ABCs of multimeter safety - Fluke Corporation

1 ABCs of multimeter safety multimeter safety and you Application Note Voltage spikes an 10,000 V spikes. These tran- unavoidable hazard sients had no mercy on early Don't overlook safety your multimeter input circuits. The life may depend on it As distribution systems and lessons learned through this loads become more complex, investigation led to significant Where safety is a concern, choosing a multi- the possibilities of transient meter is like choosing a motorcycle helmet if improvements in multimeter overvoltages increase. Motors, input protection circuits. you have a ten dollar head, choose a ten capacitors and power con- dollar helmet. If you value your head, get a version equipment, such as safe helmet.

2 The hazards of motorcycle riding Test tool safety standards variable speed drives, can be are obvious, but what's the issue with multi- prime generators of spikes. To protect you against meters? As long as you choose a multimeter Lightning strikes on outdoor transients, safety must be built with a high enough voltage rating, aren't you transmission lines also cause into the test equipment. What safe? Voltage is voltage, isn't it? extremely hazardous high- performance specification Not exactly. Engineers who analyze mul- energy transients. If you're should you look for, especially timeter safety often discover that failed units taking measurements on elec- if you know that you could were subjected to a much higher voltage trical systems, these transients be working on high-energy than the user thought he was measuring.

3 Are invisible and largely circuits? The task of defin- There are the occasional accidents when the unavoidable hazards. They ing safety standards for test meter, rated for low voltage (1000 V or less), occur regularly on low-voltage equipment is addressed by the was used to measure medium voltage, such power circuits, and can reach International Electrotechnical as 4160 V. Just as common, the knock-out peak values in the many thou- Commission (IEC). This orga- blow had nothing to do with misuse it was sands of volts. In these cases, nization develops international a momentary high-voltage spike or transient you're dependent for protection safety standards for electrical that hit the multimeter input without warning.

4 On the safety margin already test equipment. built into your meter. The Meters have been used voltage rating alone will not for years by technicians and tell you how well that meter electricians yet the fact is that was designed to survive high meters designed to the IEC 1010. transient impulses. standard offer a significantly 89. Early clues about the safety higher level of safety . Let's see hazard posed by spikes came how this is accomplished. from applications involving measurements on the supply bus of electric commuter railroads. The nominal bus voltage was only 600 V, but multi-meters rated at 1000 V. lasted only a few minutes when taking measurements while the 1 M X.

5 1. T. train was operating. A close look revealed that the train stopping and starting generated From the Fluke Digital Library @ Transient protection Understanding categories: The real issue for multimeter Location, location, location circuit protection is not just the maximum steady state volt- age range, but a combination of both steady state and transient overvoltage withstand capa- bility. 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 explo- sion, which occurs when the surrounding air becomes ionized and conductive.

6 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.). Overvoltage installation categories The most important single concept to understand about the standards is the Overvolt- Figure 1. Location, location, location. age Installation Category. The standard defines Categories I. through IV, often abbreviated as CAT I, CAT II, etc. (See Figure 1.) Overvoltage The division of a power distri- category In brief Examples bution system into categories is CAT IV Three-phase at Refers to the origin of installation, , where low-voltage based on the fact that a danger- utility connection, connection is made to utility power ous high-energy transient such any outdoor Electricity meters, primary overcurrent protection equipment as a lightning strike will be conductors Outside and service entrance, service drop from pole to building, run between meter and panel attenuated or dampened as it Overhead line to detached building, underground line to well travels through the impedance pump (ac resistance) of the system.

7 CAT III Three-phase Equipment in fixed installations, such as switchgear and A higher CAT number refers to distribution, polyphase motors an electrical environment with including single- Bus and feeder in industrial plants higher power available and phase commercial Feeders and short branch circuits, distribution panel devices higher energy transients. Thus lighting Lighting systems in larger buildings Appliance outlets with short connections to service entrance a multimeter designed to a CAT. III standard is resistant to much CAT II Single-phase Appliance, portable tools, and other household and similar receptacle loads higher energy transients than connected loads Outlet and long branch circuits one designed to CAT II standards.

8 Outlets at more than 10 meters (30 feet) from CAT III source Within a category, a higher Outlets at more that 20 meters (60 feet) from CAT IV source voltage rating denotes a higher CAT I Electronic Protected electronic equipment transient withstand rating, , Equipment connected to (source) circuits in which measures a CAT III-1000 V meter has are taken to limit transient overvoltages to an appropriately low level superior protection compared to Any high-voltage, low-energy source derived from a high- a CAT III-600 V rated meter. The winding resistance transformer, such as the high-voltage real misunderstanding occurs if section of a copier someone selects a CAT II-1000 V Table 1.

9 Overvoltage installation categories. IEC 1010 applies to low-voltage (< 1000 V) test equipment. 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.). 2 Fluke Corporation ABCs of multimeter safety It's not just the voltage level Independent testing In Figure 1, a technician work- ing on office equipment in a CAT I Independent testing is the location could actually encounter dc voltages much higher than the key to safety compliance power line ac voltages measured Look for a symbol and listing by the motor electrician in the CAT number of an independent test- III location. Yet transients in CAT I ing lab such as UL, CSA, T V or electronic circuitry, whatever the other recognized testing organi- voltage, are clearly a lesser threat, zation.

10 Beware of wording such because the energy available to as Designed to meet specifica- What does the an arc is quite limited. This does tion .. Designer's plans are CE symbol indicate? not mean that there is no electri- never a substitute for an actual A product is marked CE. cal hazard present in CAT I or CAT independent test. (Conformit Europ enne). II equipment. The primary hazard How can you tell if you're to indicate its conformance to is electric shock, not transients getting a genuine CAT III or CAT certain essential requirements and arc blast. Shocks, which will II meter? Unfortunately it's not concerning health, safety , be discussed later, can be every always that easy.