The ABCs of multimeter safety

Thursday, 21 August, 2008



In addition to multimeters being destroyed by the accidental connection of a low-voltage rated meter to a medium-voltage supply, it’s just as common to find damaged meters dealt a knock-out blow by a momentary high-voltage spike or transient that hits the multimeter input without warning.

Voltage spikes and transients

As distribution systems and loads become more complex, the possibilities of transient overvoltages increase. Motors and power conversion equipment like variable speed drives can be major spike generators, while lightning strikes on transmission lines cause hazardous high-energy transients. These ‘invisible’ and largely unavoidable transients occur regularly on low-voltage power circuits, often peaking at thousands of volts. In these cases, you’re dependent for protection on the safety margin built into your meter.

The voltage rating alone won’t state how well that meter was designed to survive transient impulses.

The real issue for multimeter circuit protection is a combination of both steady-state and transient overvoltage withstand capabilities. Transient protection is vital. When transients ride on high-energy circuits, they can 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 ionised and conductive. The result is an arc blast, which causes many electrical injuries every year.

Safety standards

IEC 1010 is the international safety standard for electrical test equipment. Locally, this was recently adopted as AS 61010. Meters designed to this new standard offer a high level of safety.

The most important concept to understand is the Overvoltage Installation Categories, or CAT I, II, III, IV, qualifying power distribution systems into categories, based on the fact that dangerous high-energy transients will be attenuated as they travel through the impedance of a system. A higher CAT number refers to an electrical environment with higher power available and higher-energy transients.

Within a category, a higher voltage rating denotes a higher transient withstand rating; eg, a CAT III-1000 V meter has superior protection compared with a CAT III-600 V rated meter. The real misunderstanding occurs if someone selects a CAT II-1000 V rated meter thinking that it is superior to a CAT III-600 V meter.

It’s not just the voltage level

A technician working on office equipment in a CAT I location could encounter DC voltages much higher than the power line voltages measured by the motor electrician in a CAT III location. Yet transients in CAT I electronic circuitry, whatever the voltage, are a lesser threat because the energy available to an arc is limited. This doesn’t mean that there’s no electrical hazard in CAT I or CAT II equipment. The primary hazard is electric shock, not transients and arc blast. Shocks can be just as lethal as an arc blast.

The rules of real estate apply equally to Overvoltage Installation Categories: location, location, location.

 
Definition Examples
CAT IV Three-phase at utility connection, any outdoor conductors
  • Origin of installation’, ie, low-voltage connection to utility power;
  • Electricity meters, primary overcurrent protection equipment;
  • Outside and service entrance, service drop from pole to building between meter and panel;
  • Overhead line to detached building.
CAT III Three-phase distribution, including single-phase commercial lighting
  • Equipment in fixed installations, eg, 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 entrance.
CAT II Single-phase receptacle connected loads
  • Appliances, portable tools, household and similar loads;
  • Outlet and long branch circuits;
  • Outlets at more than 10 m from CAT III source;
  • Outlets at more that 20 m from CAT IV source.
CAT I Electronic
  • Protected electronic equipment;
  • Equipment connected to (source) circuits where measures are taken to limit transient overvoltages to appropriately low level;
  • High-voltage, low-energy source derived from a highwinding resistance transformer, eg, high-voltage section of copier.

Table 1: Overvoltage installation categories.

Transients — the hidden danger

Consider a worst-case scenario of a technician performing measurements on a live three-phase motor circuit, using a meter with inadequate safety precautions:

  • A lightning strike causes a transient on the power line, in turn striking an arc between the input terminals inside the meter. The components to prevent this event have just failed or perhaps the meter isn’t CAT III rated, resulting in a direct short between the measurement terminals through the meter and test leads.
  • A high-fault current — possibly several thousand amps — flows in the short-circuit just created. When the arc forms inside the meter, the tech hears a loud bang and sees bright blue arc flashes at the test lead tips — fault currents superheating the probe tips, which start to burn away and draw an arc from the contact point to the probe.
  • As the tech’s hands pull back to break contact with the hot circuit, an arc is drawn from the motor terminal to each probe. If these two arcs join to form a single arc, there is now another direct phase-to-phase short, this time directly between the motor terminals.
  • This arc can approach 6000°C and as it grows, superheats the surrounding air, creating a shock blast and plasma fireball. If the technician is lucky, the blast blows him away, removing him from the proximity of the arc, only injured. In the worst case, he’s subjected to fatal burn injuries from the fierce heat of the arc or plasma blast.

Anyone working on live power circuits should also be protected with flame-resistant clothing, insulated gloves and safety glasses or, better still, a safety face shield.

Transients aren’t the only source of possible short-circuit and arc blast hazards. One of the most common misuses of handheld multimeters can cause a similar chain of events.

Let’s say a tech is making current measurements on signal circuits. The procedure is to select the ‘Amps’ function, insert the leads in the ‘mA’ or ‘Amps’ input terminals, open the circuit and take a series measurement. In a series circuit, current is always the same. The input impedance of the amps circuit must be low enough so it doesn’t affect the series circuit’s current. The input impedance on the 10 A terminal of a Fluke meter is 0.01 O, whereas the input impedance on the voltage terminals is 10 MO.

If the test leads are left in the Amps terminals and then accidentally connected across a voltage source, the low input impedance becomes a short circuit! It doesn’t matter if the selector dial is turned to ‘Volts’; the leads are still physically connected to a low-impedance circuit. That’s why the Amps terminals must be protected by fuses, the only thing standing between an inconvenience — a blown fuse — and a potential disaster. Only multimeters with high-energy fuses protecting the Amps inputs should be used. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer.

Overload protection

Overvoltage protection is provided by a circuit that clamps high voltages to an acceptable level. Additionally, a thermal protection circuit detects an overvoltage condition, protects the meter until the condition is removed and then automatically returns to normal operation. This protects the multimeter from overloads when it’s in ‘ohms’ mode. This way overload protection with automatic recovery is provided for all measurement functions as long as the leads are in the voltage input terminals.

Work safely

Combining the right tools with safe work practices gives maximum protection. Here are some tips:

  • Work on de-energised circuits whenever possible. Use proper lock-out/tagout procedures. If these procedures aren’t in place, assume the circuit is live.
  • On live circuits, use protective gear:
    • Insulated tools;
    • Safety glasses or face shield;
    • Flame-resistant clothing, not ordinary work clothes;
    • Insulated gloves (remove watches and jewellery);
    • Stand on an insulated mat.
  • When making measurements on live circuits:
    • Hook the ground clip first, then make contact with the hot lead. Remove the hot lead first, the ground lead last;
    • Hang or rest the meter if possible. Avoid holding it to minimise personal exposure to transients;
    • Use the three-point test method, especially when checking to see if a circuit is dead. First, test a known live circuit. Second, test the target circuit. Third, test the live circuit again. This verifies the meter worked properly before and after the measurement;
    • Use the old electricians’ trick of keeping one hand in a pocket, lessening the chance of a closed circuit across the chest and through the heart.

Understanding categories

Here are some quick ways to apply the concept of categories to every-day work:

  • The general rule-of-thumb is — the closer to the power source, the higher the category number and the greater the potential danger from transients;
  • The greater the short-circuit current available at a particular point, the higher the CAT number, ie, the greater the source impedance, the lower the CAT number. Source impedance is what dampens transients;
  • A transient voltage surge suppression (TVSS) device installed at a panel must have higher energy-handling capacity than one installed right at a computer. In CAT terminology, the panelboard TVSS is a CAT III application and the computer is a receptacle-connected load and therefore a CAT II installation.
Category Working voltage — V Peak impulse — V Test source
CAT I 600 2500 30Ω source
CAT I 1000 4000 30Ω source
CAT II 600 4000 12Ω source
CAT II 1000 6000 12Ω source
CAT III 600 6000 2Ω source
CAT III 1000 8000 2Ω source
CAT IV 600 8000 2Ω source

 Table 2: Transient test values for measurement categories (50/150/300 V values not included).

Multiple categories
There’s one scenario that sometimes confuses people trying to apply categories to real-world applications. In a single piece of equipment, there’s often more than one category. For example, in lighting control panels or industrial control equipment such as programmable controllers, it’s common to find electronic circuits (CAT I) and power circuits (CAT III) existing in close proximity.

In these situations, use common sense — use a meter with the higher category rating. In the interests of safety and convenience, it’s highly recommended to select a multimeter rated to the highest category in which it could possibly be used, ie, err on the side of safety.

Voltage withstand ratings
AS 61010 test procedures take into account three main criteria: steady-state voltage, peak impulse transient voltage and source impedance. These criteria together identify a multimeter’s true voltage withstand value. Refer to Table 2.

When is 600 V more than 1000 V?
Table 2 outlines an instrument’s true voltage withstand rating. Within a category, a higher ‘working voltage’ is associated with a higher transient, eg, a CAT III-600 V meter is tested with 6000 V transients whilst a CAT III-1000 V meter (eg, Fluke 289) is tested with 8000 V transients. What’s not as obvious is the difference between the 6000 V transient for CAT III-600 V (eg, Fluke 117) and the 6000 V transient for CAT II-1000 V. They aren’t the same. This is where the source impedance comes in — the 2 O test source for CAT III has six times the current of the 12 O test source for CAT II.

The CAT III-600 V meter offers superior transient protection compared to the CAT II-1000 V meter, even though its so-called ‘voltage rating’ could be perceived as being lower. It’s the working (steady-state) voltage and category combination that determines the total voltage withstand rating of the instrument, including the transient-voltage withstand rating.

The bottom line

Before deciding on a new multimeter, analyse the worst-case scenario it will be applied to and determine what category it fits into. Then identify the meters that are rated for this category. Next, look for a multimeter with a voltage rating for that category matching your needs. Test leads should also be certified to an AS 61010 category and voltage that is as high or higher than the meter. Don’t let test leads be the weak link.

Fluke Australia
http://www.fluke.com.au

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