The ABCs of multimeter safety

July 14th, 2014, Published in Articles: Vector

 

Often, users accidentally subject their multimeters to much higher voltage than they think they are measuring. This submits the units to momentary high-voltage spikes, causing them to fail.

The possibilities of transient overvoltages increase as distribution systems and loads become more complex. Motors, capacitors and power conversion equipment such as variable speed drives (VSDs) can be prime spike generators. Lightning strikes on outdoor transmission lines also cause extremely hazardous high-energy transients.

If you’re taking measurements on electrical systems, these transients are “invisible” and largely unavoidable hazards. They occur regularly on low-voltage power circuits and can reach peak values of many thousands of volt. In these cases, you depend on the safety margin already built into your meter for protection.

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 railroads.

The nominal bus voltage was only 600 V but multimeters rated at 1000 V lasted only a few minutes while taking measurements while the train was operating. A close look revealed that the train’s stopping and starting generated 10 kV spikes. This investigation led to significant improvements in multimeter input protection circuits.

New safety standards

Defining new safety standards for test equipment was addressed recently by the IEC. Industry used IEC 348 in designing equipment for a number of years but this standard was replaced by IEC61010 (EN61010) which requires a significantly higher level of safety. Let’s see how this is accomplished.

Understanding categories

Transient protection

The real issue for multimeter circuit protection is not only the maximum steady state voltage range, but a combination of both steady state and transient overvoltage withstand ability.

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 conductive. The result is an arc blast, a disastrous event which causes more electrical injuries every year than electric shock.

Overvoltage installation categories

Most important to understand about the new standards is the overvoltage installation category (CAT) concept. The new standard defines CATs I to IV (see Fig. 1).

Fig 1: The overvoltage installation category (CAT) concept: CAT I, CAT II and CAT III.

Fig 1: The overvoltage installation category (CAT) concept: CAT I, CAT II and CAT III.

Power systems are divided into these categories based on the fact that high-energy transients will be attenuated or dampened as it travels through the impedance (AC resistance) of the system.

The higher the CAT number, the higher the power available in the environment, causing higher-energy transients. A multimeter designed to a CAT III standard is resistant to much higher-energy transients than one designed to CAT II standards.

A higher voltage rating within a category denotes a higher transient withstand rating. For example, a CAT III 1000 V meter has superior protection to a CAT III 600 V meter. The real danger occurs when the user selects a CAT II 1000 V, assuming that it is superior to a CAT III 600 V meter.

Not just the voltage level

In Fig. 1, a technician working on office equipment in a CAT I location could actually encounter DC voltages much higher than the power line AC voltages measured by the motor electrician in the CAT III location.
Yet, transients in CAT I electronic circuitry, whatever the voltage, are clearly a lesser threat because the energy available to strike an arc is limited. This does not mean that there is no electrical hazard present 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 arc blast. For example, an overhead line run from a house to a detached work shed may be only 120 or 240 V, but it still technically in a CAT IV area because any outdoor conductor is subject to very high-energy lightning transients. Even an underground conductor, which will not be struck directly by lightning, is CAT IV because a lightning strike nearby can induce a transient because of the high electro-magnetic fields present.

Two major electrical hazards

Transients: the hidden danger

A worst-case scenario would be where a technician performs measurements on a live 3-phase motor control circuit, using a meter without the necessary safety precautions (see Fig. 2). Possible consequences are:

  • A lightning strike causes a transient on the power line, which in turn strikes an arc between the input terminals inside the meter. The circuits and components to prevent this event have failed or are missing. Perhaps it was not a CAT III rated meter. The result is a direct short between the two measurement terminals through the meter and the test leads.
  • A high-fault current – possibly several thousand amp – flows in this short circuit. This happens in a fraction of a second. When the arc forms inside the meter, a very high-pressure shock wave can cause a loud noise much like a gun shot. At the same instant, bright blue arc flashes occur at the test lead tips as the fault currents superheat the probe tips. These start to burn away and draw an arc from the contact point to the probe.
  • The natural reaction is to pull back to break contact with the hot circuit, but as the hands move away, an arc is drawn from the motor terminal to each probe. If these two arcs join to form a single arc, there would be another direct phase-to-phase short, this time directly between the motor terminals.
  • This arc can have a temperature up to 6000°C, which is higher than the temperature of an oxy-acetylene cutting torch. The arc is fed by short circuit current and, as it grows, it superheats the surrounding air. This creates both a shock blast and a plasma fire ball. The shock blast may blow the user away, removing him from the proximity of the arc. Though injured, his life would be saved. In the worst-case scenario, the user is subjected to fatal burn wounds from the heat of the arc or plasma blast.
Fig. 2: A worst-case scenario: potential arc blast sequence.

Fig. 2: A worst-case scenario: potential arc blast sequence.

Apart from using a correctly rated multimeter, persons working on live power circuits should be protected with flame resistant clothing, wear safety glasses or a safety face shield, as well as insulated gloves.

A lightning strike causes a transient on the power line, creating an arc between the meter’s input terminal and resulting in loud noises. Then, a high current flows Transients aren’t the only source of possible short circuits and arc blast hazard. One of the most common misuses of handheld multimeters can cause a similar chain of events.

Let us assume the user 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 that it doesn’t affect the series circuit’s current. The input impedance on the 10 A terminal of a Fluke meter is 0,01 Ω. Compare this with the input impedance on the voltage terminals of 10 MΩ.

The low input impedance becomes a short circuit if the test leads are left in the amps terminals and are then accidentally connected across a voltage source!

It doesn’t matter if the selector dial is turned to volts; the leads are still physically connected to a low-impedance circuit. This is why the amps terminals must be protected by fuses.

Use only a multimeter with amps inputs protected by high-energy fuses. Never replace a blown fuse with the wrong fuse. Use only the high-energy fuses specified by the manufacturer. These fuses are rated at a voltage and with a short circuit interrupting capacity designed for your safety.

Overload protection

Fuses protect against overcurrent. The high input impedance of the volts/ohms terminals ensures that an overcurrent condition is unlikely, so fuses aren’t necessary. Overvoltage protection, on the other hand, is required. It is provided by a protection circuit which clamps high voltages to an acceptable level. In addition, a thermal protection circuit detects an overvoltage condition, protects the meter until the condition is removed, and then automatically returns to normal operation.

The most common benefit is to protect the multimeter from overloads when it is in ohms mode. In this way, overload protection with automatic recovery is provided for all measurement functions as long as the leads are in the voltage input terminals.

Applying categories to your work

Shortcuts to understanding categories

Some quick ways to apply the concept of categories to your every-day work include:

  • The general rule is that, the closer you are to the power source, the higher the category number and the greater the danger of transients.
  • The greater the short-circuit current available at a particular point, the higher the CAT number, i.e. the greater the source impedance, the lower the CAT number. Source impedance is simply the total impedance, including the impedance of the wiring, between the point where you are measuring and the power source. This impedance is what dampens transients.

Multiple categories

There is often more than one category present in a single piece of equipment. For example, in office equipment, from the 120/240 V side of the power supply back to the receptacle is CAT II. The electronic circuitry, on the other hand, is CAT I.

It is common to find electronic circuits (CAT I) and power circuits (CAT III) existing in close proximity in building control systems such as lighting control panels or industrial control equipment.

In such cases, a meter with a higher category rating should be used. Select a multimeter rated to the highest category in which it could possibly be used. In other words, err on the side of safety.

How to evaluate a multimeter’s safety rating

Voltage withstand ratings

EN61010 test procedures take into account three main criteria: steady-state voltage, peak impulse transient voltage and source impedance.

When is 600 V more than 1000 V?

Table 2 provides an understanding of an instrument’s true voltage withstand rating:

  • Within a category, a higher working voltage (steadystate voltage) is associated with a higher transient. 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.
  • Less obvious is the difference between the 6000 V transient for CAT III  600 V and the 6000 V transient for CAT II 1000 V. 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 offers superior transient protection compared to the CAT II 1000 V meter, even though its “voltage rating” could be seen as lower. The combination of the steady-state voltage and the category determines the total voltage withstand rating of the test instrument, including the transient voltage withstand rating.
Overvoltage installation 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 Ω source
CAT I 1000 V 4000 V 30 Ω source
CAT II 600 V 4000 V 12 Ω source
CAT II 1000 V 6000 V 12 Ω source
CAT III 600 V 6000 V 2 Ω source
CAT III 1000 V 8000 V 2 Ω source
CAT IV 600 V 8000 V 2 Ω source

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

The bottom line

Before buying multimeters, the user should analyse the worst-case scenario of his work and then determine what category his 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.

EN61010 also applies to test leads – they should be certified to a category and voltage as high or higher as those of the meter.

Contact Gerrit Barnard, Comtest, Tel 011 608-8520, gbarnard@comtest.co.za