Wednesday, April 30, 2014

Think safety when selecting a multimeter

Would you just use any handheld multimeter to measure mains voltage or some other high-energy source?  Probably not! Just as different types of helmets are designed to protect against rockclimbing, bicycle and motorcycle accidents; different multimeters are designed with different levels of protection against common electrical hazards. For your own safety - and for the safety of others nearby – it’s best to choose the right multimeter that’s designed to protect against specific hazards that will be encountered.

Electricity is a familiar part of our daily lives and, perhaps because of that fact, many of us don’t give much thought to its hazards. Engineers, electricians, technicians and powerline workers are among those who may be exposed directly to electricity. Because they are at higher risk from electrical hazards, the tools they use must be designed with appropriate levels of protection to withstand the electrical hazards in their working environment.

Key characteristics to consider
There are three key characteristics that need to be considered when dealing with electrical measurements in a building. The first is voltage rating. The voltage rating in these types of applications is commonly known by electrical professionals, typically 230 or 477 V. At a minimum, the appropriate multimeter must be rated to measure the voltage that is expected to be present.

Secondly, the transient voltages must also be taken into consideration when selecting a multimeter, as the voltage rating doesn’t tell the whole story. These transient voltages come from two main sources: natural causes, such as lightning outside the building or generated by switching operations on the power distribution system, examples being switching of transformer taps, motors, inductances, sudden variation of load or disconnection of circuit breakers. They can vary from a few hundred volts peak to about 6000 V peak and tend to last 50 - 200 microseconds. If a protection margin is not built into a meter to safely withstand such peak transients, they might trigger a sequence of events that could lead to serious injury or death. Clearly, the  multimeter must be designed to safely withstand such peak transients.

Thirdly, the energy capacity of the circuit must also be considered. Higher current circuits can deliver much more energy into a fault than lower current circuits. Therefore, measurements performed on higher energy circuits are much more hazardous than measurements performed on lower energy circuits.

The energy capacity of the circuit is defined by three characteristics: operating voltage, circuit impedance and the characteristics of the circuit fuse or breaker. The closer the circuit is to the mains source, the lower the circuit impedance. In other words, the fault current is higher and extra precautions are needed.

Measurement categories
The IEC 61010 standards define three ‘measurement categories’ for mains circuits. The higher the category number, the greater the danger posed by transient voltages on the mains circuit.
  • Measurement Category I applies to everything except mains circuits. In future versions of this standard, such equipment will be considered ‘uncategorised’ instead of Category I.
  • Measurement Category II applies to cord-and-plug connected equipment, from wall sockets to the equipments’ first level of power conversion.
  • Measurement Category III applies to circuits installed completely within the building, including parts of the service panel and branch circuits.
  • Measurement Category IV applies to the source of the building electrical installation - the entrance service panel and primary mains meter.

In the current set of product safety standards, handheld multimeter manufacturers are required to mark the rated measurement category (CAT II, CAT III or CAT IV) to indicate the maximum overvoltage transient that the meter can safely withstand. Some manufacturers mark two different measurement categories, as CAT III for 1000 V mains has the same transient withstand capability as CAT IV for 600 V mains, ie, CAT III (1000 V)and CAT IV (600 V).

Remember, the transient withstand capability is only one of the characteristics of the meter that must be considered. And because of this, the meter must be designed to safely withstand certain levels of energy surges as well as voltage transients. This requirement has not yet been introduced, but some manufacturers, including Agilent, are already designing multimeters that will meet this new requirement.

Arc flash
Arc flash is the result of a very common mistake when using a multimeter that has potentially serious consequences. It’s common for an electrician to finish a current measurement and then intends to measure the circuit voltage, but forgets to switch the test leads from the current terminal to the voltage terminal. When the test leads touch a live buss bar or live terminal, a very high current will flow through the test leads and the meter. If you’re working in an area with very low circuit impedance, this fault current can be hundreds of amperes. The test leads and the meter circuitry aren’t designed to handle such high currents and the meter will cause an open circuit to occur. When it does, an arc will be established across the open part of the circuit. If it’s not immediately controlled, the arc will cause the air in the vicinity to become superheated and converted to plasma. This is called an arc flash. In extreme cases, it releases a significant amount of energy.

Manufacturers of high-quality handheld multimeters have long addressed this phenomenon by including a special high-energy fuse in the meter to control and extinguish the arc flash within the fuse, before it spreads outside the meter. Not just any fuse will do. Many meters are equipped with small glass fuses that will not always control arc flash and, in some cases, may explode themselves.
Today’s product safety standards don’t require such a fuse. For proper protection though, insist on handheld multimeters with this type of fuse on the current input circuits. The typical characteristics of these very fast-acting fuses are a 1000 Vdc or 750 Vac RMS voltage rating, 11 A operating current (may vary depending on the multimeter design) and 10,000 A breaking capacity.

Safety certification
Most products carry safety certification markings to signify that the products comply with the relevant standards and have been tested by a third-party agency. Responsible manufacturers of handheld multimeters like Agilent obtain safety certifications from third-party independent testing agencies. However, these certification bodies do not ‘approve’ products. Rather, they evaluate them for compliance to specific requirements.
The ‘CE’ mark is an abbreviation for European Conformity and is neither a mark of origin nor a quality mark. It is a mandatory conformity mark on all products marketed in the European Union. This marking symbolises conformity of the product to all of the applicable EU safety, health and environmental requirements.

When choosing a handheld multimeter that only has a CE mark, ensure that the manufacturer is known to be trustworthy and can provide a Declaration of Conformity upon request.

When measuring mains power or other high-voltage circuits, safety is of great concern. No one should compromise personal safety for any reason. To be safe, always choose a handheld multimeter with a voltage rating higher than the circuit being measured. Also, choose a CAT III rated multimeter for mains measurements; and CAT IV multimeter for measurements close to the mains source.
For protection against arc flash, be sure to select a handheld multimeter designed with a high-energy fuse on the current input circuit. Remember to check that the multimeters and probes being used are marked with third-party safety agency logos such as UL, CSA, ETL, TÜV or VDE. Don’t overlook the safety of the probes. That provides assurance that the high voltage goes into the measurement instrument instead of the operator!