Safety an inevitable feature to look for in multimeters

Protection and safety are primary concerns while selecting a multimeter. Although modern multimeters come with inbuilt safety features, users need to be aware of these features, and also know their limitations and applications.

By Richa Chakravarty

Thursday, February 23, 2012: Risks associated with multimeters

While working with multimeters, the user is always under the risk of accidents that can be fatal as well. These accidents usually occur due to misuse of the equipment. A common mistake that users commit is that, they measure the voltage while the test leads are in the current terminals. The user usually makes current measurements in low energy circuits by opening the circuit and inserting the multimeter. Although the multimeter has a fuse for protection, it is not enough. This is particularly risky when using handheld multimeters, where high energy fuses are required for protection. By using the multimeter in the wrong power category, the power can surge and blow off the fuse (which should always be replaced with another fuse from the same manufacturer). Thus, while measuring power from mains, or some other high voltage or high energy source, using a multimeter without checking on its inbuilt electronic circuitry protection could be risky.

There is another common mistake that can be made when making a resistance measurement. If the user makes a contact with a live circuit, thinking it to be dead, he can be at severe risk. However, modern multimeters generally have good overload protection and recover automatically. Nevertheless, the user should be careful.

The user should also check if the multimeter can handle transients and spikes, as the combination of high voltage spikes and high current can be dangerous. “The safety issue here is to select a multimeter that has protection against steady state voltage and transient over voltage. These spikes can be severe enough to cross the input circuitry inside the multimeter, and create a short circuit inside the multimeter,” states Praveen Kumar Gupta, director, Avante Global Services Pvt Ltd.

Safety features in multimeters

To overcome these risks, manufacturers are incorporating various safety features in multimeters that will protect the user and the electronic gadget. Let’s have a look at some of these features.

Inbuilt fuse: For typical measurement functions, manufacturers have incorporated a variety of protection features. For example, Keithley Instruments offers multimeters with inbuilt fuses on the power input module for protection from power line disturbances. “It also has fuses on the current measurement circuitry to protect from conditions exceeding the measurement range of the instrument,” says Sai Venkat Kumar, country marcomm, Tektronix/Keithley Instruments.

Many multimeters are also equipped with small glass fuses. However, these fuses do not always control arc flash, and in some cases may explode on their own.

Terminal shutters prevent erroneous insertion of test leads into current measurement terminals. If the function is switched to other than current

measurement while a test lead remains inserted in a current measurement terminal, the fuse built into the meter cannot protect the circuits. Terminal shutters prevent such accidental errors. “Multimeters with current terminal shutters prevent the user from inserting high voltage from the terminal into the internal circuit of current measuring, and also prevent him from switching to voltage measuring when inserting the test probes to current terminal,” says Kumaraswamy ML, deputy general manager, TMI and communications product division, Yokogawa India Ltd.

“Terminal shutters will prevent the wrong insertion of test leads but it will again depend upon the user to measure the same function as per the selection of terminal leads, in reverse case the problem of wrong insertion of leads remains unsolved,” adds Prashant Das, marketing director, Motwane Manufacturing Company Pvt Ltd.
Auto discharge of capacitors is a circuitry safety feature that protects the multimeter from damage. Capacitors store energy and can take a long time to dissipate that energy. Leaving a capacitor disconnected can lead to a disaster. The user should always be cautious when working with these devices. There are many different ways of discharging a capacitor, but today’s multimeters come with an inbuilt auto discharge that provides them with protection.

High energy fuse: When high current ionises the air surrounding the operator and makes it conductive, the current makes an arc across the multimeter probes, causing an explosion. When a multimeter is selected with a voltage withstand rating that is too low for its application, this kind of arc blast can occur.

 “To prevent such accidents, a special quality high energy fuse is used in Agilent multimeters to protect the inbuilt circuitry of the multimeter. This fuse is designed to control and extinguish the severe arc flash within the fuse before it spreads outside the multimeter,” explains Sadaf Arif Siddiqui, technical marketing specialist, Agilent Technologies India Pvt Ltd.

Motwane Manufacturing offers various models of multimeters with safety features keeping in mind the target applications. Shares Prashant Das, “Generally the overload protections are provided for the AC/DC voltage ranges up to 1050 V DC/AC root mean square (RMS), with the input impedance is 10 M Ohm. For AC/DC current ranges suitable mA and A high energy fuses of ratings of 250 V/500 V are provided depending on the model. For all the other parameters, protection of 440 V DC/AC rms are given.”

There are some more common risks while working with multimeters, such as stray voltages and harmonic distortion. Stray voltage is a voltage that appears in an electrical conductor such as a wire, even when the wire is disconnected from an electrical circuit. Most multimeters have high input impedance as compared to the impedance of the circuit being measured. Informs Sadaf Arif Siddiqui, “Handheld digital multimeters (DMMs) are designed to place very little load on the circuit under test and, because of this, stray voltage is measured by the digital multimeter. Typically, these types of handheld multimeters have input impedance that is greater than 1 M Ohm, and the input impedance varies depending on the digital multimeter’s design.” He explains that when working with electrical systems, low input impedance testers, such as a solenoid tester, are preferred because they do not pick up stray voltage like some of the high impedance digital multimeters. These multimeters are expensive and it’s impractical for the user to carry both—a high input impedance and a low input impedance multimeter—in order to identify stray voltage. Agilent’s U1272A, with its unique ZLow function, is a solution that allows users to switch from high input impedance mode to low input impedance mode to check for the presence of stray voltage.

Standards guarantee safety

Certifications and standards are necessary for the safety of multimeters. For a user who is unaware of the various safety features, the mark of certification signifies and guarantees protection. The biggest concern for a buyer here is to choose a multimeter with the right kind of protection against transients, and certifications that provide guarantee against such risks.

There are various testing agencies that evaluate systems and provide a compliance certificate for devices that meet specific safety requirements. International Electrotechnical Commission (IEC) 61010 is an international standard for low voltage test equipment. Other independent testing agencies such as the Canadian Standard Association (CSA) and Underwriters Laboratories (UL), TÜV provide safety certifications. Most products today carry safety certification markings to signify that they comply with relevant standards and have been tested by a third party agency.

Informs Manish Kwatra, managing director, Metro Electronic Products, “All our products adhere to CAT standards and other relevant international standards, including RoHS and CE. In fact, instruments using infrared lasers for measurement or marking are specifically graded according to human

and industry use. Also, proper insulation is provided to instruments for safety at high voltages.”

The industry today not just understands the importance of standards but also values them. Companies like Keithley Instruments offers products that are measurement Category I and II rated, as described in the IEC 60664. Similarly, devices from Yokogawa and Avante Global meet CAT III 1000 V and CAT IV 600 V safety standards. Yokogawa products are compliant to EN 61010-1 standards, and include safety shutters to prevent erroneous insertion of leads into current terminals. Other manufacturers like Agilent Technologies not only self-certify but obtain safety certifications from CSA—a third party independent testing agency. Safety standards followed by Motwane Manufacturing include latest EN 61010-1:2010, along with IP 54 protection (protection against dust and water), EN 61236:2006 EMC standards and CE certification. Digital multimeters offered by Avante Global are CAT III and CAT IV 600 certified.

Knowledge of CAT rating is critical

All manufacturers of multimeters are required to mark their products with rated measurement category (CAT II, CAT III, or CAT IV). This marking is a convenient way for users to

identify the maximum transient voltage that the multimeter can safely withstand. Shares Sai Venkat Kumar, “The CAT rating defines the level of short circuit current a multimeter is designed with. Picking a multimeter with an insufficient CAT rating could result in a dangerous situation that can cause severe personal injury or death. The user must also consider his working voltage.” The higher the CAT number, the greater is the electrical power available, and with that, the higher are the voltage transients.

“Human safety is the most important issue, which cannot be neglected. Hence it becomes important to choose proper CAT rating of the multimeter as per the application. But users are uaually unaware of the CAT ratings. There are four different CAT categories and all give protections at different levels,” says Prashant Das. These categories are:

CAT I: For instruments not connected to AC mains, typically for electronic use

CAT II: For single phase household appliances

CAT III: For applications inside the factory with three phase supply

CAT IV: Applications where three-phase supply originates from outside, like an entrance, service area, substations, etc.

How to choose a multimeter with the right safety features

When considering the safety ratings of multimeters, a user should not forget that the same ratings apply to test leads, probes, clips and even clamp-on accessories. “Before you buy a new multimeter, remember to check for the symbol of a recognised testing organisation. These symbols can only be used if the product has successfully completed testing to the agency’s standard, which is based on national/international standards,” suggests Sadaf Arif Siddiqui. Echoing similar thoughts, Sai Venkat shares, “Selecting a handheld or digital multimeter is completely dependent on a user’s requirement. For a traditional benchtop meter, which is used in conjunction with other instruments, it is not only important to understand the dynamics, but also verify and check the safety of other instruments that work together to safely deliver the necessary measurements.”

Users should be aware that certification bodies do not approve products. Rather, they evaluate products or systems for compliance to specific requirements. Thus, one should look for independent certification, not just for words saying ‘designed to meet’ or ‘conforms to IEC 61010’. These statements are no substitute for independent testing. Prashant Das adds, multimeters should have resolutions and accuracy as high as possible. All the ranges should have overload protections and it should be user friendly.

Therefore, when choosing a multimeter, first select one that meets the highest category you will be working on. If you are working in an environment that has the features of more than one category, pick the meter that is rated for the highest. Then find a meter with the voltage rating in that category which matches your requirements. Finally, get test leads that are certified to the category and voltage at the same level or greater than the multimeter you will work on.

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