Getting to grips with current measurement for maintenance, repair & operations (MRO)
01 November 2020
Office ergonomics is big business. From desks with adjustable height & optimal posture chairs to sprung arm mounts that bring monitors to the ideal height, no aspect of potential discomfort for office workers seems to have been overlooked. For maintenance engineers, however, such considerations are an enigma.
This article was originally featured in the November 2020 issue of EPDT magazine [read the digital issue]. Sign up to receive your own copy each month.
Spending their days in poorly lit electrical cabinets, atop ladders accessing cable trays, or squeezing into shafts and ducts, there is little concept of an ergonomically comfortable working environment. This doesn’t mean, however, that the tools they use daily cannot deliver some level of added convenience. Current measurement, a common analysis when monitoring equipment or locating a fault, is often challenging to undertake thanks to tightly bundled cables and cramped working conditions. Here, Harald Lehner, Head of Private Label, Brand & Support, Product & Procurement at German retailer & distributor of electronic products, Conrad Electronic reviews optimal current clamp and related tool selection for even the darkest, most awkward corners of the electrical cabinet…
Current measurement is a common capability of most multimeters, but normally requires the circuit to be broken in order to do so. Typically, their current measurement capabilities are restricted to a peak of 10 A, and the type of measurement may be further restricted by the location in the electrical system the measurement is being made. For example, measurement of equipment connected to the mains via a plug is considered to be in the CAT II safety category, while measurements made on fixed electrical equipment, such as elevator motors, are in CAT III. This CAT rating defines the highest voltage that may be measured with the multimeter, based upon the highest transient voltage that may occur due to, for example, a lightning strike on a power line supplying the site.
When undertaking current measurements on installed operational equipment, breaking the circuit to make a measurement can be both time consuming and dangerous. Current clamps enable current to be measured on live systems at low risk to the operator making the measurement. These utilise the relationship between the magnetic field generated around a conductor and the electrical current flowing through it. Current clamps can be as simple as a wire loop that is fitted around the cable being measured, to a clamp made of ferrite material that, thanks to a spring-loaded jaw construction, closes and holds itself in place around the conductor. Typically, flexible wire loops, sometimes termed flexible clamps, target busbar measurements, while ferrite-based clamps target insulated fixed wiring installations. Wire loop current clamps also enable more accurate measurement of low currents by wrapping the wire around the conductor being measured several times. The resulting measurement is then divided by the number of loops to determine the actual current.
Both types of clamps can be attached as adapters to standard multimeters to enable them to undertake high-current measurements. This is a useful approach for teams that make irregular measurements and can share equipment. Such devices are battery powered and can be provided with a calibration certificate if needed. A device such as the VC-18T provides a flexible loop of 180mm diameter and can measure currents of up to 3000 A AC at CAT III 1000V or CAT IV 600V (Figure 1).
The output is typically provided in mV/A, allowing simple measurement with a standard multimeter with resolutions lying between 1mV/A and 100mV/A depending on the measurement range. The loop can also be easily opened and closed even when wearing protective gloves during installation. For lower currents, an adapter such as the VC-511 is ideal, supporting measurements of up to 400A DC and AC at CAT III 600V or CAT IV 300V. Such adapters often provide an automatic power-off feature to save the batteries. If long-term measurements are planned, it is worth reviewing if this power-saving feature can also be overridden.
Current measurements on power-plug-connected equipment is made safer by using measurement adapters that provide access to the individual conductors of a multiwire single-phase or three-phase cable. Featuring both male and female plugs, they are inserted between the equipment to be measured and a power outlet. Access to the individual conductors (including earth) are provided by clearly marked loops to which one or more current clamps can be fitted. Some adapters also provide CAT II measurement sockets to allow voltage to be measured in parallel, such as on the DLA-3L three-phase, and DLA-1L single-phase adapters (Figure 2). Current clamps sometimes provide an in-rush capability, allowing the momentary spike of energy drawn prior to the steady-state current of a machine to be assessed.
For maintenance teams making regular current measurements, it is worthwhile investing in a fully integrated current clamp tool. Here the current clamp is integrated with a multi-digit seven-segment display and the option to switch between several measurement ranges. They may also provide the ability to measure other parameters, such as voltage (both DC and AC), continuity, frequency, resistance and capacitance via test leads. Here, it is worthwhile taking time to review not only the functionality of the test tool, but additionally assess how it will function in the conditions where it will be used. Warm environments often result in sweaty hands, from which a poorly designed device could slip. Both the form and the outer material play a role here, with grooves or rubberised elements often integrated into the product.
The legibility of the display is also important. Many multimeters only use a standard seven-segment liquid crystal display (LCD). In a dark cabinet, this makes reading of measurements a challenge. The latest generation of measurement tools have opted for negative LCD displays. These provide a black background with the segments clearly defined in white. This higher contrast makes it easier to read the display even in poorly lit locations.
This is the approach taken with devices such as the VC523, which is perfect for service technicians working on automotive and truck electronics, control cabinets and machine servicing. Should lighting in the area of work be poor, the user can also turn on the integrated LED light using the button conveniently located near their thumb. Battery life is preserved thanks to an auto-off feature that turns of both the clamp meter and LED light after 15 minutes. In the event that this not desired, the auto-off feature can also be disabled. The unit’s non-slip, rubberised coating also ensures that it stays securely in place on surfaces that are not perfectly horizontal.
Today’s industrial equipment is reliant upon solid-state power converters, such as dimmers and switch-mode power supplies, that allow machines to achieve efficiencies way beyond that of the equipment of yesteryear. Thus, where it used to be possible to assume that currents flowing were approximately sinewave in form, this is no longer a given. It should be checked that test equipment makes true RMS (root mean square) current measurements for accurate data, even when asymmetrical currents are flowing.
Tightly bundled cables are a particular challenge when using current clamps. Large clamps and thick ferrite jaws can be difficult to place around a single conductor. In such cases narrow-jawed clamps can be especially helpful. Measurement tools such as the VC-337 support conductors of up to 6mm2, and include a cable stop that ensures the conductor is captured in the jaws even when direct line-of-sight is not possible during application (Figure 4). This provides an excellent balance between size of the clamp jaws and the safety class of the device (CAT III 300 V, CAT II 600 V). The non-slip coating ensures that the device stays comfortably in the hand during long periods of use.
All of the measurement equipment high-lighted here is, like all of the Voltcraft range, calibrated during manufacturing. This ensures reliable and trustworthy measurements within the defined tolerances of the product. If required, measurement tools can also be delivered with either a DakkS or ISO calibration certificate.
Quality current measurements require durable, reliable and accurate tools that benefit the user, regardless of the difficult conditions under which they must work. Current clamps provide different clamping solutions (wire loop and clamp), whose advantages must be weighed up with the maintenance team’s typical operational requirements. The latest measurement devices provide advanced data-logging capabilities in conjunction with today’s smartphones, providing for a familiar user experience and higher comfort even under physically challenging conditions. Finally, high-contrast negative LCD displays, backlighting and integrated LED illumination, as well as non-slip coatings, ensure that eyes and hands won’t suffer during a hard day’s work.
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