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HT Instruments Clamp Meters

Hammond - PJ12124RLW N4X
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HT Instruments Clamp Meters

A Clamp Meter combines a current clamp with the basic functions of a digital multimeter, also called DMM. Clamp the "jaw" around a conductor to measure current. Over the years current clamp meters have evolved to include a host of additional features.

Take a look at this Amprobe clamp meter video. It displays Power and measures Total Harmonic Distortion (THD). Amprobe, now part of Fluke, invented the first clamp meter in 1948 and is still a leader today.
 

How to buy/choose a clamp meter
  • Choose a clamp meter that gives accurate and repeatable results.
    Make sure your clamp meter reports the true-rms reading. Otherwise noise from everything from a variable frequency drive to compact fluorescent bulbs can result in a less accurate reading. You should always make sure that the clamp meter meets the industry accuracy standard: 2% ± 5 counts. Beware of accuracies stated to ± 10 counts, as these meters can have twice the error when measuring low currents.
     
  • Do not compromise on safety
    Make sure the clamp meter has the correct rating. Check that the design will allow you to use the meter easily while wearing Personal Protective Equipment (PPE). Exciting new designs are available with remote displays and wireless capabilities to consider.
     
  • Make sure the clamp meter works where you do
    Check the specifications for the amperage and voltage range you will be working on. Also consider the ambient temperature range if you will be working in a hot environment. Is it rated only for indoor use only? Be sure the clamp meter display you select has large, easy to read characters. Some displays may seem adequate when viewed in a showroom but then fail to perform in the workplace. Real world conditions mean a wide viewing angle and backlight are a must.
     
  • Consider special features
    • Inrush. If you are working around motors and drives, an accurate inrush measurement function is a must. The Inrush function allows you to accurately measure the high current surge that flows into motors during startup. This measurement can be critical when troubleshooting problems such as nuisance trips of over current protection devices. Because it looks at the entire motor inrush period, it is far more accurate than the "MAX" function which only looks at a single point in time.
    • Autoranging Display. A measurement that displays in the correct range can be a real timesaver when working in tight spaces. Choose a clamp that automatically sets the correct measurement range so that you are not having to a adjust switch positions while trying to position the clamp and take a measurement.
    • Datalogging and PC Connectivity
    • Power and Total Harmonics Distortion (THD) Measurements
    • Flame Rectification. Some Fieldpiece clamp meters measure DC microamps (uA) for Flame rectification application, which can help determine if a furnace temperature probe is going bad
    • Wireless Now you can save your measurements to the cloud, trend data over time and work outside the arc flash zone with Fluke Connect wireless enabled test tools. Increases safety by remote readings and reduces your time in the arc flash zone.
    • Flexible Current Probes The Fluke iFlex Flexible Current Probe expands the measurement range to 2500 A ac; provides access to large conductors in tight space. See the Fluke 376 FC for details. 
Fluke370_series_img3_tn
Fluke 376 FC

How does a clamp meter work?

Clamp meters and adaptors measure this field using one of two technologies. For DC currents, "Hall Effect" is used, while for AC currents "Inductive" technology is used. Hall effect and induction are noncontact technologies based on the principle that for a given current flow, a proportional magnetic field is produced around the current-carrying conductor. Both technologies measure this magnetic field, but with different sensing methods.
hall_effect_vs_inductive

Hall Effect Technology
The Hall effect sensor consists of three basic components: the core, the Hall effect device, and signal conditioning circuitry. The current conductor passes through a magnetically permeable core that concentrates the conductor's magnetic field. The Hall effect device is carefully mounted in a small slit in the core, at a right angle to the concentrated magnetic field. A constant current in one plane excites it. When the energized Hall device is exposed to a magnetic field from the core, it produces a potential difference (voltage) that can be measured and amplified.

Inductive Technology
The ability of clamp meters to measure large ac currents is based on simple transformer action. AC current constantly changes potential from positive to negative and back again, generally at the rate of 50 Hz or 60 Hz. The expanding and collapsing magnetic field induces current in the windings. This is the principle that governs all transformers. When you clamp the instrument’s “jaws” around a conductor carrying ac current, that current is coupled through the jaws, similar to the iron core of a power transformer, and into a secondary winding which is connected across the shunt of the meter’s input. A much smaller current is delivered to the meter’s input due to the ratio of the number of secondary windings vs. the number of primary windings wrapped around the core.

Usually, the primary is represented by the one conductor around which the jaws are clamped. If the secondary has 1000 windings, then the secondary current is 1/1000 the current flowing in the primary, or in this case the conductor being measured. Thus, 1 amp of current in the conductor being measured would produce 0.001 amps or 1 milliamp of current at the input of the meter. With this technique, much larger currents can be easily measured by increasing the number of turns in the secondary.


 
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