Fluke 381 True RMS AC/DC Clamp Meter

Fluke 381 True RMS AC/DC Clamp Meter

The Fluke 381 True RMS AC/DC clamp meter does everything you would expect from a clamp meter including letting you remove the display for even more flexibility. Now one technician can do jobs that used to require two people. Clamp the Fluke 381 around a conductor, remove the display and walk across the room to operate controls or remove protective equipment, all while watching real-time readings.

The new iFlex flexible current probe (included) expands the measurement range of the Fluke 381 True RMS AC/DC clamp meter to 2500A AC while providing increased display flexibility, ability to measure awkward sized conductors and improved wire access. An integrated low pass filter and state of the art signal processing allows for use in noisy electrical environments while providing stable readings. Fluke 381 True RMS AC/DC clamp meters feature CAT IV 600V and CAT III 1000V ratings and its ergonomic design fits in your hand and can be used while wearing protective equipment.

What once took two people, now takes one

Allows one technician to do the job that would require two people. This remote display allows the user to clamp around a conductor, detach the display. Furthermore, they can walk across the room to configure controls or remove protective equipment, all while watching real-time readings.

The iFlex® Flexible Current Probe (included) expands the measurement range to 2500 A AC while providing increased display flexibility. With a cord that runs 6′, you can finally measure those tight spaces, awkward sized conductors and disorganized wires.

Measurement capability

• 1000 A AC and dc current measurement with fixed jaw
• 2500 A AC current measurement with iFlex flexible current probe
• 1000 V AC and dc voltage measurement
• True RMS ac voltage and current for accurate measurements on non-linear signals
• Frequency measurement to 500 Hz with both jaw and iFlex
• 60 kµ resistance measurement with continuity detection
• Min, max, average and inrush recording to capture variations automatically

Features

• Wireless technology allows the display to be carried up to 30′ (9.15 m) away from the point of measurement for added flexibility without interference with measurement accuracy
• iFlex™ flexible current probe expands the measurement range to 2500 A ac while providing increased display flexibility, ability to measure awkward sized conductors and improved wire access
• CAT IV 600V, CAT III 1000 V safety rating
• Integrated low pass filter and state of the art signal processing allows for use in noisy electrical environments while providing stable readings
• Proprietary inrush measurement technology to filter out noise and capture motor starting current exactly as the circuit protection sees it
• Ergonomic design fits in your hand and can be used while wearing personal protective equipment
• The removable magnetic display can be conveniently mounted where it is easily seen
• Radio transmitter automatically turns off when the display is connected to the meter
• Large, easy to read, backlight display automatically sets the correct measurement range so you do not need to change switch positions while taking a measurement
• Safety conformance: IEC/EN 61010-1:2001, 1000V CAT III, 600V CAT IV

The Fluke 381 True RMS AC/DC clamp meter ships complete with:

• 18″ iFlex current probe
• Instruction card
• Safety information sheet
• Soft carrying case
• Test leads (TL75)
• (5) AA alkaline batteries

How to measure signals using test probes

To measure ac or dc voltage:

1. Turn the meter’s dial to the proper voltage function ().
2. Connect the black test lead to the COM terminal and the red test probe to the V terminal, indicated by  on the Fluke 381.
3. Measure the voltage by touching the probes to the desired test points of the circuit.
4. View the reading in the display.

To measure resistance or continuity:

1. Turn the dial to .
2. Remove power from the circuit being tested.
3. Connect the black test probe to the COM terminal and the red test probe to  terminal.
4. Measure the resistance by touching the probes to the desired test points of the circuit.
5. View the reading on the display.

If the resistance is <30Ω, continuity is indicated by a beeper continuously sounding. If the display reads OL, the circuit is open or the resistance being measured is greater than the meters resistance range.

Visual of an AC/DC voltage measurement

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To measure frequency (on the Fluke 381):

1. Turn the dial to .
2. Center the jaw or flexible probe around the measurement source.
3. Push the yellow shift button ( on the Fluke 381) to shift to Hz.
4. View the measurement in the display.

What is True RMS?

“RMS” stands for root-mean-square. It comes from a mathematical formula that calculates the “effective” value (or heating value) of any ac wave shape. In electrical terms, the ac rms value is equivalent to the dc heating value of a particular waveform—voltage or current. For example, if a resistive heating element in an electric furnace is rated at 15 kilowatts (kW) of heat at 240 V ac rms, then we would get the same amount of heat if we applied 240 V of dc instead of ac.

Electrical power system components such as fuses, bus bars, conductors, and thermal elements of circuit breakers are rated in rms current because their main limitation has to do with heat dissipation. If we want to check an electrical circuit for overloading, we need to measure the rms current and compare the measured value to the rated value for the component in question.

If a current clamp is labeled and specified to respond to the true-rms value of current, it means that the clamp’s internal circuit calculates the heating value according to the rms formula. This method will give the correct heating value regardless of the current wave shape.

Certain low-cost current clamps, which don’t have true rms circuitry, use a short cut method to find the rms value.

These meters are specified to be “average responding-rms indicating.” These meters capture the rectified average of an ac waveform and scale the number by 1.1 to calculate the rms value. In other words, the value they display is not a true value, but rather is a calculated value based on an assumption about the wave shape. The average responding method works for pure sine waves but can lead to large reading errors up to 40 percent, when a waveform is distorted by nonlinear loads such as adjustable speed drives or computers. The table below gives some examples of the way the two different types of meters respond to different wave shapes.

Some true-rms clamp meters are ac coupled, which gives the rms value of only the ac component of a waveform. (This dates from the time when a majority of measurements in the electrical industry were predominately sinusoidal with no dc offset.) To measure the rms with an ac coupled clamp meter, first measure the rms value of the ac component. Then measure the waveform on the dc scale. Combine the ac and dc components by squaring each, adding the results, and then extracting the square root. The function AC+DC in Fluke true rms clamp meters essentially does the calculation for you.

A comparison of average responding and true-rms units