Measuring Small Currents Using a Clamp Multimeter

The typical clamp-type ammeter is designed to measure large currents. An example is the Amprobe/ Fluke clamp ammeter type AMP-330 as shown in Figure 1. This instrument is perfectly capable of measuring an AC or DC current up to 1000 A. Yet, there are times when we want to measure a much smaller current. The technique described in this post will allow you to increase the resolution and sensitivity of the meter to match the smaller currents.

Figure 1: Picture of the Amprobe AMP-330 meter with a 10-wire loop passing through the aperture. The actual reading is approximately 500 mA.

Clamp Ammeter Introduction

The clamp type ammeter is designed to measure the current passing through a wire. It does this by sensing the intensity of the magnetic field surrounding the wire. As a crude conceptualization, the ammeter may be visualized as a current transformer with a single wire passing through the primary. As the current increases in the primary (wire passing through the aperture) so too does the current detected by the secondary which is measured and then displayed by the meter. In practice, one or more Hall effect sensor(s) (semiconductor magnetic sensors) are used to measure the magnetic field.

The best attribute of the clamp style meter is its ability to measure current without disturbing the circuit. The meter’s jaw may be opened and placed directly around a large diameter wire.

Tech Tip: Remember to pass a single wire through the ammeter’s jaw. For example, we could measure either the hot or the return wire. Assuming there is no ground fault, the current will be the same in both wires. A rookie mistake is to place both wires into the jaw with a resulting meter reading of zero. In this erroneous setup, the magnetic field in each wire has opposite “polarity” resulting in a near cancelation of the field.

Measuring Small Currents

The problem with a meter designed to measure 1000 A is that is has trouble resolving low currents. The solution is to increase the current as seen by the meter. This is easily done by passing the wire through the meter’s aperture N times.

From the meter’s perspective, this is a multiplication process where the meter displays the wire’s current multiplied by the N times the wire passes through the closed jaws.

As an example,

• assume 2.5 A is passing through the wire

• pass the wire thorough the meter’s aperture four times

• the meter will display 10 A

• divide the meter reading by the number of wire passes to obtain the true reading of 2.5 A

As a practical matter, it may be easier to pass the wire through the meter 10 times as shown in Figure 1. This makes the math easier as we simply divide the meter’s reading by 10. In Figure 1 the displayed current of 4.96 A is actually 0.496 A RMS.

If you anticipate measuring the current for consumer devices, you may be interested in the line splitter shown in Figure 2. The two windows provide convenient access to a single and a ten-turn loop of wire.

Figure 2: TPI’s (Test Products Int) A202 line splitter contains the 10-turn loop making it easy to measure the current consumption of consumer devices.

Tech Tip: This multiple pass technique is generally applicable to all current measuring devices through which a wire is passed. The number of turns is governed by the limitation (saturation) of the current sensor. Do be mindful of inrush and other current spikes. Lingering magnetic hysteresis effects can cause undesirable measurement offsets.

Safety

No discussion of high-power test equipment is complete without a discussion about safety.

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For readers who are new to high power electrical systems, we should take a moment to recognize the serious nature of our actions. A mistake could cost money, production time, damage to equipment, fire, or even hurt a person.

As a starting point for all work that involves machinery or high voltage systems, there are some critical stipulations:

• you are qualified to perform the work

• you understand the risk associated with industrial systems including but not limited to electrical shock arc & blast, mechanical, entanglement, chemical, heat, fluid injection, and fire.

• you have informed your supervisor about non-standard modifications or operations on a piece of equipment

• you, along with your supervisor and other essential personnel, perform a risk assessment and safety analysis prior to performing the work

• you have proper Lock Out Tag Out (LOTO) processes in place

• you follow all applicable employer, state, and federal guidelines

Concluding Trivia

As we close this post, please take a close look at the line splitter in Figure 2. The datasheet for the splitter calls out a “test area” directly around the X1 and X10 windows.

Why?