Current measurements have been simplified by the introduction of the Clamp-On Ammeter, which allows readings to be taken without breaking the Circuit. This article outlines how such instruments work and how to get the best out of them.
Until recently the Measurement of Current was not as straightforward as that of voltage. Measurements were made by breaking the Circuit at the Point where the Current was to be measured and inserting a Current Meter. For low Current Circuits less than 10 or 20 Amperes this is still the only practical method. For High-Current A.C. Circuits, a Current Meter usually employed a Current Transformer to give a known, scaled down, representation of the measured Current.
A natural development of this technique was A.C. Clampmeters, which are now manufactured in a variety of different models, makes and types. The basic principle is very simple: a Current Transformer whose single turn primary is formed by the lead carrying the Current to be measured. The Clamp Jaws, which form the Iron Core of the Transformer, carry the Secondary Winding. This Winding normally drives Electronic Circuit and in turn gives a Signal to a Display which reads the Current flowing in the Conductor. Ranging is usually achieved by appropriate shunting the Instrument.
The Instrument is usually held and operated by one hand. The Jaws are opened to Clamp around the Lead being measured by finger pressure on a ‘Trigger’ and a Meter Data Hold Button is often provided to enable Measurements to be made in a Location where it is difficult to see the Meter Display.
Clamp Multimeter has multiple Ranges up to 200A and 1kA. Some Instruments have Range up to 3000 Amperes. Range changing can be achieved by a Thumb Wheel Switch.
Accuracy
As with all Measurements there are a number of variables which can affect the Accuracy. The first effect to consider is, in common with all Measurements, the effect that Insertion of the Meter has on the Circuit. In this case, the reflected Impedance of the Meter and the Circuit through the Current Transformer will usually reduce the Current being measured. But, in most practical cases, since the Meter and Diode Impedances are divided by the Square of the turns ratio of the Current Transformer, the reflected Impedance is very small and has a negligible effect.
The second effect to be considered is that of the Position of the Conductor being measured within the Ring of the Clampmeter Jaws. If the Conductor is small in comparison to the Jaw Ring, the most accurate reading is obtained with the Conductor at the Centre. For a well designed Clampmeter, moving the Conductor radially towards the edge of the Ring has only a very small effect caused by Leakage Flux. This would have the greatest effect when the Conductor is next to the Jaw Aperture.
The third effect is due to the gap between the Jaws when closed. This is caused by the Matting Surfaces not being perfectly flat. This causes Leakage Flux and hence poor Coupling between the Primary and Secondary Windings of the Current Transformer, which results in a reduced reading. But again, this loss is small as the Jaw Faces are usually ground in order to minimise the air gap at the opening point.
A fourth effect is caused by the presence of any A.C. magnetic field, such as those from large Transformers or Electric Motors. Such magnetic fields would not have an effect for for the Jaw Gap. Alignment of the Jaws with the magnetic field will affect the error and hence, if possible, the Clampmeter should be rotated to keep this to a minimum and be situated as far as possible away from sources of A.C. fields.
Another effect is caused by the iron magnetisation. This is a Non-Linear, particularly at the initial magnetisation stage when Low-Level Currents are being measured. At the other extreme, when large Currents are to be measured, core saturation can occur. However, for the Range specified, accuracy is not usually affected.
Also the Frequency of the Current being measured has an effect on the reading. Clampmeters are usually calibrated at the Frequency at which they are to be used. Such as 50 or 60 Hz. Frequency has effects in several ways, for example, leakage resistance, eddy Currents and core saturation. The final effect which need to be considered is Ambient Temperature. This will change the Iron Losses, Jaw Gap and Circuit Impedances. The temperature coefficient in often quoted.
Different Types
As already stated, there are a variety of types on the Market. Some are very simple, having a Low-Current Range which are 25A and 100A models. They also have a voltage measurement facility for normal mains supplies. More sophisticated models have several Ranges of Current, Voltage, Resistance, Temperature, Frequency, Capacitance etc. Data Hold Function enables the readings to be held firm. This is particularly useful where the Clampmeter has to be used in positions which put the meter display out of view of the Operator. Digital models also provide for greater resolution and less ambiguity of reading.
One further type, known as Leakage Clampmeters, are capable of resolving current readings down to 1mA, and can be used for either single or three-phase supplies. For single-phase, if the Meter is clamped around the live and neutral cables together, the Meter will indicate the leakage current to earth. For three-phase work, operation is the same except that the Meter is clamped around all three phases cables together. In this way the Clampmeter will indicate Current in the Neutral Cable and Earth. If the neutral is also clamped, the resultant reading is the Earth-Leakage Current. These can also be used to measure current at low levels in single Conductor.
High Current D.C. measurement has become practical with a Clampmeter. This works on a very different principle. Since Transformer action is not possible for D.C., a Hall Effect Device is used to measure the Magnetic Flux in the Clamp Jaws. This is done by fitting the Hall Effect Device in a gap in the magnetic circuit of the Clampmeter. A reference current is applied across one plate of the Device and another potential is generated across the perpendicular plate. This output potential is proportional to the Reference Current and the Magnetic Flux level, which again is proportional to the Current being measured. Hall Effect Devices work equally well with A.C., but then the output potential is also A.C. The output is measured, using a D.C. or A.C. Digital Meter. All the possible errors discussed above also apply to this type of Meter.
Attachments
Since most electrical engineers and electricians have either an Analogue or Digital Multimeter, Clamp-On attachments for these Instruments have been developed. There are either A.C. only or combined A.C./D.C. types employing the principles already mentioned. In place of the display these attachments are fitted with output terminals for connecting to the Multimeter.
Wattmeters must measure both Current and Voltage simultaneously. Obviously a similar principle to the Clampmeter can be employed to measure the Current. Voltage, however is measured in the a normal way using a pair of Probes and a Voltage Divider. The information from the Current and Voltage measurements has to be processed to give a reading of power (VIcosf).
The simplest type of Instruments use the normal Wattmeter Principle employing a Digital Circuit. The output of the Clamp Circuit and the Voltage Measurement Circuit are used to drive these Circuit and give a display proportional to power.
An alternative method uses Electronic Circuits to process the Current and Voltage Measurements to give an indication of power. A Phase Sensitive Detector can sense the In-Phase Component of Voltage with respect to Current and a Multiplier is used to give the product of this Component and the Current. The output of the Electronic Processor can drive a Digital Display with the aid of an Analogue-to-Digital Convertor.
Another electronic method is to use a Microprocessor to calculate cosf from the sampled readings of Voltage and Current and then calculate the product VIcosf. The output of the Microprocessor is Digital and can drive a Digital Display. Either of the Electronic Methods can also be used to give a reading of the Phase Angle between the Measured Voltage and Current.
Safety
A Final Point which must be considered concerns safety. When considering safety, it is normal to look for a relevant standard specification such as ‘’UL’’, IEC348 or IEC414. For Safety – Double Insulation per UL/IEC/EN61010-1 Ed. 3.0, IEC/EN61010-2-033 Ed. 1.0, CAN/CSA C22.2 No. 61010-1 Ed. 3.0, IEC/EN61010-2-032 Ed. 3.0 and IEC/EN61010-031 Ed. 1.1 to CAT IV 1000V and AC and DC. EMC Meets EN61326-1.2006. For Clampmeters to conform to these specifications they should meet the requirements for Double Insulation and hence, as a rule of thumb, they should withstand a Flash Test of four times the Working Voltage, plus 2kV. This is twice the Voltage required for single Insulation. The Clampmeters should also have Transient Protection of 6-12 KV for Safety of the Instrument and the User. Flash Tests should be carried out between the Jaws and the rest of the Instrument wrapped in Conductive Foil. If there are any Leads of Voltage or Resistance Measurements, they should also withstand the Flash Test from themselves to the Clampmeter body and to the Jaws. If the Working Voltage or Flash Test Voltage is quoted it should be for Double Insulation, and if not quoted, taken as the Single Insulation Figure, for safety.
