There are many ways to measure current. The traditional methods of taking current measurements by inserting a shunt resistor or using a current transformer have the disadvantage of having to break the circuit conductor. For current transformer there are also models that can be installed in place without cutting the cable (more expensive and usually less accurate). It is also possible to use Rogowski coil or Hall sensor to measure current.
Conventional current clamp meters have been available for many years and are an accepted method of non-intrusive current measurement. However these instruments can only measure current in a single core cable. If you have current flowing on multi-core cable, you need to do usually more or less intrusive split (remove cable cover) before a core can be measured.
I have seen articles and advertisements on multi core clamp meter that can measure current form cables with 2 or 3 cores passing through it (two cores carry the current). They have been much more expensive than traditional clamp meters (that can be nowadays bought very cheaply). I have wondered about their working principle of those multi core clamp meters.
Multi-Core Digital Clampmeter is designed to be used anywhere where you want to measure current without breaking a circuit or splitting cables. Megger MMC850 has a complex array of Planer magnetic sensor coils to calculate the current flowing in the conductors of multicore cables with either 2 or 3 cores.
Suparule web page AC Current Measurement in Multi-Core Cable gives an introduction how SMF Flexiclamp technology can measure the current in each conductor. Here is an illustrative picture from that web page.
Flexiclamp Technology current sensing that by sensing the magnetic field at multiple points, and combining this information with the information about the distance between the conductors, as well as the distance from each sensor to each conductor. Flexiclamp Technology white paper shows also picture of the sensor. It resembles quite much on the sensor technology used inside open jaw meters.
Then for some time I have wondered if there is a DIY method that could be used to measure current on mains cables like. I have made some experimenting with placing some sensing coils to different places around cable, but I have not came out with anything specifically useful or consistent.
Wouldn’t it be nice if you could measure current without having to mess with cords or plugs at all. There are small hall effect sensors with linear output that can be used to measure magnetic fields. I thought that it might be possible to use these to measure the current in common electrical wires such as lamp cords.
The wires on the cable create opposite magnetic fields (that pretty much cancel each other at long distance). The trick used here is to to get the sensor physically closer to one wire than to the conductor.
Two linear hall effect sensors, in surface mount packages are mounted in close proximity to the conductors. They are oriented so that each sensor is closer to one of the wires in the cable. The AC voltage from the sensors is then subtracted, so that common mode noise will drop out, and filtered with a low-pass filter to eliminate any signals greater than the power line frequency.
The output of the whole sensors is a DC voltage that is proportional to the current flowing in the power cable. The technology implementation used on this sensor seems to be quite different than used on patented Flexiclamp Technology. I consider this as a good thing because if this sensor works well there are no patent lawyers trying to stop you from using in in some real application.
The sensor will sense 10 watts at the low end (83 mA @ 120V) or 1500 watts (12.5 A @ 120 volts) on the high end.
The sensitivity of the meter looks quite OK to me. It is in line with many cheaper clamp meters and open jaw meters.
Here is picture from Current Sensor blog article to illustrate how to use the sensor:
Modern Device shop sells the current sensor device with following description: This a little current sensor we designed with the idea of providing an isolated current sensor for mains voltages. It uses some Hall Effect Sensors with a lot of gain applied, to sense the magnetic field generated by current in a wire. The sensor output is just an analog voltage which is proportional to the current sensed. This sensor runs at either 3.3 or 5 volts and is easy to calibrate. The output is fairly linear.
This looks like product that I need to check out and test how well it performs.