In electrical testing we would use a clamp meter for the safe testing of live current flow through live circuits and conductors, usually in the form of wires and cables. The clamp meter works by using the integrated hinged jaws to detect the magnetic field produced by flowing current, the measurement of which is then displayed instantly on an LCD display screen. Some dedicated clamp meters only measure AC current, while others are effectively multimeters which can measure both AC and DC current while also carrying out resistance, continuity and frequency tests.
Clamp meters that test both AC and DC current are also known as Hall Effect Clamp Meters, and are named after a method of detecting AC and DC current. The jaws of some clamp meters are wrapped in copper wire, such as a current transformer clamp meter, but the AC/DC clamp meter is different. Instead, the magnetic field produced by the conductor is focused across small gaps in the core after the jaws are clamped around the wire or cable, ie. there are small gaps when the jaws are fully closed and the jaws don’t close flush.
Understanding how an AC/DC clamp meter works is about understanding the “Hall Effect” principle. The gap in the closed jaws creates an air pocket which the magnetic field is then required to jump. This gap creates a limit to the magnetic flux and means the core cannot saturate. With an AC-only clamp meter the jaws are flush when closed and when they are open the tip of the jaws are bare metal, but with an AC/DC “Hall Effect” clamp meter the jaws around the gap are covered in a plastic moulding which houses a semi-conductor known as the “Hall Effect” sensor.
The Hall Effect sensor varies the output voltage in response to the magnetic fields present – in this case the conductor or wire being measured – in order that it can measure the magnetic flux directly. When this is done the output voltage is amplified and scaled to represent the current flowing through the conductor inside the jaws. And because the gap is small, this ensures that the magnetic field remains concentrated across this gap.
Using this method of AC/DC current testing is the safest method available, because it is quick, simple and non-contact, which means the electrical tester can work efficiently while also causing minimal disruption to working practices.
A key feature of an AC/DC current clamp meter is that it is a handheld instrument, and hence it is light and portable and easy to use. It has a pair of hinged jaws, a trigger to operate the jaws, a power switch, operating buttons and a large LCD screen to view results. A dedicated AC/DC current clamp meter such as the TIS 257 is characterised by a large rotary dial on the front which is used to select different measurement modes. There are other clamp meters supplied by Test Instrument Solutions which are dedicated to solar PV testing and specifically for measuring earth leakage, while other models are installed with TRMS for supreme accuracy.
This instrument is widely used by electrical installation engineers and is also used in building maintenance and construction. It is commonly used for the installation, test and maintenance of telephone lines.
Below is a product demonstration video on the TIS 570 AC/DC earth leakage clamp meter:
Please note that this section is for information purposes only. Anyone using equipment referred to in this section must be suitably qualified and/or experienced within the respective field. If in doubt before use, please consult a qualified electrician or engineer & thoroughly read all instruction booklets.
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