Current Transformers

Current Transformers (CTs) are use to measure AC amperage in an electrical circuit. CTs are installed around an energized conductor and sense the magnetic field generated by the current flowing in the circuit. The CT's output signal is proportional to the current flowing in the circuit.

Current Transformers are used with the FlexSmart True RMS Module and the Energy Logger Pro to measure current (amps) and with the WattNode to measure power and energy (kW and kWh).

All of the CTs that we sell feature “safe”, low-voltage outputs. This type of CT contains a built-in burden resistor and is calibrated to output exactly 333 mV at the CT’s rated full-scale input current.

Potential Transformer (PTs) are also available for measuring voltage up to 600 Vac. These PTs also feature a 333 mV full scale output voltage which is measured using the FlexSmart True RMS Module.

Two styles of opening CTs are stocked - Split Core and Clamp On Probe. Two additional styles of CTs - Custom Split Core and Toroidal (donuts) - are also available but not stocked.

Select a CT with an opening size large enough to fit around the wire. Our standard CTs are available in three opening sizes and each size is available in various full-scale current ranges.

Select a CT with a current range equal to or greater than the maximum current being measured. Make sure that the minimum current being measured is more then 10% of the CT's rated current. Measurements below 10% of the CT's full-scale rating are not accurate.

For assistance in selecting CTs for your application please call 800-450-4454 or email us at sales@datalogger.com

Split Core CTs are the most popular style that we sell. They are smaller then the Clamp On Probe style CTs and cost considerably less. They work best in crowed electrical panel where space is offen limited. Split core CTs offer the widest range of current ratings and opening sizes. Accuracy is ±1% from 10% to 130% of the rated current.

Clamp-on probe style opening CTs have handles which allow easy installation in an electrical panel using just one hand, which is major safety feature. Handles make the probe style CTs larger than split-core CTs, so they may not fit in some panels. They also cost more, and are only available in three current ranges.

For applicatins requiring larger opening sizes, Custom Size CTs are available. These opening style CTs are made to order in virtually any opening size and current range. They are typically used in bus bar installations. They cost a little more then standard styles but lead-times are typically 4-6 weeks. Call 800-450-4454 or email us at sales@datalogger.com for ordering assistance.

Toroidal (donut) style non-opening CTs are the most accurate, smallest size, and cost less then opening CTs. Their main disadvantage is that installation requires that the circuit being measured be disconnected and the wire threaded through the CT opening. For metering class accuracy (0.25%) watt measurements, toroidal CTs can be supplied with custom calibrated WattNodes. Call 800-450-4454 or email us at sales@datalogger.com for ordering assistance.

CAUTION

Note that most common industrial CTs have a 5 or 10 amp current output that can generate lethal high voltage levels. Only use CTs with a 333 mV full scale voltage output. Using any other type of CT can damage your equipment!

When CTs are used with True RMS Modules to measure current, they can be installed in any physical orientation and it does not matter which color lead wire connects to which terminal. Just wire the black and white leads in any order to the input terminals of the module.

When CTs are used with a WattNode transducer they must be installed in the proper physical orientation and the lead wires must be connected with the correct polarity (color) to the appropriate WattNode terminals.

Note that Magnelab split core CTs have a label that says "THIS SIDE TOWARD SOURCE". When using these CTs with a WattNode, make sure that the label is facing towards the power source typically the circuit breaker or the utility's meter box. Other styles of CT may have an arrow or some other marking to indicate proper polarity. For additional information read the installation guide.

Potential Transformers (PTs) are use with the FlexSmart True RMS Module and the Energy Logger Pro to measure AC voltages in electric systems.

PTs are a precision type of step down transformer with an input and output winding. The input or primary (high voltage) winding is connected between the two phases conductors or between an a phase conductor and neutral of the circuit being measured. The output or secondary (low voltage) winding produces output a signal that is proportional to the primary voltage being measured.

Potential Transformers, like all the CTs that we offer, feature “safe”, low-voltage outputs. Accuracy is ±1% from 10% to 130% of the rated voltage.

Split-core CTs are opened by pulling the removable "I" section straight out this may require a strong pull. When reinstalling the removable portion of the CT, check that the ends mate correctly. Note that the "tongue and groove" ends of the exposed laminated steel core will only mate in one direction. When pressing the CT back together, if it seems to jam and will not close, the steel core pieces are probably not aligned correctly. Do Not Use Excessive Force! Instead, reposition or rock the removable portion until the CT snaps together easily. After a split-core CT has been placed around a wire, a nylon cable tie may be secured around the CT to prevent accidental opening.

Currents lower than 10% of the CT’s full scale rating can be measured more accurately by passing the current carrying wire through the CT’s core more than once. For example, to measure currents up to 1 amp with a 5 amp CT, pass the wire through the CT once, then loop back around the outside of the CT, and pass the wire through the CT again. Repeat until the wire passes through the CT five times. With five primary turns on the CT, its current rating is now one amp. The rule is: divide the full-scale current rating of the CT by the number of turns passing through its core.

Keep the metal pole pieces of current transformers and probes clean at all times! Rust or a dirt particle trapped between the pole pieces creates an air gaps between the jaws, which can result in measurement errors. In damp environments, a light coating of oil or rust inhibitor applied to the mating surface of a CT helps to prevent rusting.

For maximum accuracy, always keep the two halves of a split core CT together when not in use – they are a matched set. The “C” shaped body and the removable “I” section of the CT are calibrated as a single unit. These parts should never be interchanged with another CT.