Summary
The Magnelab SCT series of current transformers (CT’s), also known as current transducers, are designed to produce a safe usable output when properly installed around a primary conductor carrying AC current. In most cases the output is pre-burdened to a small output voltage linearly proportional to the primary current. Although a non-burden version is also available if the end user wants to burden to output. These outputs are then connected to various types of measurement or data collection devices.

Magnelab current transformers are an important component when measuring AC currents. They are a safe non-invasive way of measuring the amount of current in each conductor, or multiple conductors of the same phase, to determine power consumption, power usage or signal quality.

The SCT series is an opening style of CT so the circuit to be measured does not need to be interrupted. And because the output is preburdened to a safe low voltage there is no danger when assembling them onto an active conductor with the secondary leads open.

The SCT line comes in 5 different sizes to fit a large range of conductor sizes and current ratings. See Table 1 for the 5 sizes, current rating and turns ratio. This will help determine the proper SCT for the conditions of the measurement requirements.

Please refer to the individual specification sheets for mechanical specifications.

Table 1
SCT Size Turns ratio Max Rated Current (A)
SCT-0400 3000:1 75
SCT-0750 7500:1 200
SCT-1250 7500:1 600
SCT-2000 7500:1 1500
SCT-3000 8400:1 3000

The operation of a typical current transformer is simple magnetically coupled windings with a one turn primary, the conductor passing through it, and an output current determined by the primary current divided by the number of turns of the secondary winding. Due to the high turns’ ratio of the SCT’s, the currents in the secondary windings are exceedingly small.

Using the output current determined by the ratio, Magnelab can then select the proper resistance for the burden to set the output voltage to the desired level. Care must be used to ensure the power rating of the resistor circuit is much higher than the power dissipated in the circuit to make the SCT safe in an over current situation. The windings are made using fine wires and will open if the current in the secondary approaches 150% of the maximum rated current, but the resistor circuit must be able to handle the power levels until the winding opens.

There is a non-burdened option used by some customers when the voltage is to be controlled by the measuring device. In this instance, a bi-directional diode is installed across the output internal to the SCT to clamp the voltage to 22 volts maximum.