Many meter manufacturers and others ask why use a Magnelab standard, pre-burdened voltage output current transformer (CT) for monitoring power instead of the traditional 5 amp output current transformer (CT) that’s been used for decades. There are several similarities and differences between the two CT types–the similarities are that both CT’s operate under the same principles of magnetic theory, they both provide isolation between the primary and secondary conductors, and both generally use a single turn primary winding. Because the differences are more complex, I’ll expand on the characteristics of each.

Accuracy:
The accuracy of a 5 amp output CT depends solely on the number of turns on the secondary winding. There are no adjustments that can be made to the CT once the winding is complete. Any adjustment for accuracy must be made in the measuring device. This means that the turns must have a zero tolerance and the measuring device must have a high accuracy shunt or be tunable in order to achieve accurate readings.

The accuracy of a voltage output CT is adjusted at the manufacturer’s site using calibrated equipment. Both voltage and phase shift can be trimmed to the specified tolerance so no field calibration is required.

Safety:

The 5 amp output CT presents a safety hazard if the secondary is open while the primary is active. This condition will produce a high voltage spike across the secondary winding. A shorting block is required when installing the unit to avoid this large voltage potential on the secondary.

The voltage output CT is internally shunted and will only produce the rated voltage (0.333V) across the secondary, even when installed on an active primary with the secondary open. This protects the operator from high voltage spikes.

Low Current:

In general, 5 Amp output devices are only usable down to around 50 amps without multiple turns on the primary winding. Anything lower will usually causes issues with accuracy.

A voltage output CT can accurately measure down to fractions of an amp depending on the design. The secondary turns can be adjusted along with the burden shunt to create a highly calibrated signal voltage output for precision measurement.

Lead extension:

The lead-length and/or wire gauge can be an issue on a 5 amp output CT. The gauge of the leads must be large enough to carry 5 amps without developing a significant resistance causing a large voltage on the leads. This voltage will increase the magnetic flux levels in the core of the CT which will increase losses and error.

The lead-length of a voltage output CT has little to no effect on the output signal. Because the voltage is set within CT, the leads carry very low current levels and therefore very little voltage is developed.

Eric Christensen

Magnelab, Inc.

Director of Engineering and Sales