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Open Source Meter Application Note

Open Source Metering Inc., provides the SCT/UCT-XXX-XXX-4-20B loop powered bundle also known as 4-20B as an offering that allows you to couple a high quality true RMS to 4-20mA process control transducer transmitter to a wide selection of split-core and solid-core current transformers. The paired current transformers have a wide range of current from 1A or less to 3000A or more. In our split-core transformer series there are 5 different sizes that range from an opening of 0.4 inches to a maximum opening of 2.5 x 5 inches. Unlike other 4-20mA current transducers the 4-20B bundles are an off-the-shelf stock item that can ship quickly.

The 4-20B bundle connects a 0.333V RMS output transformer to a transducer that is then converted to a robust 4-20mA processes signal. Typically 4-20mA signals are not affected by capacitive and induced transients and likewise are very resistance to RFI. In general 4-20mA process signals can be used in wire runs in excess of 1000 feet from the power source without experience voltage drops that require corrective action. As wire run length increase it may become more of an issue to manage voltage drops as a result of longer wire runs, these same issues of voltage drop can also be seen in 5A secondary transformer outputs to a bigger extent. With the 4-20B bundle connection from the current transformer to the transducer is a voltage output and represents a very small and safe load. This configuration will not be affected by voltage drops over longer wires runs.

    Some typical applications for the 4-20B bundle are:

  • General RMS Load monitoring
  • Variable Frequency Drive monitoring
  • Fan and Motor monitoring
  • Product can be ordered with SCR support as a custom option.
  • Process Relay Load Control for load shedding applications
  • Process Relay Load Control for protective relay applications

One of the best features of using a 4-20mA loop powered 4-20B bundle is that the power and signal to the transducer is a 2 wire cable, No additional supply cables need to be provided to the transducer. Low cost monitoring systems are commonly found with an excitation voltage for 4-20mA transducers. This allows for a larger number of offerings to meet a wide number of needs.

The typical setup to do General RMS Load Monitoring and RMS voltage monitoring is a Current Transformer or a Voltage Transformer with a 333mV output connected to a 4-20B transducer. The signal power is derived from current or voltage transformer. The distance of the current or voltage transformer to transducer should be kept to a distance of less than 100 feet. The transducer translates the RMS signal to a 4-20mA process control signal that can be transmitted over a 1000 feet distance. The transducers are loop powered and can be used with a loop powered display.
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