RACO RTU Analog 4-20ma Loop Wiring

This FAQ provides examples and general guidance for connecting a RACO RTU to an analog 4-20ma loop circuit. There are a myriad of devices, systems, and connection options and while it's impossible to cover every variation, these examples should be sufficient to successfully wire your system.

The information shown below is intended to serve as a guide. Please consult a systems integrator that's familiar with the existing loop configuration and abide by all safety and building codes.

4-20ma Current Loop Connection Principles

Active and Passive devices: Every 4-20ma loop requires a power source. The power source can be supplied by a standalone power supply, by the sensor's transmitter, or by any of the devices in the loop. (For simplicity, a sensor plus its transmitter will be referred to simply as a sensor). A device that can provide loop power is called an active device. Devices that don't provide power are passive devices. A loop can only have one power source which can be either a power supply or an active device. Connecting multiple active devices or a power supply in addition to an active device can cause malfunctions and even equipment damage.

A loop can have many passive devices wired in series as long as the power source can supply sufficient voltage. The voltage required is equal to 20ma divided by the total loop resistance. The loop resistance is equal to the sum of the resistances of all the loop devices plus the resistance of the wire in the loop. Vps = 20ma / (loop devices resistance + loop wire resistance). Typically the voltage is in the 12 to 24vdc range.

Loop inputs: A passive device can have either a single ended or differential input. A single-ended input has one of its input terminals connected to ground. Since that terminal cannot supply current it must be the last device in the loop. It must be connected directly to the power supply or active device's common (-) terminal.

All of the RACO RTUs are single-ended inputs except for the Verbatim. Connecting the RTU's common (C) terminal to anything other than the loop's power source will cause incorrect reading on any devices in the loop and may lead to equipment damage.

A differential input does not have either terminal connected to ground and thus can be located anywhere in the loop. A variation on the differential design is an isolated input. In this device, the current loop is electrically isolated from the rest of the electronics in the device. It works similarly to a standard opto-isolator.

single_ended_input

Differential Input

Grounding: Since the last device in the loop should be grounded, the RACO common is connected to chassis ground. If the power supply is also grounded, this could generate ground loops and lead to erratic and erroneous reading. It is recommended that the power supply common be lifted from ground to prevent that.

Wiring Diagram Examples

These diagrams show the typical wiring for common configurations. A Catalyst is shown but this wiring will work with any RACO RTU. The common (C) of the RTU must always be connected to the minus of the power supply or active device, except for Verbatims.

Verbatim RTU: The Verbatim RTU is unique among RACO RTUs as it has differential inputs. This adds flexibility the Verbatim to be installed anywhere in the loop.

Active Sensor: This is the simplest system which is composed of an active sensor connected to an RTU.

Active Sensor + Passive PLC: This arrangement includes a passive PLC.

active_sensor_passive

Passive Sensor + Power Supply + PLC: This is similar to the previous example but uses a passive sensor and gets power from a power supply. Notice that the connections to the sensor have reversed. The general guideline is that the active device (+) goes to the passive device (+) and likewise their (-) connections are connected together. Passive devices are connected in series with the (+) of one device connected to the (-) of the next passive device in daisy-chain fashion.

Active PLC + Passive Sensor: This uses an active PLC and passive sensor.

active_plc

Examples of Incorrect Wiring: These are some examples of wiring that WILL NOT work with RTUs except for the Verbatim. Due to its differential inputs it can be placed anywhere in the loop as a passive device.

incorrect01

RTU Connections and Resistance

These diagrams detail the wiring connections for each specific RTU model. The red and black wires here represent the red and black wires in the connection examples.

rtu_connections