Question: We have an existing continuous level gauge on a twisted pair with 4-20 mA. How can we add a separate transmitter to get overfill protection using the same cable?

Answer: Acromag offers two basic 4-20mA alarms that can solve the continuous level overfill application. The first, and most direct, approach is to use a loop-powered alarm, This type of 4-20 mA alarm is powered by the existing wires and offers one SPDT relay output. It's installed in series and can be placed anywhere in the loop. The unit is inherently isolated and failsafe (relay is de-energized in alarm condition and loss of mA signal). Because of its design, the device does not require any additional power wiring and will only add a 225 ohm load to the existing loop.

If the continuous level signal cannot drive a 225 ohm load, then an AC or DC-powered device is recommended. By using an alarm with a separate power source, the user can benefit from a current input with a lower input load (i.e., 50 ohm) or a voltage input (no additional loop load by sensing across an existing device). Acromag models 461A or 361A offer these advantages and more. The alarms feature both voltage and current inputs, one or two 5 A relays, failsafe or non-failsafe relay actuation, full isolation, and an AC or DC power option.