What is an “isolated” 4-20mA signal vs non-isolated?

Problems can occur when non-isolated 4-20mA signal wires have ground paths at both ends of the loop.

When the voltages (V) at the two ground points are different, a circulating, closed current (I) path is formed by the copper wires used for the 4-20mA signal and the ground (Figure 1). When this happens, a ground loop current is introduced into the loop, causing measurement error.

A ground loop forms when three conditions are present:

1) There are two grounds, 2) The grounds are at different potentials, and 3) There is a galvanic path between the grounds.

Eliminating any one of these three conditions will remove the ground loop. However, because the number of grounds cannot always be controlled, and because and it is often impossible to simply “lift” a ground, the first and second conditions are not plausible candidates for elimination.

A signal isolator can be used to “break” the galvanic path between the two grounds (Figure 2). When the conductive path between the differential voltages is broken, the ground loop has been eliminated.

Breaking the Galvanic Path

A galvanic path is a direct electrical connection between two or more electrical circuits that allow current to flow. The job of an isolator is to break the galvanic path between circuits that are tied or “grounded” to different potentials. Breaking this galvanic path can be accomplished by any number of means including electromagnetic, optic, capacitive, inductive and even acoustic methods.

Blue-White products typically use the electromagnetic and optical methods.

Optical Isolation—Optical isolation uses light to transfer a signal between elements of a circuit.

An optical isolation circuit is comprised of two basic parts: a light source (usually a LED- Light Emitting Diode, acting as the transmitter) and a photo-sensitive detector (usually a phototransistor, acting as the receiver). The output signal of the opto-coupler is proportional to the light intensity of the source. The insulating air gap between the LED and the phototransistor serves as the galvanic separation between the circuits.

Optical isolation has better common-mode noise rejection, is usually seen in digital circuits, is not frequency sensitive, is smaller, and can sometimes provide higher levels of isolation than transformer isolation.

Electromagnetic Isolation— Electromagnetic isolation, often referred to as transformer isolation, uses a transformer to electromagnetically couple the desired signal across an air gap or non-conductive isolation gap. The electromagnetic field intensity is proportional to the input signal applied to the transformer. Transformers are very efficient and fast at transferring AC (alternating current) signals.