The short answer
Signal Isolation
A signal isolator passes a 4 to 20 milliamp or voltage signal from one circuit to another with no electrical connection between them, so that a ground potential difference, a common-mode voltage from a drive, or a fault on one side does not appear on the other. It comes in several forms: a loop-powered isolator that takes its power from the receiving loop and passes a transmitter signal into it; an output-powered isolator that powers a two-wire transmitter on its input side and drives the output from the panel supply; a four-wire isolator with its own supply that isolates input, output, and power from each other; a splitter that takes one signal and produces two isolated outputs for two receivers; and converters that also change the signal form. Each adds a small error and a small delay, needs mounting, power, and fusing, and has an isolation rating that must exceed the voltage it will see. Where the controller input module has isolated channels, the isolator is unnecessary for the ground loop case; it remains the answer for feeding two receivers, for a grounded transmitter into a non-isolated input, and for signals from drives and other panels.
Key points
- An isolator passes the signal and blocks the ground path; it cures ground loops, common-mode voltage, and fault propagation.
- Choose the form by who powers what: loop-powered, output-powered for a two-wire transmitter, or four-wire with its own supply.
- A splitter feeds two receivers from one transmitter without putting them in series.
- Isolation adds error and delay; check the accuracy and response time against the loop needs.
- Isolated input channels remove the need for an isolator in the simple ground loop case.
Forms
| Type | Power | Use |
|---|---|---|
| Loop-powered isolator | From the receiving loop; no separate supply | A transmitter signal from a field or another panel into a panel loop; simplest to install |
| Output-powered isolator | Panel supply on the output side; provides loop power to a two-wire transmitter on the input side | A grounded or remote two-wire transmitter feeding a non-isolated input |
| Four-wire isolator | Its own supply, isolated from input and output | Signals from drives and instruments with their own power; full three-way isolation |
| Signal splitter | Panel supply | One transmitter feeding a controller and a chart recorder, or two controllers, each isolated |
| Converter | Panel supply | Current to voltage, voltage to current, or a range change, with isolation |
| Intrinsically safe isolator | Panel supply | Galvanic isolation and energy limiting for a hazardous area loop |
What it solves
- Ground loops
- A transmitter grounded at its mounting and a receiver grounded at the panel share a ground path through the earth; the potential difference drives a current that adds to the signal. The isolator breaks the path.
- Common-mode voltage
- A signal from a drive or a device with its own electronics sits at a voltage above the panel common; a non-isolated input reads the voltage as signal or is damaged. The isolator floats the input side.
- Fault propagation
- A surge or a short on the field side stays on the field side; the isolator sacrifices itself before the input module.
- Two receivers
- Putting two receivers in series on one loop works until the loop voltage runs out or one receiver is grounded; a splitter gives each its own isolated loop.
- Signal form
- A device with a voltage output feeding a current input, or the reverse, through a converter that isolates as well.
Costs
- Accuracy: a tenth of a percent of span is typical; check it against the loop tolerance, and include it in the loop error budget.
- Response time: milliseconds to tens of milliseconds; irrelevant for level and flow, relevant for a fast pressure loop.
- Power: a supply and a fuse per isolator, and heat in the panel.
- Failure: one more device that can fail open, short, or drift; a calibration point on the schedule.
- Isolation rating: the voltage between sides the isolator withstands; specify above the worst common-mode voltage or surge it will see.
Installation
- 1
Where
At the panel where the signal enters, on the rail, near the terminal strip of the loop, after the surge protector if the loop leaves the building.
- 2
Which side is grounded
Decide and draw it: one ground per side, and the isolator between. An isolator with both sides grounded at the same point isolates nothing.
- 3
Power
From the panel 24 volt supply through a fused terminal; the loop it powers is then a panel loop.
- 4
Range
Configure or order the input and output ranges; a 4 to 20 into 4 to 20 isolator is the common case, a range change is a converter.
- 5
Calibrate
Two points through the isolator, with a calibrator on the input and a meter on the output, at commissioning and on the loop schedule.
- 6
Document
The isolator on the loop drawing and the I/O list, with its type, its power source, and which side is grounded.
Diagnosing an isolator
- Output at 4 milliamps with the input live: the isolator has no power or has failed; check the fuse and the supply.
- Output tracks the input with an offset: calibration, or the isolator drifting; recalibrate and watch.
- Output correct on the bench and wrong in the panel: the isolation is bypassed by a second ground, or the common-mode voltage exceeds the rating.
- Output noisy: the input side is picking up noise the isolator passes; the isolator does not filter, and the cable and shield are the fix.
Frequently asked questions
- Do I need an isolator on every loop?
- No. Loops powered from the panel, grounded only at the panel, into a non-isolated module with all channels on the same common, work without one. Isolate the loops that cross grounds, come from drives or other panels, or need two receivers.
- Can I put the isolator at the transmitter end?
- A field-mounted isolator or an isolated transmitter output does the same job at the other end and is used where the panel cannot be changed or the common-mode problem originates at the panel. It needs power and an enclosure in the field.
- What is the difference between an isolator and a surge protector?
- A surge protector clamps transients to ground and is connected to ground on purpose; an isolator blocks the ground path. A loop that leaves the building has both: the protector at the entrance, the isolator after it.
- Why does the reading change when I ground the shield at the field end?
- The shield became the ground loop path; a shield is grounded at one end only, and the reading change proves the loop needs isolation or a correct shield ground rather than a second one.
Related topics
- Ground Loops in InstrumentationTwo grounds at different potentials, one signal circuit between them, and a reading that will not sit still. How to find it and how to prevent it.
- IsolationIsolating the controller from the field: why grounds differ and what that does to signals, signal isolators and isolated channels, isolation transformers and their limits, relays as isolation, fiber as the ultimate isolator, and when isolation is worth it.
- 4-20 mA Signals from the Instrument SideHow a 4-20 mA current loop is powered and wired at the transmitter, why the loop resistance budget matters, and how to check a loop with a meter.
- How to Diagnose a Ground LoopProve that a noisy or offset analog signal is a ground loop, find the second ground, and fix it the right way: remove the extra ground, isolate the signal, or correct the shield termination. With the measurements that make the diagnosis certain.
- Surge Protection for Signal CircuitsProtecting 4-20 mA loops, discrete inputs, network cables, and radio feeds from lightning and switching surges: where surges enter, the device types and how they clamp, the grounding that makes them work, the resistance and capacitance they add, and where to put them at a remote site.
- 4-20 mA Signal Is UnstableAn analog reading that will not settle. How to separate a genuine process swing from induced noise, a ground loop, or a loop running out of voltage.
Direct contact
Have a controls question?
Reach Eric Sullivan directly about anything on this site, a controls or automation topic, or one of his personal projects.