The short answer
Ground Loops in Instrumentation
A ground loop occurs when a signal circuit is connected to earth at more than one point and those points sit at slightly different potentials. The difference drives current through the signal wiring, adding an error that typically appears as noise, drift, or an offset that changes when nearby equipment runs. The fix is to establish a single ground reference for each signal circuit.
Key points
- Two earth connections at different potentials drive current through your signal path.
- Ground a cable shield at one end only, normally the panel end.
- Symptoms often correlate with a VFD, a large motor, or welding nearby.
- Isolators break the loop when you cannot remove the second ground.
- Never lift a protective equipment ground to fix a signal problem.
Why two grounds are a problem
Earth is not a single equipotential surface. The ground at a lift station two hundred feet from the plant is not at the same potential as the ground at the motor control center, and the difference varies with load, soil moisture, and whatever else is drawing current. It may be millivolts. During a fault or a large motor start it can be volts.
Connect a signal circuit to earth at both ends and you have created a conductive path between two points that disagree about what zero is. Current flows through the signal wiring to equalize them, and that current is superimposed on your measurement.
How to recognize one
| Symptom | Points toward a ground loop when |
|---|---|
| Reading noisy or wandering | Noise appears or worsens when a nearby drive or motor runs |
| Fixed offset error | The instrument reads correctly on a bench but not when installed |
| Error changes with weather | Wet soil changes ground resistance and shifts the reading |
| Multiple loops affected at once | Several signals from the same remote panel misbehave together |
| Improves when a wire is lifted | Disconnecting one end of a shield changes the reading |
| Worse on long runs | Distant panels have the largest ground potential difference |
Finding it
- 1
Characterize when it happens
Trend the signal and correlate it against drive run status, motor starts, and time of day. A noise source that switches on and off is far easier to identify than one you observe only once.
- 2
Measure potential between grounds
With a meter on AC volts, measure between the panel ground bar and the ground at the far end of the circuit. Anything more than a few hundred millivolts is worth pursuing. Do this carefully and treat both points as potentially energized.
- 3
Look for the second ground
Trace the signal circuit end to end. Common culprits: shield landed at both ends, a transmitter case bonded to a grounded pipe while its signal common is also grounded, a surge protector referencing local ground, or a spare conductor bonded at a junction box.
- 4
Test by removing, not by adding
Lift the suspected second ground on the signal circuit and watch the reading. If it settles, you have found it. Never lift a protective equipment ground as part of this test.
- 5
Confirm the fix under load
The condition that caused the problem has to be running. A loop that is quiet with the drive stopped proves nothing.
Preventing it in design
- Ground each cable shield at exactly one end, normally the panel end where the reference is established. Insulate and terminate the far end so it cannot contact anything.
- Establish a single signal reference point per panel, and land analog commons there rather than at scattered points.
- Keep instrument cable out of trays and conduits carrying drive output conductors. Cross at right angles where crossing is unavoidable.
- Use twisted shielded pair for analog signals. The twist rejects magnetically coupled noise; the shield handles capacitive coupling.
- Specify isolated analog input channels where the signal originates in a remote panel with its own ground system.
- Use signal isolators on any circuit crossing between separately grounded structures.
Isolators
A loop isolator galvanically separates the input side from the output side, usually with an optical or transformer coupling, and passes the 4-20 mA value across without a shared conductive path. There is no ground loop because there is no continuous circuit.
Use one where the second ground cannot be removed: a transmitter bonded to a grounded process pipe, a signal arriving from another building, or equipment supplied by a vendor whose internal grounding you do not control. They are inexpensive relative to the time spent chasing an intermittent reading, and they should be on the standard bill of materials for any signal leaving the building.
Frequently asked questions
- Should a cable shield be grounded at one end or both?
- One end, for analog instrumentation signals, normally the panel end. Grounding both ends creates exactly the path this article is about. High-frequency network cabling follows different rules; do not apply this guidance to Ethernet.
- Which end should the shield be grounded at?
- The end where the signal reference is established, which for a loop-powered transmitter is the panel supplying the loop. Be consistent across the site and document it, because mixed practice is worse than either convention.
- Can a ground loop damage equipment?
- Usually it degrades a reading rather than causing damage. During a fault or a lightning event, however, the same path can carry very large current and destroy analog inputs. Surge protection and isolation on circuits leaving a building are worthwhile for this reason.
- How do I tell a ground loop from induced VFD noise?
- They often occur together and both correlate with drive operation. Induced noise couples into the cable and responds to routing, distance, and shielding. A ground loop responds to how many points the circuit is earthed at. Lift one shield end: if the reading settles, it was a loop.
Related topics
- Signal IsolationIsolating analog signals: loop isolators and their variants, splitters that feed two receivers, converters, the common-mode problem isolation solves, ratings, accuracy and response costs, and when an isolated input module makes the isolator unnecessary.
- 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.
- VFD Noise on Analog SignalsAnalog readings that jump or wander when a drive runs, and speed references that make a drive hunt: how the drive output couples into signal wiring, why the input turns kilohertz noise into a slow wander, the tests that pin it on the drive, and the fixes.
- Shield Grounded at Both EndsA shield connected to ground at the panel and again in the field becomes a conductor between two grounds, and its current becomes noise on the pair inside. How to find the second connection, what to measure before lifting it, and when both ends are correct.
- HARTDigital data riding on the 4-20 mA loop: how HART signaling works, what a handheld or a HART-enabled input can read, multidrop and burst modes, and the wiring conditions that make HART fail while the current loop keeps working.
- Pulse SignalsPulse outputs for totalizing: flowmeters, energy meters, rain gauges, and feeders, output types, scaling in volume per pulse, counter inputs, pulse width and rate, debouncing reeds, totalizer retention and rollover, and verifying against the register.
Direct contact
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