Symptom
Noise on an analog signal or errors on a communication link that appear when a particular motor, drive, or contactor operates; signals that were clean until a new cable was pulled in the same conduit or tray; a panel where the analog wiring runs through the same wireway as the motor leads.
The short answer
Cable Routing Problems
A signal cable picks up noise from a nearby power or drive cable through capacitance, which couples voltage changes, and through mutual inductance, which couples current changes; both grow with the length of the parallel run and shrink with separation. The fix is separation by signal class: low-level analog and communication cables in their own conduit, tray, or wireway, control wiring in another, power in another, and drive motor cables kept farthest of all, with crossings at right angles. Find the problem by correlating the noise with the equipment that produces it and then walking the cable route to find where the two share a path, including inside the panel.
Key points
- Coupling grows with parallel length and shrinks with distance; a long run in a shared tray is the worst case.
- Drive output cables are the noisiest thing in the plant; keep them in their own conduit and far from everything.
- Inside the panel counts: the last three feet of analog wiring next to the motor leads undoes a clean route outside.
- Cross at right angles where crossing is unavoidable; parallel runs are where the coupling happens.
- Steel conduit and separate trays are cheaper than filters and isolators added afterward.
Possible causes and what to check
| Possible cause | What to check |
|---|---|
| Signal cable in the same conduit as power or motor conductors | Pull the conduit schedule or look at the conduit entries; a signal cable entering the panel through the same conduit as the motor leads is the answer. |
| Long parallel run in a shared tray | Walk the tray. Analog and communication cables laid on top of power cables for tens or hundreds of feet couple enough to matter even with shields. |
| Drive motor cable routed with signal cables | Trace the motor cable from the drive to the motor; every foot it shares with a signal cable is a coupling path. Noise that changes with drive carrier frequency confirms it. |
| Signal wiring bundled with motor leads or contactor wiring inside the panel | Open the wireway covers. Analog wiring in the same wireway as the drive output or the motor starter wiring is common in panels built for space rather than noise. |
| Crossing at a shallow angle or running alongside for a short distance at a transition | Look at where cables enter and leave the panel and where trays merge; short parallel runs at transitions add up. |
| Signal cable passing close to a transformer, a large contactor, or a drive | Magnetic fields from these fall off quickly with distance; a cable routed across the back of a transformer or a drive heat sink picks them up. |
| Unshielded or untwisted signal cable making the routing matter more | A shielded twisted pair tolerates a poor route that an unshielded one does not; the routing fault is still the cause. |
How a cable couples into another
Two conductors running side by side form a capacitor, and a fast voltage change on one drives current into the other. The output of a drive switches hundreds of volts in a fraction of a microsecond, thousands of times per second, and a signal cable beside it receives a train of current pulses that the analog input reads as noise. Two conductors also form a transformer: current changes in one induce voltage in the other, which is how a motor starting current or a contactor closing shows up as a spike on a nearby signal. Both effects are proportional to the length of the run where the cables are close and fall off with the distance between them. That is why the rules are about separation and parallel length, and why the first question is always where do the cables share a path.
Separation by class
| Class | What is in it | Route it |
|---|---|---|
| Low-level signal | Analog 4-20 mA and voltage signals, RTDs and thermocouples, communication cables, encoder and pulse signals | Own conduit, tray, or wireway; shielded twisted pair; farthest from drive cables |
| Control | 24 VDC and 120 VAC discrete I/O, relay and contactor coils, pilot lights | Separate from low-level signal; may share with other control wiring |
| Power | Feeders, branch circuits, motor leads from across-the-line starters | Separate from signal and control; steel conduit helps |
| Drive output | Motor cables from drives, and drive input cables where filters are not fitted | Own steel conduit or shielded cable, separated from everything by the largest practical distance |
Manufacturers publish minimum spacing tables that depend on the voltage, the cable type, and whether the cables are in steel conduit, and the numbers range from a few inches for control wiring beside signal wiring to a foot or more for drive output cables in parallel with signal cables over long runs. The drawing for the project should state them. Where it does not, the working rule is: separate conduits or trays for each class, drive cables as far from signal cables as the building allows, and a right-angle crossing where paths must intersect.
Diagnostic procedure
- 1
Correlate the noise
Trend the affected signal at a fast rate and note when the noise appears: when a drive runs, when a pump starts, when a heater cycles. The equipment that correlates is the source, and its cables are what you trace.
- 2
Trace both cables
On the drawings and then in the field, follow the signal cable from the device to the panel and the source cable from its panel to its load. Mark every place they share a conduit, a tray, a wireway, a junction box, or a panel entry.
- 3
Look inside the panel
Open the wireway covers and follow the signal wiring from the terminal strip to the input card. It should not share a wireway with motor leads, drive output wiring, or contactor wiring. Panels that were clean when built accumulate shared routes with every modification.
- 4
Test the suspicion
Where practical, temporarily reroute the signal cable outside the shared path, lying on the floor if necessary, and watch the noise. Or shut the source down briefly. A clean signal during the test confirms the route.
- 5
Separate permanently
Pull the signal cable into its own conduit or tray, move the drive cable, or add a barrier in the tray. Inside the panel, reroute the signal wiring through its own wireway, even if that means a longer path.
- 6
Verify
Trend again with the source running under the conditions that produced the noise. Then record the route on the drawings so the next modification does not undo it.
When separation is not possible
Existing conduit that cannot be added to, a tray that is full, a building that gives no other path: the options are to reduce what couples rather than the distance. Shielded twisted pair with the shield grounded at the panel cuts capacitive coupling by a large factor. Steel conduit around the signal cable or around the drive cable attenuates both coupling mechanisms. Converting the signal to 4-20 mA if it is a voltage signal, adding a filter at the input, or moving the signal to a digital link or fiber removes the sensitivity. And lowering the drive carrier frequency or adding an output filter at the drive reduces the source. Each of these is a compromise compared with a separate route, and the drawing should say which was chosen.
Frequently asked questions
- Can a shielded cable share a conduit with power?
- Codes allow certain combinations, and the shield helps, but it does not make it a good idea. A shield reduces capacitive coupling and does little against magnetic coupling from motor current, and a drive cable in the same conduit will still be heard. Treat a shared conduit as a last resort and keep the drive cables out of it entirely.
- How far apart do drive cables and signal cables need to be?
- Follow the drive manufacturer table, which usually gives spacing as a function of parallel length and cable type, and the values are commonly a foot or more for open runs of any length, less in steel conduit. When in doubt, more. The cost of distance is a few feet of cable; the cost of too little is an isolator on every loop.
- The noise only started after a new cable was pulled. Nothing else changed.
- Then the new cable either is the source, such as a new drive cable pulled through a tray of signal cables, or is the victim, such as a new analog cable pulled through the nearest conduit that happened to carry motor leads. Trace it. Pulling a cable through whatever conduit has room is the most common way a clean plant gets noisy.
- Does crossing at a right angle really matter?
- Yes, because coupling is proportional to the length over which the cables run parallel. At a right angle that length is essentially the cable diameter. A crossing at a shallow angle, or a run that drifts alongside for a few feet before crossing, is a short parallel run, and several of them add up.
Related topics
- 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.
- Unshielded Cable in a Noisy RunThe signal was run in plain conductors through a route full of power and drive cables, and now it is noisy. What unshielded cable cannot do, how to prove the cable is the problem before pulling a new one, and the options when replacing it is not possible.
- Drive Causes Instrument NoiseWhen many instruments became noisy after a drive was installed, the drive installation is the place to look: the checklist for motor cable, shielding, grounding, filters, and carrier frequency that decides how much noise a drive puts into the plant.
- Component LayoutArranging a panel so it is safe to work in, cool enough to run, quiet enough for its signals, and possible to modify: power and control separation, heat, wireways, terminals, and the working space rules.
- 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.
- 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.
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