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Panel & Electrical·7 min read·

Electrical noise: where it comes from and how to stop it

Capacitive and inductive coupling, the separation that works, screen termination, and the analogue input that reads correctly until the drive starts.

Short answer

Separate signal cable from power and motor cable, ideally by at least 100 mm and in different trunking, crossing at right angles where they must cross. Use screened cable for analogue and network, and terminate the screen over its full circumference at the panel entry rather than as a pigtail. Where separation is impossible, an isolated input or a signal isolator solves it properly.

Sinking and sourcingSINKING (NPN)INPUT CARD+current flows inSOURCING (PNP)INPUT CARD0Vcurrent flows out

Noise problems present as intermittent faults with no pattern, and they are diagnosed last because nothing looks broken.

The mechanisms are few and well understood, and knowing them turns a fortnight of elimination into an afternoon.

The two couplings

Capacitive. Two conductors near each other form a capacitor. A fast changing voltage on one drives a current into the other. Proportional to voltage and to the rate of change, so a drive output, switching hundreds of volts in tens of nanoseconds, is the dominant source in most panels.

Inductive. A changing current produces a magnetic field which induces a voltage in a nearby loop. Proportional to current and its rate of change, and worse for large loop areas, which is why a signal and its return should run together.

Both fall off with distance and both are worse for longer parallel runs. Those two facts are the whole of segregation practice.

Separation, in numbers

As a working rule for panel and machine wiring:

  • Mains and motor cable to signal cable: at least 100 mm, more where the run is long.
  • Drive output cable to anything: as much as possible, and it deserves its own route.
  • Where crossing is unavoidable, cross at 90 degrees. The coupling length is then almost nothing.
  • Separate trunking, not separate bundles in the same trunking.
A perpendicular crossing is fine. A metre of parallel run in the same trunking is not, and the difference is not a matter of degree.

Screen termination

The place most installations lose most of the benefit they paid for.

A screen works by giving induced current a path back to its source that is not the signal conductor. That requires a low impedance connection over the full circumference of the screen.

A pigtail, where the braid is gathered into a twisted tail and run to a terminal, is an inductor in series with the screen. At the frequencies that matter it is close to an open circuit, and the screen stops working.

Use an EMC gland or a screen clamp at the point the cable enters the enclosure. Both ends, for high frequency.

The one end argument

The advice to earth a screen at one end only is not wrong, it is answering a different question.

It addresses low frequency earth loops, mains hum coupling into an audio circuit. It costs you all high frequency screening, because a screen earthed at one end is not a screen above a few kilohertz.

In an industrial panel with drives in it, high frequency is the problem. Terminate both ends. If an earth loop then causes an error, break the loop with an isolated input or a signal isolator, which solves both problems rather than trading one for the other.

Differential and isolated inputs

Two increasingly capable answers, and both are cheaper than chasing a noise problem for a week.

Differential inputs measure the difference between two conductors and reject anything common to both, which is most coupled noise.

Isolated inputs additionally have no galvanic connection to the rest of the system, so no earth loop can form. On a long run to a remote instrument this is the right specification from the start.

Diagnosing one

A sequence that gets there quickly.

  • Does it correlate with a drive, a contactor or a welder starting? Note exactly what.
  • Disconnect the field cable at the input and short the terminals. Does the reading go stable? If yes, the problem is on the cable. If no, it is in the panel or the card.
  • Look at the routing of that one cable for its whole length, particularly where it leaves the panel and where it passes a drive.
  • Check the screen termination at both ends. Physically look at it.
  • Only then consider filters and isolators. They are a fix, and a fix applied before the cause is understood tends to be applied to the wrong cable.

Common questions

Should a cable screen be earthed at one end or both?
For high frequency noise, which is what drives produce, both ends, terminated over the full circumference. The old advice to earth one end only addresses low frequency earth loops and leaves the screen ineffective above a few kilohertz. Where an earth loop is a genuine problem, break it with an isolator rather than by lifting the screen.
Why does my analogue input drift when the motor runs?
Coupling from the motor cable, which is switching thousands of times a second with fast edges. Check the separation first, then the screen termination. A screen terminated as a pigtail at a terminal rather than clamped at entry is the single most common cause.
What is a ground loop?
A circuit formed when a signal cable's screen or common is earthed at two points that are at different potentials. Current flows through the screen, and on a single ended analogue input that current appears as a signal error. Isolated inputs and signal isolators break the loop.

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