Grounding, Shielding and Noise
Protective earth and functional earth, how noise couples, routing and separation, shield termination and suppressing loads.
Two jobs for earth
“Earth” in a panel does two different jobs, and mixing them up causes both safety problems and noise problems.
| Kind | Purpose | Design rule |
|---|---|---|
| Protective earth (PE) | Safety: connects every exposed metal part to the supply earth, so that a fault trips the protective device quickly and touch voltages stay low | Heavy conductors, low impedance, tested; never switched or fused; sized by the code |
| Functional earth (FE) and reference | Function: a reference for signals, shield terminations and EMC filters, and a path for high-frequency noise currents | Short, wide, low impedance at high frequency (a metal plate, a strap, not a long wire) |
| 0 V of the 24 V supply | The return of the control circuits | Connected to PE at one point only, usually at the supply, unless the system is deliberately isolated or monitored |
- Bond every metal part (door, plate, enclosure, rails, cable trays, motor frames) to the PE bar with a short conductor, or by the metal-to-metal contact of bare mounting surfaces. Paint and anodizing are insulators: scrape or use star washers where they are in the path.
- Use one earth system for the machine. A second “clean earth” connected separately to the building earth creates a loop and a dangerous potential difference.
- Star or tree layout: run PE conductors from each device to the PE bar, not daisy-chained from device to device, so that removing one device does not break the earth of the next.
- Do not use a shield as the protective earth.
Worked example: the size of the protective conductor
| Phase conductor S | Minimum PE conductor (typical rule, check the code) |
|---|---|
| 16 mm² or less | same as the phase conductor |
| Over 16 up to 35 mm² | 16 mm² |
| Over 35 mm² | half the phase conductor |
A 25 mm² phase conductor therefore needs a PE of 16 mm² (the rule for 16 to 35 mm²). The PE of a small panel is often larger than the minimum simply because it is easier to fit the same conductor everywhere.
Protective earth and the fault loop
In a TN supply, an earth fault is a short circuit through the earth conductor, and the protective device must trip within the required time (0.4 s for 230 V final circuits, typically 5 s for distribution). It does so only if the fault current is high enough, which depends on the earth-fault loop impedance Zs: the impedance of the supply, the phase conductor and the earth conductor in series.
Zs ≤ U0 ÷ Ia (Ia = the current that trips the device within the required time)
Worked example: how high may the loop impedance be?
| Device | Ia (instantaneous trip, 5 × or 10 × In) | Maximum Zs at 230 V |
|---|---|---|
| 10 A type B | 50 A | 4.60 Ω |
| 16 A type B | 80 A | 2.88 Ω |
| 16 A type C | 160 A | 1.44 Ω |
| 20 A type C | 200 A | 1.15 Ω |
| 32 A type D | 640 A | 0.36 Ω |
- A 16 A type C breaker trips instantly at about 160 A. With a measured loop impedance of 0.8 Ω the fault current is 230 ÷ 0.8 = 288 A, which is above 160 A: it trips at once. Longer cables and thin conductors raise Zs (see Wiring, Cables and Voltage Drop).
- Measure it with a loop tester at the farthest point (see Testing, Commissioning and Acceptance) and compare with the table; if the result is too high, use a larger conductor, a shorter run, or an RCD.
- During the fault, the exposed metal rises to about the voltage drop along the earth conductor: with an earth path that is 20 % of the loop impedance, it is 46 V for the short time before the device trips. Keeping the PE conductor short and thick keeps that low.
RCDs and drives
A residual-current device (RCD) trips on a small imbalance between the phase and neutral currents, which means current leaking to earth. It protects people (30 mA) and, at higher ratings, against fire. But drives and their EMC filters leak by design: the filter capacitors connect to earth.
Worked example: why the 30 mA RCD trips
| Source | Leakage current |
|---|---|
| Drive 4 kW with EMC filter | 30 mA |
| Drive 1.5 kW with EMC filter | 15 mA |
| Switched-mode supplies (three) | 3 mA |
| Total | 48 mA |
An RCD should not carry more than about 30 % of its rated residual current in normal operation, or it will trip on spikes and starts. For 48 mA of normal leakage the RCD must be rated at least 48 ÷ 0.3 = 160 mA: a 30 mA RCD would trip all the time, and the next standard size, 300 mA, is not a personal-protection device. The usual answer is a type B (or B+) RCD sized for the leakage, or removing the filter, or using the protection of the TN system with a fast breaker, in agreement with the drive maker and the code.
How noise couples
| Path | What carries it | What helps |
|---|---|---|
| Conductive (shared impedance) | A current returning through a conductor or earth that the signal also uses | Separate returns, a star earth, a low-impedance plate, isolation |
| Capacitive | Fast voltage edges (dv/dt) and the stray capacitance between conductors | Shield (earthed), distance, lower impedance on the victim, lower dv/dt |
| Inductive | Current changes (di/dt) and the mutual inductance of loops | Twisted pairs, small loop area, distance, magnetic shielding, differential inputs |
| Radiated | High-frequency fields from fast edges, long cables and slots | Shielded enclosure, short cables, filters, ferrites |
Putting numbers on it
Worked example: the size of the noise
- Capacitive. A 230 V line runs beside a high-impedance signal wire with 2 pF of coupling to it, and the wire has 100 pF to earth. The wire picks up V × Cc ÷ (Cc + Cl) = 230 × 2 ÷ 102 = 4.5 V. An earthed shield that takes the coupling away cuts it by a factor of 100 (40 dB) to 0.045 V. A low-impedance (current) input is far less sensitive than a voltage input.
- Inductive. Two parallel runs form a mutual inductance of 2 µH. A current ripple of 2 A in 50 µs induces V = M di/dt = 2 µH × 40 000 A/s = 80 mV in the loop. Twisting the pair cancels most of it: at 30 dB it is 2.5 mV. Against a 4 V signal span (4 to 20 mA across 250 Ω), 80 mV is 2 %.
- Common mode. A differential input with a common-mode rejection ratio of 80 dB turns 1 V of common-mode noise into 0.1 mV of error (0.003 % of a 4 V span): use differential or isolated inputs for long analog runs.
- Ground loop. If the two ends of a shield are earthed at points that differ by 0.5 V (large machines, long cable runs, a welder nearby), a loop current of 0.5 ÷ 1.0 Ω = 0.5 A flows in the shield, and with a transfer impedance of 10 mΩ per meter over 50 m it puts 0.25 V of noise on the signal. Earth an analog shield at one end only, or use isolation.
Routing and separation
| Cable class | Examples | Keep away from |
|---|---|---|
| 1: sensitive | Analog signals, encoders, thermocouples, microphones | Class 3 and 4: by at least 200 mm in a parallel run |
| 2: control and I/O | 24 V DC I/O, relays, field buses (shielded) | Class 4: by at least 100 mm |
| 3: noisy control | AC contactor coils, unfiltered DC valves, switching outputs | Class 1: by at least 200 mm |
| 4: power | Mains, drive input and motor cables, welders | Class 1 and 2: by distance or a metal divider |
Worked example: how much does distance buy?
In the near field, the coupling between parallel runs falls about with the square of the separation. Moving a signal cable from 20 mm to 100 mm from a power cable cuts the coupling by about (100 ÷ 20)² = 25 times, 28 dB (a rough rule, but the direction is certain). A grounded metal divider between them does more than the same distance.
- Separate cable routes and ducts inside the panel and on the machine (see Panel Layout, Heat and Enclosures and Wiring, Cables and Voltage Drop).
- Cross at right angles where a signal cable must cross a power cable.
- Keep loops small: run the outgoing and return conductors together, in one cable.
- Metal trays and conduit act as shields when they are continuous and earthed at both ends; use them for power and noisy cables.
- Short cables: every meter of cable is an antenna and a coupling path.
Shields and how to terminate them
| Cable | Terminate the shield… | Why |
|---|---|---|
| Motor cable from a drive | At both ends, 360° through a clamp to the plate and the motor frame | It carries the high-frequency return current; a pigtail makes it useless |
| Analog signal (4–20 mA, 0–10 V, thermocouple) | At one end (usually the controller end), the other end insulated | Avoids a loop current in the shield |
| Encoder and fast digital signals | At both ends, as the maker says | High-frequency noise needs a path at both ends |
| Industrial Ethernet and fieldbus | At both ends through the connector shell, as the network guideline says | Keep the loop small with an earth bond along the cable; see Industrial Communication |
| Unshielded 24 V control | Not applicable: route away from noise | Cheap and fine if it is short and separated |
- Use a 360° clamp, never a wire “pigtail” from the shield to a terminal: a few centimeters of wire have the impedance to defeat the shield at high frequency.
- Do not interrupt the shield in a connector or terminal block; if it must be broken, bridge it with a wide strap and a shield terminal.
- One shield, one purpose: do not use the shield of a signal cable to carry a supply or a return current.
- Check the shield with a continuity test from end to end and to the plate (see Testing, Commissioning and Acceptance).
- Where a long cable runs between buildings or earth systems, use fiber, an isolator or a repeater to break the loop. A copper Ethernet port has about 1 500 V of isolation through its transformer, but that is not a solution for high potential differences.
Suppressing the noise at its source
| Source | Suppressor | Note |
|---|---|---|
| DC coil (relay, valve) | Flyback diode across the coil (or a diode with a Zener, or a varistor) | The diode makes the release a little slower; fit it at the coil |
| AC coil (contactor) | RC snubber or varistor across the coil | Often supplied as a clip-on block |
| Motor and drive output | EMC filter, output reactor, short screened cable | As in the drive manual |
| Switching supplies | Input filter; keep the wiring short | Buy supplies with the right EMC rating |
| Arcs and brush motors | RC networks, ferrites | At the source, not the victim |
Worked example: the energy in a relay coil
A 24 V, 2 W relay coil draws 83.3 mA. With an inductance of about 2 H the stored energy is ½ L I² = ½ × 2 × 0.083² = 6.9 mJ. Opening the contact that feeds it forces that energy somewhere: without a diode it appears as a spike of hundreds of volts and an arc at the contact, which couples into nearby wiring and wears the contact. The diode lets the current decay safely in the coil and removes the spike.
Drives and motor cables
- Follow the drive maker’s EMC instructions exactly: the right filter, the cable type, the shield terminations, the distance to other cables, and the earthing of the filter and the drive on the same bare plate.
- Keep the motor cable short; where long, use an output reactor or a dv/dt or sine filter (below), because the fast edges and the cable capacitance create reflections and bearing currents.
- Separate the motor cable from every other cable, by 200 mm or a divider, and never run it in the same duct as signals.
- Use a separate earth conductor inside the motor cable (three phases, PE and an overall screen, or symmetrical cable with three PE).
- Bearing currents from drive common-mode voltages can damage motor bearings: insulated bearings or shaft grounding rings for large motors.
| Motor cable length (typical, check the drive) | Output filter |
|---|---|
| up to 25 m | no output filter needed (check the drive) |
| 25 to 100 m | output reactor or dv/dt filter |
| over 100 m | sine filter, and check the motor insulation |
Related
- System Integration: the overview of this guide.
- Wiring, Cables and Voltage Drop: cable choices and shields in the wiring.
- Panel Layout, Heat and Enclosures: the layout zones that keep noise sources apart.
- Testing, Commissioning and Acceptance: earth and shield tests.
- Electronics Fundamentals and Industrial Communication (cabling and noise).
FAQ
Should I earth a shield at one end or both?
Both for the motor cables and for high-frequency signals such as encoders and fast buses; one end (the controller end) for low-frequency analog signals, to avoid a loop current in the shield. Follow the device maker’s instruction.
Why does my analog reading wander when a drive starts?
Noise from the drive is coupling in: capacitively, inductively or through a shared earth. Separate the cables, shield and twist the signal, earth the shield correctly, use a differential or isolated input, and fit the drive’s filter.
Why does the 30 mA RCD keep tripping on my drive?
The drive’s EMC filter leaks current to earth by design. Size the RCD for the leakage (at least three times the normal leakage) and use a type B device, or follow the drive maker’s guidance.
What is the difference between protective and functional earth?
Protective earth is for safety: a heavy, low-impedance path that trips the protective device on a fault. Functional earth is for performance: a reference and a path for high-frequency noise. They connect to the same system but have different design rules.
References
- IEC 60204-1 (equipotential bonding, protective bonding, EMC); IEC 61000-6-2 and -6-4 (EMC generic standards); IEC 61800-3 (EMC of adjustable-speed drives); IEC 60364-4-41 (protection against electric shock).
- Ott, H. W., Electromagnetic Compatibility Engineering, Wiley; Morrison, R., Grounding and Shielding, Wiley.
- Drive and PLC manufacturers’ EMC installation guides; PROFIBUS and PROFINET installation guidelines for shields and earthing.
Last reviewed: October 2026. Impedances, couplings and distances are illustrative: follow the device and drive makers’ instructions and the standards.