PLC Analog Wiring Best Practices: Termination & Shielding

1. Overview and Scope

Industrial control cabinets combine logic-level signals (24 VDC discrete), analog signals (0-10 V, ±10 V, 0-20 mA, 4-20 mA, RTD, thermocouple), high-voltage AC (120/240/480 VAC), and switched-mode power wiring in close quarters. Analog circuits operate on millivolt resolution at the ADC; even a few hundred microamps of stray current through a cable shield can saturate or offset a measurement. This reference consolidates field-proven wiring practice for analog I/O on Siemens S7-200 and S7-400 PLCs (and analogous systems) covering conductor termination, shield bonding, segregation, current-loop design, and the specific problem of sharing one analog transmitter across two CPUs when the communication ports are already saturated.

Two recurring failure modes drive every recommendation below:

1. Intermittent connections from stressed wire strands at terminal screws that present months or years after commissioning.
2. Common-mode and normal-mode noise injection through improperly bonded cable shields and lack of physical segregation from VFD and AC power runs.

Both modes can usually be traced to a specific panel-build decision, and both are preventable at install time.

2. Wire Selection and Color Coding

Stranded tinned copper (Type III or V per ASTM B8) is the default for cabinet wiring because it survives vibration better than solid. For most analog runs, 18 AWG (0.823 mm²) or 20 AWG (0.518 mm²) is adequate; derate to 16 AWG (1.31 mm²) when the run exceeds 100 m or carries a loop-powered 4-20 mA signal at the high end of the supply budget.

Color coding must match the cabinet standard and the inspection regime. The most common conventions are:

3. Termination Methods: Bare Wire vs Insulated Ferrules

Two termination styles dominate European and North American panel building:

1. Bare stranded wire inserted directly under the terminal screw or spring clamp.
2. Insulated bootlace ferrule (crimp-on connector) crimped with a square or trapezoidal crimp tool.

Both are accepted; both fail when applied incorrectly.

3.1 Bare Wire Pitfalls

• Twisting the strands before insertion concentrates bending stress on the outer strands. The bundle becomes round; the terminal saddle is flat. The strands are pinched and fatigue over thermal cycles. Typical failure mode: one strand breaks inside the jacket, contact area decreases, resistance rises, the analog channel reads high-impedance or noisy.
• Stripping too long leaves bare copper exposed past the terminal block, increasing the chance of a stray strand bridging adjacent terminals.
• Stripping too short results in the screw clamping on insulation rather than copper, leading to a high-resistance joint that may pass continuity at install and fail in service.

2.2 Insulated Ferrule Advantages

A properly crimped ferrule (e.g., Phoenix Contact AI 0.5-8 WH, Weidmüller 0.5-2.5mm², Wago 216 series) produces a square, gas-tight end that:

• Eliminates stray "hairs" — the plastic collar confines every strand.
• Maximizes contact area under the terminal saddle.
• Prevents the installer from clamping on insulation because the ferrule collar acts as a positive stop.
• Is mechanically repeatable with a calibrated four-indent crimper (e.g., Knipex 97 53 09, Weidmüller PZ 4).

Crimp failure modes exist but are different from bare-wire failure:

• Crimp on insulation (ferrule too long for strip length) — the plastic crimps flat but the copper is loose inside. Always strip to the ferrule barrel length exactly.
• Strand breakage inside the insulation from using ratcheting pliers instead of a proper square crimper. The break is invisible; resistance rises over time.
• Bending stress concentration at the ferrule shoulder if the wire is unsupported. This is a real failure mode in high-traffic areas where operators reach into the panel, pull on wires, and then re-seat them. Mitigate with cable duct and tie-downs.

3.3 Ferrule Selection Table

4. Shield Grounding and Drain Wire Termination

For low-frequency analog signals (DC to a few kHz), the shield provides protection against capacitive (electric-field) coupling only when it carries no current. The rule:

Reasons in detail:

1. Ground loops. If both ends of a shield are bonded to different ground references (even if they measure only a few millivolts apart), the small potential drives current through the shield. The shield now has current-induced voltage that magnetically couples into the inner conductors via mutual inductance. Result: 50/60 Hz hum or low-frequency drift on the analog channel.
2. Noise injection. Any transient current on the shield (from a VFD, a relay coil, or a nearby lightning strike) is inductively coupled into the signal pair. Magnitude scales with dI/dt and the mutual inductance, often 0.5-2 µH per meter of parallel run.
3. Shield continuity is preserved by capacitance. Even when the ungrounded end is cut flush and covered with heat shrink, the foil plus drain wire continue to act as a Faraday cage at high frequencies. The ungrounded end should not contact any conductor or chassis; heat shrink or insulating boot is mandatory.

4.1 Termination Procedure at the Cabinet End

1. Strip 50 mm of jacket from the analog cable.
2. Fold the foil shield back over the jacket.
3. Twist the drain wire (if present) with the bare shield braid.
4. Land the combined drain on a dedicated shield terminal block (e.g., Phoenix Contact USLKG 4 or Weidmüller WPE 4) that is bonded to the cabinet ground bar via a short (≤50 mm) green/yellow jumper.
5. Apply a cable tie at the gland plate so the cable cannot be pulled out and the shield bond cannot be strained.

4.2 Termination Procedure at the Field End

1. Cut the drain wire and foil shield flush with the cable jacket.
2. Cover with adhesive-lined heat shrink (e.g., Raychem ATUM-A-12/3) so no stray strand can contact a terminal or chassis.
3. Verify with a megohmmeter that the field-end shield is > 1 MΩ to chassis ground and to the signal pair.

5. Cable Segregation and Routing in Panels

Per NFPA 70 (NEC) Article 725.55 and NFPA 79 (Electrical Standard for Industrial Machinery), conductors of Class 1 (120/240/480 VAC power) and Class 2/3 (analog and low-voltage signal) circuits must be physically segregated. The intent is to keep the magnetic field from a power conductor (and especially from a VFD output cable) from inducing common-mode voltage in a signal pair.

Inside cabinets where space is tight, use split wire duct with a metal divider (e.g., Panduit FS2X2LG6NM with FSD2-L divider). Inside the panel:

• Color-code the wire duct by voltage class: white for < 50 V signal, grey for 120 VAC, black for 480 VAC.
• Mount the white duct on one side, black on the opposite side, and let grey occupy the middle band.
• Route analog and communication wiring on the side furthest from VFD input/output wiring. If the VFD is mounted in the same cabinet, route analog through duct along the back wall while VFD input power runs down the side gutter.

6. Twisted Pair, Shielded Pair, and Noise Rejection Math

Twisted pair cable is the foundation of any analog run. The twist enforces a uniform geometric relationship between the two conductors so that any external magnetic field induces equal voltages in both wires. The differential receiver at the PLC cancels the induced common-mode voltage because the signal is taken as V+ minus V−.

6.1 Twist Rate

6.2 Cable Specifications for 4-20 mA

A typical Belden 8761 (or equivalent) 1-pair shielded instrument cable has:

• 22 AWG (0.324 mm²) tinned copper, 7-strand
• Twist rate 4 turns/ft (13 turns/m)
• Aluminum-polyester foil shield + drain wire
• Capacitance: 130 pF/m conductor-to-conductor, 230 pF/m conductor-to-shield
• Resistance: 53 Ω/km loop (round trip)

6.3 Common-Mode Rejection Ratio (CMRR)

A S7-400 SM331 analog input module with 16-bit resolution has a CMRR of ≥ 80 dB at DC and ≥ 60 dB at 50/60 Hz. To preserve this in the field:

• Use shielded twisted pair with one-end shield grounding (Section 4).
• Keep the signal pair within the same cable run; never pull only the +ve signal and use a chassis return for the −ve signal.
• Any unused pair in a multi-pair cable must be grounded at one end only and insulated at the other end, exactly like the active pair, to prevent accidental coupling.

6.4 Voltage Drop in a 4-20 mA Loop

For a 2-wire loop (transmitter powered from the receiver side), the loop power budget must satisfy:

V_supply > V_drop + V_burden + V_transmitter_min

Voltage drop formula:

V_drop = 2 × L × R_per_km × I_max / 1000

where L is one-way length in meters, R_per_km is the loop resistance per km, I_max is 0.020 A for a 4-20 mA signal, and the factor 2 accounts for the return path.

Worked example using Belden 8761 (53 Ω/km loop) over 500 m at 20 mA:

V_drop = 2 × 500 × 53 × 0.020 / 1000
        = 1.06 V

With a 24 VDC supply and a S7-400 SM331 burden of 250 Ω (5.0 V at 20 mA), the transmitter needs a minimum of 9 VDC headroom. A 4-20 mA loop transmitter such as a Rosemount 3051 typically needs ≥ 10.5 VDC. The 24 VDC supply minus 5.0 V burden minus 1.06 V drop equals 17.94 V available, which satisfies the transmitter minimum with 7.4 V margin. If the run were 1500 m, V_drop would rise to 3.18 V and the margin would shrink to 5.76 V — still adequate but approaching the limit. Above 2000 m with 22 AWG, derate to 16 AWG (13.2 Ω/km loop) to recover budget.

7. 4-20 mA Current Loop Wiring Topologies

7.1 2-Wire vs 4-Wire Transmitter

Always re-confirm conductor count at the device terminal. A 4-wire transmitter mistakenly wired as 2-wire will not power up; a 2-wire transmitter wired with separate power will saturate the input.

8. Sharing One Analog Signal Between an S7-200 and an S7-400

The recurring field problem: a single field instrument (sensor or transmitter) must be read by two Siemens CPUs in the same cabinet. The DP/MPI/Ethernet ports on both PLCs are already used for HMI and other peer traffic, so a digital peer link is not available without adding hardware. Three engineering options are workable without major rebuild:

8.1 Option A — Analog Re-driver via S7-200 AO

1. Read the sensor into the S7-200 via an analog input module (e.g., 6ES7 231-0HC22-0XA0 EM231, 4AI, 0-10 V / 0-20 mA).
2. Scale the engineering value in the S7-200 ladder or STEP 7-Micro/WIN program and write it to an analog output module (e.g., 6ES7 232-0HD22-0XA0 EM232, 4AO, 0-10 V or 0-20 mA).
3. Wire the EM232 output terminals (M0, V0, I0 referenced as M, V, I) to a S7-400 analog input module (e.g., 6ES7 331-7KF02-0AB0 SM331, 8AI, 13-bit resolution).
4. Configure the SM331 input range to match the EM232 output: 0-10 V, ±10 V, 0-20 mA, or 4-20 mA depending on wiring.

This re-driver approach has real limits:

• Accuracy stack-up. Each ADC and DAC has a quantisation error plus zero/span drift. Two conversions reduce the effective resolution.
• Common-mode voltage. Both PLCs must reference the same 24 VDC return, otherwise the EM232 output common-mode voltage can exceed the SM331 input range. Tie M of the EM232 to M of the SM331 with a dedicated 2.5 mm² jumper to the cabinet ground bar.
• Single point of failure. If the S7-200 stops, the S7-400 loses the signal. Acceptable for non-critical mirroring; unacceptable for safety interlocks.
• Update latency. EM232 conversion time is typically 1 ms; SM331 conversion time for the 8AI variant is 10-100 ms per channel depending on integration mode. End-to-end latency for the loop is 50-200 ms.

8.2 Option B — Add a PROFIBUS Slave Module to the S7-200

The 6ES7 277-0AA22-0XA0 EM277 PROFIBUS-DP slave module lets the S7-200 appear as a DP slave on the S7-400 PROFIBUS network. The S7-200 reads the sensor internally and exposes its data as PROFIBUS I/O. The S7-400 reads it over the existing DP network using standard I/O mapping — no analog re-driver, no extra copper run. Configuration steps:

1. Mount EM277 on the S7-200 expansion bus.
2. Set the PROFIBUS address on the EM277 rotary switches (range 1-99, default 2).
3. Import the EM277 GSD file (siem808d.gsd) into the STEP 7 HW Config on the S7-400 side.
4. Configure I/O bytes (typically 8-32 bytes in/out) to expose the analog value.
5. On the S7-200 side, ladder logic moves the AIW register value into the V-memory area mapped to the EM277 output buffer.

This is the cleanest solution if a PROFIBUS port or a free DP node address is available.

8.3 Option C — Industrial Signal Splitter / Isolation Amplifier

When the S7-200 and S7-400 must remain galvanically isolated (different grounding systems, different cabinet sections, or different plant areas), use a dedicated 4-20 mA splitter or isolation amplifier:

These accept one 4-20 mA input and provide two independent 4-20 mA outputs, each isolated to 1.5-2.5 kV. The S7-200 reads output 1; the S7-400 reads output 2; either CPU failure does not affect the other. Loop burden is typically 50-100 Ω, accuracy 0.1 % of span.

8.4 Option D — Wire the Single Sensor into Both PLCs in Parallel

For 0-10 V or 4-20 mA sources with high drive capability, it is sometimes possible to wire the sensor output to two analog input channels in parallel:

Sensor + ──┬── AI channel 0 of S7-200 EM231
           └── AI channel 0 of S7-400 SM331
Sensor − ──┬── M of EM231
           └── M of SM331

Caveats: the source must be capable of driving both burdens in parallel; the two ADC grounds must be at the same potential within the input common-mode range of the SM331 (typically ±5 V). A small (10 Ω) series resistor in each leg limits cross-talk and helps the source drive the additional load. This arrangement is the cheapest but the most fragile; use only when both PLCs share a common ground bus and the source is a true voltage or current source with low output impedance.

8.5 Decision Matrix

9. Mechanical Integrity: Torque, Strain Relief, Bend Radius

A correctly torqued terminal screw is as important as the wire prep. Under-torque produces a high-resistance joint that drifts with thermal cycling; over-torque strips the screw or deforms the saddle. Always use a calibrated torque-limiting screwdriver.

Bend radius for analog cables should be ≥ 6× the cable OD; for PROFIBUS cables, ≥ 8× OD per PROFIBUS installation guideline. Sharp 90° bends at terminal landings fatigue the copper and breach the shield. Use cable duct with adequate depth (≥ 60 mm for a 40-conductor run).

Strain relief: every cable entering the cabinet via a gland should be clamped within 150 mm of the gland so that pulling the cable outside does not transfer force to the terminal block. Use multi-hole cable glands (e.g., Murrplastik M20 × 4) to keep power and signal cables separated at the gland plate.

10. Commissioning and Verification Procedure

1. Visual inspection. With the cabinet de-energized, walk every analog terminal. Verify the ferrule is fully seated, no copper is exposed past the terminal, and the saddle is on copper, not insulation. Verify every drain wire is landed on a single shield terminal and not in series with another shield.
2. Continuity. With a low-voltage ohmmeter, ring out each analog pair end-to-end. Acceptable reading: 50-60 Ω/km at 20°C for 22 AWG. Open circuit indicates a broken ferrule or a missed termination.
3. Shield-to-ground resistance. With a megohmmeter at 100 V or 500 V, measure between the cabinet shield terminal and the cabinet ground bar. Reading should be ≤ 1 Ω, indicating a low-impedance bond. Measure between the field-end shield and any conductor at the field instrument; reading should be ≥ 1 MΩ, indicating the field end is floating.
4. Loop current verification. Connect a calibrated mA meter in series with the loop at the transmitter terminals. Force 4.000 mA from the transmitter (if supported) or apply a controlled pressure/temperature and verify 4.000 mA. Then force the full-scale input and verify 20.000 mA. Verify the PLC ADC reads the corresponding engineering value (e.g., 0 °C at 4 mA, 100 °C at 20 mA).
5. Noise check. With the process at a steady state, log the analog value over 5 minutes. Peak-to-peak noise should be ≤ 3× the ADC LSB. Higher noise suggests an ungrounded shield, a missing shield bond, or a routing violation. For a S7-400 SM331 in 16-bit mode over 0-10 V, the LSB is 10 V / 65535 ≈ 0.15 mV; noise > 0.5 mV is a concern.
6. Functional verification. Force the PLC output to 0 % and 100 % from the HMI; verify the field device (valve, VFD speed reference) responds linearly. For S7-200 → S7-400 re-driver, force the S7-200 AIW, observe the S7-200 AQW, then observe the S7-400 AIW.
7. Documentation. Mark up the as-built drawing with final wire colors, ferrule crimp tool setting, torque value applied, and shield bond location. File the commissioning checklist in the maintenance system.

11. Troubleshooting Matrix

12. Applicable Standards

• IEC 61131-2:2013 — Programmable controllers: equipment requirements and tests. Defines EMC immunity and emission limits that the cabinet wiring practice must support.
• IEC 60204-1:2016 — Safety of machinery: electrical equipment of machines. Defines wire colors, segregation, and protective bonding.
• IEC 60446:2007 — Basic and safety principles for man-machine interface, marking and identification of conductors.
• NFPA 70 (NEC 2023) Article 725 — Class 1, 2, and 3 remote-control, signaling, and power-limited circuits. Mandates segregation between power and signal conductors.
• NFPA 79:2024 — Electrical standard for industrial machinery.
• UL 508A:2018 — Industrial control panels. Required for North American panel listings.
• ISA-RP7.7 — Recommended practice for producing instrument loop diagrams.
• IEEE 518:2018 — Guide for the installation of electrical equipment to minimize electrical noise inputs to controllers from external sources.