S7-300 CPU and CP Separate 24VDC: Ground Loop Prevention

Overview

When an S7-300 CPU (such as a CPU 312, 314, 315, 316, or 318) and a Communication Processor (CP) — for example a CP 340, CP 341, CP 342-5, or CP 343-1 — occupy the same mounting rack but receive 24 VDC from physically separate switched-mode power supplies, the system is exposed to equipotential currents. These currents do not originate in the PLC logic; they originate in the metallic path formed between two grounds of slightly different potential. Siemens classifies this strictly as an EMC and installation-grounding issue and resolves it with equipotential bonding, not with extra regulation of the 24 VDC rails.

The corrective procedure is documented in the Siemens S7-300 Installation Manual, entry ID 13008499, in chapter A.2 Protection against electromagnetic interference, section A.2.7 Equipotential bonding. The same rules apply to S7-400 and ET 200M distributed I/O stations attached by PROFIBUS or PROFINET, because the ground-loop hazard exists at every chassis-to-chassis transition.

The Electrical Phenomenon: Ground Loops in 24 VDC Systems

A ground loop forms whenever two points in a low-voltage circuit are referenced to earth at more than one location, and those two earth references are not at the same potential. In a 24 VDC PLC installation the path is built from:

1. The 24 V return (M / negative) terminal of power supply A, bonded to chassis and to earth at point G1.
2. The 24 V return terminal of power supply B, bonded to chassis and to earth at point G2.
3. A signal or PROFIBUS cable between the CPU rack and the CP (or between the CPU rack and an ET 200M), which provides a low-impedance metallic return from one ground system to the other.

Even a 50 mV potential difference between G1 and G2 will drive a continuous current through the loop. With loop impedances as low as 0.5 to 2 ohm — typical for shielded PROFIBUS cable with the shield landed at both ends — the resulting current is 25 mA to 100 mA. That current does no useful work; instead it flows through the input protection diodes of signal modules, through the bus connector backplane, and across the M terminal of isolated modules. Symptoms progress from intermittent serial errors, to CP loss-of-link faults, to chronic damage of 24 V I/O groups.

The cause, as Siemens states explicitly, is "differences in the power supplies" — not switching noise, not inrush, not load transients. Any path that ties two grounds together is a ground loop by definition.

Connecting the Negative Poles Is Not Bonding

A common field assumption is that running a single wire between the M (negative) terminals of the two 24 VDC supplies, or between the M terminals of two CPU racks, fixes the problem. It does not. That single conductor, unless sized and routed as an equipotential bonding conductor, has higher impedance than the loop formed by the shielded PROFIBUS or PROFINET cable. Most of the unwanted current simply bypasses the bonding wire and continues to flow through the data cable shield and the signal conductors.

Siemens Equipotential Bonding: Sizing and Routing

Section A.2.7 of the S7-300 installation manual requires a dedicated bonding conductor between any two system components whose 24 VDC supplies are referenced to different earth points. Apply the following rules when laying out the conductor:

The bonding conductor must be bonded to the chassis ground stud of each rack — not to the 24 V M terminal, not to a DIN-rail mounting bolt that carries paint. A chassis ground stud is a threaded post welded to the subpanel and is the only guaranteed low-impedance connection to the rack frame.

Cable Shield Termination Rules

Bonds and shields work together. Both ends of a PROFIBUS or PROFINET cable shield are normally landed to provide an HF return path. That intentional shield termination is what creates the DC ground loop if the two chassis are at different potentials. The bonding conductor bleeds off the DC potential so the shield no longer carries DC current.

1. Land the shield at both ends with 360-degree low-impedance terminations (PROFIBUS connectors with shield clamp, not the pigtail type).
2. Use shield bus bars or EMC backshells to spread the termination area; do not rely on a single drain wire.
3. Run the bonding conductor between the two chassis ground studs, in the same cable tray as the PROFIBUS cable.
4. If the total span exceeds 30 m, evaluate intermediate equipotential bonding points; do not simply increase conductor cross-section.

The S7-300 manual explicitly warns: "Potential differences can occur between separate system elements. This can result in high equipotential currents, e.g. if the cable shielding is terminated at both ends and grounded to different system components." The bonding conductor is what prevents those currents from exceeding the rating of the I/O protection paths.

Case 1 — New Plant: Clean Earth Wiring

The simplest installation is a greenfield plant where the protective earth grid is designed before the I/O list is locked. The Siemens-recommended approach is a "clean Earth" or instrumentation-ground system, separate from the power protective earth (PE) but bonded to it at a single point.

1. Drive ground rods (copper-clad steel, 16 mm diameter, 2.4 m length) at 5 to 10 m intervals around the plant perimeter.
2. Connect the rods with bare tinned copper conductor, minimum 35 mm², exothermically welded at each joint.
3. Bond every PLC chassis, CP chassis, ET 200M station, and field instrument to the clean Earth grid using 10 to 16 mm² stranded copper.
4. Bond the clean Earth grid to the plant PE at exactly one point, normally at the main incoming 400 V distribution panel.
5. Run the clean Earth conductor in a dedicated tray, separated from 400 V power conductors by at least 200 mm or by a magnetic divider.

With all chassis referenced to the same ground grid, the potential difference between any two racks is reduced to a few millivolts. The bonding conductor can still be installed for redundancy, but its steady-state current will be near zero.

Case 2 — Existing Plant Retrofit

Retrofit is harder because the existing earth grid may be inaccessible, may carry high fault currents, or may have been installed without low-resistance copper. Apply the following sequence:

1. Measure the existing earth-resistance at each affected rack with a four-pole ground tester (e.g., Megger DET4TDR or Fluke 1625). Document the value; reject any reading above 1 ohm relative to the main ground rod.
2. Install an equipotential bonding conductor between the chassis ground studs of the CPU rack and the CP rack as the immediate fix. This corrects the fault without civil works.
3. If the loop current, measured with a clamp-on ground meter (Fluke 360 or AEMC 3711), exceeds 200 mA, investigate the supply paths. One of the 24 VDC supplies is almost certainly referenced to a different ground electrode than the other.
4. Where the two supplies cannot be brought to a common electrode, route a dedicated insulated 25 mm² copper bonding conductor back to the main grounding bus bar (MGB) of the building. Avoid the existing cable trays of AC power circuits.
5. Document the bonding path on the as-built grounding diagram; future maintenance must not break the bond when replacing a single module.

ET 200M and Distributed I/O Considerations

The same bonding rules apply to ET 200M stations, IM 153 interface modules, and PROFIBUS slaves that have their own 24 VDC supplies. A common error is to bond only the CPU chassis and leave the ET 200M chassis referenced to a separate distribution panel in a different building or even a different floor. The PROFIBUS cable shield then carries the loop current directly into the IM 153 and into the backplane connector of every digital and analog input module on that station.

Where the distance between CPU and ET 200M exceeds the reach of copper PROFIBUS (typically 100 m at 1.5 Mbps to 12 Mbps), an optical link (OLM, OBT, or SCALANCE) eliminates the bonding requirement entirely. The same applies to PROFINET when converted to fiber via SCALANCE XF208 or similar media converters.

When Two CPUs Share a Rack but Different Supplies

A second CPU in slot 1, with the original CPU in slot 2, is occasionally seen in redundant S7-400 designs. On an S7-300 rack the second slot is normally used for the CP, but the bonding rule is the same regardless of which module sits there:

1. Bond the rack chassis to the clean Earth or MGB before either 24 VDC supply is energized.
2. Verify with an ohmmeter that the resistance between the chassis ground stud and the MGB is below 0.5 ohm, including the conductor itself and the lug-to-stud contact.
3. Energize supply A first, then supply B. Monitor the loop current at the bonding conductor with a clamp-on ammeter. A reading below 50 mA confirms correct installation.
4. If the loop current is above 200 mA with no load connected, the two supplies are referenced to separate ground electrodes — return to the retrofit procedure.

PS 305 and PS 307 Power Supply Considerations

The PS 305 and PS 307 load power supplies do not break the safety earth path internally. The PE terminal of each supply is bonded to chassis, and the 24 V negative (M) is referenced to chassis through the input filter network. Two PS 307s on the same rack therefore present a Y-connected set of chassis grounds. If the upstream AC feeds are from different isolation transformers or different 400 V bus sections, the chassis grounds can sit at different potentials even with both supplies mounted in the same rack frame.

Two PS 307s in the same rack fed from the same 230 VAC distribution typically show less than 50 mV between M terminals and exhibit no measurable loop current. Two PS 307s fed from different 230 VAC sources, or a PS 305 fed from a separate 110 VDC battery plant, are the configuration that drives the bonding requirement.

Verification Procedure

After installation of the bonding conductor, perform the following checks before powering the CPU and CP together:

1. Visual inspection: Confirm 360° shield terminations on every PROFIBUS / PROFINET connector. Confirm bonding conductor is bonded at chassis ground stud, not at the 24 V M terminal.
2. Resistance check: Measure the resistance between each rack's chassis ground stud and the bonding-conductor termination. Reading must be below 0.5 ohm end-to-end.
3. Voltage check (supplies off): Measure AC and DC voltage between the chassis ground studs of the two racks. A reading above 100 mV AC or above 50 mV DC indicates the racks are not on a common ground reference; investigate before energizing.
4. Voltage check (supplies on, no load): Measure AC and DC voltage between the chassis ground studs with both 24 VDC supplies energized. Target: below 50 mV AC and below 20 mV DC. Higher values mean the bonding is inadequate or another parallel ground path exists.
5. Loop current check: Clamp a clamp-on ground ammeter (resolution 1 mA) onto the bonding conductor. Steady-state reading below 50 mA confirms safe operation; 50 to 200 mA warrants investigation; above 200 mA indicates a separate ground electrode on one supply.
6. Functional check: Bring up the CPU and CP. Monitor the diagnostic buffer of the CPU for SF, BF, or short-to-ground faults on the SM modules. Any persistent fault after the bonding installation indicates damaged hardware that must be replaced.

Common Field Mistakes

Standards and Reference Documents

The bonding requirements for industrial control equipment are derived from generic EMC and installation standards. Verify each clause against the current revision applicable to your jurisdiction:

• IEC 61918:2018, Industrial communication networks — Installation of communication networks in industrial premises, clause 6.6 — equipotential bonding of cable shields.
• IEC 61000-5-2:1997, Electromagnetic compatibility (EMC) — Installation and mitigation guidelines — Earthing and cabling, clause 6 — multi-grounded systems and equipotential bonding.
• IEC 60364-4-41 / IEC 60364-5-54, protective earthing and equipotential bonding for electrical installations.
• Siemens S7-300 Automation System, CPU 31xC and CPU 31x: Installation Manual, chapter A.2.7.
• Siemens EMC Installation Guideline for SIMATIC NET, PROFIBUS Networks, document 6GK1970-0AA00-0AA1.

Module-Level Symptom Matrix

If bonding was not performed at original installation, the following faults will emerge predictably over months rather than at first commissioning:

Spare-Parts Strategy After a Ground Loop Incident

Once a ground loop has been confirmed, replace any module whose input or output has been exposed. The on-state current of typical S7-300 inputs is 7 mA; an internal protection path rated at 200 mA continuous will survive a 50 mA loop indefinitely but will not survive a 300 mA loop for more than a few hours. Triage the replacement list as follows:

1. Replace every module that shows a diagnostic buffer fault related to short-circuit or channel fault.
2. Replace every analog module whose zero-point calibration has shifted by more than 0.05% of full scale; do not attempt to recalibrate in the field.
3. Replace any CP that has logged BF or SF events during the suspected period, even if it currently communicates correctly — the protection diodes may be partially damaged and will fail under the next surge.
4. Do not reuse the original 24 VDC supplies without measuring their M-to-PE voltage under load; if it is outside ±50 mV, the supply is part of the fault.

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