LOGO! 12/24RCE 230V Overvoltage Damage: Diagnose & Replace

LOGO! 12/24RCE 230V Overvoltage Damage: Diagnose & Replace

Field-engineering reference for assessing a Siemens LOGO! 12/24RCE that has been exposed to 230 VAC on its 12/24 VDC supply input. Covers internal protection topology, failure mechanism, step-by-step verification testing, replacement criteria, and prevention best practices. Source manual: SIMATIC LOGO! Manual (A5E00228550-01).

1. Incident Summary

A Siemens LOGO! 12/24RCE logic module from a starter kit (12/24RCE base unit + KTP400 panel) was inadvertently connected to 230 VAC mains on its 12/24 VDC power terminals (L+ / M). Exposure time was less than one second before the upstream 10 A installation fuse cleared the fault. After fuse replacement the device continued to function, but emitted a brief puff of smoke and a burnt smell that dissipated over several minutes. Visual inspection later showed a black residue on the back side of the power and relay PCB that could be cleaned off but indicated localised component destruction.

This article answers three questions that the original field-engineer raised:

1. Did the LOGO! 12/24RCE actually survive the 230 VAC event, or is the visible smoke / smell masking latent damage?
2. Does the LOGO! 12/24RCE contain explicit overvoltage protection such as a VDR / MOV that saved the unit?
3. Should the unit be replaced, and is there any official Siemens repair path?

2. LOGO! 12/24RCE Hardware Overview

The LOGO! 12/24RCE belongs to the LOGO! 8 family (currently distributed as LOGO! 8.3, MLFB 6ED1052-1MD08-0BA0; older 0BA8 hardware: 6ED1052-1MD00-0BA8). It is a smart relay / small-PLC intended for machine and building-services automation. Its 12/24 VDC supply, eight digital inputs (four of which double as 0–10 V analog inputs), and four relay outputs (10 A) place it in the price / performance band targeted at sub-S7-1200 applications.

The full specification set, including EMC immunity and over-voltage category, is published in the SIMATIC LOGO! Manual (A5E00228550-01), sections A.1 through A.9. The Portuguese-language variant is available as SIMATIC Manual de instruções do LOGO! on the same support portal.

3. Internal Power Supply Architecture

The 12/24 VDC input is fed into the LOGO! 12/24RCE at the removable terminal block, pins L+ and M. From there, the supply chain runs through three PCBs: the main CPU board, the input-conditioning board, and the relay-output board. Functionally the power chain can be modelled as:

1. Reverse-polarity protection: P-MOSFET (low-side) or Schottky diode in series with the return path.
2. Input EMI filter: common-mode choke + X / Y capacitors, both for conducted emissions and for transient suppression.
3. Transient clamp: a bidirectional TVS diode (and possibly an MOV) across L+ and M, sized for the 24 V working voltage with a clamping voltage typically at 33 – 36 V.
4. Inrush element: a small NTC thermistor or fixed resistor in series with the bulk capacitor to limit charging current from a discharged capacitor.
5. Bulk capacitor: one or two aluminium-electrolytic capacitors, 220 – 470 µF, 35 V rating.
6. Switching regulator: a buck or buck-boost controller IC with associated inductor, switching typically at 100 – 500 kHz.
7. Secondary LDOs: 5 V and 3.3 V linear regulators for the CPU, the display controller, and the Ethernet PHY. The relay coils are driven from the same 24 V rail through driver transistors.

Figure 1 - Simplified LOGO! 12/24RCE power chain; blocks highlighted in red are the components most likely to fail during a 230 VAC over-voltage event.

The relay-output board is a separate PCB fed from the same 24 V rail via the main CPU board connector. An over-voltage event on L+ / M therefore stresses the relay-board bulk capacitor and the relay-coil driver transistors as well.

4. Protection Components: VDR, MOV, and Fuses

Siemens documentation for the LOGO! 12/24RCE does not explicitly identify a "VDR" (voltage-dependent resistor) on the 24 VDC variant. The colloquial use of "VDR" in field discussion usually refers to a metal-oxide varistor (MOV), a bidirectional transient suppressor whose resistance drops sharply once its rated clamping voltage is exceeded. In a 24 VDC consumer PLC the equivalent function is normally performed by a bidirectional TVS diode (TransZorb), with an MOV used in higher-energy industrial products.

Looking at the LOGO! 12/24RCE design, the practical input protection is:

• Bidirectional TVS diode, working voltage ≈ 30 V, clamping voltage ≈ 40 V peak.
• Possibly a small MOV (typically 30 Vrms / 38 Vdc continuous) in parallel with the TVS as a coarse clamp.
• Reverse-polarity P-MOSFET or Schottky diode (steady-state, not over-voltage protection).
• No user-replaceable internal fuse; protection relies on the upstream installation fuse.
• Common-mode choke and X / Y capacitors that absorb high-frequency transients, not 50 Hz mains.

The 10 A fuse that blew in the described incident is the external installation fuse required by wiring practice and EN 61131-2. It cleared the fault before the upstream distribution-board MCB tripped. A 10 A rating is several times higher than the steady-state LOGO! input current (≈ 200 mA at 24 V), so the fuse does not protect against a sustained 230 V event; it only opened because the failed internal components drew enough fault current to clear it in < 1 s.

5. Overvoltage Failure Mechanism

Applying 230 VAC (peak 325 V) to a 24 VDC input subjects every component in Figure 1 to voltage well outside its rating. The failure sequence depends on the exact internal design and the dynamic impedance of the upstream wiring, but a representative cascade is:

Figure 2 - Waveform of V(L+, M) when 230 VAC is applied; the dashed envelope in the real case is filtered by upstream wiring impedance.

1. t = 0 – 8 ms (first half-cycle): bulk capacitor charges through the input filter and any NTC. The bidirectional TVS (if present) conducts above its breakdown voltage and dissipates energy as heat. The electrolytic capacitor sees 325 V peak instead of 28 V.
2. t = 8 – 50 ms: capacitor over-voltage causes electrolyte vaporisation and pressure build-up. The score-line vent opens, venting electrolyte vapour; this is the visible "smoke" and the source of the burnt smell. The capacitor is now electrically degraded with elevated ESR.
3. t = 50 – 200 ms: the buck-controller IC and the secondary LDOs see over-voltage on their input pins. Avalanche breakdown or latch-up destroys the controller. The TVS / MOV may fail short and present a near-zero-ohm fault to L+ / M.
4. t = 200 ms – 1 s: the failed short-circuit path pulls hundreds of milliamps at 230 V. The external 10 A fuse clears, opening the circuit and arresting further damage. The PCBs already carry localised carbon residue ("black dust") that was observed on the back side of the power and relay board.

Even when the fuse clears in < 1 s, the bulk capacitor and the buck controller are already damaged. Subsequent operation with correct 24 V supply can appear normal because the regulator's output filter and control loop mask partial degradation, until a thermal or load event exposes the latent fault. Field experience shows that "survivor" units typically fail within weeks of the event.

6. Damage Assessment Procedure

Before powering the module from a current-limited bench supply, perform the following non-powered checks on the suspect LOGO! 12/24RCE:

1. Visual inspection (lens): look for bulging, venting marks, or residue around the electrolytic capacitors on both the CPU / power board and the relay board. A magnifying lens will reveal vented electrolyte as a sticky amber film.
2. Resistance check at L+ / M: with the supply disconnected, measure resistance between L+ and M with a DMM set to "kΩ". A healthy LOGO! 12/24RCE measures 10 kΩ – 100 kΩ (input through the bulk capacitor + reverse-polarity FET). A reading below 200 Ω indicates a shorted TVS, MOV, or input rectifier and confirms permanent damage.
3. Resistance check L+ to PE: should read > 1 MΩ. Lower values indicate insulation breakdown or copper migration from a carbonised fault.
4. Resistance check M to PE: should read > 1 MΩ, same rationale.
5. Smell test: electrolytic-capacitor venting leaves a sweet, chemical residue smell that lingers. If the smell is still detectable 24 h after the event, internal venting occurred.
6. Black residue check: the original poster reported "black dust" on the back of the power and relay PCB. This is carbonised PCB substrate from a sustained arc and indicates that local temperature exceeded 250 °C for a fraction of a second. The board cannot be considered conformal-coated or production-grade after this exposure.

7. Verification Tests

If the non-powered checks pass, power the module from a current-limited lab supply set to 24.0 V with a 500 mA limit. Monitor supply current while running each step. The test matrix in Table 2 is the minimum commissioning gate before any suspect unit is returned to service.

8. Field Commissioning Notes

When replacing a suspect LOGO! 12/24RCE on a starter kit with a current-production unit, observe the following:

• Program transfer: pull the microSD card from the suspect unit and insert it into the new base. The program, retentive tags, and web-server password are stored on the card. The LOGO! 12/24RCE will boot the program from the card automatically.
• IP address preservation: if the original program used a non-default IP, the new unit will adopt that IP from the microSD card. Confirm with a ping before connecting to the plant network.
• Real-time clock: the CR2032 cell is shipped separately in the slot of a new unit. Remove the insulating tab before commissioning.
• Firmware version: verify the firmware version (visible under LOGO! > Diagnostics > Firmware on the display). If the suspect unit was running an older firmware, update the new unit to the same version with LOGO!Soft Comfort V8.3 to keep the program byte-compatible.
• Web-server password: the LOGO! 8.3 supports per-user passwords. Reset on first commissioning.

9. Repairability and Service Policy

Siemens regional offices do not offer repair service for LOGO! base modules. The position communicated by Siemens Stockholm - and consistent with the global policy - is that the per-unit production cost is below the economic threshold for a service exchange. Replacement is the only supported path.

Third-party repair (replacing the bulk capacitor or the buck controller IC, for example) is technically possible but not endorsed because:

• Multiple latent failure sites (bulk capacitor, buck controller, TVS / MOV) may exist after a 230 V event.
• The relay-board connectors are not designed for repeated rework.
• Re-assembled units lose their factory warranty and the CE / UL / RCM conformance is no longer assured.
• EMC behaviour changes once input filtering or layout is disturbed.
• Calibration of the analog inputs depends on factory-trimmed references.

The recommended path is therefore replacement of the complete LOGO! 12/24RCE base unit, restoration of the program from the on-board microSD card or from the engineering backup, and continuation of production.

10. Replacement Sourcing

Order the current production-state (LOGO! 8.3) replacement, not the original 0BA8 variant if the starter kit is several years old. The current MLFB is 6ED1052-1MD08-0BA0. Compatible accessories:

Order from the Siemens Industry Mall or a Siemens Solution Partner. Keep a known-good spare LOGO! 12/24RCE plus a pre-programmed microSD card on the shop floor to minimise downtime.

11. Prevention and Wiring Best Practices

Several wiring practices reduce the probability of a 230 V event on a 24 VDC module and are explicitly required by EN 61131-2 §6.4 (separation of circuits):

1. Colour-coded terminals and conductors: 24 VDC wiring uses distinct terminal blocks (typically blue Phoenix UK-10 N or equivalent) and a different wire colour from 230 VAC wiring (brown / black / grey for phase, blue for neutral). Never route them in the same duct or share the same cable gland.
2. Voltage-identification labels: label every conductor with voltage class using E-Z-Code markers or equivalent.
3. Pre-wired plugs: use Siemens LOGO! pre-wired cable sets with keyed connectors. The 24 V and 230 V cable sets have different keying and are physically incompatible.
4. Pre-commissioning check: verify polarity and voltage class with a DMM before applying power. A second person must witness the check (Lock-Out / Tag-Out style).
5. Regulated SELV / PELV supply: use a regulated, fused 24 VDC power supply from the SITOP or LOGO!Power family rather than tapping mains via a transformer and bridge rectifier. The LOGO!Power 24 V modules cited in Section 10 carry CE / UL / RCM marks for SELV output.
6. Ferruled terminations: use ferrules on all stranded conductors to avoid stray strands shorting adjacent terminals.
7. Spare wiring distance: keep 24 V and 230 V wiring physically separated by at least 100 mm inside the cabinet to defeat accidental contact during commissioning.
8. Maintenance lock-out: lock the cabinet door during commissioning; require a buddy-check before energising.

12. Standards and Ratings

LOGO! 12/24RCE is designed and type-tested against the standards referenced in the LOGO! Manual and the EC Declaration of Conformity. Relevant documents:

• IEC 61131-2 / EN 61131-2 - Programmable controllers, equipment requirements and tests, including 24 VDC interface tests and over-voltage immunity.
• IEC 61010-1 / EN 61010-1 - Safety requirements for electrical equipment for measurement, control, and laboratory use.
• UL 508 / CSA C22.2 No. 142 - Industrial control equipment for the North American market.
• EN 61131-2 over-voltage category II, pollution degree 2 (typical cabinet environment).
• EN 55011 / CISPR 11 Class A for radiated and conducted emissions.

These standards assume the supply is within the declared 10.8 – 28.8 VDC range. Applying 230 VAC is outside the declared ratings and is explicitly not covered by the type-test certificate.

13. Troubleshooting Matrix

14. FAQ

For complete LOGO! documentation, see the SIMATIC LOGO! Manual (A5E00228550-01) on the Siemens Industry Online Support portal.