Introduction: why the modernization of relay systems is an urgent task
Relay logic remains a working solution at hundreds of Ukrainian enterprises. KMY contactors, RPU-2 intermediate relays, PE-6 timers — these components perform their function for decades. But "works" does not mean "effective". The average downtime due to the failure of one relay in the interlocking chain is from 2 to 8 hours. At the cost of an hour of downtime of UAH 15,000–45,000 (for an average machine-building enterprise), even two accidents per year turn into significant losses.
Additional factors accelerating migration:
- DSTU EN 61508:2015 requirements for functional safety — SIL1 relay circuits do not meet modern SIL2/SIL3 requirements without significant redundancy.
- EU Directive 2012/27/EU (Energy Efficiency Directive) and DSTU ISO 50001:2020 — mandatory energy audit for enterprises exporting to the EU.
- Physical aging of the component base: relay coils degrade after 10⁷ cycles, contacts after 10⁶ switching under load.
- No diagnostics: the relay circuit does not report degradation until the moment of failure.
This article is a practical engineering guide to planning and executing a relay logic to programmable logic controller (PLC) migration with minimal impact on the manufacturing process.
Evaluation of the existing relay system
Before starting the design, it is necessary to perform a full audit of the existing system. Below is a structured table of evaluation criteria:
| Criterion | What to evaluate | Method | An acceptable result |
|---|---|---|---|
| Number of inputs/outputs | Discrete I/O, analog signals 4-20 mA | Recalculation according to the scheme + physical verification | Full compliance with the scheme |
| Status of contact groups | Transient resistance of contacts (mΩ) | Microohmmeter, thermal imaging examination | <50 mΩ for power, <100 mΩ for signal ones |
| The logic of blocking | Full state diagram (state diagram) | Reverse development from the scheme + interviews with operators | Documented logic without "gray areas" |
| Cable infrastructure | The state of insulation, marking, tracing | Megohmmeter (1000 V), visual inspection | Rizol >1 MΩ, marking >80% |
| Electromagnetic compatibility | Level of interference on signal lines | Oscilloscope, spectrum analyzer | Compliance with DSTU EN 61000-6-2 |
| Security | SIL level of existing protections | Analysis according to DSTU EN 62061 | PL/SIL level defined |
| Feeding | Network quality 24 V DC / 220 V AC | Power quality analyzer | Deviation <±10%, ripple <5% |
A typical mistake is to ignore "unofficial" modifications. In practice, 20–40% of relay circuits have changes not reflected in the documentation. Each additional wire, jumper or shunt contact must be documented prior to migration.
Modern alternatives: a comparison of technologies
Comparison table of relay logic and PLC solutions for a typical conveyor line control task (32 DI / 24 DO / 4 AI / 2 AO):
| Parameter | Relay logic (existing) | PLC (modern solution) |
|---|---|---|
| Number of components in the cabinet | 80–120 relays, timers, counters | 1 CPU + 4-6 I/O modules |
| Power consumption of the control cabinet | 800–1200 W | 80–150 W |
| Event reaction time | 50–200 ms (depends on the number of cascades) | 1-10 ms (typical scan cycle) |
| MTBF of the system | 8,000–15,000 hours | 60,000–150,000 hours |
| Fault diagnosis time | 1-8 hours (manual search) | 5–30 minutes (built-in diagnostics) |
| The possibility of changing the logic | Reassembly, 4-16 hours | Program change, 15–60 minutes |
| Compliance with DSTU EN 61131-3 | Ні | Yes |
| Integration with SCADA/MES | Impossible without additional equipment | Native (Ethernet, Profibus, Modbus) |
| Cost of components (cabinet) | ~45,000 hryvnias | ~120,000 hryvnias |
| Annual maintenance costs | 35,000–60,000 UAH | 5,000–12,000 UAH |
For the connection of pneumatic and hydraulic actuators with the new control system, the quality of pipeline communications is critically important. Pipe Parker 837PU6-RED-RL (polyurethane, outer diameter 6 mm, pressure up to 10 bar at 20°C, temperature range from -40°C to +60°C) is the optimal solution for pneumatic signal lines in modernized cabinets. PU material provides resistance to repeated bending (minimum radius 9 mm) and chemical resistance to grease and condensate. The red color corresponds to the marking of emergency/signal lines according to DSTU EN ISO 4414:2012.
Calculating ROI: The Real Numbers
Input data for calculation (typical machine-building enterprise, Zaporizhzhia, 2024):
- The cost of an hour of an idle line: UAH 25,000 (including loss of products, payment of personnel, fines)
- Number of emergency stops due to relay failure: 6 per year
- Average length of stay: 4 hours
- The cost of electricity: UAH 4.2/kWh (II class)
- The rate of an electrical engineer: UAH 350/hour
- External service rate: UAH 1,200/hour
Modernization costs (one-off)
| Article | Amount, UAH |
|---|---|
| PLC (CPU + I/O modules + HMI panel 7") | 135,000 |
| Control cabinet (housing, DIN rails, terminals, labeling) | 28,000 |
| Parker pneumatic tubes 837PU6-RED-RL (200 m) + fittings | 12,500 |
| Cable products (LIYCY, ÖLFLEX) | 18,000 |
| Design and programming | 85,000 |
| Installation and commissioning | 55,000 |
| Staff training (2 engineers × 40 hours) | 32,000 |
| TOGETHER | 365 500 |
Savings (annual)
| Source of savings | Calculation | Amount, UAH/year |
|---|---|---|
| Reduction of downtime | (6 stops × 4 h - 1 stop × 0.5 h) × 25,000 | 587,500 |
| Energy saving | (1.0 kW - 0.12 kW) × 8,760 h × UAH 4.2 | 32,360 |
| Reduction of costs for personal protective equipment | Relays, timers, contactors — UAH 35,000/year → UAH 5,000 | 30,000 |
| Reduction of labor costs for maintenance | (120 h - 20 h) × UAH 350 | 35,000 |
| TOGETHER | 684 860 |
Payback period: 365,500 / 684,860 = 6.4 months.
Even under a conservative scenario (3 accidents per year instead of 6), the payback period does not exceed 13 months.
The answer to the objection "the old system still works"
Total cost of ownership (TCO) over 5 years:
- Relay system: 45,000 (initial) + 5 × 95,000 (maintenance + downtime) = 520,000 UAH
- PLC system: 365,500 (modernization) + 5 × 17,000 (maintenance) = 450,500 UAH
The difference is UAH 69,500 in favor of modernization, even without taking into account the increase in productivity and the improvement of product quality.
Implementation Roadmap
Phase 1: Preparation (Weeks 1–4)
- Full audit of existing system (see criteria table above)
- Reverse development of relay logic → functional specification
- Determination of safety requirements (DSTU EN 62061, Performance Level according to EN ISO 13849-1)
- Choice of PLC hardware platform (number of I/O with 20% reserve)
- Ordering components through UNITEC-D (PLC modules, Parker 837PU6-RED-RL pneumatic tubes, terminal blocks, cables)
Phase 2: Design (Weeks 5–10)
- Development of a PLC program according to DSTU EN 61131-3 (Structured Text + Ladder Diagram for operators)
- Control cabinet design (EPLAN, layout according to DSTU EN 61439-1)
- FAT (Factory Acceptance Test) on the stand — checking all locks
- Development of commissioning and rollback plan procedures
Phase 3: Installation (Weeks 11–12, during scheduled downtime)
- Installation of a new cabinet parallel to the existing one
- Laying of new cables and air lines (Parker 837PU6-RED-RL for signal air lines)
- Connection to existing sensors and actuators
- Power switching is a critical moment (plan for the weekend)
Phase 4: Commissioning (Weeks 13-14)
- Channel-by-channel I/O test (100% test)
- Checking of safety interlocks (E-Stop, protective fences, SIL-chains)
- Functional tests according to scenarios (normal mode, emergency situations)
- 72-hour test run under load
- Commissioning, signing of the act
Technical challenges during retrofit
1. Inconsistency of signal levels
Problem: The old sensors output 220V AC and the PLC inputs 24V DC.
Solution: intermediate relays with galvanic isolation or sensor replacement. For critical points — installation of spark safety barriers according to DSTU EN 60079-11.
2. Electromagnetic interference
Problem: commutation of inductive loads (contactors, solenoids) creates impulse disturbances of 1–5 kV.
Solution: RC circuits (snubbers) on coils, shielded cables for analog signals, compliance with DSTU EN 61000-6-2 (interference immunity) and DSTU EN 61000-6-4 (emission).
3. Loss of "tribal knowledge"
The problem: the logic of operation is known only to experienced operators who are retiring.
Solution: video recording of system operation in all modes, structured interview with operators, creation of state diagrams (state machine) before programming.
4. Pneumatic connections
Problem: Old 6mm copper tubes with union nuts have microcracks, air leaks 15-25% of total consumption.
Solution: Complete replacement with Parker 837PU6-RED-RL polyurethane tubing with push-in fittings. Reduction of leakage to <2%. Installation is 3 times faster than with copper pipes.
Case: modernization of the packaging line, Dnipro
Initial state
- Bulk materials packaging line, put into operation in 2003.
- Control: 96 RPU-2 relays, 12 PE-6 timers, 8 SI-206 counters
- Productivity: 12 tons/shift (project — 18 tons/shift)
- Emergency stops: 8 per year, average duration 5.2 hours
- Compressed air consumption: 2.8 m³/min (norm — 1.9 m³/min due to leaks)
- MTBF: 1,100 hours
Completed modernization
- Siemens S7-1200 PLC installed (CPU 1214C + 3 SM1223 modules)
- KTP700 Basic HMI panel for visualization
- Replacement of pneumatic lines with Parker 837PU6-RED-RL (total length 180 m)
- New pneumatic distributors with electromagnetic control 24 V DC
- Integration with existing SCADA via Profinet
Results (after 6 months of operation)
| KPI | Before modernization | After modernization | Change |
|---|---|---|---|
| Productivity | 12 tons/shift | 16.8 t/shift | +40% |
| Emergency stops | 8/year | 1/year | -87.5% |
| Mean Time to Recovery (MTTR) | 5.2 hours | 0.8 hours | -85% |
| MTBF | 1,100 hours | 8,200 hours | +645% |
| Compressed air consumption | 2.8 m³/min | 1.7 m³/min | -39% |
| Power consumption of the control cabinet | 980 W | 110 W | -89% |
| Expenses for personal protective equipment | UAH 42,000/year | UAH 6,500/year | -85% |
Actual payback period: 5.8 months. Additional profit from increased productivity — 1.2 million hryvnias/year.
Commissioning and validation
Procedure of acceptance tests
- I/O verification (100% test): forced activation of each input, verification of the response of each output. Protocol in the form according to DSTU EN 61131-2.
- Safety interlocks test: simulation of each emergency situation. The activation time of the emergency stop is no more than 100 ms (requirement of DSTU EN ISO 13850).
- Power failure test: 24V DC power off/on, check for correct restart. The state of the outputs when communication with the CPU is lost is safe (all outputs = 0).
- Pneumosystem test: tightness check at 6.3 bar for 30 minutes. The permissible pressure drop is no more than 0.1 bar (EN ISO 4414).
- Load test (72 hours): continuous operation in automatic mode. The acceptance criterion is no unscheduled stop.
- Documentation check: complete set - electrical circuits, PLC program (source code + comments), operator's manual, test reports.
Acceptance criteria
The system is considered accepted if: all functional tests are passed without comments; PLC cycle time does not exceed 10 ms; all safety protections meet the defined PL/SIL level; the documentation is complete and up-to-date.
Summary
Migrating from relay logic to PLC is not a question of "if", but a question of "when". Each month of delay is UAH 57,000 lost (according to our calculations). Key success factors:
- A thorough audit of the existing system — 80% of project success lies in the preparation stage
- Parallel installation - the new cabinet is installed next to the old one, switching in 4-8 hours
- Quality components - from PLC to pneumatic tubes (Parker 837PU6-RED-RL for signal lines)
- Fallback plan — the ability to return to the relay scheme within 2 hours in case of critical problems
UNITEC-D GmbH provides a full range of components for retrofit projects, from PLC modules and industrial cables to Parker pneumatic tubes and fittings. The catalog is available for Ukrainian enterprises with technical support and consultations on selection.
Select the components for your modernization project in the UNITEC-D E-Catalog.
List of used standards and sources
- DSTU EN 61131-2:2015 — Programmable controllers. Equipment and testing requirements
- DSTU EN 61131-3:2016 — Programmable controllers. Programming languages
- DSTU EN 61508:2015 — Functional safety of electrical/electronic systems
- DSTU EN 62061:2015 — Machine safety. Functional safety of control systems
- EN ISO 13849-1:2015 - Safety of machines. Safety-related parts of control systems
- DSTU EN ISO 4414:2012 — Pneumatic actuators. General rules and safety requirements
- DSTU EN 61000-6-2:2017 — Electromagnetic compatibility. Immunity for industrial environments
- DSTU EN 61439-1:2014 — Low-voltage complete distribution and control devices
- DSTU ISO 50001:2020 — Energy management systems
- Directive 2012/27/EU — Energy Efficiency Directive
- Parker Legris Technical Manual—Polyurethane Tubing, Series 837PU
- Siemens Migration Guide: Relay Logic to S7-1200 (Application Note A5E03461440)
