Calculating Electrical Power Loads and Thermal Dissipation for PCS 7 Control Panels

To calculate the total electrical load list for a SIMATIC PCS 7 control panel, split the load calculation into three distinct domains: 24 VDC internal system electronics load (CPU 410-5H dual power supplies, interface modules, SCALANCE switches), 24 VDC field I/O sensor/actuator load (transmitters, valves, relays), and 230 VAC auxiliary equipment (cooling units, lighting, socket outlets). Use the TIA Selection Tool to export baseline hardware power ratings, apply standard coincidence factors (0.7 to 0.8 on digital outputs), calculate total thermal dissipation in Watts, and size redundant SITOP power supplies with a minimum 20% future expansion margin.

Executive Summary

Electrical engineering and hardware panel design for SIMATIC PCS 7 (V8.0 through V9.1) systems require rigorous power load calculations to ensure uninterrupted operational reliability and adequate panel thermal management. A common engineering challenge during the front-end engineering design (FEED) phase is sizing 24 VDC power supplies, mains circuit breakers, and enclosure climate control systems for panels housing redundant CPU 410-5H controllers, distributed I/O (ET 200SP HA / ET 200M), interposing relays, and field instrumentation. This article provides a comprehensive, step-by-step engineering methodology for electrical load profiling, coincidence factor application, and thermal dissipation calculations.

Architecture and Fundamental Mechanics

Electrical Load Profiling and Thermal Architecture in PCS 7 Panels:

Division of Power Distribution Domains:
1. System Power (24 VDC Internal Electronics):
- Powers backplane buses, CPU logic, communication processors (CP 443-1), distributed I/O interface modules (IM 155-6 HA / IM 153-2), and SCALANCE network switches.
- Requires ultra-stable, electrically isolated power; sensitive to field short circuits.
  2. Field Load Power (24 VDC I/O Circuits - L+):
- Powers 2-wire analog transmitters (20–25 mA per loop), digital input wetting current (3–7 mA per channel), solenoid valves (0.1–0.5 A each), and interposing relays (typically 15–30 mA per coil).
- Must be segregated from system power using electronic selectivity modules (e.g. SITOP PSE200U / SEL1200) to isolate field faults without resetting the CPU.
  3. Mains Auxiliary Power (230 VAC / 120 VAC):
- Feeds the 24 VDC SITOP power supplies, enclosure cooling units/air conditioners, heat exchangers, internal cabinet lights, and utility receptacles.
Coincidence Factor (Diversity Factor - $k_g$):
Not all digital outputs, relays, or solenoid valves are energized simultaneously.
Engineering standard: Apply a coincidence factor of $k_g = 0.7 \text{ to } 0.8$ for DO circuits, while maintaining $k_g = 1.0$ for analog inputs, controllers, and communication infrastructure.
Thermal Heat Dissipation ($P_v$ in Watts):
Every Watt of electrical power consumed by internal equipment transforms into heat inside the enclosure.
Enclosure climate control sizing requires calculating total dissipated heat:
$$P_{v,\text{total}} = \sum P_{v,\text{AS}} + \sum P_{v,\text{IO}} + \sum P_{v,\text{SITOP}} + \sum P_{v,\text{Relays}}$$
Power supply efficiency must be factored: a 40 A SITOP PSU running at 94% efficiency dissipates approximately 60 W of internal heat under full load.

Step-by-Step Implementation Procedure

Step-by-Step Procedure to Calculate DCS Panel Load List and Sizing:

Generate Hardware Baseline in TIA Selection Tool:
- Open Siemens TIA Selection Tool (cloud or desktop version).
- Configure the complete distributed I/O architecture (ET 200SP HA / ET 200M modules, interface modules, terminal blocks).
- Export the 'Power Demand and Calculation' report; note the backplane and sensor supply currents per rack.
Calculate System Power Demand ($I_{\text{sys}}$):
- Dual CPU 410-5H Power Supplies (e.g. PS 407 10A / PS 405 24V): ~2.0 A to 4.0 A at 24 VDC.
- SCALANCE Switches (e.g. XC-208 / XC-216): ~0.5 A to 1.0 A per switch.
- Distributed I/O Interface Modules (IMs): ~0.3 A to 0.5 A per redundant IM pair.
- Sum total: $I_{\text{sys}} = \sum I_{\text{components}}$.
Calculate Field I/O Load Demand ($I_{\text{field}}$):
- Digital Inputs: Number of active DIs $\times 0.005\text{ A} \times 0.8$ (coincidence).
- Digital Outputs: Number of active DOs $\times I_{\text{load}} \times 0.7$ (coincidence).
- Analog Inputs (2-wire): Number of active AIs $\times 0.025\text{ A} \times 1.0$.
- Analog Outputs: Number of active AOs $\times 0.025\text{ A} \times 1.0$.
- Relays / Indicators: Number of energized coils $\times 0.020\text{ A} \times 0.7$.
- Total Field Current: $I_{\text{field}} = I_{\text{DI}} + I_{\text{DO}} + I_{\text{AI}} + I_{\text{AO}} + I_{\text{relays}}$.
Size 24 VDC Power Supply and Apply Expansion Margin:
- Total 24 VDC Load: $I_{\text{total}} = (I_{\text{sys}} + I_{\text{field}}) \times 1.20$ (20% future spare capacity margin).
- Select redundant SITOP PSU8200 power supply units (e.g. dual 20 A or dual 40 A) paired with SITOP PSE202U redundancy modules.
Calculate Enclosure Heat Dissipation and Cooling Capacity:
- Sum the power loss ratings from component technical datasheets (in Watts).
- Use manufacturer enclosure thermal tools (e.g. Rittal Therm) to calculate whether natural convection is sufficient or if forced fan cooling / an air conditioner (e.g. 1500 W cooling unit) is required.

Verification

Verification Procedure:
1. Spreadsheet Validation: Cross-check the tabulated load list against hardware catalog datasheets to verify that no modules or communication processors were omitted.
2. Physical Current Audit: During factory acceptance testing (FAT), measure total incoming 24 VDC current on system and field branches under maximum energized conditions using a calibrated clamp meter. Confirm measured current does not exceed 80% of rated PSU capacity.
3. Thermal Camera Audit: Perform infrared thermal scanning inside the panel during continuous full-load testing; verify that internal ambient temperatures remain well below component thresholds (typically < 55 °C).

Limitations and Operational Precautions

Operational Precautions and Electrical Design Rules:
- Inrush Current Management: Power supplies and upstream circuit breakers must accommodate high inrush currents during cabinet power-up. Use Type C or Type D miniature circuit breakers for AC feeds.
- Battery Buffering (DC-UPS): Critical DCS controllers require continuous operation during power transitions; incorporate a SITOP DC-UPS (UPS1600 with battery modules) to bridge mains interruptions.
- Commercial and Procurement Validation: Always confirm specific part numbers, redundancy module models, and selectivity breakers with an authorized Siemens distributor or through the Siemens Industry Mall.

Engineer FAQ

What software tool helps calculate Siemens distributed I/O power requirements?

The Siemens TIA Selection Tool automatically calculates backplane and sensor current requirements for ET 200 distributed I/O modules.

What is a typical coincidence factor for digital output circuits in DCS panels?

A coincidence factor between 0.7 and 0.8 is standard industry practice for digital outputs, acknowledging that not all loads are active simultaneously.

Why should system electronics power be separated from field I/O power?

Separating system power from field power prevents field short circuits or inductive voltage spikes from causing CPU resets or communication module crashes.

What spare capacity margin should be included when sizing power supplies?

A minimum future expansion margin of 20% to 25% should always be added to the calculated load.

How is thermal dissipation in Watts converted to cooling requirements?

Total electrical power loss inside the cabinet equals heat dissipation in Watts. Cabinet surface area, ambient temperature, and heat loss determine whether ventilation or an air conditioner is required.