Deep Technical Chapters · Electrical, Controls & Commissioning
Wiring Architecture — Before You Touch It
Walk up to unfamiliar equipment and mentally separate it into five circuit types — power, control, inputs, outputs, and communication. Don't memorise the machine. Understand the architecture.
When you're new, a control panel looks like a spaghetti bowl of wires. Once you understand the architecture, you start seeing patterns: power → control → input → logic → output. Suddenly it's not 100 mysterious wires — it's a handful of different circuit types doing different jobs. That's the skill: get familiar with the architecture, don't memorise the machine.
Before you disconnect or reconnect anything, you should be able to answer: what circuit is this wire part of, what is it carrying, and what is the controller expecting to see?
1. Power
Power is the supply that runs everything. Before touching equipment, identify where the power enters and how it's distributed.
- Single-phase or three-phase supply
- Protective devices — circuit breakers, fuses, overloads
- Contactors and relays switching power to loads
- Motors, compressors, heaters, fans and other power-consuming equipment
2. Control
Control is the logic layer that decides what runs and when. It sits between the power and the outputs.
- Control voltage (transformer-derived 24V, or line voltage control)
- Relays and contactors acting on control signals
- Interlocks — sequences that must be satisfied before equipment runs
- Safeties — protective devices that break the control circuit
- Enable and run signals from controllers or PLCs
3. Inputs
Inputs are what the controller or safety circuit uses to make decisions. Every input tells the system something about its current state.
- Pressure switches (LP, HP, oil pressure)
- Float switches (condensate, liquid level)
- Door switches and interlocks
- Thermostats and temperature switches
- Thermistors and temperature sensors
- Pressure transducers
- Humidity and other sensors
4. Outputs
Outputs are the commands the controller sends to make things happen. When you're tracing a fault, the output is where the controller's decision meets the real world.
- Compressor command
- Fan command
- Solenoid valves (liquid line, hot gas, defrost, pump-down)
- Heaters (defrost, crankcase)
- Valve actuators
- Alarms and indicators
5. Communication / Analogue
Modern equipment increasingly uses variable signals and digital communication instead of simple on/off switching. These circuits carry information, not just power.
- 0–10 V signals (damper actuators, variable fans)
- 4–20 mA loops (pressure transducers, level sensors)
- Communication networks (Modbus, BACnet, proprietary)
- PLC I/O
- Proprietary OEM control signals
The "five wires" idea
When you understand the architecture, a control panel stops being 100 mysterious wires and becomes a handful of different circuit types doing different jobs. You might have your three-phase power conductors, control wiring, a switch or safety returning to a controller, and sensor or communication wiring. That mental map transfers between a coldroom, condensing unit, rack, packaged system, AHU, PLC-controlled plant or OEM PCB-controlled equipment.
The "lost wire" situation
If a wire gets disconnected, damaged or you're unsure where it belongs — don't guess. Use the schematic, identify the circuit, isolate safely, and bell the circuit out where appropriate.
- Identify the wire from the schematic
- Find its termination points
- Confirm continuity
- Identify whether you're actually on the correct conductor
- Check for unexpected parallel paths that could give a misleading continuity result
- Label wires before disconnecting them
- Photograph and document connections before disturbing them
- Verify the circuit again after reconnection
A continuity beep doesn't prove you've found the right circuit
A continuity beep tells you there's a path between your two probes. It does not tell you that path is the conductor you think it is. Parallel paths through other components, shared neutrals, or common rails can all give you a misleading beep. You need to understand what you're measuring and what else is connected to it before you trust the result.
Apprentice exercise — "Before You Touch It: Identify the Five"
Before working on unfamiliar equipment, look at the schematic and physically identify these five circuit types. Then trace them through the machine.
- Where is the power?
- Where is the control?
- What are the inputs?
- What are the outputs?
- What is communicating or providing a variable signal?
The bigger lesson
You don't need to know every machine. You need to understand how machines communicate — how power, control, inputs, outputs and communication fit together. That architecture is the same whether you're looking at a coldroom, a rack, a packaged unit or a PLC-controlled plant. Learn the pattern and every new machine becomes readable.
Field Rules
- 01
"Don't memorise the machine. Understand the architecture."
- 02
"Before you touch a wire, know what circuit it's part of, what it's carrying, and what the controller expects to see."
- 03
"A continuity beep doesn't prove you've found the right circuit. Understand what you're measuring."
- 04
"Don't guess a lost wire. Use the schematic, isolate safely, and bell it out."
Diagnosis at a glance
Common mistakes & callbacks
- •Guessing where a disconnected wire goes instead of reading the schematic.
- •Trusting a continuity beep without checking for parallel paths.
- •Not labelling or photographing connections before disturbing them.
- •Treating every machine as unique instead of recognising the common architecture.
