Cold commissioning groups the inspections and tests done with the power circuits de-energized: insulation, ratios, wiring, secondary injection, controls and interlocks. Hot commissioning starts with energization and checks what only exists with voltage and load: phase sequence and phasing, synchronism, protection directionality, real measurements and thermal behavior. Whatever can be tested cold should be tested cold.
The two halves of commissioning
The commissioning of a substation, an industrial plant or a solar plant naturally splits into two parts depending on the state of the power circuits:
- Cold commissioning: the power circuits are de-energized. Equipment is tested one item at a time, and the protection and control systems are tested with test sources and station service.
- Hot commissioning: the installation is already energized. What does not exist without voltage and load is verified.
The two terms do not come from a standard: they are project terminology, widely used in industrial plants, solar energy and the process industry. What matters is the rule behind them: whatever can be tested cold should be tested cold, because when hot, a mistake is paid for with a trip, damage or a risk to people.
What is tested cold
| Area | Examples of de-energized checks |
|---|---|
| Primary equipment | Insulation, ratio, winding and contact resistance, breaker timing |
| Instrument transformers | CT ratio, polarity, excitation and burden; VT ratio and polarity |
| Wiring | Continuity, insulation and point-to-point checks of current, voltage, trip and signal circuits |
| Protection | Secondary injection with the approved settings, logic, actual trip to the breakers |
| Control | Local and remote control, interlocks, signals to the HMI and the control center |
| Grounding | Grid resistance and continuity of the equipment ground connections |
| Station service | DC system, batteries, chargers and AC transfers |
The switching planned for energization is also rehearsed cold: with breakers and disconnect switches operating without voltage, each control is confirmed to work, and the interlocks are confirmed to allow the planned sequence and to block it when they should.
Going from cold to hot
The change of state is the riskiest moment of the project. Up to that point the installation was treated as a construction site; from energization onward, any work requires a clearance, verification of absence of voltage and grounding. Before energizing, cold commissioning should be formally closed: nonconformities resolved or accepted in writing, test switches and test modes back to normal, temporary grounds removed and counted, areas signposted and a single person in charge of switching. That closure usually ends in a “ready to energize” certificate. The sequence of the first energization is described in first energization of a new substation.
What can only be verified hot
Phasing and synchronism
With real voltage, the phase sequence and the phasing between sources, buses and transformers are checked. If two sources will operate in parallel, the synchronism check must receive the right voltages, and the angle measured across both sides must make sense before the first parallel closing.
Directionality and differentials
Secondary injection proves that the relay works, but it cannot prove that the real CTs and VTs are wired with the right polarity in the installation. With load flowing, the currents and voltages and their angles are read in the relay, and the power flow is confirmed to appear in the expected direction. This validates the directionality of the 67 and 21 functions, the behavior of the zero-sequence directional elements and the residual differential current of the 87 functions, which must be small under load.
Real measurements and signals
The measurements in SCADA, revenue meters and recorders are compared with those of the relays and with an independent instrument. Ratio, scaling and power sign errors appear here.
Thermal and mechanical behavior
With current flowing, thermography detects poorly torqued connections. Transformer noise and temperature and cooling are observed and, in industry, motor rotation and starting.
- Does it need real voltage or current in the power circuit?
- NoDone cold, before energizing
- YesDoes it depend on the real angle or direction of power flow?
- YesHot with load: directionality, differentials, power sign
- NoHot at no load: phase sequence and phasing, synchronism, voltage readings
In each type of installation
- Substation. Cold work takes most of the time: primary tests, protection, control and end-to-end. Hot commissioning starts with an energization coordinated with the utility and continues with load pickup, following the utility’s requirements.
- Solar plant. Cold: strings, combiner boxes and DC-side insulation are inspected and measured, along with the protection at the point of interconnection. Hot: the inverters are started, power export is verified and the interconnection functions required by the utility are tested.
- Industrial plant. Cold: switchgear, panels, motors and drives are tested uncoupled; hot: no-load starts, direction of rotation, and then operation with the process, coordinated with mechanical and instrumentation; see electrical commissioning of an industrial plant.
- Battery storage. The safety systems and the BMS are tested before the batteries are energized; with the grid, charge and discharge cycles. See BESS commissioning.
Common mistakes
- Leaving for hot commissioning what could be done cold, such as point-to-point wiring checks or trip tests.
- Energizing with protection disabled “so it does not trip”. Energization is exactly when all of it must be active.
- Skipping the on-load check because energization went well. A differential with a reversed CT does not trip at no load, but it does on the first external fault.
- Not recording the angle and magnitude readings taken hot: they are the evidence that polarity and directionality are right.
Cold and hot commissioning with Tiza Group
Our crew performs both halves of commissioning in medium, high and extra-high voltage substations, industrial plants and solar plants, in Colombia, or elsewhere on request. Contact us about your project.
Related guides
- Synchronism check 25
- Interlocking tests
- Pre-commissioning, commissioning and start-up
- Ground directional 67N (PTOC + RDIR): how it works
- First energization of a new substation
References
- ANSI/NETA ATS-2025 and ANSI/NETA ECS-2024.
- IEEE C37.233-2023, IEEE Guide for Power System Protection Testing.
- IEC 62446-1:2016+AMD1:2018, Photovoltaic (PV) systems – Requirements for testing, documentation and maintenance – Part 1.
- IEC 62337:2012, Commissioning of electrical, instrumentation and control systems in the process industry – Specific phases and milestones.
Frequently asked questions
What is cold commissioning?
It is the part of commissioning done with the power circuits de-energized. It includes primary equipment tests, wiring verification, secondary injection of the protection and tests of controls and interlocks. Station service is usually energized.
Which tests can only be done hot?
Phasing and phase sequence with real voltage, synchronism check between sources, protection directionality and differential stability under load, consistency of the measurements in SCADA, and thermography with current flowing.
Is cold commissioning the same as pre-commissioning?
They are close, but not always the same. Pre-commissioning is usually limited to inspections and static tests of each piece of equipment; cold commissioning also includes functional tests with auxiliaries energized, such as secondary injection and controls, before the power circuits are energized.