Limited Power Source vs PS2: What Changed Under IEC 62368-1?
A Limited Power Source and a PS2 circuit are not interchangeable labels. LPS is a defined set of power-limiting requirements, while PS2 is one of the IEC 62368-1 electrical power-source classifications used to decide fire safeguards. A circuit can satisfy both descriptions, but one result does not prove the other unless the safety report evaluates both requirements for the same circuit and conditions.
How do LPS and PS2 fit into IEC 62368-1?
IEC 62368-1 classifies energy sources and then assigns safeguards based on the source, the people who can access it, and the operating conditions. PS1, PS2, and PS3 belong to the electrically caused fire analysis. The circuit is classified by the maximum power available from the source under the prescribed normal, abnormal, and single-fault conditions.
LPS is a separate defined requirement. In IEC 62368-1:2023, Annex Q covers circuits intended for interconnection with building wiring and includes tables for inherently limited sources and sources limited by an overcurrent protective device. The applicable voltage, current, apparent-power, protection, and test conditions must be read from the adopted edition.
The current IEC 62368-1 publication record identifies Edition 4 as valid and notes a corrected version dated 2025-08. It also points to Annex Q and the comparison annex for terms used in earlier standards. Record the exact edition and national adoption in the project file.
What are the six practical differences?
| Decision point | LPS | PS2 |
|---|---|---|
| Primary purpose | Qualifies a limited power source for the use covered by the relevant clause or annex | Classifies available electrical power for fire-safeguard decisions |
| Result form | Passes or fails the applicable LPS limits | Falls into PS1, PS2, or PS3 |
| Limit structure | Depends on voltage and whether limitation is inherent or protective-device based | Depends on maximum available power and time under prescribed conditions |
| Fault method | Applies the LPS test path and protection assumptions | Includes worst-case load and power-source fault evaluation |
| Design consequence | Can affect wiring, interconnection, and downstream construction requirements | Drives safeguards for combustible materials, potential ignition sources, wiring, and fire enclosures |
| Evidence needed | Annex or clause result tied to the exact output | PS classification and measurement tied to the exact circuit |
UL’s IEC 62368-1 engineering Q&A cautions that “power-limited” is not itself a defined classification. It explains that a designer may mean a PS2 output or an LPS output, and that LPS limits vary with voltage and protection method. That is why the buyer should request the actual classification instead of accepting “limited power” in a sales table.
Why can the distinction change the end-product design?
A PS2 classification can trigger material, separation, wiring, and fire-spread safeguards even when the circuit also meets another limited-power requirement. The end-product assessor needs to know the circuit class, potential ignition-source analysis, accessible materials, wiring path, connector, enclosure, and fault behavior.
Conversely, a statement that an output is PS2 does not automatically demonstrate the Annex Q LPS limits. The safety report must show whether the LPS evaluation was performed and which limitation method was used. This becomes especially important when a protective device, electronic limiter, foldback behavior, multiple outputs, or an adaptive voltage profile affects the available energy.
Treat a power-supply certificate as one input to the system review. The finished device may apply the adapter output to internal wiring, external building wiring, batteries, motors, heaters, or communication ports with different requirements. The IEC 60601-1 versus IEC 62368-1 guide also shows why the end-product standard must be selected before relying on an adapter classification.
What should the buyer ask the supplier and test lab?
Ask for a circuit-level answer, not a family-level slogan:
- Which standard, edition, national adoption, clause, and annex were used?
- Which exact model, output, voltage profile, connector, and cable were evaluated?
- Is the output PS1, PS2, or PS3 under normal operation, abnormal operation, worst-case load fault, and power-source fault?
- Was LPS compliance evaluated separately, and was limitation inherent or dependent on an overcurrent protective device?
- Which component establishes the limit, and what happens if it fails?
- Are multiple outputs tested individually and in the worst permitted combination?
- Which wiring, material, enclosure, or installation conditions remain for the end product?
- Does any condition of acceptability restrict load, ambient temperature, mounting, or downstream protection?
Keep the circuit diagram, test points, loads, fault assumptions, waveforms or readings, protection-device data, report pages, and model mapping in the qualification file. If a supplier changes the controller, current-sense network, fuse, transformer, firmware, output connector, or parallel-port behavior, repeat the classification review before approving the change.
For an adapter assessment, send Merryking the end-product standard, output profile, normal and fault loads, cable and connector, external-wiring use, enclosure materials, and required market approvals. The resulting proposal should state which classifications have evidence and which conclusions still require the end-product test laboratory.