LPS stands for Limited Power Source. It's a circuit classification that first came from IEC 60950-1: if the main circuit of a piece of equipment keeps its voltage, apparent power, and fault current low enough to be unlikely to start a fire — even under fault conditions — it qualifies as LPS. The practical payoff was real money and design freedom. A product that met the LPS limits didn't need a full flame-retardant fire enclosure. Instead, manufacturers could get away with a UL94 HB-rated case, which is cheaper to produce, easier to mold, and generally kinder to environmental compliance than the higher-flammability-class materials a fire enclosure would otherwise require.
Update (2026)
IEC 60950-1 has since been withdrawn. It's been fully replaced by IEC 62368-1, which takes a different approach to safety — instead of table-based voltage/current limits, it classifies circuits by the actual hazard they pose. For fire risk specifically, that means Clause 6 and its Power Source (PS) classes:
- PS1 — power stays below ignition-risk levels; little to no extra fire safeguarding needed.
- PS2 — an intermediate tier that may call for additional safeguards.
- PS3 — real ignition risk; usually needs a fire enclosure or another supplementary safeguard.
In other words, the job LPS used to do — letting a product skip the fire enclosure and use a lower-cost, lower-flammability-rated case — now falls to a PS1 classification instead.
One thing worth being careful about: LPS and PS1 aren't just two names for the same thing. CSA's guide to IEC 62368-1 Edition 4 is explicit that using a limited power source doesn't automatically mean a circuit meets PS2 compliance either — the two frameworks test and measure things differently, so a circuit that was LPS-rated under the old standard still needs to be evaluated on its own terms against IEC 62368-1's PS test conditions, not assumed to carry the rating over.
The same kind of gap shows up on the voltage side. Under 60950-1, LPS circuits were typically paired with SELV-classified outputs. IEC 62368-1 doesn't use the term SELV at all — the closest equivalent is ES1 (a Class 1 electrical energy source). TC108, the committee behind the standard, has said it's fine to treat ES1 as offering the same level of touch safety as SELV to smooth the transition between standards. But they're not a perfect match: SELV is defined by voltage alone, while ES1 accounts for both voltage and current. So SELV/LPS and ES1/PS1 are useful shorthand pairings, not formal substitutions.