Lower explosive limit

The lowest concentration of a fuel in an oxidiser that will support combustion.

Abbreviated LEL. For hydrogen in air the figure is around four per cent by volume, and hydrogen in oxygen behaves comparably. Below the limit the mixture is too lean to propagate a flame.

In electrolyser operation the relevant measurement is hydrogen content in the oxygen stream, and plants do not run to the limit itself. Safety systems typically intervene at a fraction of it, commonly around half, so that instrument error, sensor lag and transient excursions all remain inside the margin.

That intervention point is what sets minimum stable load in most electrolysers. It is a safety threshold rather than an equipment limit, which is why the usable operating range narrows as a separator ages and baseline crossover rises.