Purple-K, also known as PKP, is a dry-chemical fire suppression agent used in some dry chemical fire extinguishers. It is the second most effective dry chemical in fighting class B (flammable liquid) fires after Monnex (potassium allophanate), and can be used against some energized electrical equipment fires (USA class C fires). It has about 4–5 times more effectiveness against class B fires than carbon dioxide, and more than twice that of sodium bicarbonate. Some fire extinguishers are capable of operation in temperatures down to −54 °C or up to +49 °C. Dry chemical works by directly inhibiting the chemical chain reaction which forms one of the four sides of the fire tetrahedron (heat + oxygen + fuel + chemical chain reaction = fire). To a much smaller degree it also has a smothering effect by excluding oxygen from the fire. "Dry chemical" extinguishers, such as Purple-K, are different from "dry powder" extinguishers that are used to fight Class D flammable metal fires.