The most common type portable extinguisher for commercial use in the
This means you are probably cleaning the residue from such an extinguisher. To be sure you can look at the label on the spent extinguisher; the contents are listed there. The ammonium phosphate is that fine, yellow powdery fallout that was blasted over a surprisingly large area. Ammonium phosphate softens and sticks when in contact with hot surfaces. While this characteristic makes it a great extinguishing agent, it sometimes makes clean up very difficult. ABC dry chem can do serious damage to delicate equipment and corrode metal, especially if it is not thoroughly cleaned. That is why we have recommended to our customers; sweep or vacuum the free chemical, then scrub (perhaps using a strong detergent and firm brush), mop, and inspect to make sure no residue remains.
From the forgoing it becomes clear why extinguisher manufacturers and the NFPA require dry chemical extinguishers not be installed to protect delicate electronic equipment. Clean agent extinguishers are required for hazards involving this kind of equipment.
NFPA 10 ANNEX