J120-524 United Electric Pressure XP 1/2IN FNPT/BUNA/SETPNT RNG/2.5TO50 PSI. The United Electric J120 and H121 series explosion proof air and water pressure switch monitors air and water pressure in a hazardous environment. The J120 series have a single SPDT contact. The H121 series has a single SPDT contact and reference dial for local indication.