What’s Wrong Here? Hint: One of These Things Is Not Like the Other
Hint: One of these things is not like the other.
While it may or may not be a violation for this installation, the fact that the middle fuse is a Class H fuse with an interrupting rating of only 10,000A while the other two fuses are Class K type with a 50,000A interrupting rating provides me with an opportunity to discuss the importance of knowing the available fault current.
Section 110.9 requires fuses, circuit breakers and other equipment intended to interrupt current at fault levels to have a rating equal to or greater than the available fault current available at the line side terminals of the equipment. If the available fault current here is 10,000A or less, there is no Code violation or safety concern with having fuses of different AIC ratings. However, if the available fault current is greater than 10,000A, there are Code violations and serious safety concerns. Installers must know the available fault current in order to choose the correct equipment to be installed. If the available fault current here is 50,000A, the fuse with the 10,000 AIC rating could fail violently and catastrophically if exposed to that high fault current. A person turning this disconnect switch on during that fault condition could be severely injured or worse if that fuse exploded. Many Code Sections require the available fault current to be marked or documented for equipment including Sections 110.24, 408.6, and 409.22 among others.
About the Author
Russ LeBlanc
Owner
Russ started in the electrical trade as an apprentice in 1985. He worked his way up to become a Journeyman Electrician and then eventually became a Master Electrician and Licensed Construction Supervisor. In 1999 Russ become an Electrical Instructor for The Peterson School of Engineering in Massachusetts where he developed his passion for teaching, and quickly became Department Head of Electrical Instruction. Russ has taught thousands of apprentices, electricians, engineers, inspectors, and other electrical professionals during his career as an instructor. He continues to provide electrical professionals with Electrical Code seminars, Arc-Flash Awareness training seminars and educational material through his LeBlanc Consulting Services in North Reading, MA whose specialty is educating electricians. He has been an active member of the NFPA Electrical Section and has authored hundreds of National Electrical Code proposals and comments which have become Code rules to improve the safety for the electrical industry. Russ is also an IAEI certified Electrical Inspector.
Please visit www.russleblanc.net for more information.