Forensic Engineering Investigation Of Lawnmower Seat Switch Malfunction

Abstract

Examination Of A Commercial Riding Mower Found Intermittent Function Of Seat Interlock Switch In That Powered Rotation Of Blades And Drive Wheels Would Occasionally Occur When Operator Left Seat. Testing Confirmed That This Magneto Shorting Seat Switch Would Occasionally Show Open Or High Resistance Across Contacts When Plunger Was Released By Seat. Evidence Of Opposing Expert Alleged Switch Plunger Binding Due To Galling Was Not Revealed During Testing. The Mower Design Resulted In Leaking Battery Acid Contaminating The Seat Switch Terminals. Extensive Switch Testing And Analysis (Including Dynamic x-Ray Viewing, Sem Imaging And Energy Dispersion, And Chemical Sampling) Revealed Sulfuric Acid Contamination And Corrosion Of Switch Internal Contacts That Inhibited Consistent Electrical Continuity. Concluded Was That Mower Design And Maintenance, Rather Than Off The Shelf Switch Design Flaw Was Cause Of Accident, Which Affected The Civil Trial Outcome.