Measurement of potential magnetic field interference with implanted cardioverter defibrillators or pacemakers

The return to active employment in a Central Florida phosphate mine of people with recently implanted cardioverter defibrillators raised a serious concern about possible electromagnetic interference. The implantable cardioverter defibrillator as well as the pacemaker has magnetically actuated switches to enable the physician to program and test the device. The draglines used in strip mining use large DC motors driven by AC to DC motor generator sets. There are no catalog data on the fringing fields near these electrical machines. A one week measurement effort found several regions near machines where the DC magnetic fields exceeded limits set for the implanted devices. These were found to be a function of the placement and installation of the machines. It is necessary to make the actual measurements to locate possible unsafe regions. The DC magnetic fields were measured with a Hall Effect Gaussmeter. Equipment complying with IEEE Std. 644-1994 was used to measure 60 Hz AC magnetic and electric fields. No unsafe regions were found below 230 kV three-phase bulk power transmission lines or near any of the outdoor substations. Some regions near 480 V AC equipment cabinets were found to have excessive 60 Hz magnetic fields. This is a function of the bus configuration inside the cabinets. These measurements at one CF Industries facility allowed one supervisory employee to resume his duties with very few restrictions based on his implanted defibrillator; however, the measurements in no way can be intrepreted to apply to other facilities.