What is a GFCI or GFI Breaker or Outlet and why might it trip when used with an electric kiln?

What is a GFCI or a GFI Breaker or Outlet?

  • A 'GFCI' or Ground Fault Circuit Interrupter, sometimes called a 'GFI' or Ground Fault Interrupter is part of either an outlet or a circuit breaker.
  • It is typically required for added safety on any circuit that may come in contact with water, like outdoors, or kitchen, or bathroom.
  • A GFCI measures the amperage on both the hot and the neutral wires in a 120 volt circuit, or it measures amperage on both hot wires in a 240 volt circuit. 
  • If there is more than about a 5 milliamp difference (5 thousandths of an amp difference) between the amperage readings of the wires the GFCI acts like a circuit breaker and shuts the power off. 

Why might it trip when used with an electric kiln?

  • In a kiln, the firebrick is not a perfect insulator, so a tiny bit of the current leaks from the elements to the safety ground and unbalances the current in the 2 hot wires. 
  • The resistance of the brick goes down as the temperature goes up so the kiln may be able to start a firing and but the GFCI would probably trip as the temperature went up. 
  • Above 1700 F is when the firebrick has a significant drop in resistance. 
  • Due to the nature of the firebrick in a kiln, GFCI protected circuits are not recommended for use with the power circuits in electric kilns without special element insulators.