Protecting Silicon Carbide Furnaces with Fuses

Carbolite frequently receives questions from customers regarding the power required to operate their high temperature furnaces.  Furnaces can be provided to operate on 208 or 240V, single phase or three phase supplies.  The supply must also be sized to the amperage draw of the furnace.  This value is easily calculated for furnaces with resistive wire heaters.  The amperage is determined by dividing the furnace wattage by the operating voltage (A =  W / V).  A different method of calculation is necessary for Carbolite furnaces heated by silicon carbide elements.  Because the power characteristics of these furnaces are different, fuses are recommended for protection devices in the customer's main power panel, rather than a circuit breaker.

Carbolite silicon carbide furnaces are controller by electronic power limiting.  Each model will vary, but typically the power is supplied to the elements in bursts of about 1/3 of a second, for 50 to 80% of the time.  This produces two amp figures; the maximum amperage which occurs for the 1/3 second, and the heating effect amps, which is the average current.

Calculation of the maximum amp is determined by dividing the volts by the resistance (A=V/R).  To obtain the average amps, you cannot divide wattage by volts. Average amps will not be determined by calculations using the voltage value applied for 1/3 second intervals.  One must use tangible quantities, such as watts and resistance.  To calculate the average current or heating amps, the following calculation may be used:

 Heating Amps = sq. root of (watts/resistance)

Carbolite also factors in a tolerance figure on the resistance when deterring the amperage to rate the furnace.  Rather than definitively explain the tolerance factor calculations, it is easiest to check the calculated heating effect amps figure on the Furnace Data Plate.

Fuses

Fuses are affected by the heating effect amperage (average current).  All the wiring inside the furnace, internal components (except the SSRs), wiring from the customer's power supply to the furnace, and fuses can be sized for the heating amps.

Circuit Breakers

Circuit breakers work on maximum amps.

 Example

Take a furnace with a 60 amp heating effect amps and a 74 amp maximum amps.  The furnace internal components are rated at 60 amps (actually 63 amps, a standard rating value)

If a customer wires the furnace to suit 60 amps, and installs 60 amp fuses, then everything is properly matched.  In a fault condition, the fuses will blow before the other components fail. 

If a customer installs 60 amp circuit breakers, they can trip in the first 1/3 second, preventing use of the furnace. 

If the customer installs 74 amp circuit breakers, the furnace will work, but the components with not be properly protected.  In the event of a fault, the 60 amp rated components may fail before the circuit breaker trips. 

Hopefully, the above explanation will help you to understand why Carbolite recommends that fuses should be used instead of circuit breakers for Carbolite furnaces with silicon carbide heating elements.

Return