Patent ID: 8405946

Claim:
A wire protection method of protecting a wire used for supplying electric power from a power source to a load, the method comprising the steps of: detecting an applied-current to the load every predetermined time; calculating a temperature change in the wire per the predetermined time using the detected applied-current; estimating a temperature of the wire, wherein the estimating includes calculating a rise in temperature of the wire using the wire temperature change per the predetermined time and adding the wire rise temperature to a reference temperature; determining whether the estimated wire temperature is lower than a predetermined upper limit temperature; newly estimating the wire temperature upon determination in the determining step that the estimated wire temperature is lower than the predetermined upper limit temperature, wherein the newly estimating includes calculating the wire temperature change per the next predetermined time, newly calculating the wire rise temperature using the calculated wire temperature change per the predetermined time, and adding the new wire rise temperature to the reference temperature; and stopping the electric power supply from the power source to the load upon determination in the determining step that the estimated wire temperature is equal to or higher than the predetermined upper limit temperature, wherein: the estimating the wire temperature includes using the detected applied-current in a relational expression and calculating the wire rise temperature from the relational expression, the relational expression being related to heat generation and heat radiation of the wire; and the estimating the wire temperature includes calculating the wire rise temperature using a following formula as the relational expression: Δ Tw ( n )=Δ Tw ( n− 1)*exp(−Δ t/τw )+ Rthw*Rw ( n− 1)* I ( n− 1) 2 *(1−exp(−Δ t/τw )), where: I(n) is a detected applied-current value (A) in the n-th detection, n being an integral number equal to or larger than 1; ΔTw(n) is the wire rise temperature (° C.) in the n-th detection; Rw(n) is a resistance (Ω) of the wire in the n-th detection, wherein Rw(n)=Rw(0)*(1+κw*(Tw−To)); Rw(0) is a resistance (Ω) of the wire at a temperature To; Rthw is a heat resistance (° C./W) of the wire; τw is a heat-radiation time constant (s) of the wire; κw is a resistance temperature coefficient (/° C.) of the wire; Δt is the predetermined time (s); and Tw=[reference temperature]+ΔTw(n), wherein Tw is the wire temperature (° C.) in the n-th detection.