Protection apparatus of load circuit

1. A protection apparatus of a load circuit, comprising:

a timer configured to count an elapsed time of a current flowing through a load via an electric wire;

a current detection unit configured to detect the current;

a switch portion configured to switch supply and interruption of the current;

a temperature estimation unit configured to estimate a temperature of the electric wire based on a pseudo-temperature arithmetic expression; and

a breaking control unit configured to break the switch portion when the temperature estimated by the temperature estimation unit has reached an allowed temperature of the electric wire,

wherein the pseudo-temperature arithmetic expression is set in such a manner that, in a temperature arithmetic expression of the electric wire, the temperature arithmetic expression using the elapsed time counted by the timer, the current detected by the current detection unit, and a heat capacity and conductor resistance of the electric wire, a pseudo-heat capacity smaller than the heat capacity of the electric wire is assigned to the heat capacity, and a pseudo-conductor resistance larger than the conductor resistance of the electric wire is assigned to the conductor resistance.

2. The protection apparatus of a load circuit according to claim 1 , wherein the breaking control unit is further configured to turn the switch portion to a connection-enabled state when the temperature estimated by the temperature estimation unit has dropped to an ambient temperature or lower after the switch portion was broken.

3. The protection apparatus of a load circuit according to claim 1 , wherein the pseudo-conductor resistance and the pseudo-heat capacity are set so that current/breaking time characteristics of the electric wire, which are based on the pseudo-temperature arithmetic expression, can be lower than current/breaking time characteristics of an electric wire smaller in diameter than the electric wire for use in the load circuit.

4. The protection apparatus of a load circuit according to claim 1 , wherein the pseudo-conductor resistance and the pseudo-heat capacity are set so that current/breaking time characteristics which are based on the pseudo-temperature arithmetic expression can be located between minimum current/breaking time characteristics of a fuse protecting the electric wire and maximum current/breaking time characteristics of the fuse.

5. The protection apparatus of a load circuit according to claim 1 , wherein the temperature arithmetic expression of the electric wire is:

T 2 =T 1 +I 1 2 rR{ 1−exp[− t /( C·R )]}  (1)

T 2 =T 1 +I 2 2 rR {exp[− t /( C·R )]}  (2)

in which Expression (1) is a temperature arithmetic expression at a time of heat generation, and Expression (2) is a temperature arithmetic expression at a time of heat radiation,

where T 1 is an ambient temperature [° C.], T 2 is an estimated temperature [° C.] of the electric wire, I 1 and I 2 are energization currents [A], r is an electric wire conductor resistance [Ω], R is a thermal resistance [° C./W], C is a heat capacity [J/° C.], and t is a time [sec].

6. A protection apparatus of a load circuit, comprising:

a timer configured to count an elapsed time of a current flowing through a load via an electric wire;

a current detection unit configured to detect the current;

a first temperature arithmetic operation unit configured to estimate a temperature of the electric wire based on a first pseudo-heat generation temperature arithmetic expression and a first pseudo-heat radiation temperature arithmetic expression;

a second temperature arithmetic operation unit configured to estimate the temperature of the electric wire based on a second pseudo-heat generation temperature arithmetic expression and a second pseudo-heat radiation temperature arithmetic expression;

an overcurrent determination unit configured to determine that, when the current detected by the current detection unit is smaller than a preset predetermined threshold current, and an estimated temperature calculated by the first temperature arithmetic operation unit has reached a first allowed temperature, the current thus detected is an overcurrent, and when the current detected by the current detection unit is equal to or larger than the threshold current, and an estimated temperature calculated by the second temperature arithmetic operation unit has reached a second allowed temperature, the current thus detected is an over current; and

a breaking control unit configured to break the switch portion when the overcurrent determination unit has determined that the detected current is the overcurrent,

wherein the first pseudo-heat generation temperature arithmetic expression and the first pseudo-heat radiation temperature arithmetic expression are set in such a manner that, in a temperature arithmetic expression of the electric wire, the temperature arithmetic expression using the elapsed time counted by the timer, the current detected by the current detection unit, and a heat capacity and conductor resistance of the electric wire, a first pseudo-heat capacity is assigned to the heat capacity,

the second pseudo-heat generation temperature arithmetic expression and the second pseudo-heat radiation temperature arithmetic expression are set in such a manner that a second pseudo-heat capacity is assigned to the heat capacity in the temperature arithmetic expression of the electric wire, and

the first and second pseudo-heat capacities are different from each other;

wherein a value of the first pseudo-heat capacity is smaller than a value of the heat capacity of the electric wire, and

a value of the second pseudo-heat capacity is smaller than a value of the first pseudo-heat capacity.

7. The protection apparatus of a load circuit according to claim 6 ,

wherein the breaking control unit is further configured to turn the switch portion to a connection-enabled state when the temperatures of the electric wire, which are estimated by the first temperature arithmetic operation unit and the second temperature arithmetic operation unit, have dropped to an ambient temperature after the switch portion was broken.

8. The protection apparatus of a load circuit according to claim 6 ,

wherein the first pseudo-heat capacity is set so that current/breaking time characteristics which are based on the first pseudo-heat generation temperature arithmetic expression and the first pseudo-heat radiation arithmetic expression can be located between minimum current/breaking time characteristics of a fuse protecting the electric wire and maximum current/breaking time characteristics of the fuse, and

the second pseudo-heat capacity is set so that current/breaking time characteristics which are based on the second pseudo-heat generation temperature arithmetic expression and the second pseudo-heat radiation arithmetic expression can be lower than current/breaking time characteristics of an electronic switch through which the current flows, the electronic switch being used for the load circuit.

9. The protection apparatus of a load circuit according to claim 6 ,

wherein the temperature arithmetic expression of the electric wire is:

T 2 =T 1 +I 1 2 rR{ 1−exp[− t /( C·R )]}  (1)

T 2 =T 1 +I 2 2 rR {exp[− t /( C·R )]}  (2)

in which Expression (1) is a temperature arithmetic expression at a time of heat generation, and Expression (2) is a temperature arithmetic expression at a time of heat radiation,

where T 1 is an ambient temperature [° C.], T 2 is an estimated temperature [° C.] of the electric wire, I 1 and I 2 are energization currents [A], r is an electric wire conductor resistance [Ω], R is a thermal resistance [° C./W], C is a heat capacity [J/° C.], and t is a time [sec].

10. The protection apparatus of a load circuit according to claim 6 ,

wherein the first allowed temperature is lower than the second allowed temperature.