Protection device for load circuits
A protection device protecting a load circuit by shutting off a power thereof depending on an estimated present temperature of a wire provided in the load circuit to connect a load. The protection device has a first temperature estimation device estimating a rising temperature of a conductor including the wire based on both a current in the conductor and thermal properties of the conductor, a second temperature estimation device estimating a falling temperature of the conductor based on the thermal properties, a third temperature estimation device estimating an arc-induced rising temperature of the conductor when arcing occurs in the conductor, and a fourth temperature estimation device estimating the present temperature of the conductor deduced from the above estimated temperatures. If the estimated temperature exceeds a predetermined allowable temperature, the protection device shut off the power of the load circuit.
1. A protection device for a load circuit including a power supply, a switch and a load, comprising:
a current detection device detecting and measuring a current in a conductor connected between the power supply and the load;
a first temperature estimation device that estimates a rising temperature of the conductor, when the current is detected by the current detection device;
a second temperature estimation device that estimates a falling temperature of the conductor when the current detection device fails to detect the current or detects that the current is being decreased,
a third temperature estimation device that estimates an arc-induced rising temperature of the conductor that is induced by arcing that occurs just after the current has returned to a normal current;
a fourth temperature estimation device that estimates a present temperature of the conductor based on the estimated rising, falling, and arc-induced rising temperatures;
a temperature determination device that determines whether the present temperature of the conductor estimated by the fourth temperature estimation device exceeds a predetermined threshold temperature;
a shutoff control device that shuts off power to the load circuit when the temperature determination device determines that the present temperature of the conductor exceeds the predetermined threshold temperature, and
wherein the rising temperature is estimated based on both the current measured by the current detection device and thermal properties of the conductor;
the falling temperature of the conductor is estimated based on the thermal properties of the conductor; and
the arc-induced rising temperature is estimated based on the current just before the current returns to the normal current.
2. The protection device according to claim 1 , wherein the thermal properties are thermal resistivity R and heat capacity C.
3. The protection device according to claim 2 , wherein the rising temperature estimated by the first temperature estimation device is determined by the following formula;
T
1
=
T
2
+
i
2
r
·
R
·
(
1
-
ⅇ
-
1
C
·
R
·
t
)
,
where T1 is a temperature of the conductor [°C], T2 is an ambient temperature [°C], i is the current [A], r is the electrical resistance [Ω] of the conductor, R is the thermal resistivity of the conductor [°C./W], C is the heat capacity of the conductor [J/°C or W-sec/°C], and t is transit time [sec].
4. The protection device according to claim 2 , wherein the falling temperature estimated by the second temperature estimation device is determined by the following formula;
T
1
=
T
2
+
i
2
r
·
R
·
ⅇ
-
1
C
·
R
·
t
,
where T1 is a temperature of the conductor [°C], T2 is an ambient temperature [°C], i is the current [A] required to saturate the temperature of the conductor at which the current is not detected or the current decreases, r is the electrical resistance [Ω] of the conductor, R is the thermal resistivity of the conductor [°C/W], C is the heat capacity of the conductor [J/°C or W-sec/°C], t is transit time [sec].