Cooling system for cooling interior of substantially airtight housing

A cooling system for cooling an interior of a substantially airtight housing, said cooling system comprising:

• an internal-air passage having an internal-air inlet and an internal-air outlet, both said internal-air inlet and said internal-air outlet communicating with an interior of said housing;
• an external-air passage having an external-air inlet and an external-air outlet, both said external-air inlet and said external-air outlet communicating with an exterior of said housing;
• an internal-air fan that takes in internal air from said interior of said housing through said internal-air inlet and expels said internal air into said interior of said housing through said internal-air outlet;
• an external-air fan that takes in external air from said exterior of said housing through said external-air inlet and expels said external air to said exterior of said housing through said external-air outlet;
• a heat exchanger that exchanges heat between said internal air introduced into said internal-air passage and said external air introduced into said external-air passage;
• and
• a heater unit that heats said internal air introduced into said internal-air passage, said heater unit including:
• an electric heater that generates heat upon energization;
• a power supply circuit that supplies power to said electric heater;
• a heater protective device that is interposed in said power supply circuit, said heater protective device including a first thermostat and a second thermostat, said first thermostat being interposed in said power supply circuit on a first side of said electric heater, said second thermostat being interposed in said power supply circuit on a second side of said electric heater that is opposite to said first side of said electric heater, said first thermostat and said second thermostat being arranged to open substantially at a first predetermined temperature or higher to disconnect said power supply circuit;
• and
• a heat receiving plate that receives heat from said electric heater and conducts said heat of said electric heater to said heater protective device, said first thermostat and said second thermostat being secured to said heat receiving plate.