C.  Alarm Cutout Control Card
(1)  The alarm cutout control card is a small printed circuit card which plugs into the compartment overheat accessory unit and is held in place by screws. The control card provides turn-on and cutout control of the alarm bell and master fire warning lights when controlled from the wheel well control unit and bell cutout switches (Fig. 2).
D.  Test Relay
(1)  A wheel well overheat test relay is used in the wheel well test circuit. The relay parallels the sensing circuit to the overheat control unit. When the test circuit is operated, the relay closes and connects a resistor and ground to the sensing circuits, thus simulating a fire condition.
4.  Compartment Overheat Accessory Unit Components (Fig. 1) (Airplanes with Fenwal electronic compartment overheat accessory unit)
A.  The compartment overheat accessory unit is an electronic equipment rack E3 in the electronic compartment.
B.  The compartment overheat accessory unit has a cover plate which contains a numeric LED display, a maintenance advisory LED, and four BITE pushbutton controls identified as; MEM READ, MEM CLEAR, LOC TEST, DISP TEST.
C.  The compartment overheat accessory unit contains a power supply card, a control card and a relay card. The compartment overheat accessory unit also serves the functions of the wing and lower aft body overheat detection system (Ref 26-18-00).
(1)  
The power supply card receives 28-v dc from the battery bus and provides 5-v dc, 19-v dc, and 20-v dc to the control card.

(2)  
The control card contains a microprocessor and memory, which performs fault detection and BITE functions. If a fault is detected, a record is made in memory to be recalled as required.

(3)  
The relay card contains three relays, which control the warning indications and test circuit.

5.  Alarm Bell
A.  The alarm bell warns of an overheat condition in the wheel well area. The bell is controlled by the overheat control unit through the alarm cutout control card in the compartment overheat accessory unit (Fig. 2). The alarm bell is located in the aural warning devices box.
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6.  Master Fire Warning Lights
A.  The master fire warning lights are located on the lightshield module (P7). When an overheat condition occurs, the fire warning lights will alert the crew to the type of problem that exists.
B.  The master fire warning light will alert the crew of a wheel well overheat condition. The master fire warning lights are controlled by the wheel well overheat control unit alarm cutout card. Either light may be pressed for bell cutout and for extinguishing the master fire warning lights (Fig. 2).
7.  Sensing Element
A.  A sensing element provides overheat detection in the main wheel well. The sensing element is attached to structure in the ceiling of the wheel well. The element is attached at various intervals by quick-release fasteners.
B.  The sensing element is a thermistor type device, which has a resistance inversely proportional to temperature. As the element temperature rises above the alarm temperature, its resistance is lowered. The sensing element consists of a single nickel wire embedded in insulation, which is impregnated with a selected salt compound and sheathed in an Inconel tube.
C.  If any portion of the sensing element is subjected to an overheat condition, the element resistance drops sharply from a relatively high to an extremely low level. The length of element heater or the rate of temperature rise has little effect on the ability of the system to alarm at a fixed temperature. The single wire embedded in the center of the core is the power lead and the outside tubing is grounded. The insulating resistance of the core material decreases abruptly at the alarm temperature. Current flows through the core material to ground when the alarm temperature is sensed.
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8.  Operation
A.  Normal Operation
(1)  
Normal overheat detection is automatic when power is applied to the airplane and the overheat detection and aural warning circuit breakers are closed.

(2)  
If a fire condition occurs in the wheel well area, the sensing element is subjected to a temperature rise. The temperature rise causes the resistance value of the element to decrease. At a resistance value corresponding to the alarm temperature, the overheat control unit and alarm control card will trigger an overheat warning signal. The signals will activate the fire alarm bell, turn on the wheel well overheat light on the fire protection panel and illuminate the fire warning lights on P7. When the overheat condition is removed, the temperature and sensing element resistance values return to normal. The overheat signal will be removed and the wheel well overheat indicating components will go off. The alarm bell can be silenced when either the bell cutout switch or one of the master fire warning lights are momentarily pressed.
　
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