MIL-PRF-24775A(SH)
heater. The thermostat shall be accessible through an access panel.
3.12.5 Heater operation. The heater shall provide continuous operation under normal shipboard environmental
conditions, such as vibration and the presence of moisture in the air stream. Heaters shall operate in an environment
up to 100 percent relative humidity and shall have 1 increment of heat. Heaters shall be designed for 3-phase
operation and shall have 3, or a multiple of 3 elements of equal wattage so that a balanced 3-phase load is provided.
Heaters shall provide continuous operation at an airflow of 80 to 125 percent of the fan-coil unit rated airflow as
specified in Table I.
3.12.5.1 Heating elements. Heating elements shall be of the enclosed type in a hermetically sealed, finned
sheath. Maximum surface temperature shall not exceed 750 °F at rated airflow. Element shall meet the
requirements of 3.12.5.2 and 3.12.5.3 after the element has been completely immersed in water while under a
hydrostatic gauge pressure of at least 60 pounds per square inch for a period of at least 20 minutes. Element shall be
capable of continuous operation at rated kW with rated airflow as specified in 3.7. Element watt density shall not
exceed 55 watts per square inch of sheath area (excluding fin area).
3.12.5.2 Insulation resistance. Insulation resistance of the element at normal operating temperature (see
3.11.13.1) shall be not less than 25 megohms. Insulation resistance when each heating element (excluding wiring) is
at ambient room temperature shall be not less than 200 megohms.
3.12.5.3 Dielectric strength. Heating element shall withstand a dielectric test of twice the rated line voltage
plus 1000 volts for a period of 1 minute. The voltage wave shall approximate a sine wave, and frequency shall not
be less than 60 Hz.
3.12.5.4 Moisture resistance. Heater element shall be immersed in water and while immersed subjected to a
hydrostatic gage pressure of at least 60 PSI for a period of at least 20 minutes. Terminals of the element shall be
wiped dry and subjected to dielectric test and the insulation resistance test (see 4.5.2.11).
3.12.5.5 Heater life expectancy. An accelerated life test by fusion shall verify life expectancy of 21,000 hours
as described in 4.5.2.11.1.
3.12.5.6 Heater element creepage and clearance.
a.
Creepage distance is the shortest distance between energized parts or between an uninsulated energized
part and ground, along the surface of an insulating material. When necessary, insulating barriers may
be used to interrupt continuous electrical creepage paths. Cemented or butted joints shall not be used.
b.
Creepage distance ¼ inch maximum.
c.
Clearance distance is the shortest point-to-point distance in air between uninsulated energized parts or
between an energized part and ground.
Clearance distance ⅛ inch minimum.
d.
3.12.6 Electrical requirements.
3.12.6.1 Electrical power cable entrance. A blank, gasketed plate shall be provided on each side of the fan-coil
unit in the general area of the control terminal strip to permit installation of shipboard cable by the installing
activity.
3.12.6.2 Electrical wiring and connections. Wiring shall be neat and tied or clamped in a manner that supports
and prevents chafing of the wire insulation because of vibration and shock. There shall be no splices in the wire and
the connections shall be at the terminals of the devices or terminal strips. Wiring shall be clear of access panels and
located out of the air stream whenever possible. Wiring shall be clear of areas that may require maintenance and
shall be so routed within the cabinet that no damage will occur when drilling or assembling the units.
3.12.6.3 Grounding connections for electromagnetic compatibility. Fan-coil units shall be grounded and
bonded in accordance with IEEE STD 45.
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