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Environmental Stress Screening Objective of ESS

5.1.2 Atmospheric:

1. Moisture/ Humidity tests
2. Sand and Dust Test
3. Salt Fog
4. Rain
5. Fungus

1. Moisture/ Humidity tests: Humidity/ Moisture tests are conducted during the development and evaluation or manufacture of equipment to ascertain its ability to resist the effects of humid atmosphere on the product. The humidity ranges from 5% to 98% and generally combined with various temperatures to make the test more effective. The effects of Moisture/ Humidity are
1. Loss of dielectric strength
2. Corrosion
3. Aids fungus growth

The corrective actions include applying adequate seals, moisture resistant parts and conformal coating.

2. Sand and Dust Test:Sand and Dust is a test used during the development and evaluation of equipment to ascertain its ability to resist the effects of a dry fine sand laden atmosphere. This test simulates the effect of sharp-edged dust particles, which may penetrate into cracks, bearings, and joints, and cause a variety of damage such as fouling moving parts, making relays inoperative, forming electrically conductive bridges with resulting “shorts” and acting as a nucleus for the collection of water vapor, and hence a source of possible corrosion and malfunction of equipment. This test is applicable to all mechanical, electrical, electronic, electromechanical devices for which exposure to the effects of a dry duct (fine sand) laden atmosphere is anticipated.

3. Salt Fog: The salt fog test is conducted to determine the resistance of the equipment to the effects of a salt atmosphere. Damages to be expected from exposure to salt fog is primarily corrosion of metals, although in some instances salt deposits may result in clogging or binding of moving parts. In order to accelerate this test and thereby reduce testing time, the specified concentration of moisture and salt is greater than is found in service. The test is applicable to any equipment exposed to salt fog conditions in service. It is also useful for evaluating the uniformity ( that is thickness and degree of porosity) of protective coatings.

4. Rain:The rain test is conducted to determine the effectiveness of protective covers or cases to shield equipment from rain. This test is applicable to equipment that may be exposed to rain under service conditions. Since any test procedure involved would be contingent on requirements peculiar to the test item and the facility employed, a standardized test procedure for rain erosion is not included in this test method.

5. Fungus Test: The fungus test is conducted to determine the resistance of equipment to fungi. Fungi secretes enzymes that can destroy most organic substances and many of their derivatives. Typical material that support fungi and are damaged are cotton, wood, linen leather, paper, and cardboard. For fungus test, the test items are tested for several days(typically 28 days) at 300C and 95% humidity.

5.1.3 Radiation:

1.Solarradiation: The sunshine test is conducted to determine the effect of solar radiation energy on equipment in the earth’s atmosphere. For the purpose of this test, only the terrestrial portion of the solar spectrum is considered. The specified limits and energy levels provide the simulated effects of natural sunshine. The ultraviolet portion simulates natural sunshine in a general way, and is considered representative of irradiation in most geographical locations. Sunshine causes heating of equipment and photo-degradation, such as fading of fabric colours, checking of paints, and deterioration of natural rubber and plastics. Sunshine tests are applicable to equipment that may be exposed to solar radiation during service or unsheltered storage at the earth’s surface or in the lower atmosphere.

2. Electromagnetic radiation:The equipment are tested for

1. Generation of interference to other equipment/ objects
2. Susceptibility to interference from other equipment / sources

Corrective actions include

1. Grounding
2. Shielding and filtering
3. Power isolation and conditioning

Some of the examples of equipment that generate electromagnetic interference are printers, relays etc.

5.1.4 Dynamic Testing

Dynamic testing may broadly be classified into
1. Vibration testing
     i) Sinusoidal Vibration
     ii)Random Vibration
2. Mechanical Shock

1. Vibration testing Vibration is considered second highest precipitator of faults only after temperature cycling. In sinusoidal vibration, only a pure (ideally) sine wave excitation is given to the test item, whereas in random vibration, the excitation contains all frequencies in the given bandwidth. Random vibration is considered to be more effective because all the frequencies are present at the same time in the desired bandwidth (same as excitation noise bandwidth). The principal effects of vibration are
                                                1. Resonance damage
                                                2. Fatigue damage
These can be limited by stiffening the mechanical structure, controlling resonances.

2. Mechanical Shock Mechanical shock test is used determine the ability of the product to withstand the shock that it might undergo during its transportation and/or service environment

Environmental Stress Screening Objective of ESS
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