01 March 2005
Table 3-3. Effects of Moisture and Fungi on Various Materials
PART OR MATERIAL
Moisture causes swelling that causes the support to misalign, resulting in
Fiber: washers, supports
binding of supported parts. Destroyed by fungi.
Moisture forms electrical leakage paths, causing flashovers and crosstalk.
Insulating properties are lost. Destroyed by fungi.
Moisture causes insulating properties to be lost. Leakage paths cause
Laminated plastics: terminal strips, boards,
flashovers and crosstalk. Delamination occurs and fungi grow on surface and
switchboard panels, tube sockets and coil
around edges. Expansion and contraction under extreme temperature
forms, and connectors
Molded plastics: terminal boards,
Machined, sawed, or ground edges of surfaces are supporters of fungi,
switchboards panels, connector, tube
causing shorts and flashovers. Fungal growth reduces resistance between
sockets and coil forms
parts mounted on plastic to such an extent that the parts are useless.
Cotton linen, paper, and cellulose
Moisture causes loss or impairement of insulating and dielectric properties,
derivatives: insulation, coverings, webbing,
causing arcing, flashovers, and crosstalk. Destroyed by fungi.
belting, laminations, dielectrics
Wood: cases, houses and housings, plastic
Dry rot, swelling, and delamination are caused by moisture and fungi.
Moisture and fungi destroy tanning and protective materials, causing
Leather: straps, cases, gaskets
Fungi grow on organic dust, insect track, insect feces and dead insects.
Glass: lenses, windows
Dead mites and fungal growth on glass obscure visibility and corrode nearby
Fungi-inhibiting waxes that are not clean support the growth of fungi, cause
Wax: for impregnation
destruction of insulating and protective qualities, and permit entrance of
moisture that destroys parts and unbalances electrical circuits.
High temperature and moisture vapor cause rapid corrosion. Fungus and
bacterial growth produce acid and other products that speed corrosion,
etching of surfaces, and oxidation.This interferes with the operation of moving
parts, screws, and causes dust between terminals, capacitors, plates or air
condensers, which in turn may cause noise, loss in sensitivity, and arc-overs.
Metals may have different corrosion potentials. When moisture is present,
one of the metals (anode) corrodes.
Residual soldering flux on terminal boards holds moisture, which speeds up
corrosion and growth of fungi.
or by drying the interiors of compartments with
present. The metabolism of these bacteria requires
desiccants. However, some moisture-proof coatings
them to obtain food sources by oxidizing inorganic
are attacked by microorganisms, especially if the
compounds such as iron, sulfur, hydrogen, and carbon
surface on which they are used is already contaminated.
monoxide. The resultant chemical reactions cause
Some microorganisms can survive in spore form for
long periods while dry, and can become active when
moisture is available. When desiccants become
3-14.2.2. Microbial Growth Requirements. Fungi make
saturated and unable to absorb moisture in the affected
up one class of microorganism that feeds on organic
area, microorganisms can begin to grow. Dirt, dust,
matter. Low humidity levels will inhibit the growth of
most species of fungi and bacteria. Ideal growth
recognized contributors to microbial attack. Unnoticed,
conditions for most fungi and bacteria are temperatures
small amounts of airborne debris may be sufficient to
between 68° and 104°F (20° and 40°C) and relative
promote fungal growth.
humidities between 85 and 100%. It was formerly
believed that microbial attack could be prevented by
applying moisture-proof coatings to nutrient materials