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13. GLOSSARY OF TERMS
ALTERNATING CURRENT (AC) - A current which periodically reverses in direction and changes its magnitude
as it flows through a conductor or electrical circuit. The magnitude of an alternating current rises from zero to
maximum value in one direction, returns to zero, and then follows the same variation in the opposite direction.
One complete alternation is one cycle or 360 electrical degrees. In the case of 50 cycle alternating current the
cycle is completed 50 times per second.
AMBIENT TEMPERATURE - The air temperature of the surroundings in which the generating system operates.
This may be expressed in degrees Celsius or Fahrenheit.
AMPERE (A) - The unit of measurement of electric flow. One ampere of current will flow when one volt is applied
across a resistance of one ohm.
APPARENT POWER (kVA, VA)- A term used when the current and voltage are not in phase i.e. voltage and
current do not reach corresponding values at the same instant. The resultant product of current and voltage is
the apparent power and is expressed in kVA.
AUTOMATIC SYNCHRONIZER - This device in its simplest form is a magnetic type control relay which will
automatically close the generator switch when the conditions for paralleling are satisfied.
BREAK MEAN EFFECTIVE PRESSURE (BMEP) - This is the theoretical average pressure on the piston of an
engine during the power stroke when the engine is producing a given number of horsepower. It is usually
expressed in pounds/inch
2
. The value is strictly a calculation as it cannot be measured, since the actual cylinder
pressure is constantly changing. The mean or average pressure is used to compare engines on assumption that
the lower the BMEP, the greater the expected engine life and reliability. In practice, it is not a reliable indicator of
engine performance for the following reasons.:
The formula favours older design engines with relatively low power output per cubic inch of displacement in
comparison with more modern designs. Modern engines do operate with higher average cylinder pressures, but
bearings and other engine parts are designed to withstand these higher pressures and to still provide equal or
greater life and reliability than the older designs. The formula also implies greater reliability when the same engine
produces the same power at a higher speed. Other things being equal, it is unlikely that a 60 Hz generating set
operating at 1800 RPM is more reliable than a comparable 50 Hz generating set operating at 1500 RPM. Also it is
doubtful that a generator operating at 3000 RPM will be more reliable than one operating at 1500 RPM even if the
latter engine has a significantly higher BMEP. The BMEP for any given generating set will vary with the rating
which changes depending on fuel, altitude and temperature. The BMEP is also affected by generator efficiency
which varies with voltage and load.
CAPACITANCE (C)- If a voltage is applied to two conductors separated by an insulator, the insulator will take
an electrical charge. Expressed in micro-farads (µf).
CIRCUIT BREAKER - A protective switching device capable of interrupting current flow at a pre-determined
value.
CONTINUOUS LOAD - Any load up to and including full rated load that the generating set is capable of
delivering for an indefinitely long period, except for shut down for normal preventive maintenance.
CONTINUOUS RATING - The load rating of an electric generating system which is capable of supplying
without exceeding its specified maximum temperature rise limits.