Rockwell Automation 1426 PowerMonitor 5000 Unit - Manual (Page 385)

Rockwell Automation Publication 1426-UM001F-EN-P - November 2013 385
Glossary
Neutral The conductor chosen as the return path for the current from the load to the
source. It is also a voltage reference point in a power system.
Ohm The unit of electrical resistance. One ohm is the value of resistance through
which a potential difference of one volt maintains a current flow of one ampere.
Peak Demand The highest average load over a utility specified time interval during a billing
period. If there is no ratchet clause in the rate schedule, then the peak demand is
also the billing demand.
Polyphase Having or utilizing several phases. A polyphase power circuit has several
(typically three) phases of alternating current with a fixed phase angle between
phases.
Potential Transformer (PT) An transformer with the primary winding connected in parallel with the circuit
whose voltage is to be measured or controlled. PT's are normally used to step
down high-voltage potentials to lower levels acceptable to measuring
instruments. Also known as voltage transformer (VT).
Potential Transformer Ratio The ratio of primary voltage divided by secondary voltage.
Power Factor The ratio of real power in watts of an alternating current circuit to the apparent
power in volt-amperes. Also expressed as the cosine of the phase angle between
the fundamental voltage applied to a load and the current passing through it.
Power Factor Correction Steps taken to raise the power factor by closely aligning the current to be in phase
with the applied voltage. Most frequently this consists of added capacitance to
increase the lagging power factor of inductive circuits.
Power Factor Penalty The charge utilities impose for operating at power factor below some rate
schedule-specified level. This level ranges from a lagging power factor of 0.80 to
unity. There are innumerable ways by which utilities calculate power factor
penalties.
Ratchet Clause A rate schedule clause that states that billing demand can be based on current
month peak demand or on historical peak demand, depending on relative
magnitude. Usually the historical period is the past eleven months, although it
can be for the life of the contract. Billing demand is either the current month
peak demand or some percentage (75% is typical) of the highest historical peak
demand, depending on which is largest. It is designed to compensate the electric
utility for maintaining equipment not fully utilized.
Reactance The opposition to the flow of alternating current. Capacitive reactance is the
opposition offered by capacitors and inductive reactance is the opposition offered
by an inductive load. Both reactances are measured in ohms.