Autonomy
This is the time that the battery must support the load and is also often called back-up or discharge time
Blackout
A total loss of electrical power.
Brownout
A low voltage condition over an extended period of time.
Bypass
An ac power path around a UPS.
An automatic bypass is controlled by a UPS and operated under fault or overload conditions to maintain power to the protected load.
A manual bypass is a user switch on a UPS allowing a complete electrical bypass of the unit, used when there is a total UPS failure or when carrying out maintenance or repair without shutting down the load. Some parts of the UPS – the input and output terminations for example – may remain live.
A service or maintenance bypass is a manual bypass allowing complete isolation, maintenance, or removal of the UPS without shutting down the load.
Common Mode Noise
Disturbances between the neutral and earth or grounding conductors. Noise may result from injection into the neutral or grounding wires, wiring faults, or overloaded power circuits.
Crest Factor
Expressed as the ratio between the crest (peak or maximum) value of a current to the root-mean-square (rms) value. A square wave of current has a crest factor of one. A sine wave has a crest factor of 1.414. Typical computer power supplies draw current with a crest factor of between two and three.
Current Limit
The function of a circuit or system that maintains a current within its prescribed limits. UPS systems have an electrical current limit that regulates the output current to a value within the UPS limits.
Differential Mode Noise
Describes noise which occurs between the live and the neutral conductors. Caused by load switching locally.
Dip
A transient voltage decrease.
Distortion
Difference between the actual ac voltage waveform delivered and an ideal sine wave.
EMI
Electro-Magnetic Interference
Electrically induced noise or transients.
Filter
An electronic device that allows only certain frequencies to pass.
Float Charging
A battery charging system suitable for UPS batteries. A float charger maintains a voltage on the battery, known as the float voltage, which is the ideal voltage for the battery and maximises battery life. At float voltage, a current flows into the battery which exactly cancels the battery’s internal self discharge current.
Ground Fault
An undesirable connection that allows current to flow from a conductor to ground.
Harmonic
Voltage or current signals that are not at the desired 50Hz fundamental frequency, but are at some multiple frequency, such as the third harmonic, 150Hz and the fifth harmonic, 250Hz. Excessive harmonic voltages can have serious effects on modern equipment power supplies and may cause overheating.
Harmonic Distortion
Excessive harmonic (a frequency that is a multiple of the fundamental frequency) content that distorts the normal sine wave.
Inverter
Part of a UPS that converts the dc into ac power.
kVA
Thousand VA.
Load
Any electrical device connected to a power source is a “load”. For a UPS, the load is the amount of current/power required by the attached electronic equipment.
MOV – Metal Oxide Varistor
A voltage clamping device capable of absorbing very large currents without damage.
Noise
Any undesirable electrical signal.
Over Voltage
An abnormally high voltage sustained for an extended period.
Phase
Load current is drawn from a voltage source. In ac systems, the voltage is a sine wave and for a purely resistive load, the current drawn is also a sine wave aligned perfectly (in phase) with the voltage sine wave. Most loads, however, are not purely resistive and the current drawn is delayed and lags behind the voltage sine wave (out of phase). The lag is measured in degrees. Power factor is equal to the cosine of this phase difference.
Power Factor
The relationship between actual power (W) and apparent power (VA). Calculated by dividing Watts by Volt-Amperes (W/VA) and usually in the range of 0.75 to 0.85.
PWM – Pulse-Width Modulation
Process of varying the width of a train of pulses to adjust the rms voltage and frequency and modify the waveshape, typically to sinusoidal.
Rectifier / Charger
Part of a UPS that converts the incoming ac utility power to dc power for the inverter and to charge the batteries.
RFI – Radio-Frequency Interference
Electrical noise resulting from some parts of the equipment or wiring acting as a radio antenna. This noise may be large enough to disrupt communications or cause computing error.
Sine Wave
A fundamental waveform produced by periodic, regular oscillation that expresses the sine or cosine or a linear function of time or space or both.
SNMP
Simple Network Management Protocol is a set of protocols for managing complex networks. SNMP works by sending messages, called protocol data units (PDUs), to different parts of a network. SNMP-compliant devices store data about themselves in Management Information Bases (MIBs) and return this data to the SNMP requesters.
Static Switch
A static switch is an “intelligent switch” used to select either the UPS’s inverter output voltage or the raw mains to supply the load. The selection is made by control logic which continually monitors the bypass (raw mains) and inverter voltages.
Surge
An abnormally high voltage lasting for a short period of time.
Three Phase
An electrical system with three different voltage lines with sine wave waveforms that are 120 degrees out of phase from one another.
Total Harmonic Distortion
The ratio of the rms sum of all the harmonic components and the fundamental signal.
Under Voltage
An abnormal low voltage lasting for a longer period of time than a sag.
VRLA – Valve Regulated Lead Acid Batteries
Valve regulated batteries emit virtually no gas, require no topping up and need no special ventilation other than that required by the local building codes.
Watts
The unit of actual power. Compare with Volt-Amperes (VA), which is the unit of measure of apparent power.
