It is generally accepted there are 10 forms of power disturbance (IEC62040-3):
| # | Problem | Definition | Duration | Causes | Effects |
| 1 | Mains Failure | A complete drop in the power supply. |
>10msec | Numerous, including weather, system faults, human error, circuit breaker tripping, accidental disconnection etc. |
Equipment fails to operate |
| 2 | Voltage Fluctuations | Momentary drop or raise in supply voltage |
<16msec | Large loads starting up, high source impedance, overload of supply network |
Equipment resets, crashes, hardware damage |
| 3 | Voltage Transients | Momentary rise (or fall) in supply voltage. Can be catastrophic |
<16msec | Two fold, by equipment switching off, especially inductive machines, or by lightning strikes |
Equipment damage |
| 4 | Undervoltage or Brown-Out | Prolonged undervoltage | Potentially Continuous | High loading or overload on network, intentional supply reduction |
Equipment malfunction, failure to operate, crashes, hardware damage |
| 5 | Overvoltage or White-Out | Prolonged overvoltage | Potentially Continuous | Low loading on network, faulty supply regulation |
Hardware damage |
| 6 | Lightning Effects | Effects due to lightning | <1ms, Sporadic | Nearby Lightning Strikes, Strikes to power lines. |
Hardware damage, equipment malfunction |
| 7 | Voltage Surges/Spike | Large Magnitude Over or Under Voltage |
<4ms | Inductive machinery switching, lightning |
Hardware damage, equipment malfunction. |
| 8 | Frequency Fluctuation | Variation from the supply frequency | Periodic | Generator loading. | Hardware damage, equipment malfunction |
| 9 | Voltage Bursts | Repetitive High Frequency Transients | Periodic | Electronic Equipment, machinery | Hardware damage, equipment malfunction |
| 10 | Harmonic Distortion | Distortion of the power waveform from pure sinewave |
Potentially Continuous | Non linear loads, eg computer power supplies, rectifiers, variable speed drives, etc. |
Equipment Damage, overheating in equipment and supply cables. Nuisance breaker tripping. |
So depending upon what level of problem you are experiencing, or what your risk analysis tells you what you should protect against determines the choice of product used:
| # | Problem | Method used to eradicate |
| 1 | Mains Failure | A UPS system – VFD, VI, or VFI |
| 2 | Voltage Fluctuations | A Voltage Regulator, or VI/VFI UPS |
| 3 | Voltage Transients | A Surge Supressor and Filter |
| 4 | Undervoltage | A Voltage Regulator or VI/VFI UPS |
| 5 | Overvoltage | A Voltage Regulator or VI/VFI UPS |
| 6 | Lightning Effects | A Surge Suppressor with filter or power conditioner |
| 7 | Voltage Surges/Spike | A Surge Supressor and Filter |
| 8 | Frequency Variation | A VFI UPS system |
| 9 | Voltage Bursts | A filter, surge supressor (with filter) or power conditioner |
| 10 | Harmonic Distortion | A VFI UPS system (protects load) |
Since most UPS systems will contain a surge supressor and filter, a UPS is the ideal power protection solution, however consider a power conditioner if you do not require any form of battery backup. The choice of system will be made according to the following table:
|
#
|
Problem |
PC
|
VFD
|
VI
|
VFI
|
| 1 | Mains Failure |
N
|
Y
|
Y
|
Y
|
| 2 | Voltage Fluctuations |
N
|
Y
|
Y
|
Y
|
| 3 | Voltage Transients |
Y
|
?
|
?
|
Y
|
| 4 | Undervoltage |
N
|
N
|
Y
|
Y
|
| 5 | Overvoltage |
N
|
N
|
Y
|
Y
|
| 6 | Lightning Effects |
Y
|
?
|
?
|
Y
|
| 7 | Voltage Surges/Spike |
Y
|
N
|
N
|
Y
|
| 8 | Frequency Variation |
N
|
N
|
N
|
Y
|
| 9 | Voltage Bursts |
Y
|
?
|
?
|
Y
|
| 10 | Harmonic Distortion |
N
|
N
|
N
|
Y
|
Note that 3, 6 and 9 are grey areas, VFD and VI may provide protection against these, dependent upon type of UPS and level of protection employed.
Other Considerations
Once you have decided on the minimum level of technology needed you will need to look at the options you require, and if there are any special requirements. For example, you may need the unit to be situated in a rack server, or the UPS may need to be used in a patient vicinity environment and therefore have special certification requirements. You may have long run time requirements, and need to choose a UPS that can deal with this.
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