UPS - UNINTERRUPTIBLE POWER SYSTEMS
STANDBY VS. CONTINUOUS

DEFINITIONS:

1. UPS - Uninterruptible power system or supply which provides a steady source of electric energy to a piece of equipment. The Industrial and Commercial Power Committee of the IEEE defines a UPS as a system which is designed to provide power during all periods wherein the normal or prime source of power is outside acceptable limits, without causing disruption of the flow of acceptable power to the load.

2. Continuous On-Line UPS - A UPS system for which the load is normally continually drawing power through the batteries, battery charger, and inverter and not directly from the normal AC supply.

3. Standby Off-Line UPS - A UPS system which normally connects your equipment to the normal AC line with the batteries and inverter in standby mode. When the line is weak or down it transfers the load to the batteries and inverter without any load malfunction and without any user action. When the line returns to normal the load is automatically retransferred back to the AC line.

DISCUSSION:

There is much debate concerning UPSs as to whether a continuous on-line UPS or standby off-line UPS is required for personal and small business computers. I have worked at trade shows and had some people loudly insist it must be a continuous UPS and that you should never use a standby UPS. Also they argue that a standby UPS is not a "true" UPS and that only a continuous UPS is a "true" UPS and that the term UPS should only be used for the latter. The facts are that the term UPS which means Uninterruptible Power System (Supply) refers to a power system which delivers energy to a critical piece of equipment which must be kept operating. The Industrial and Commercial Power Committee of the IEEE defines a UPS as a system which is designed to provide power during all periods wherein the normal or prime source of power is outside acceptable limits, without causing disruption of the flow of acceptable power to the load. The term implies that if the normal power goes off, the system keeps working. So simple. Why all the esoterics? But argue people will over semantics. Well, these kind of arguments can rapidly boil down to the philosophical extremes such as how many angels can dance on the head of a pin. As I point out to these people, not even the normal AC power is continuous as far as the power supply to the computer equipment is concerned. It is going on and off 120 times per second. At every zero voltage crossing point, or thereabouts, in the AC sine wave the equipment's power supply is coasting and being drawn down, since no energy can be drawn from the AC line at and about the zero voltage crossing point. In fact, this dead band can be 2-4 milliseconds wide during which time the computer's power supply is being discharged and not being charged by the normal AC line voltage. As all engineers know, the charging takes place at the peaks of the AC line. The real world test of whether the system is uninterruptible is whether, when the normal AC voltage fails or is weak, does the alternate source of energy keep the critical computer system running long enough to safely accomplish the users objectives under those conditions. Since modern, well designed, standby emergency power systems can transfer from off to on in 1 to 4 milliseconds, for all practical purposes they are uninterruptible sources of energy as seen by the equipment since the equipment lives with 2 millisecond interruptions 120 times per second normally anyway. And since continuous UPSs are 3 to 4 times more expensive than standby UPSs for a given VA rating and reliability rating, the personal and small business user will find no conflict in the term standby UPS. This is why most manufacturers label their equipment UPSs and add the prefix continuous or standby, or on- line or off-line, to tell the user which type is being offered.

SUMMARY:

So when shopping for a UPS be sure you ask what type they are quoting and why. In this authors opinion modern standby UPSs with <=6ms transfer time are as effective as a continuous UPS, cost only 1/3 as much, and thus represent the best buy for the personal and small business computer user.