The off-line unit is designed to pass through the input line voltage and frequency to the load. (Note: the power passed through is non-conditioned utility power.) While this may be fine for office environments, it is not acceptable for industrial settings with periodic voltage and frequency deviations. Due to the design of off-line systems, some of the deviations will be passed directly to the loads, causing loads to drop and/or loss of data. Off-line UPS suppliers could tighten input parameters so these levels of voltage and frequency are not passed through. However, this would require the systemsÅ batteries to assume the load more frequently. As you will see in point #2, this can also have some troubling results.

If the input voltage and frequency deviate outside of acceptable limits, the systemsÅ batteries will automatically assume the supply of the charger/inverter. While this mode of operation rectifies the problem of voltage and frequency pass through, It can cause other serious problems.

First, if the voltage and frequency deviate (for example every time a motor or pump starts up) then the system will be operating on its batteries. The batteries supplied with off-line systems are a valve-regulated "maintenance free" type. These batteries are very sensitive to cycling. (Cycling is defined as any time that the battery supplies current to the load.) Cycling is not time dependent, so a one minute discharge is just as bad as a ten minute discharge. A battery is designed to supply only a certain number of cycles over life. (Note: valve-regulated batteries have a limited number of cycles, even less than other battery types.) Therefore, itÅs not hard to imagine what will happen when an off-line system is supplied for an industrial setting. The constant starting of motors, pumps and other electrical devices will result in voltage deviations outside the limits of the pass through logic. This will result in the system's batteries being cycled each time it occurs. Eventually, you will exceed the limited number of cycles available and the batteries will fail. This will undoubtedly happen when you least expect it, and you will probably not be aware of the condition. The result is that you will drop your critical load and also have to replace your batteries.

Another problem associated with the battery pickup feature is that even if the batteries are functioning normally failure may occur because of a lack of recharge current. Typically, off-line UPS systems are not supplied with fully-rated chargers. Instead the systems are supplied with a "trickle chargers." These "trickle chargers" are not designed to quickly recharge the systemÅs batteries after a discharge. If the batteries are being cycled often, the result is that the "trickle charger" may not be able to fully recharge the batteries in-between discharges. The batteries can be discharged to a state from which they can no longer supply the required current to your critical loads. Not only does this damage the systemÅs batteries, but it will also drop critical loads.

Off-line systems do not handle nonlinear (crest factor) loads well. Therefore, in order to supply these types of loads, off-line systems must often be oversized. (Note: Typical nonlinear loads are DCS systems and computer loads.) If the systems are not oversized to handle these types of loads, they will deprive the load of necessary current, resulting in the "flat topping" of the current wave form. The result would be loss of data and/or system failures.

We feel that the above points illustrate the failings of a off-line systems in an industrial atmosphere. It is not our intent to say that the off-line system are not a viable design. What we are saying is that we do feel that off-line designs are not reliable for long term operation in industrial environments. Off-line systems are typically manufactured for office type environments which do not place the demands on the system that an industrial environment does.

Solidstate Controls systems differ from off-line designs in the following ways: