Handbook of Local Area Networks, 1998 Edition:LAN Basics
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Stage 2: Power Conditioning to Reduce Random System Abends
Harsh loads actually create electrical transients as a normal by-product of operation. Whether these harsh loads are on the same branch circuit as electronic systems is, to some degree, irrelevant. All branch circuits are electrically connected to each other at the source or at the distribution panel. For this reason, specific power conditioning devices are recommended as a second-stage technique for improving reliability of electronic devices by isolating them from the natural building electrical environment.
For supporting distributed digital computing systems, conditioning the electrical supply has traditionally focused on two areas: maintaining a consistent AC line voltage (i.e., voltage regulation) and protecting the computer system from hard failure caused by lightning-induced power surges.
With respect to supporting distributed computing devices that are integral to a network and that have high reliability requirements, a third area of concern must be addressed, isolating the system from lower levels of high-frequency voltage transients (commonly referred to as electrical noise or ground noise).
Voltage Regulation
The power supplies in most microcomputers are very effective at blocking fluctuations in AC line voltage. In fact, these switching power supplies are typically more voltage tolerant than line voltage-regulating devices. Most system experts agree that supplemental voltage regulation for microcomputers is redundant.
Transient Protection
With respect to the issues of lighting protection and noise isolation, there is less consensus among experts. The debate can be distilled to two subtle issues: controlling peak voltage and the rise time of conducted transient voltage events, and providing this control in both normal (i.e., line-neutral) and common (i.e., neutral-ground) modes.
The most thorough and the simplest way to provide this control is through the use of a full output isolating transformer. This methodology was developed in the 1970s to provide the proper electrical environment for reliable operation of mainframe systems in raised floor computer room facilities.
This proven methodology is being applied in microcomputer-based LAN systems because the evolution of Novell’s NetWare and the movement toward a standard UNIX have made microcomputers the platform of choice for many critical path business systems. In these environments, certain spurious hardware interrupts (e.g., the nonmaskable and general protection interrupt errors in NetWare) an cause an abend, or system crash. These errors are often symptoms of subtle power quality defects that are not corrected by surge suppression devices alone. A better choice is to use small power conditioning systems that employ the full output isolation and high-frequency grounding principles defined in Federal Information Processing Standards (FIPS) 94 for computer room facilities.
Stage 3: Deciding on an Uninterruptible Power System
Even one power failure-related system crash in a file server or other critical device can be damaging and costly enough to justify the ongoing expense of a UPS. Key issues in deciding on a UPS include how much time it will have to supply power, and what level of system interface should be implemented. If power conditioning was not implemented before UPS installation, power conditioning capabilities of the UPS become an important consideration. Some UPS designs combine effective power conditioning based on full output transformer isolation with reliable battery backup in one integrated device.
Backup Power for Simple System Shutdown
If the primary concern is to protect the integrity of data files during an uncontrolled system shutdown, then the key question is: How much time does the system take to shut down in a controlled fashion? In most NetWare environments, shutting down the server takes less than 2 minutes. In a busy UNIX system, this can take as long as 5 minutes. For a UPS, the minimum time acceptable is twice that required for controlled shutdown.
Twice the shutdown time is necessary because UPS batteries deteriorate with age. A new system that provides twice the minimum runtime can be expected to have a life of at least 12 months. After 12 months, batteries should be tested and replaced when they can no longer deliver the minimum runtime required.
It may also be wise to buy enough battery backup capacity to keep the network functioning through brief power outages. If after 2 to 4 minutes, the Ac is still off, 2 to 5 minutes of reserve time are still necessary for a controlled shutdown. The calculation for total reserve time under these circumstances is 2 × [(server shutdown time) + (reserve for operating through brief outages)].
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