UPS & Surge Protection

Terminology

Digital electronics circuits are very sensitive to power fluctuations.  Such fluctuations can easily cause a PC to shut down unexpectedly, or damage circuits.

• Spike: a large, transient voltage change lasting nanoseconds (billionths of a second).  These can be caused when connecting or disconnecting devices, by switching nearby equipment on or off, or by nearby lightning strikes.
• Surge: an increase in voltage lasting only a few milliseconds (thousands of a second).
• Sag: a decrease in voltage lasting only a few milliseconds.
• Brownout: a decrease in voltage lasting for a protracted period.  Its name comes from the fact that conventional light bulbs emit a brownish glow when underpowered.
• Blackout: a complete failure of power for a protracted period.

Brownouts and blackouts are easy to identify because they last a long time.  Fast-acting disturbances are harder to spot, and in such circumstances the symptoms may vary from random reboots to data corruption, or repeated component failure.

Surge Suppressors

Surge suppressors, also called anti-surge devices, clean up the power supply voltage by filtering out spikes and small surges.

They work in a variety of ways:

• A fuse or circuit breaker protects against longer surges.
• Some transformer designs transmit AC power but are unable to transmit sudden surges.
• An MOV is a small device that short-circuits at a high voltage, hopefully passing the spike / surge to earth.  Unfortunately, they are destroyed in the process.  Cheap surge protectors use only MOVs and will need to be replaced after tripping.
• A zener diode protects against normal spikes in a circuit, especially motor controllers.
• A gas discharge tube works like an MOV but can pass much more power without destruction, but cannot react to sudden spikes.

There are two factors to consider when choosing a surge suppressor:

• Clamping speed: how quickly the protective circuitry reacts to changes in the incoming power level and attempts to limit the change.
• Clamping voltage: the voltage level that the device will attempt to maintain.

Surge suppressors are useful for protecting systems from spikes and surges but offer no protection against brownouts or blackouts.  Additionally, a large surge will blow MOVs, fuses or trip circuit breakers — thus switching the computer off suddenly, which could result in the loss of data.

Uninterruptible Power Supplies (UPS)

A UPS provides a battery back-up power supply when the mains sags or fails.

Batteries use dc (direct current) electricity so an inverter is used to convert the battery's dc to the 230V ac (alternating current) required to power a PC.

If the battery starts to run low the UPS sends a shutdown signal (via a separate cable connected to the PC by USB or RS232 serial port) to tell the operating system to shut down gracefully and save all open files before power runs out.

Offline ("Standby") UPS

An offline UPS monitors the input power: when a problem occurs, the supply is switched to a pre-charged battery.

This is the cheapest type of UPS.

Line-interactive UPS

With a line-interactive UPS the inverter is always connected to the output of the UPS.  While mains power is supplied the inverter operates in reverse to charge the battery.

This type of UPS is often used for small servers.

Online UPS

An online UPS is considered to be a "true" uninterruptible supply because the PC is continually powered from the battery.  If mains power fails there is no switching delay.

An online UPS is expensive but gives better protection and power conditioning.

UPS ratings

When buying a UPS the most important factor is its volt-ampere (VA) rating.

This should not be confused with wattage (W), often calculated as voltage times current.  With ac, voltage and current are often out-of-phase — which means that there is a difference between the instantaneous voltage and current and the average measurement used for wattage calculations.  This difference is given by a power factor.

• Rule of thumb: for PCs the power factor is approximately 0.6.  For safety, technicians usually subtract 5% from the wattage.  This would mean a 600 VA UPS should not be used with PCs rated at more than 340 W.

Another important factor is the length of time they can supply power, measured in ampere-hours (Ah).

Using a UPS

A UPS is designed to provide continual power for critical devices.  Therefore they should not be used to power printers or other non-essential equipment.  Additionally, laser printers can exert a heavy drain and should never be attached to a UPS.

The UPS data cable should connect directly to the PC; in a power failure devices like USB hubs might fail to pass signals through to the PC.

The software accompanying a UPS may log significant disturbances; therefore a UPS can also be a useful tool for checking for repeated power disruption in the absence of more specialist voltage-measurement equipment.

Many UPS users recommend a full battery run-down at six-month intervals to extend the lifespan of the battery.