Electric power in the AI Center

Artificial Intelligence Center
The University of Georgia

Last revised by Michael A. Covington, 2008 Feb. 18

Contents
Introduction
Volts, amps, watts, and other terminology
Amount of power used by common devices
What can go wrong with electric power
Some standard tests that we perform
Map of our circuits

Introduction

Although intended mainly for use by lab technicians, this document is made available to all users of the AI Center's laboratories because all computer professionals need to know something about electric power and safety.

Volts, amps, watts, and other terminology

Voltage is the pressure that makes electric current flow. It is present even when current is not flowing. American power outlets normally deliver 120 volts.

Amps (amperes) measure the rate at which current is actually flowing. Our circuits can deliver up to 20 amps each. Most circuits in private homes can only deliver 15 amps.

Watts measure the rate of energy consumption. Wattage is related to voltage and amperage by the equation:

Watts = Volts × Amps

It follows that a 120-volt, 20-amp circuit can deliver a maximum of 2400 watts.

Some equipment is rated in volt-amperes (VA) instead of watts. For our purposes they are the same thing. Volt-ampere ratings are more commonly used on electromagnetic equipment where the alternations of current and voltage are not perfectly in phase.

All modern power distribution is based on the idea of constant voltage, load-dependent current. That is, the power station tries to deliver 120 volts all the time, and the number of amps depends on what is actually plugged in and turned on.

If a circuit is overloaded — that is, if you ask it to deliver more amps than it is designed to — then the wires will overheat and the building can catch fire. To keep this from happening, every circuit is fed power through a circuit breaker, which is a switch that should open automatically if the circuit is overloaded.

AC versus DC

American power outlets deliver alternating current (AC) which constantly changes direction at a frequency of 60 Hertz (60 Hz = 60 cycles per second). This frequency will not change unless something goes wrong at the power generating station.

The reasons for using alternating current instead of direct current are:

It is easier to make (by just twirling a magnet at 60 revolutions per second inside a coil);
Transformers (which trade volts for amps) can be used with AC but not DC;
AC motors are simpler than DC motors (and 100 years ago, motors were the second most important use for electricity, after light bulbs).

AC is changed to DC, generally at a different voltage, by the power supply inside a computer or other piece of apparatus.

Amount of power used by various common devices

Radio	5 watts	0.04 amp
Laptop computer	75 watts	0.6 amp
Desk lamp	100 watts	0.8 amp
Desktop computer	180-300 watts	1.5-2.5 amps
Laser printer	500-1000 watts	4-8 amps
Vacuum cleaner	500-1000 watts	4-8 amps
Space heater	500-1500 watts	4-12 amps
Hair dryer	1200 watts	10 amps
Coffee maker	1400 watts	12 amps

The lesson is that it is easy to overload a circuit with heaters, coffee makers, or laser printers. Remember that a whole circuit is only good for 20 amps (here) or 15 amps (at a typical residence). Don't just assume a circuit has enough capacity — know what is plugged into it and how much power is being demanded.

What can go wrong with electric power

Brownout — a prolonged period of reduced voltage (110 V or less) due to problems at the generating station or excessive demand by the whole town. Brownouts are very uncommon in Georgia.

During a brownout, most loads (e.g., lights) draw less current than normal (thanks to Ohm's Law) and this reduces the demand for power. Paradoxically, though, computers draw more amps during a brownout because their regulated power supplies efficiently trade amps for volts.

Surge (spike) — a momentary excessive voltage (like 500 V for 1 millisecond), caused by nearby lightning, or by the magnetic field in a big motor that is suddenly turned off.

A surge protector is a device that attempts to absorb surges. Surge protectors are usually built into UPSes and may also be built into some power strips. There is no way to test whether they are working. They do wear out, with age.

Power outage (power failure) — a period of no incoming power, lasting from a few seconds to a few days. In Georgia, short power failures (under 5 minutes) are moderately common; longer ones are not.

We protect computers from power outages using UPSes (uninterruptible power supplies). Inside a UPS is a large battery, a battery charger, and a circuit for converting the battery voltage to 120-volt AC. Normally, the output of the UPS is connected to the input. When the input voltage goes away, the UPS switches over and generates output power from the battery.

The main thing to know about UPSes is that each can only deliver a certain number of watts or volt-amps, and the batteries deteriorate with age, especially if not used. (A UPS in use will last longer than one that is on the shelf.) Periodically, UPSes should be tested to see if they will still run the computer long enough to get through a typical power outage.

UPSes do not work magic. In particular, they do not "fix" the problem if a circuit is overloaded or has low voltage.

A power strip usually contains a 15-amp circuit breaker. It may or may not also contain surge protectors, which may or may not work. A power strip does not fix any kind of power problem — it just outputs what is fed into it.

Some standard tests that we perform

Voltage — Use the Fluke true-RMS voltmeter set to AC voltage (V~). Voltmeters that are not true-RMS-responding will be less accurate because they are thrown off by noise on the line.

If the voltage is not between 115 and 125 volts, or is more than 1.5 V lower than other outlet voltages in the vicinity, please notify Dr. Covington. If it is outside the range 110-130 V or is more than 3 V lower than other nearby outlets, we have a problem and if the source of the problem is not obvious, you should get help.

Grounding — To check that an outlet is wired correctly, use a receptacle tester like the one shown. You should get 2 amber lights and no red light. If you find a problem, please notify Dr. Covington.

The button on top is not used with our configuration.

The AI Center does not presently own this tool. It can be borrowed from Dr. Covington.

You can also test grounding with a voltmeter, by taking 3 readings on each outlet. They should be:

Wide slot to narrow slot — 120 volts AC (V~).
Ground pin to narrow slot — 120 volts AC (V~).
Wide slot to ground pin — Under 2 volts AC.

Identifying what is on a circuit — A circuit comprises everything that is fed from a single circuit breaker.

To trace circuits, use a transmitter and receiver like the ones shown. Plug the transmitter in an outlet. The receiver will respond to other outlets on the same circuit and (usually) to the appropriate circuit breaker (in the breaker box in the outer hall, next to room 183).

Shown is a Sperry CS-550A. If the receiver is too sensitive, you can calibrate it by holding it next to the (working) transmitter and pressing the calibration button (the trigger underneath it). To return it to maximum sensitivity, turn it off and on again. Other tools of the same type may work differently.

Contrary to the instructions, you do not have to unplug or turn off other devices on the circuit. Also, you can usually get away with plugging the transmitter into an extension cord or power strip if the outlet is not directly accessible.

The AI Center does not presently own this tool. It can be borrowed from Dr. Covington.

A less reliable way to trace circuits is to assume that if two outlets show exactly the same voltage, they are probably on the same circuit.