no. 768 ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^M

1 ggg U.S. Department of Commerce

P ^ National Institute of Standards and Technology
Infor

Aposter prepared by the NIST Power

Quality Committee to inform users of

sensitive equipment about problems

with and solutions for protecting their equipment

from power disturbances.

This poster contains:

• Answers to seven common questions about power

quality that should help you pinpoint problems

and solutions related to power disturbances.

• A chart describing the types of power disturbances, the equipment affected, and a brief

summary of the types of protection equipment that

is effective against the disturbance.

• A glossary of common power terms.
Glossary

critical load— critical load is equipment that is sensitive to power

disturbances, (also referred to as a

sensitive load)

common mode noise—Electrical noise between the power conductors and ground, i.e., between

line and ground or between neutral

and ground.

impulse—^See spike.

inverter—^An inverter takes dc

power and converts it into ac power.

kilovolt-ampere (kVA)—An electrical unit related to the power

rating of a piece of equipment. It is calculated by multiplying the rated

voltage of equipment by the current required (or produced). For resistive loads 1 kilovolt-ampere equals 1 kilowatt.

line—A designation of one or more

power-carrying conductors for power distribution. The black (or

red or blue) wire is the line conductor, the white wire is the

neutral, and the green wire is ground. The voltage difference

between the Une conductor and the

neutral is the supply voltage, i.e., 120 volts.

line conditioner—A line conditioner contains multiple protection

devices in one package to provide,

for example, electrical noise

isolation and voltage regulation.

momentary overvoltage—A momentary overvoltage (or "swell") is an increase in voltage outside the

normal tolerance for a few seconds

or less. Voltage swells are often

caused by sudden load decreases or

turn-off of heavy equipment.

motor generator—A motor

generator consists of an ac motor

coupled to a generator. The utility

power energizes the motor to drive

the generator, which powers the

critical load. Motor generators

provide protecHon against noise

and spikes, and, if equipped with a heavy flywheel, they may also protect against sags and swells.

neutral—A designation of one of the two power carrying conductors

for power distribution. This is the

white wire and is normally at or near the voltage of the ground wire.

Thus the line or black conductor is at a high voltage (i.e., 120 volts)

with respect to the neutral and

ground wires.

noise—^Electrical noise is a

distortion of the normal sinewave

power and can be caused by radar

and radio transmitters, fluorescent

lights, power electronics control

circuits, arcing utility and industrial

equipment, and loads with solidstate rectifiers.

outage—An outage is a complete

loss of power that may last from

several milliseconds to several hours and may be caused by power

system faults, accidents involving

power lines, transformer failures,

and generator failures. Some

sensitive equipment may be

disrupted by outages as short as 15

milliseconds.

power conditioner—See line conditioner.

sag—A voltage sag is a momentary

(less than 2 seconds) decrease in voltage outside the normal

tolerance. Voltage sags are often

caused by stating heavy loads, such

as motors or welding equipment,

and by power system faults.

spike (or impulse, switching surge, lightning surge)—These terms refer

to a voltage increase of very short

duration (microsecond to milli- second). Spikes can range in amplitude from 200 volts to 6,000

volts and are caused by lightning,

svdtching of heavy loads, and short

circuits or power system faults.

spike suppressor—An inexpensive

device that provides protection

against short duration (microsecond

to millisecond) voltage increases

known as spikes, impulses,

transients, or high-frequency

surges.

standby power supply (SPS)—See

uninterruptible power supply.

surge—The word "surge" has

different meanings in different engineering communities. To the

protection engineer a "surge" is a transient overvoltage with a duration of a few microseconds, i.e., a

spike. To others a "surge" is a momentary overvoltage lasting up

to a few seconds. To avoid confusion we will use the word "swell"

as an abbreviation for momentary

overvoltage.

surge suppressor—See spike

suppressor (and "surge" above).

swell—See momentary overvoltage.

uninterruptible power supply

(UPS) (see also motor-generator)

Non-mechanical (static) uninterruptible power supplies can provide

protection against all power dis- turbances. As on-line or "true" UPS

converts the utility ac power to dc

and uses the dc to charge a battery

and to power an inverter that delivers power to the critical load. An off-line UPS, more properly

called a Standby Power Supply

(SPS), supplies the utility power

directly to the critical load and

transfers the load to a batterypowered inverter to supply power

during outages.

utility power—Alternating current supplied to the user by the (usually

commercial) electrical utility. May

be subject to spikes, sags, swells,

electrical noise, and outages.

voltage regulator—Voltage regulators control the output voltage,

eliminating voltage sags and swells

in the input voltage that last from 15

milliseconds to one-half second.

They are typically relatively

inexpensive feedback controlled

transformers.
^1
National Institute of Standards and Technology

Informational Poster on Power Quality

Aposter prepared by the NIST Power

Quality Committee to inform users of

sensitive equipment about problems

with and solutions for protecting their equipment

from power disturbances.

This poster contains;

• Answers to seven common questions about power

quality that should help you pinpoint problems and solutions related to power disturbances.

• A chart describing the tvpes of power distur- bances, the equipment affected, and a brief summary of the types of protection equipment that

is effective against the disturbance.

• A glossary of common power terms.

Who is this poster intended for?

Anyone who is using a compute

other equipment that may be ser

to power-line disturbances or to momentary power outages.

Why should I be

interested?

In some locations, the number of power outages experienced by users is increasing each year. For example, data taken over a period of II years at a particular location on the NIST

Gailhersburg campus show an

increase in the inadence of major power interruptions.

NIST Gaithersburg Number of Gross Electrical Outages

This chart shows only those outages

lasting long enough to disturb electro- mechanical equipment. Electronic equipment can be disturbed by much

shorter (and more frequent) outages. Furthermore, the power quality Eroblem has become more visible ecause of the recent increase in the use of computers for experiment

control ana data and word processing,

as well as the proliferation of switched- mode power supplies, which are more

sensitive lo power disturbances. The

use of the wrong kind of protection equipment, or no protection equipment at all, may cause: • lost or scrambled data, • damaged equipment, or • ruined experiments. Many electronic systems wili safely

restart following an outage, but may

be damaged or severely disturbed if a second outage occurs during the

restart period.

Why can't the power company

supply clean power?

Power quality is a problem th.it involves the supplvinj; eluctnc utility, end users operatinj; M'nsilivu equipment, and their physii.jl and

electrical neighbors. Many distur- bances are created between the utility and the user or even pass from one

usee to another and cannot be controlled by the utility. Such sources of power disturbances may include light- ning striking near a power line, tree limbs falling on power lines, motors

starting or stopping, and operating

industrial processes such as welding. Even seemingly innocuous actions such as turning on or off a small appliance can upset sensitive operations. A personal computer user at NIST discovered this when a coffee pot was plugged into the same receptacle as the computer. The computer cor- rupted the data every time the coffee pot heater cycled on or off. For any organization attempting lo provide a centralized, site-wide

solution, the results would be extremely costly and would not be

effective. Not all equipment needs the same level of protection, and

providing "better" protection than

required is unnecessarily costly. Providing protection at a central location does not prolf " ' "

their neighbors. For cc protection, the power

equipment that is appi

sensitive equipment o

should be installed near tnai sensitive equipment and suppit/ power only to llial equipment.

iiplet

used.

By the waif. i.

I elfcl pur^Oi iabon

imporlaii' < distribiitivi

What is NIST doing about power quality?

To address these concerns, the NIST

Executive Board has appointed a committee lo investigate the quality and reliability of the ac power being

delivered lo ihe NIST Gaithersburg

(Ud^elheet

personnel

plished thrc

lems and ac

quality of If

ind to equipment and

.ks will be accomimbi nation of

their piHver probThe i;i'.il li In pnn'ide infor he Liters l.ir mjlvini; miorme.

s oi disturbances and

uld be of ameliorating

information to d decipi.wler is vt inform- msand



Whatcanldo I hei

elii Bui

here area number o( ways you can reduce or eliminate the effects of power disturbances. But first you should be sure you have a power

problem. Start with the five steps below.

1. Document the problem. Exactly how did the equipment

malfunction? Did the computer reset itself? Was there losi or scrambled data? Was there physical damage to the equip- ment? Was a fuse blown? Did anything else happen at the same time? Did the lights blink? Was there a thunderstorm

going on? Keepinga log of the symptoms will be an

invaluable aid if you need to call someone in to help with your problem. By continuing the log even after protection equipment is added you can teU if the solution is effective.

2. Check the wiring. Although buildin : I miv

have been installed correctly, worn i.m

d

connections, and improper use ot nitiple outlet strips, or other "temporary' l U'times cause problems. Make sure that all etjuiprnL-nt properly

installed and grounded and thai plups fit .snugly in their receptacles, if possible, measure the voltage at the receptacle. It should be within 108 to 132 volts for a nominal 120

volt circuit.

3. Relocate the equipment to

a different electrical circuit or

receptacle. Two pieces of equipment operating on the same

circuit may inlerfert- with each other Try to isolate sensitive ei]uipmenl tci ils own receptacle. Avoid plugging photocopy nvichints it machine tools into the same

reicptflcle as

a compulor

4 Nfi. KU' ihe char! below that describes the types of disiutl->ances and the protection equipment available and then

consider purchasing protection equipment.

5. Periodically test and maintain your power-protection equipment.

How much is power

protection going to cost me?

The exact cost of power protection will, of course, depend on Ihe sensitivity and power consumption of the equipment being protected and the

level of protection required, The

problem can be reduced to an

economic tradeoff of how much is the

iiHhli IS Invi ii ifiL- equipment fails oris

iliMuilt il' Mini, levels of protection

111 nil lins i,u-,| li.isiL protection to

a

LiimprLliensivu solution are discussed.

NOTE: The costs shmon bclmi> apply for small loads of less than 2 kilowlf-amperes

(kVA). Loads of this sort include two or Ihrw desktop computers and a laser printer.

Spike Suppressor I Spikes, also known as ^ "mpuises. transients, c ' high-frequency surges,

short duration '(microsecond to i f voltage increases, Spike

suppressors arc inexpensive [ devices often packaged as a I power strip or extension

that provide protection I against these spikes Spikes

$300-800

Voltage Regulator

Voltage regulators

control the output

voltage, eliminating

voltage sags and

overvoltages in "

input voltage. Regulators do

not protect against spikes,

noise, or outages '

of more thane

half second duration. Some voltage regulators are built using

electrostatically shielded transformers

to eliminate common mode noise. Line conditioners or power conditioners are voltage regulator systems

that often contain multiple devices in one package and usually cost less than

the combination of the individual

devices- Although according to many

experts, line conditioners should

isolate electrical noise and regulate

voltage,a numl>er of products do not accomplish all these functions. For

applications of low power ratings where switched-mode power supplies

are used in the load equipment the

voltage regulation function is not

essential. However, voltage regulation should explicitly be a feature of larger

line conditioners.

$iooo-$4oao UPS

Uninterruptible power

supplies (UPS) typically used for small loads are non-mechanical static devices as opposed to rotating motor generators- The static UPS can be one

of two basic types. A "true" or on-line UPS

rectifies the utility ac power

(o dc and uses the dc to charge a battery and power

an inverter Under normal

conditions the inverter converts the dc

back to ac to supply the sensitive equip- ment load. When an outage occurs, the battery supplies the inverter with no

interruption of the output power. The second type of UPS is an off-line UPS and is more properly called a Standby Power Supply (SPS). Under

normal operating conditions this type

of UPS supplies the utility power

directly to the sensitive load with perhaps some filtering or noise

isolation and also charges

a battery. When an outage is sensed, the load is quickly transferred (in about 4

milliseconds) to the output of an

inverter powered by the battery. When

utility power is restored the load is transferred back to the utility power

This type of system is somewnat less expensive than a true UPS but may

introducea brief power interruption during Ihe transfer. Botn systems typically allow 5 to 15 minutes for the user to shut down the

critical system in an orderly manner oi ride through the outage, depending upon its duration and the size of the

battery and the load. The true UPS

protects against voltage spikes, noise

sags, surges, and outages, while the SPS generally protects against only

NOTE: Uninterruptible power

supplies contain

batteries titat mvst

be periodically

tested. Stidang

another piece of equipment and

forgetting about it isa guarantee

that it will not work when needed. Test the UPS every month by interrupting the incoming power and making sure the

sensitive equipment continues to function

properly. Follow the manufacturers recommendation on replacing the batteriesi but if no recommendation is given, replace them every 2 years.

Don't forget . . .

In addition to purchase cost, then may be installation, maintenance,

efficiency costs (UPS systems deli' less power than they consume).

Also the reliability of the protection equipment should be considered; in some cases, redundant

systems may be required.

And Isgonerallv 1. spikes. Bui, mtisr , tnputerd,-itai.ir.iii> , , . progr-ims, Splku suppressors aiv a bj.su lir>t line di-fi-nse and should be seriously considered lobe minimum pintivlion

for desktop mmputiTs ,>nil pt-nphirjls IwiiUBe of theit low >i.-,i liiv;li .u.ul-

•ibility, and easy insi.ill.iiion A SPikr suppretsor w\tl\ KMI fdirnm;. IwcAawii d» a po\ixr sinp. i* curredllv muilflWe in the NIST storrnxims. ^1

Types of Disturbances and Protection Equipment Effectiveness

SplkM, linpulHi.8urs**

Type of Disturbance

n>nJitii.<n> o( Abnonrully ttlgt* ot lent

voltajiy Idsting for more ihtn

n tew

wcurid» They are cju.smi by cinrult overlo4d». poor vo\tage rvgulAtion. and inii-nijonal reductions by the

ulilify ibnnvn out).

\'i)ltflge MR if

A momcnliiry (Im* Itinn

i) few srconds) dmeist in rlic vuttdgr uulsidc Itii' normal tolerance. (SwvlU

arv <\irivspi)nding voliji^ lnrrwa,-un-) Wtlt.iso AK often caused when

twMvv lo.li!* .itv stditcd. hv li^hlnin^ jnd tiv (Hiv-i-r -y^icm faulb- 'VoUige

siiiVUv ,irv iitlrn laused by ludden

liwd dcawttei or lum-otf ol ItiNivy i-qulpment.

Spikes (impulses, switching iurgn.

Undei\\)]taff: anil owtwltage drtect a\\ equipment, alihoui; 

most vqtilpmenl

h

designeii to tolerate 130 V s 10%.

iSt*

Sajp affect power-down

vnsing arcuilry on

tomputei and large ninlroUcnandain

cause itfuipmeni lo shutdown, "Sivell^' can damage equiprnrnt

linctuding spike tupptvMors) thM liave insufftdenl toI«nincv,

Spikes can destroy

electronic loacb and

breakdown Iransfomter or motor Instilaliun.

conditioner, or uniniemipHble

Spike MipprvMOr

(alM> called surge suppfTssofX or som

line ronditionen

An outage is

n complete Iom of power

thdi may Lul Inim several milliMV ond% Co M-vrral liours and may Iw

caused by powvr syxlom fault*. accidenl\ intxilving power lines, Iransformer failurc*. and );ener«loT

Ifliliireri Some trnsiiive equipment

m.iv be disrupted by outageA

u sihori

a.- 15 millinecondi.

Electrical

nn of the

UninlerruptibUpoistrr supply (UPS)

or ifandby powrr

supply (SK)

Filter isolation

oninleinipHble power supply (UPS).

Approximate purchase cost

for large capacity protection systems versus their power

capacity.

spike xuppreNNur*

"T—I—rill

—r*

Kilovalt-Ampcre* (kVAt

'11)0 purchau cont of aplke tupprvMor) Is independenl ol the lize (il the load being

protected. I ligh power spike »uppresaor« de»igncd for installation in bulld/ng entrance nervlce panels are lypically available (or SIOdD.

Where do I go for more

technical information?

Federal Information Processing Standanls PublicatiDn (FIPS PUB) 94. "Guideline on Electrical R)wer for ADP Installations" provides

information for ADP installahons on

electricity. FIPS PUB 94 is available Jrom the National Technical Information Service (NTIS), U.S. Department ot Commerce, Springfield, VA 22161, (703)487-4650, for $19.95 in unit quantities plus shipping.

In addition,

a bibliography of technical papers on power quality and power

conditioning has been compiled and is available in limited quantities from Ihe NIST Hjwer Quality Committee, Room BIW BIdg. 220, Gaithersburg, MD

2t(.S99. For technical advice concerning the power distribuliorVconditioning system In your building, contact:

iMued luly, 1989