Significance:

Part 5 – Monitoring instruments, laboratory measurements, and test methods

First part of a two-part trade magazine version of the IEEE paper “Power Quality Site Surveys: Facts,

Fiction, and Fallacies.” (See file “PQ surveys FFF” for the original IEEE paper and file “Site surveys” for

the second part of the trade magazine publication.)

Provides background on the motivation for site surveys and a brief description of the types of monitors

that have been used, concluding with remarks on how future site surveys should be conducted.

Introduces to the trade literature the term of “swell” – as opposed to “surge” to descrive a momentary

overvoltage at the power frequency.
Monitoring 

power qualm* 

Fran~ois D. Martzloff . 

National Institute of Standards and 

Technol y Gaithersburg, 8aryland Thomas M. Gruzs 

Liebert Corporation Columbus, Ohio 

Power disturbances that affect 

sensitive loads have a variety of 

sources. Results of site surveys can 

assist in determining which line 

conditioning methods are most 

appropriate. 

ite surveys are generally initiated to evaluate the 

quality of the power available at a specific location with the aim of avoiding equipment disturbances in a planned installation or of explaining 

(and correcting) disturbances in an existing 

installation. In either case, survey results constitute one 

of the inputs in the decision-making process of providing 

supplementary line conditioning equipment, either before 

or after power disturbances have become a problem. 

Depending on the reliability requirements of the load 

equipment, its susceptibility, and the severity of the disturbances, various line conditioning methods have been 

proposed: surge suppressor (with or without filter), isolation transformer, voltage regulator, magnetic synthesizer, 

motor-generator set, or uninterruptible power supply 

(UPS). ,---, 

Because this additional line conditioning equipment 

may require significant capital investment, the choice of 

corrective measures is generally made by economic tradeoff which is the prerogative and responsibility of the end 

user. However, if technical inputs to this trade-off are 

incorrect because erroneous conclusions were drawn as a 

result of a faulty site survey, the whole process is worthless 

- or worse yet, misleading. 

For this reason, a good understanding of the merits and 

limitations of site surveys is essential for reconciling 

expectation with reality before expensive line conditioning equipment is called for; one should deal, not with 

fiction or fallacies, but with facts. 

Power disturbances that affect sensitive electronic loads 

have a variety of sources. Lightning, utility switching, 

and utility outages are often-cited sources of power disturbances. However, power disturbances are frequently 

caused by users themselves, through switching of loads, 

ground faults, or normal operation of equipment. As one 

example, computer systems are not only sensitive loads, 

but can also generate disturbances themselves. Their nonlinear load characteristics can cause interactions with the 

power system such as unusual voltage drops, overloaded 

neutral conductors, or distortion of the line voltage. 

Utility systems are designed to provide reliable bulk 

power. However, it is not feasible for them to provide 

continuous power of the quality required for a completely 

undisturbed computer operation. Because normal use of 

electricity generates disturbances, and because unexpected 

power system failures will occur, every site will experignce 

some power disturbances, but their nature, severity, and 

incidence rates will vary from site to site. 

Confusion In Term Definitions 

As will become painfully apparent in next month's 

review of site surveys, the terms used by the workers 

reporting their measurements do not have common definitions. An effort is being made within the IEEE to resolve 

this problem, as described later in this paper, but consensus has yet to be reached. In this paper, terms describing disturbances are consistent with the IEEE Standard 

Dictionary of Electrical and Electronics Terms 111 and with 

established usage within the community of surge protective devices engineers. 

The generally accepted meaning of surge voltage, in the 

context of power systems, is a short-duration overvoltage, 

typically less than 1 ms or less than one half-cycle of the 

t' wer frequency. This meaning is not that which has 

een established by manufacturers and users of monitoring 

instruments and line conditioners. This unfortunate second meaning is a momentary overvoltage at the fundamental frequency with a duration of typically a few cycles. 

22 FEBRUARY 1990 + POWERTECHNICS MAGAZINE 
Power ally accepted as meaning a momentary voltage reduction 

at the ac power frequency. However, details (threshold, 

duration, etc.) of what characterizes a sag are not well 

WAVEFORM DISTORTION 

Figure 1. 

Graphcc dep~ctions of disturbances with multiple meanings 

- 

defined. 

Motivation For Site Surveys 

With the increasing dependency on computerbased systems for industry, commerce, and consumers, disruptions from power disturbances are 

less and less acceptable. Operational problems 

such as hardware damage, system crashes, and processing errors are the most visible indications of 

power disturbances. Some computer system users 

may accept (albeit reluctantly) such problems 

because they see them as unavoidable. Others 

may be unaware that otherwise invisible power disturbances could be the cause of these problems. A 

single power disturbance can cost more in downtime and hardware damage than the investment in 

power protection that would have prevented the 

effect of the disturbance. Nearly all sites can benefit from a reduction of operational problems by 

improving the quality of power supplied to the 

computer systems [2]. 

Power line monitoring with sophisticated power 

disturbance recorders has often been advocated as J a way of determining if any line conditioning is 

required. While monitoring appears to be a logical 

first step, it has limitations. For example, severe 

disturbances occur infrequently or on a seasonal 

basis. Therefore, monitoring periods of less than a 

In this article, this second meaning of the word "surge" (a year might not produce an accurate profile of power disturmomentary overvoltage) will be signaled by the use of bance, and most users are unwilling to wait at least a year 

quotation marks. What power engineers call surge is to characterize the problem. Also, power line monitoring 

referred to as "impulse" or "spike" by the monitoring only produces data on past performance. Changes within 

instrument community. Figure .. 1 shows . by graphic the site (or at neighboring sites) or by the utility can 

descriprions the contusion created by the 

dual meaning of the word surge. ~cknowledging the desire of users for terse labels, we 

propose for consideration the word "swell" 

instead of "surge" for a momentary overvoltage. 

The term "outage" is another example of 

confusion created by unsettled definitions. 

Most users agree that it means a complete 

loss of line voltage, but the duration of an 

outage can be quite different when defined 

by computer users (as short as one half-cycle) 

or power engineers (seconds, perhaps minutes). Part of the problem may be that the 

definition of "outage" has regulatory implications for evaluating the performance of public utility companies. Users and 

manufacturers of line conditioners do not 

make a clear distinction between complete 

loss of line voltage (zero voltage condition), 

severe undervoltages ("deep sags"), or the 

single-phasing of polyphase power systems. 

For example, a momentary flicker of fluorescent lighting caused by a brief loss of voltage 

m~ght be considered an outage; however, a 

brief sag to less than 80 percent of nominal 

voltage will produce the same visible effect. 

Some UPS manufacturers consider input 

voltage sags that cause transfer to the battery 

backup operation as outages. 

The term "sag" has not yet been defined 

in the lEEE Dictionary, but it is now generPeripheral Remote element 

Separate signal cables 

power introduce an 

~UPP~Y additional ground 2 1- loop Electromagnetic field couples into loop = 

and induces common mode disturbance Composite line 

on input port 

Impinging common mode 

disturbance is blocked by 

an isolation transformer G 

neglecting interwinding 

capacitance 

Ground current from other systems 

couple common mode disturbance 

into grounding conductor of power 

supply 

Figure 2. 

Origins of common mode disturbances, including ground cuwent om 

other systems (I), electromagnetic field coupling into loop to i f we 

input port disturbance (2), and introduction of an additional ground 

loop from remote peripherals (3) 

POWERTECHNICS MAGAZINE + FEBRUARY 1990 23 

- - -- -- - - 
Power Monitoring three-phase grounded-wye power supplies typical of large 

computer systems, common mode disturbances can also be 

defined as the potential difference between neutral and 

Nonlinear or switching loads 

interact with power system 

source impedance to cause 

disturbance on bus bars (B) 

. , \supplying load (A) (Bltiag on local system oher loads 

1 lmplnglng disturbance t- A - ?----? 

: t Normal mode (lightning, remote I 

swltch~ng) :----J ~~~d (A) disturbance I / I 

Normal mode 

disturbances are coupled I 

through an isolation I 

transformer by I 

magnetlc coupltng '- ! ,-I 

~o;clearl~ - - defined 

Figure 3. 

Ongins of normal mode disturbances include impinging 

disturbances such as lightning (I), switching of other loads on 

local systems (2), and interaction between nonlinearlswitching 

loads with pwer system source impedance (3) 

drastically alter the power disturbance profile in ways that 

line monitoring cannot predict. 

While exact prediction of the disturbances to be 

expected at a specific location is almost impossible - and 

attempting it would be a fallacy - general guidelines can 

be formulated. An attempt has been made by standardswriting groups to provide guidance [3] or specifications 

reflecting expected disturbances [4-61. However, users will 

still seek specific data for their particular case, and site 

surveys will still be necessary. Another fallacy would be 

to attempt correction of power line disturbances revealed 

by monitoring and then to expect operational problems of 

equipment to disappear without having first determined 

the exact susceptibility of the equipment. Operational 

roblems are the results of more than just power disturEances. 

Types Of Disturbances 

Power line disturbances can be classified into two categories: common mode and normal mode disturbances. 

The two terms were first defined in the context of communication circuits; a recent IEEE Guide [7] has established an expanded definition which is used in this article, 

as outlined in the following paragraphs. The IEEE Dictionary [I] and the IEC Dictionary [8] define symmetrical 

and asymmetrical voltages akin to, but not interchangeable with, the definitions of normal mode and common 

mode, respectively. 

Common mode disturbances are defined as unwanted 

potential differences between any or all current-carrying 

conductors and the grounding conductor or earth. In 

ground. 

Two different types of common mode disturbances can affect a sensitive load. The first type is a 

disturbance on the input power conductors relative 

to the input power grounding conductor. Points 1 

and 2 of Figure 2 show examples of the origins for 

these disturbances. This type of disturbance can be 

limited somewhat by a line conditioner, but it is 

also influenced by the location of the line conditioner and the wiring practices. 

The second type is a ground potential difference 

between elements of the computer or remote 

peripherals connected to the computer. Point 3 of 

Figure 2 is an example of this type of disturbance, 

which is more difficult to limit because it is influenced by factors such as system configuration and 

the impedance of the grounding system. These two 

factors are generally beyond the direct control of the 

user except in the construction of a new facility. 

Because of the broad frequency band involved, 

wiring resonances can make equalizing ground 

potentials difficult. Proper computer system grounding, including a signal reference grid, has been 

found to be effective against most common mode 

disturbances [9]. However, when remote elements 

are connected to the computer systems by data 

cables, large ground potential differences are possible. Proper surge protection of the power supply 

and proper grounding of data cables will help eliminate hardware damage but might not prevent data 

corruption. When dealing with the situation of 

example 3 in Figure 2, fiber optic links are very 

effective because they provide complete metallic 

se~aration of the various elements in the svstem - a 

separation that might not be sufficiently achieved by the 

discrete opto-isolation devices sometimes proposed for that 

function [lo]. 

Normal mode disturbances are defined as unwanted 

potential differences between any two current-carrying circuit conductors. Figure 3 shows three examples of the 

origins of such disturbances. Usually a sine wave of nominal voltage is desired for a computer power supply; any 

deviation from this sine wave is a normal mode disturbance. Computer users and monitoring instruments 

designers characterize these disturbances by a variety of 

terms not always clearly defined such as sags, surges 

("swells"), outages, impulses, ringing transients, waveform 

distortion, and high-frequency noise. Unfortunately, 

there is no consensus at the present time on the exact 

meaning of these terms and their underlying quantitative 

definitions such as amplitude, duration, and thresholds. 

Types Of Monitors 

The instruments used in the various surveys reflect 

technological progress as well as logistics constraints 

resulting in a diversity of approaches. Nevertheless, all 

monitoring instruments used in past surveys were voltmeters (with one exception, combining voltage and current 

measurements) from which disturbance parameters were 

derived. Some of the monitors recorded a single parameter such as the actual voltage peak or the fact that the 

voltage exceeded a preset threshold. Other monitors combined time with voltage measurements describing voltage 

waveforms. The recording functions of instruments used 

in the surveys may be classified in broad categories: 

FEBRUARY 1990 + POWERTECHNICS MAGAZINE 
Threshold counters - The surge is 

applied to a calibrated voltage 

divider, triggering a counter each 

time a preset threshold is exceeded. 

The early types were analog; more ments are useful (and inexpensive) 

recent types are digital. indicators of frequent disturbances; 

Digital peak recorders - The surge is other, more sophisticated (and more 

converted to a digital value which is expensive) instruments can provide 

Figure 4. 

Effect upon voltage peak and waveforms 

with varistor placed upstream of monitor 

recorded in a buffer memory for later 

playback or printed out immediately 

after it occurs. In the early types of 

. recorders, only the peak was 

recorded; in later types, the duration 

of the surge was also recorded, opening the way to the more complex digital waveform recorders now 

available. 

Oscilloscope with camera - The surge 

triggers a single sweep on the oscilloscope's CRT, which is recorded as it 

occurs by a shutterless camera with 

automatic film advance. The oscilloscopes available at that time (the 

early 1960s) did not allow differential 

measurements. 

Screen storage oscilloscope - The 

surge is displayed and stored on the 

cathode ray tube. The writing-speed 

capability of these oscilloscopes was a 

limitation in the late 1960s. 

Digital storage oscilloscope - The 

surge is digitized and stored in a shift 

register for subsequent playback and 

display whenever a preset threshold is 

exceeded. An important feature is 

the capability of displaying events 

prior to the beginning of the surge. 

Digital waveform recorder - The 

surge is digitized and stored in a manner similar to the digital storage 

oscilloscope, but additional data processing functions are incorporated in 

the instrument, allowing reports of 

many different parameters of the disturbance relating voltage to time. 

Although some surveys might aim 

at great accuracy, the real world 

experiences such a variety of disturbances that any attempt to describe 

them in fine detail only restricts general usefulness of the data. Seeking 

such fine and definitive detail is 

another fallacy. Some simple instruquite comprehensive data 

on disturbances (but only 

on past events from which 

future disturbances can be 

extrapolated only by assuming that the causes will 

remain unchanged). Thus 

there is a practical limit to 

the amount of detail that a 

survey can yield, and unrealistic expectations of very 

precise information should 

be avoided. 

Previous And Future 

Surge Recordings 

Before attempting yet another 

broad survey of power quality, wouldbe surveyors need to consider not 

only improvements in instrumentation but also changes that have 

occurred in modem power systems - 

in particular the proliferation of surge 

protective devices. These two differences between earlier surveys and 

the more recent surveys should be 

kept in mind when comparing results 

and when planning future surveys. 

Prior to the proliferation of surge 

protective devices in low-voltage systems (those defined by IEEE and IEC 

as 1,000V or less), a limitation had 

already been recognized [4] for peak 

voltages: the flashover of clearances, 

tvvicallv between 2 and 8 kV for lowvgltage ' wiring devices. For 

that reason the expected 

maximum value cited in the 

IEEE Guide on Surge Voltages 

[3] reflects this possible truncation of the distribution 

around 6 kV. Unfortunately, 

some readers of this Guide 

interpreted the upper practical limit of 6 kV as the basis 

for a withstand requirement, 

and they have included a 

6-kV test requirement in 

their performance specifications. A new version of this 

Guide, currently under reparation as a Recommende c! Pracrice, will attempt to avoid this 

misinterpretation. 

The number of surge-protective devices such as varistors used in the United States 

since their introduction in 

1972 on low-voltage ac power 

circuits may he estimated at 

500 million. Therefore, a new limitation exists in the voltage surges 

that will be recorded. A surge-recording instrument installed at a random location might be close to a 

varistor connected near the point 

being monitored. Such a proximity 

of surge protective devices and recording instruments may impact present and future measurement 

campaigns. Four are outlined below. 

1) Locations where voltage surges 

were previously identified - 

assuming no change in the source 

of the surges - are now likely to 

experience lower voltage surges, 

while current surges will occur in 

the newly installed protective 

devices. 

2) Not only will the peaks of the 

observed voltages be changed, but 

also their waveforms will be 

affected by the presence of nearby 

varistors as illustrated in Figures 4, 

5, and 6. 

A) If a varistor is located between 

the source of the surge and the 

recording instrument (Figure 

4), the instrument will record 

the clamping voltage of the 

varistor. This voltage will 

have lower peaks but longer 

time to half-peak than the 

original surge. 

B) If the instrument is located 

between the source of the surge 

and a varistor, or if a parallel 

branch circuit contains a varistor (Figure 5), the instrument 

Figure 5. 

Effect upon volta e peak and waveforms 

with varistor p k ed downstream of 

recorder 

POWERTECHNICS MAGAZINE + FEBRUARY 1990 25 

-- - - - 
Power Monitoring 

will now record the clamping voltage of the varistor, 

preceded by a spike corresponding to the inductive 

drop in the line feeding the surge current to the 

varistor. 

C) If a varistor is connected between the line and neutral conductors, and the surge is impinging between 

line and neutral at the service entrance (normal 

mode), a new situation is created, as shown in Figure 6. The line-to-neutral voltage is clamped as 

intended, but the inductive drop in the neutral conductor returning the surge current to the service 

entrance produces a surge voltage between the neutral and the grounding conductors at the point of 

connection of the varistor and any downstream 

point supplied by the same neutral. Because this 

surge has a short duration, it will be enhanced by 

the open-end transmission line effect between the 

neutral and grounding conductors [ll]. 

3) The surge voltage limitation function previously performed by flashover of clearances is now more likely to 

be assumed by the new surge protective devices that are 

constantly being added to the systems. 

4) These three situations will produce a significant reduction in the mean of voltage-surge recordings from the 

total population of different locations as more and more 

varistors are installed. However, the upper limit will 

remain the same for locations where no varistors have 

been installed. Focusing on the mean of voltage surges 

recorded in power systems can create a false sense of 

security and an incorrect description of the environment. Furthermore, the need for adequate surge current handling capability of a proposed suppresser with 

lower clamping voltage might be underestimated 

because some diversion is alrehdy being performed. 

Part 11 of this article, a detailed review of several site surveys, will appear in the March issue of Powertechnics. This 

article was based on "Power Quality Site Surveys: Facts, 

Fiction, and Fallacies," by F.D. Martzloff and T.M. Gruzs, 

which first ameared in lEEE Transactions on Industrv 

Applications, \id. IA-24, No. 6, pp. 1005-1018, ~ov./~ei 

1988. 1988 IEEE. 

References 

[I] lEEE Standard Dictionary of Electrical and Electronics 

Terms, ANSIIIEEE Std. 100, 1988. 

[2] T.M. Gruzs, "Power Disturbances and Computer Systems: A Comparison of the Allen-Segall and the 

Goldstein-Speranza Power Line Monitoring Studies," in Proc. 1986 Electrical Overstress Exposition, 

Nelson Publishing Co., Nokomis, FL, Jan. 1986. 

[3] Guide on Surge Voltages in Low-voltage AC Power Circuits, ANSIIIEEE C62.41, 1980. 

[4] lnsulation Coordination within Low-Voltage Systems, 

lncluding Clearances and Creepage Distances for Equipment. EIC 664. 1980. 

N 

N-G 

Figure 6. 

Protective device installed between line and neutral conductors, for 

surge applied in normal mode at service entrance and varistor 

connected at end of 75-m branch circuit (a); surge applied at 

"send" end, 3 kV initial peak, 100 kHz oscillation (b);(c) shows 

surge clamped at L-N at end of line, maximum voltage 400 V 

(same time scale as b, but different amplitude scak); 2500 spike at 

N-G at end of line (d), created by surge current in neutral 

conductor (same scales as b) (d) will experience short duration 

surge 

[5] ~lectrima~netic Emission and Susceptibility 

Requirements for the Control of Electromagnetic Interference, DOD-STD-461B. 

[6] interface Standard for Shipboard Systems, 

DOD-STD-1399, Sec. 300, 1978. 

[7] lEEE Guide on Surge Testing for Equipment 

Connected to Low-Voltage AC Power Circuits, ANSIIIEEE C62.45, 1987. 

[8] IEC Multilingual Dictionary of Electricity, published by IEEE and distributed in cooperation with Wiley-Imerscience. 

[9] Guideline on Electrical Power for ADP Installations, FlPS Pub 94, National Bureau of 

Standards, 1983. 

[lo] F.D. Martzloff, "The Protection of Computer and Electronic Systems Against 

Power Supply and Data Line Disturbances," General Electric Co., Schenectady, NY, Report 85CRD084, 1985. 

[ll] F.D. Martzloff and H.A. Gauper, "Surge 

and High-Frequency Propagation in 

Industrial Power Lines," IEEE Trans. Ind. 

Appl., vol. 1A-22, no. 4., JulyIAug. 

1986. 

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