Monitoring of Computer Installations for Power Line Disturbances

George W. Allen and Donald Segall

International Business Machines Corporation

Reprint, with special permission, of IEEE Conference Paper C 74-199-6

IEEE PES Winter Meeting, New York, 1974

With discussion submitted in 1974 by

Harold A. Gauper and François D. Martzloff

General Electric Company

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Significance:

Part 3: Recorded surge occurrences and surveys 

Part 5: Monitoring instruments

This conference paper (which was not later published in the IEEE Transactions, now difficult to retrieve and therefore 

reproduced here in extenso with discussion submitted to IEEE but also unpublished) presents the result of monitoring 

what became known as “Power Quality” at selected IBM computer installations. The authors classified disturbances 

in three categories:

1. “Voltage Spikes” – now labeled surges

2. “Oscillatory decaying disturbances” – now labeled switching surges

3. “Undervoltages and occasional overvoltages” now labeled sags or dips and swells

In the context of an anthology on surge protection and the recording of surges, the paper and discussion are included 

for three reasons :

Instrumentation limitation

The authors report only low “spike” levels in the period of monitoring (starting in 1969 and concluded in 1972), 

compared with other contemporary surveys (all before the introduction and proliferation of low-voltage metal-oxide 

varistors). Because the initial measurements were based on an expectation of moderate surges, the instrumentation 

range did not include higher values and therefore would miss them or incorrectly record them if and when they would 

occur, leading to a self-fulfilling prophesy. The discussion – submitted to IEEE immediately after the presentation of 

the paper but not published because of the lingering Conference Paper status – offered a possible explanation for 

that apparent discrepancy as being a limitation of the amplitude and sweep speed ranges selected for the storage 

oscilloscopes. 

Importance of threshold selection

Some statistical reports on powerline disturbances have attempted to rank disturbances by order of frequency of 

occurrence, sometimes presenting pie charts on respective percentages of surges, sags, swells, etc. within the total 

of recorded disturbances. With hindsight, this approach neglected the fact that the number of occurrences included 

in these statistics is strongly influenced by the threshold at which the counting begins. Capsule quotations appeared 

in published articles creating the impression that one type of disturbance is more frequent than another – the winner 

in the number game being the disturbance for which the threshold was set the lowest. Depending on the field of 

interest of the researchers conducting the monitoring, the thresholds were generally set logically and understandably 

at a level where the disturbances begin being a problem for the specific category of interest, hence a disparity of 

threshold selection among different surveys.

Misquotation of the paper

The conclusions correctly emphasize the significance of sags, swells, and switching surges, but with little attention 

given to “spikes” (surges). Nevertheless, this paper has been cited as evidence that larges surges do not happen, or 

hardly ever occur, hence the need to include the discussion in the anthology and maintain proper perspective.
MONITORING OF COMPUTER INSTALLATIONS FOR POWER LINE DISTURBANCES 

George W. Allen Donald Segall 

International Business Machines Corporation 

System Development Division 

Kingston, New York 12401 

ABSTRACT 

A study was made of three-phase, 208 and 240 volt power 

sources supplying typical data processing equipment. Monitors 

capable of detecting a wide range of voltage conditions, including 

those most likely to affect the operation of the equipment, were 

installed at representative data processing installations. Duration and 

amplitude plots of three types of disturbances are presented. Average 

incident rates are computed, and incident-rate versus number-of-sites 

plots are included. 

INTRODUCTION 

Data processing equipment is being used more and more for 

real-time data operations such as process controls, reservation 

systems, and time sharing applications; it is also employed for such 

critical business uses as inventory control and check processing. Such 

increased dependence on data processing has led to increased interest 

in power-line disturbances and their possible effects on data 

processing equipment. 

Papers that discuss the sources and types of disturbances existing 

on power lines are available (see References 1 to 5). They cover the 

following general types of short-term voltage variations that power 

lines experience. 

Voltage spikes due to lightning, power-network switching, 

and the operation of customer equipment. These spikes are 

typically of short duration (1 0 to 100 microseconds); bursts 

of such pulses may also occur with each burst lasting as long 

as a millisecond or more. Voltage changes of 150% or more 

of peak nominal ac voltage may be experienced. Spikes of 

nanosecond duration may also occur. 

Oscillatory, decaying disturbances due to power-factorcorrection-capacitor-switching and other network or load 

switching. These disturbances have a frequency range of 

from 400 Hz to 5 kHz or more, with beginning amplitudes 

of up to 100% or more of nominal power peak voltage. 

Normally they will decay to zero within one cycle of the ac 

power voltage. 

Undervoltages (occasionally overvoltages) of greater 

duration than 112 cycle of the power voltage caused by 

faults and the resulting actions of the fault-clearing devices; 

CONERNE PAPER 

C 74 199-6. A paper recarunended by the IEEE Sme 

Protective Devices Cannittee of the IFEE Power wineering Society for mesentation at the IEEE PES Winter Meeting, New York, N.Y., Jan- 27-February 1, 

1974. Manuscript suhitted August 31, 1973 ; made 

available for printing December 7, 1973. 

Price: Members $1.50 All Rights 

Normembers $2.00 Reserved 

At Meeting: $1.00 by= 

or abnormalities caused by large load changes and the 

resulting actions of the utility-regulating devices and 

procedures. 

Even though information on causes of power-line disturbances is 

available, monitoring results at user locations, showing range of 

voltage deviation and disturbance duration, are not available (to the 

best of our knowledge) in the literature. However, one report on the 

results of monitoring for voltage spikes is avai1able.1~1 

All three types of disturbances can cause the malfunction of 

data processing equipment. Hence. information concerning the 

voltage profile, duration profile, and frequency of occurrence are of 

direct interest in determining power system design criteria. 

MONITORS USED 

To cover the wide range of time durations involved in power-line 

disturbances, it was necessary to devise special monitoring 

eq~xipment.[7] For preliminary monitoring (phase one), two 

automatic recording cathode-ray oscilloscope and 35mm camera 

combination monitors were developed (analog monitors). Two 

oscillograph-type recording monitors, manufactured by the 

Hathaway Corporation, of Denver, Col.. were also used. 

For final monitoring (phase two), a smaller monitor requiring 

less servicing and no analysis of waveforms was developed. 

PHASE ONE MONITORING 

The objectives of phase one monitoring were: 

To verify the three types of disturbances described in the 

introduction. 

To determine appropriate characteristics for an improved 

monitor. 

To obtain preliminary information on the magnitude, 

duration, and rates of occurrence of power-line 

disturbances. 

Phase one of the monitoring was begun in July 1969 and was 

completed after the first week of November 1970. Approximately 38 

monitor months of data were recorded during this 16-month period. 

The four monitors, two analog and two oscillograph (Hathaway), 

were used to monitor a series of locations expected to produce the 

three types of power-line disturbances described in the introduction. 

Phase One Data 

In compiling the data from phase one monitoring, certain 

ground rules were used. These rules are given below for each of the 

three classes of disturbances. 

Overvoltage or Undervoltage Longer Than 112 Cycle of the Power 

Frequency 
Only the largest undervoltage incident within any 2-minute 

period was counted. 

Unless a voltage dip exceeded 10%,, it was not plotted. 

Two types of points were used in the plot. 

"0" points were used when all the line-to-line voltages 

dipped more than 20%. 

"x" points were used when at least one of the 

line-to-line voltages did not dip more than 2070. 

Both "0" dips and "x" dips were plotted at the value of the 

greatest line-to-line dip. 

Figure 1 shows the plot of all undervoltage disturbances from 

phase one monitoring. No short-term overvoltages greater than 10% 

were observed. This figure presents a visual picture of the density 

function (magnitude and duration) of the disturbances recorded. 

Oscillatory Decaying Disturbances (Switching) 

Thc following rules were established. 

(1) It was not considered necessary to note the duration of the 

oscillatory disturbances. (Only one was observed to last 

longer than 16 milliseconds.) 

(2) Only disturbances geater than 10% amplitude wcre plotted. 

(3) When disturbances appeared on more than one phase, only 

the greatest of the three line-to-line disturbances was 

plotted. 

The results are shown in Figure 2. This figure presents a visual 

picture of the density function in magnitude of the disturbances 

recorded. 

'Voltage Spike Disturbances 

For the voltage spike disturbances, only two sites were 

monitored at sweep speeds fast enough to determine spike width. No 

disturbances of less than 10% were plotted. The plots of the spikes 

monitored are shown in Figure 3. 

Incident-Per-Month Plots 

Another useful presentation of the data is a plot of the number 

of sites versus their recorded incident rate per month. These plots are 

shown for undervoltage disturbances in Figure 4 and for oscillatory 

disturbances in Figure 5. It was assumed that the rate observed 

would continue throughout the year. This is not true, but the site did 

experience the rate plotted for the monitored period, and, thus, the 

rates plotted are actual rates experienced. 

These plots showed a higher-than-expected incident rate (Figures 

4 and 5). The rates exceeding the 1 level were the following. 

a. Wndervoltages: Average of data, 5.66 per month: range, 

0-12.3 per month (median, 3.5). 

b. Oscillatory decaying: Average of data. 42.2 per month. 

range, 1.00-1 10 per month. 

c. Voltage spikes: Only two sites were monitored; the rates 

were 2.00 incidents per month and 23.0 incidents per 

month. 

Results of Phase One Monitoring 

Phase one of the monitoring did verify the occurrence of 

the three types of disturbances described in the 

introduction. 

Phase one of the monitoring program did produce a 

substantial amount of data on rate of occurrence. 

magnitude. and duration of power line disturbances. 

Phase one monitoring resulted in the establishment of the 

following operating and physical requirements for phase 

two monitors. 

Operation for extended per~ods of tlme without 

maintenance. 

No film-developing requirement. 

No analysis-of-waveform requirements. (Disturbances 

must be separated into type, magnitude, and duration. 

as required; hence the name "Digital Monitors".) 

A hard copy output from the monitor. 

The monitor must be small and light in weight. 

PHASE TWO MONITORING 

The objectives of phase two monitoring were: 

(1) To obtain a representative sample of typical data processing 

installation input power disturbances. 

(2) To obtain information on the magnitude. duration. and 

rates of occurrence of these disturbances. 

Phase two monitoring began in June 1970 and had accumulated 

approximately 109 monitor months of data from the digital 

monitors when completed in July 1972. 

In order to obtain as representative a sample of monitored 

locations as possible, the following factors were considered in the 

selection of monitored sites. 

A climatic and geographic cross section of the U.S 

A cross section of the major types of utility loads (heavy 

industry, light industry, office and retail stores, residential. 

mixed). 

A cross section of the major types of utility transniission 

and distribution networks (isolated. power pool, overhead 

distribution, underground distribution. single-feeder, 

dual-feeder). 

A cross section of data processing system density 
Phase Two Data C. Incident Rate Plots 

In compiling the data from phase two monitoring, a slightly 

different format was required because the data had already been 

grouped into ranges by the monitors. The settings of the various 

sense levels of the monitors determined the ranges used to present 

the data. 

Wherever possible, the ground rules used were the same as those 

for phase one monitoring. The particulars for each of the three types 

7f disturbances are listed below. 

Overvoltages or Undervoltages Longer Than 112 Cycle of the Power 

Frequency 

All the ground rules were the same as those for phase one, 

except for the addition of the following rules involving overvoltages. 

For overvoltages, the only sense level was +lo%. Thus: 

"0" points are those where all line-to-line overvoltages were 

more than 10% in magnitude. 

"x" points are those where at least one of the line-to-line 

overvoltages did not exceed 10% in magnitude. 

Figure 6 shows a plot of all overvoltage and undervoltage 

disturbances from phase two monitoring. This figure presents a visual 

picture of the density function (magnitude and duration) of the 

disturbances recorded. 

Oscillatory Decaying Disturbances (Switching) 

The ground rules far the oscillatory disturbances were the same, 

except that disturbances below 15% were ignored, and the ranges for 

grouping the disturbances were determined by the monitor settings 

and were different from those for phase one. The summary plot of 

phase two monitoring is shown in Figure 7. The number of 

disturbances recorded is shown inside the block, while the number of 

monitor days needed to accumulate the data is shown outside the 

block, at the top. 

Voltage Spike Disturbances 

The voltage spike disturbances were grouped into ranges by the 

monitor settings. The summary plot of phase two monitoring is 

shown in Figure 7. The number of disturbances recorded is shown 

inside the block, while the number of monitor days needed to 

accumulate the data is shown outside the block, at the top. 

Incident-Per-Month Plots 

The plots for phase two of the monitoring of the number of sites 

versus their recorded incident rate per month for the three types of 

disturbances are shown in Figures 8, 9, and 10. 

Results of Phase Two Monitoring 

A. Following the ground rules, a series of sites was selected and 

moni tored. 

These plots have provided very interesting data. Continued 

high rates of incidence were observed as indicated in phase 

one of the monitoring (Figures 8, 9, and 10). The rates 

were: 

a. Undervoltages: Average of data, 14.4 per month; range, 

0-1 17 per month. Median of data, 4.5 per month. 

Moving the undervoltage threshold to 20% undervoltage 

changes this average to 2.09 per month and range to 

0-8.2 per month. Removing the 10 to 20% 

undervoltages eliminates the high number of 

occurrences monitored at locations which operated for 

extended periods of time near the 10% undervoltage 

threshold. 

b. Oscillatory Decaying: Average of data, 62.6 per month; 

range, 0.395-185.0 per month (at 15% threshold). 

c. Voltage Spikes: Average of data, 50.7 per month; 

range, 1.19-95.1 per month (at 25% threshold). 

d. Monthly Distribution of Power Line Disturbance 

Incident Rates 

Figure 11 illustrates how the incident rates vary 

throughout the year. The plot represents an average of 

the monthly incidents from two consecutive years to 

try to minimize effects of unusual storm activity during 

one of the periods. Voltage dips between 10% and 20% 

are not included here because large numbers of them 

came from very few locations and would have biased 

this plot for the months that these few locations were 

monitored. As one might expect, the summer months, 

with their lightning storms, produced the highest 

incident rates. 

e. Outages vs Duration (Figure 12) 

This plot, "Cumulative Percent of Outages vs Duration 

in Minutes," is based on 65 outages monitored during a 

total of 109 monitor months at 29 locations. It is 

extremely useful for estimating the effectiveness of a 

certain number of minutes of power buffering. The 

average outage rate was 0.6 incidentslmonth. 

D. Monitor Data by Location 

There is a tremendous variation in the disturbances recorded 

at the different locations. In general, city locations, with 

their underground distribution systems and networks, have 

relatively few severe undervoltages or outages but have a 

substantially higher number of oscillatory disturbances. A 

few locations showing an extremely high rate of 10% - 2Wo 

undervoltages were found to be actually operating at a low 

steady-state voltage during certain periods of the day. In 

two cases, the data processing customer took corrective 

action by having transformer taps changed after being 

informed of the monitoring results. 

B. Incident magnitude plots for the three types of disturbances 

are shown in Figures 6 and 7. 
Comparison of Undervoltage Results from Phase One and Phase Two 

Monitoring 

One would expect the incident rates from the phase one 

monitoring to be substantially higher than phase two because the 

sites had been selected in anticipation of the presence of certain 

types of disturbances. Therefore, it was surprising to find the average 

incident rates higher for the phase two monitoring. All of this 

increase in undervoltage incident rates can be attributed to those few 

locations already mentioned that were operating at low voltage and 

exhibited a very high number of 10% - 20% undervoltages. The 

sensor circuits used on the analog monitors were designed so they 

would not be triggered on gradual changes and, therefore, would not 

have indicated many of these incidents even if they were present at 

the phase one locations. 

Because severe undervoltages are very infrequent, their study 

requires as large a data base as possible. For this reasm, we have 

combined the undervoltage data from 109 minitor months of the 

phase two study with 38.4 monitor months of the phase one study 

and an additional 24.3 monitor months of data gathering with the 

analog monitors following completion of the phase one study. This 

171.7 monitor months of data from 49 locations is presented on 

Figure 13 as a density plot of "Undervoltage Incidents vs Cycles 

Duration."' 

Distribution of Disturbances by Shift 

All monitoring was done on a 24hour-per-day, 7-day-per-week 

basis. 

A study of a sample of the data to determine the distribution of 

undervoltage incidents by shift revealed that 42% of the incidents 

occurred in the first shift (9:OO a.m. to 5:00 p.m.), with 28% in the 

second shift (5:OO p.m. to 1:00 a.m.) and 30% in the third shift 

(1:OO a.m. to 9:00 a.m.). About 60% of the disturbances exceeding 

43% magnitude occurred during the first-shift hours, with the 

remainder about equally divided between the second- and third-shift 

hours. 

STATISTICAL CONSIDERATIONS 

The statistical validity of the data is increased when a relatively 

large number of monitoring locations is chosen. Also, an extended 

period of monitoring is necessary to determine the rate for less 

frequent disturbances. However, given the number of monitors 

available and a limited time period for the monitoring project, the 

estimated actual population mean incident rates computed by the 

usual statistical procedures for the 90% confidence level is 

significant. 

For the combined undervoltage data, the computed 90% 

confidence level of the actual total mean, derived from the observed 

means of 43 monitored locations, is 11.4 +5 incidents per month. A 

sample size four times as large would be necessary to halve the spread 

at the 9Wo confidence level. 

For the digital oscillatory disturbances exceeding the 30% 

threshold, the computed 90% confidence level of the actual total 

mean, derived from the observed means of 19 monitored locations, is 

34.6 A14.7 incidents per month. The sample size for the digital 

voltage spike data is 18 locations. The computed 90% confidence 

level of the total mean for voltage spikes exceeding the 50% 

threshold is 17.1 +2.4 incidents per month. For both of the above, a 

sample size four times as large would also halve the spread at the 90% 

confidence level. 

Inherent in the above calculations of the expected total means 

computed from the sample means are three important assumptions 

that are necessary in applying the usual statistical procedures. 

(1) The seasonal variation is similar at all locations. 

(2) There is no variation in the data from year to year. 

(3) The sample is statistically random. 

We know that the third assumption is not valid because an effort 

was made to make the sample representative, rather than random. 

This effort will probably result in the sample being closer to the 

population than a limited random sample would be. Of course, there 

is no positive assurance that the sample selected is really 

representative, in spite of efforts to make it so. 

We also know that the second assumption is not precise, and we 

know of no generally accepted way of estimating the year-to-year 

variation. Also it seems reasonable to suspect that the first 

assumption is not precise. The effect of the actual imprecise nature 

of the assumptions is to introduce a possible bias in the calculated 

results. 

In spite of the possible bias, we are unaware of any collection of 

data better tailored to the needs of data processing and control 

equipment manufacturers. We can only hope that, in the future, 

additional data will be available to help increase the sample size and 

to help answer the questions about year-to-year and seasonal 

variations. 

CONCLUSION 

UPS equipment is required for reliable continuous operation 

of electronic equipment through the wide range of disturbances 

found on power lines. 

The monitoring data presented can be used by industry or 

professional society committees to determine appropriate design 

criteria to make electronic equipment less sensitive to power-line 

disturbances. Both types of data plots used in this paper (magnitude 

vs duration, and incident rate) are useful. The magnitude and 

duration plots can be used for setting appropriate design limits and 

determining the number of incidents that lie outside proposed or 

selected design limits. Plots of incident rate versus sites can show the 

effect of selected design limits across a profile of customer locations. 

In other words, the data can be used to provide information about 

the probable conditions at "worst site" installation, as well as the 

overall average effects of selected or proposed design criteria. 

Until appropriate standards are established, the design susceptability limits of electronic equipment must remain the individual 

responsibility of each manufacturer. 

REFERENCES 

(1) "Switching Surges, Part I," IEEE Committee Report, June I96 1. 

(2) "Switching Surges, Part 11," IEEE Committee Report, October 

1966. 

(3) "Five Year Field Investigation of Lightning Effects on 

Transmission Lines," IEEE Winter Power Meeting, December 

1964. 
(4) T. S. Lauder, "Determining Maximum Overvoltages Produced by 

EHV Circuit Breaker Closure," IEEE Transaction on Power 

Apparatus, April 1968. 

(5) W. E. Master and A. Hauspurg, "What the Industrialist Should 

Know and Do When Arranging to Obtain Utility Power," AIEE 

General Meeting, Conference Paper Number CP61-829, June 

1961. 

(6) Francois P. Martzloff and Gerald J. Hahn, "Surge Voltages in 

Residential and Industrial Power Circuits," IEEE Transactions 

on Power Apparatus and Systems, Vol. PAS-89, No. 6, 

July/August 1970. 

(7) George W. Allen. "lksign of Power-L~nc Monitoring 

Equipment." IEEE Transactions on Power Apparatus and 

Systems. Vol. PAS-90. No. 6. Noven~ber!Deccnibcr 1471 

ACKNOWLEDGMENT 

The authors wish to thank all thaw ~molved in this iiat'tcollection process. espec~nlly Mr. R. E. McKinne) and Mr. J. K. 

Lowney. We also wish to thank Dr. Yen Bm Chcn for hi5 help ib1t11 

statistical data. 

Cycles Durat~on 4 

. . A h) w P U1o.UW.00 

- F3 o p bb~bo'9- N w P U, 0.~0390 o 0 o o ooooo 

11 70 Monitor Days 

42 o Disturbances (Including 

NOTES. 

Number next to plotted point ~ndicates several incidents hav~ng same voltage and durat~on. 

Arrow next toplotted point indicates durat~on exceeded the record length. 

Figure 1. Plot of Undervoltages From Phase 

One 
440 Monitor Days 

9 Locationr 

610 Disturbances 

"%of 60 Hz Peak, superimposed on 60 Hz 

Figure 2. Plot of Switching Disturbances 

From Phase One Monitoring 

Dumtion Microseconds 

of Bore of Spike 

Figure 3. Plot of Spike Disturbance From 

Phase one 

Incidents/Mo. Osc . Dist ./Mo. 

Figure 4. Incident Rate Plot For Under- Figure 5. Incident Rate Plot For Switchinq, 

voltage, Phase One Phase One 
Inf. 

l lo 

100 

inom) 

90 

80 

70 

57 

20 

Cycler Duration 

inf. 

Key 030 > I 10°o 

Figure 6. Under and Over Voltages, Digital 0-30 < ROO0 

x others 

Monitors 

356 1523 1433 

Doyr Days Days vp 100% 

60"~- 

30'" 

Days Doyr Days Monitor Months -. 109 

Undervoltager = 1,569 

Overvoltages = 103 

Outager = 65 

50'. Sw. Disturb. = 2,831 

H.F. Dirtuio. = 1,676 rota PLD rn 

Oscillatory Dirt. Voltage Spike Dist 

Figure 7. Switching and Spike Data From 

Digital Monitors 

Undervoltage Incidents/Mo. Exceeding 20% Threshold 

IncidentdMo. trceedinq 10% Threshold 

Figure 8. Undervoltage Incident Rate Plot, 

Diqital Yonitors 

7 
Sw. Dist/Mo., Exceeding 60°h Threshold 

Sw. Dirt/Mo., Exceeding 30% Threshold 

Sw. Dist/Mo., Exceeding 15% Threshold 

Figure 9. Switching Incident Rate Plot, 

Digital Monitors 

Spiker/Mo., Exceeding 100% Threshold 

l2 r 

Spikas/Mo., Exceeding Threshold 

Spiker/Mo., Exceeding 25% Threshold 

Figure 10. Voltage Spike Incident Rate 

Plot, Digital Monitors 

Jan. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nov. Dec. 

Month of Year 

- Avg. 2.09 

1.51 Outages 

, 1 .o 

2 0.5 -- -- - --- - Avg. 

- 0 Jon. Feb. Mar. Apr. May Jun. Jul. Aug. Sep. Oct. Nav. Dec. 

Month of Year 

Figure 11. Monthly Distribution of PLD 

Incident Rates From Domestic 

Digital Monitor Data 
lnf. 

Figure 12. Cumulative Percent of Outages 

vs Duration in Minutes 

xr16 x-ll x=13 x=8 x=26 x=14 x~6 x=6 x--3 x=5 x-9 x=13 x=8 X= I 

O= 1 0-1 0=3 0-3 0=2 0=4 0=2 0=1 0=3 O= 1 

x=3 x=7 x=3 x=3 x=9 x=13 x=12 x=2 x=2 x=3 x=6 x=4 x=4 

0=1 0=2 0=3 0=1 0=1 0-7 0=1 

=22 Dur. Unk. 

x=l 

0=1 

x= l 

0=8 

Outages vs Duration Domestic Digital Monitor Data 

109 Monitor Months 

65 Outages 

29 Locations 

0 1 2 3 4 5 6 7 8 9 10 15 30 120900 Inf. 

Cycles Duration 

U/Z/ Disturbances 0 = 38 <80% 

x = Others Monitor Months = 171.7 

Undervoltages = 1,848 

Outages = I16 

x=l 

0=1 

0=1 

Mean Outage Time 1.0 Min. 

Median Outage Time 0.1 Min. 

0 I I 111111 I I ,11111 I I 1 1 1111 I I 1 ,1111 

.01 0.1 1 .o 10 100 

Duration in Minutes 

0=1 

Figure 13. Total Undervoltage Data Plot 

x=l 

0=1 

0=2 

x=5 

0=3 

x=l 

x=l 

O= 1 

x=l 

0=2 

x=3 

0=1 0=3 

x=l 

0=3 

0-6 

x=l 

0=2 

0=7 

x=l 

0% 

x= 1 

0=16 

0=2 

0=8 0=41 0- 
Discussion of 

Paper C74 199-6 

Monitoring of Computer Installation 

for Power Line Disturbances 

George W. Allen & Donald Segal 

Discussed by 

Harold A. Gauper, Jr., Senior Member 

and 

Francois D. Martzloff, Member 

Corporate Research .& Development 

General Electric Company 

P.O. Box 43 

Schenectady , New York 12301 
DISCUSSION 

The contribution by the authors of the data presented in their paper 

is significant in an area where much too little is available; however the 

data presented may be biased by instrumentation limitations for the recording of very short duration spikes, especially if their peaks exceeded the 

expectations implied in the instrument settings. This may be a partial explanation for the results showing lower peaks than the Reference 6 paper. 

Another explanation may be that thesystems monitored in this study have 

lower impedance, and consequently would not allow the voltage caused by load 

switching rise to the levels recorded in the "softern residential circuits. 

The nature of the distribution system (overhead versus underground) may also 

have an effect on the levels of spikes due to lightning remnants. To address 

this question, it would be useful if the authors could give a brief description of the type of power systems monitored. This would also provide the 

beginning of an appraisal of the energy levels involved, corresponding to 

the reported voltage measurements. 

The paper describes two phases of monitoring for spikes, the first with 

storage oscilloscopes, the second with a digital instrument receiving its 

information from a band-pass filter. A brief discussion of possible pitfalls 

or limitations in the case of short duration spike follows, for each type of 

instrument. 

A possible shortcoming of the storage oscilloscope, when used as described, 

is illustrated in the three oscillograms. Oscillogram #1 shows the recording 

of a random spike occurring in a 120 volts AC system, with the controls of a 

Tektronix 549 oscilloscope set,approximately as described in the paper presentation: 200 volts/division, 0.2 millisecond/division, and a zero trace. Inspection of the recording shows the occurrence of a 300 volts, positive spike, 

or about 1.8 per unit. 

However, if the oscilloscope had been set to record higher spikes, as in 

the case of oscillogram #2, where the only change has been to set the voltage 

sensitivity to 1 kV/division, the same spike occurrence is now recorded as 

shown, where the 2200 volts negative spike is not lost by excessive beam speed, 

as was the case in oscillogram #l. Oscillogram #3 shows a detailed recording 

of the same spike using a conventional oscilloscope (1 kV/div., 5 ps/div.). 

200 ps/div. 200 ps/div. 5 ps/div. 

Figure 1. Spike Recording with Storage and Conventional Oscilloscopes 
The question is then raised, whether the reported recording might have 

missed fast spikes at levels beyond the expectations of the observer. The 

peculiarities of the response to short spikes of a band pass filter, as 

discussed below, may unwittingly have confirmed this finding by attenuating 

the levels passed on to the threshold detection. Furthermore, the paper 

does not mention threshold settings higher than loo%, so that no evidence 

could be obtained for the higher levels. 

The digital monitor (described in Reference 6 paper) may not respond 

as anticipated to the voltage spikes of 10 to 100 microseconds duration. 

The first item for examination is the transfer function for the transformer 

used at the input of the digital monitor. A normal 60 Hz instrumentation 

transformer could be expected to behave like a transformer up through 5 

kilohertz. It would thus be useful for the authors to discuss the transformer behavior to inputs of the voltage spikes of interest. 

The second item is the behavior of the 3-30 kHz bandpass filter to 

voltage spikes with the short duration of interest. 

The peak output voltage of a bandpass filter is directly proportional 

to the bandwidth of the filter and the magnitude of the peak voltage spectral intensity of the voltage spike within the passband of the filter. 

The filter response has been calculated assuming 1.) that the bandpass 

filter has unity gain over the passband and zero gain at all other frequencies, 2) that the voltage spikes are square with a rise and fall time much 

shorter than their duration. 

," 

Fig. 2 is a plot of the peak voltage at the filter output as seen by 

the peak voltage detector as a percentage of the assumed square wave input 

voltage vs. the width of the square wave. The response may be even less for 

the typical power system voltage spikes. 

We thus request the authors to further discuss the recording characteristics of both described spike monitoring systems as a function of pulse 

width. 

Fiqure 2. Filter Response