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SYSTEM RESPONSE TO RELAY CHATTER (U)by

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G. A. Antaki and J. A. Radder

Westinghouse Savannah River CompanySavannah River SiteAiken, South Carolina 29808

A paper proposed for presentation at thePressure Vessels and Piping ConferenceNew Orleans, LouisianaJune 22 - 26, 1992

and for publication in the proceedings

The information contained in this article was developed during the course of work done underContract No. DE-AC09-89SR 18035 with the U.S. Department of Energy. By acceptance of thispaper, the publisher and/or recipient acknowledges the U.S. Government's right to retain anonexclusive, royalty-free license in and to any copyright covering this paper along with the rightto reproduce and to authorize others to reproduce ali or part of the copyrighted paper.

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SYSTEM RESPONSE TO RELAY CHATTERG.A. ANTAKI & J.A. RADDER

WESTINGHOUSE SAVANNAH RIVER COMPANYAIKEN, SOUTH CAROLINA

INTRODUCTION

An important aspect of the qualification of safety related systems, is theassessment of the possibility and consequences of seismic or vibration inducedrelay chatter. The current rules, contained in ANSI/IEEE C37.98 [1] and theSQUG GIP [2], consider as seismic induced "failure" of a relay, a chatter contactin excess of 2 milliseconds. While this rule is clear in defining "failure" of asingle relay, the definition of "failure" becomes more problematic when weconsider (a) the alignment of an assembly of relays in series or parallel, and(b) the effects of chatter on the function of the system in which the relays aremounted.

Irl this paper, we report a method, based on system test and probabilitiesanalysis, used to assess "failure" by relay chatter ill an assembly of relayswhich control an electric motor.

SYSTEM DESCRIPTION

Consider the relay arrangement _llustrated in figure 1. Power is supplied to amotor which can drive, up or down, a weight W. The table in figure 1indicates the single relay alignment configuration which can drive theweight W up: K1, K2 and K3 closed; K4A open; and K4B closed. Any other relayalignment would either drive the weight down or _.ould interrupt the motion.

In our case, driving the weight W down or maintaining it stationary isacceptable. Therefore, "failure" only occurs if the relay chatter results inupward motion of the weight W.

MOTOR WIRE

WEIGHTW

K1 K2 K3 K4A

K4B

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RELAY K1 K2 K3 K4

Open No No No K4 Noncontact =Movement Movement Movement No Movement

- Closed Movement Movement Movement KaA closed = V"downSee K4 See K4 See K4 K4B closed = Wup

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FIGURE - 1: RELAY ALIGNMENT LOGIC ANDMOTOR, WIRE AND WEIGHT ASSEMBLY

,, RELAY CHATTER TEST

1 A relay chatter test was conducted to study its effect on the l"esponse of the_' motor drive, in an attempt to predict the maximum "tailure" rate, that is the

maximum upward motion which may reeult from sinusoidan vibration in therange of 0.5 hertz to 20 hertz. To this end, relays Kl, K2, and K3 were aligned inthe "upward" configuration. Due to equipment limitations, a sinusoidalforcing function could not be created, instead K4 was subjected to analternating, periodic step function as shown in figure 2.

CONTACTMOVEMENT ] CONTACTMOVEMENT

K4A CONTACT .....,ql-- tc-.l_

I _ TIME

K4B CONTACT

FIGURE.2: CONTACTTIMEtcFORASINUSOIDALFORCINGFUNCTIONANDTIMESTEPSIMULATION

The time of contact T/2 of the step function excitation, overpredicts the time ofcontact tc which would be achieved in a sinusoidal excitation. This stepfimction ignores the time it takes the relay to travel between open and closedco_',tact and therefore overpredicts the upward movement,which isappropriate to estimate a maximum upward motion.

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Twenty K4 relays were tested. The frequency 1/T of the step function wasvaried, for each tested relay, from 0.5 hertz to 20 hertz in 0.5 hertz incrementsEach test run lasted 20 seconds and was repeated twice. Therefore each relaywas subjected twice to 40 step function test runs, for a total of 20 relays X 2 testseries/relays X 40 test runs/test series = 1600 test l_,is, of 20 seconds each.

In only 7 of the 1600 test runs, (which occured in 2 of the 20 relays), the motordrove the weight W upward. In ali other cases, the motor response time did notfollow the fast changing relay signals and did not last a sufficient time toovercome the weight's downward inertia.

We have therefore at this stage made two important observations:

1- Relay contact by chatter is a necessary, but not sufficient,condition to cause "failure" of the system function; which isdefined here as an upward motion of the weight W. In our case,the response characteristic and inertia of the motor and weightare too slow to follow the relay chatter "upward" signal.

2- Upward motion occurred for only 2 of 20, or 10%, of the testedrelays, and in only 7 of 1600 test runs.

PROBABILISTIC ASSESSMENT

To now assess the probable response of a system of relays K1 to K4, we callconsider that the relay contacts of K1, K2 and K3 will spend 50% of the time inan open position and 50% of tim time in a closed position. This approachigno'es the amount of time it takes the centacts to travel between open andclosed position, and therefore tends to favor the alignments which result inmovement of the weight W.

The contacts of relays K 1, K2 and K3 will not exhibit synchronous behavior,because the relays have different spring tensions and different contact sets.The occurrence of contact in relays KI, K2 and K3 is therefore independent,and the probability of the K1 = K2 = K3 = closed alignment can be calculated as:

P(K1K2K3) = P(K1) x P(K2) x P(K3)P(K1K2K3) = 0.50 x 0.50 x 0.50P(K1K2K3) = 0.125

Multiplying the P(K1K2K3) probability by the probability of K4 chatter tocause upward motion P(K4) = 2/20 = 10%, we obtain the probability of upwardmovement P (t,p):

P(up) = P(K1K2K3) x P(K4) = 0.125 x 0.10 = 1.25%

This series of tests and probablistic assessment indicate the difference between"failure" of a single relay as defined in ANSI/IEEE C37.98 (2 msec contact) andthe actual probability of failure of the system to perform its fimction.

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IMPORTANT SAFETY CONSIDERATIONS

The relay test and evaluation performed point to several important safetyconsiderations of relay chatter in a system, as opposed to individual relaychatter:

(a) When the motor in figure 1, received single frequency chatter signalson the upper end of the test frequency range (close to 20 hz), it wasforced to continuously and quickly change driving direction (up anddown). As a result, in several cases, the motor blew a fuse and becameinoperable . While the continuous single frequency reversible testloading (figure 2) is more severe on the motor than a randomearthquake forcing function, the possibility of electrical damage to thedownstream component appears to be an important consideration inrelay hatter analysis.

(b) In the test frequency range between 15 hz and 20 hz, electrica I sparkswere observed across the relay contacts. This important phenomenon,which is a function of several system variables, can result in thecontacts being welded closed from the heat generated by the electricalsparks. The electrical sparks, which depend on the current acrosscontacts could therefore result in a different configuration thanreported in the GI _ [2].

(c) In ali tests conducted between 17 hertz and 19 hertz, as the relay chattersends alternating signals to the motor to change driving direction (upand down), the motor + wire + weight assembly (figure 1) was driveninto visible and audible resonance.

The vibration could have damaged the zssembly. Again, the 20 secondsof continous periodic excitation achieved in the test allow for thisbuildup of resonance, which is expected to be much less prominent inthe more random response to an actual earthquake.

CONCLUSION

This series of tests and probablistic eva iuations indicate that the assessment ofrelay chatter is more complex when one considers the common case of a seriesof relays and their interaction with a downstream motor package.

In this case, "failure" has to be defined in terms of the function of thedownstream system. This system related definition, together with thep_obability 9f worst case relay alignments, allow for a quantitative analysis ofthe failure mode, as described in this paper.

REFERENCES

1) ANSI/IEEE C37.98, ._....;,.:...f_fc _1"7_.17'_c)( _'+_ , I'_6_-7.2) Seismic Qualification Utilities Group (SQUG) Generic Implementation

Procedure (GIP) for Seismic Verification of Nuclear Plant Eqaipment.Revision _, Corrected June 28, 1991.

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