On Relay Chatter Circuit Analysis
Presented by
James C. Lin
PSA 2015
International Topical Meeting on
Probabilistic Safety Assessment and Analysis
Sun Valley, Idaho
April 26-30, 2015
Outline of Presentation
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Introduction
Seismic relay chatter circuit analysis
Selected relay chatter considerations
Selected vulnerable circuits
Non-relay contact devices
Conclusions
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Introduction
• Functions of relays in control circuits
• Types of relays
– Control and auxiliary relays
• Electro-mechanical, pneumatic timing, and solid state relays
– Protective relays
• Electro-mechanical, solid state, and overload relays
• Relays are typically not structurally damaged during a seismic event
– Due to the low seismic ruggedness of the relay contacts, vibration induced
actuation of certain relays could occur during an earthquake potentially resulting in
an undesired equipment operation or change in equipment position/state
• Inadvertently operate equipment
• Could lock out electrical buses or DGs
– The adverse effect of relay chatter is one of the causes of seismic induced failures
evaluated in SPRA
• To develop a comprehensive SPRA, the potential effects of relays on
the performance of the accident mitigation components in response to
the earthquake must be analyzed
– An essential part of a rigorous SPRA
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Seismic Relay Chatter Circuit Analysis
• Identify relays or contact devices whose chatter during an
earthquake can cause an adverse impact on the accident
mitigation functions
– Should evaluate the control circuits for all accident mitigation equipment
considered for the response to seismic initiating events
– Relay chatter-induced unavailability of the PRA components on the SEL
• Should be performed by PRA components on the SEL
– Evaluate control circuits associated with each of these components for chatter
induced adverse impacts
– Generally assumed that circuits that require simultaneous chatters of two or
more relays in unison in order to re-position or change the state of a
component are deemed improbable
– In some circuits, contact chatter during the earthquake may temporarily cause
selected mitigation equipment unavailable
• Once the earthquake is over, the chatter would stop and the mitigation function
would be automatically restored
• Due to the insignificant impact on the PRA mitigation function, this type of relay
chatter can be screened for further consideration.
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Selected Relay Chatter Considerations
• Solid state and timing relays
– Solid state relays are not prone to chatter due to the lack of mechanically moving parts
– Timing relays with settings greater than 1 second are not affected by chatter of upstream relays
• Relay energy state
– The seismic capacity of an energized relay is significantly stronger than that of de-energized relays
– Normally energized relays can typically be screened out in the relay chatter analysis
• Chatter frequency
– If the chatter frequency of a contact device upstream of a relay is reasonably high, the duration in which the
circuit path is complete may not be sufficiently long to allow the downstream device to actuate (e.g., relay to
energize and its contacts to move, breaker to trip open, MOV to change position, etc.)
– The amount of time required to energize the downstream device (e.g., breaker trip coil, motor winding, etc.),
build up the electromagnetic field, trip open the breaker, cause the motor to move, change the MOV
position, etc., is likely to be longer than the circuit path complete time during the contact chatter cycles
– The circuits that typically would cause adverse effect on the PRA equipment mitigation functions are those that
involve a seal-in circuit or lockout relays
• Most likely adverse impacts
– MOVs or SOVs inadvertently changed to undesired positions, tripping open of bus or equipment (e.g., pump)
breakers, spurious opening of Safety Relief Valves/Power Operated Relief Valves (SRV/PORV), blocking of
breaker closure or equipment start, and lockout of an essential bus or a diesel generator
– In addition to tripping open the breakers, overcurrent protection may sometimes (e.g., diesel output breaker or
essential bus overcurrent) also include lockout function by blocking the breaker from closure if the lockout relays
are not reset
– The typical overcurrent protection design for a piece of equipment such as a medium voltage pump includes
ground fault overcurrent, instantaneous overcurrent, phase time overcurrent, etc.
– The overcurrent protection for a diesel generator breaker could also include such other protection function as
differential overcurrent, etc.
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Selected Vulnerable Circuit
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MOV seal-in circuit
Large pump overcurrent circuit
Bus lockout circuit
Emergency diesel generator lockout circuit
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Example MOV Control Circuit
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Example Control Circuit for 4.16-kV Pump
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Example Circuit for Undervoltage Signal
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Example Circuit for Bus Overcurrent
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Chatter Vulnerability for Non-Relay Contact Devices
• Circuit breakers
– The main contacts are designed to withstand the magnetic forces resulting from very
high short circuit current
– The breaker main contacts are mechanically actuated by a linkage from the breaker
and have not shown chatter vulnerability
– The breaker auxiliary contacts are also typically not vulnerable to chatter because
they are built into and mechanically linked to the breaker main contacts (except for a
few models of low/medium voltage switchgear breakers)
• Electro-mechanical contactors and motor starters
– The main contacts of an electro-mechanical contactor/motor starter are designed to
withstand the magnetic forces resulting from very high short circuit current, and have
not shown chatter vulnerability
– The auxiliary contacts for electro-mechanical contactors/motor starters are not
designed to withstand short circuit conditions. However, they are mechanically
actuated contacts (i.e., mechanically opened/closed when the contactor is actuated)
which are inherently rugged and not prone to chatter (except for a few models)
• Manually operated control switches
– Rotary control switches are not vulnerable to chatter because vertical and horizontal
seismic motion cannot cause the contacts to rotate
– The non-rotary control switches are also not vulnerable to chatter because the
contacts of these manually operated control switches are mechanically actuated
which is inherently rugged for chatter
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Chatter Vulnerability for Non-Relay Contact Devices
• Limit, torque, and position switches
– Actuation of a contact on Limitorque motor operators with a rotary shaft
requires the rotary shaft to turn 90 degrees and thus are not prone to seismic
chatter
– A torque switch is a mechanical device and not prone to chatter
– The position switch contacts for circuit breakers are mechanically operated and
not prone to chatter (except for a few models)
• Mechanical sensor switches
– Include mechanical pressure, level, flow, and temperature switches
– Are generally not vulnerable to chatter because they require the application of
a reasonable force to change state
– Mercury and sudden pressure switches on large power transformers are
vulnerable to chatter
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Summary of Non-Relay Contact Devices
• Most of the non-relay contact devices are not prone to chatter and do not need to
be evaluated in the relay chatter analysis
• Chatter of only the following contact devices need to be considered
– Mercury and sudden pressure switches on large power transformers
– Small sensitive switches (e.g., operator micro switches)
– Auxiliary contacts for motor contactors and low/medium voltage switchgear circuit breakers for a
few models
• Sudden pressure switches are mostly located in the control circuits for nonessential equipment
– Chatter is, in general, not likely to result in adverse impacts on the accident mitigation
equipment
– However, sudden pressure relays can be activated as a result of earthquake induced pressure
pulses causing lockout of auxiliary transformers and possibly loss of offsite power supply
• Compile a list of all mercury switches (e.g., Mercoid and Magnetrol switches),
sudden pressure switches, and micro switches in use at the plant
– Switches associated with any components in systems not credited in the SPRA and switches
that are located in non-seismically designed area of the plant can be screened out
– For those switches retained, a circuit analysis similar to that for relays should be performed
– Those switches with no adverse impact on the PRA components can also be screened
– Only switches with non-negligible impacts on the PRA mitigation functions require a fragility
analysis
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Conclusions
• The most significant adverse impacts can result from control
circuits with seal-in and lockout designs
– Seal-in feature can cause the MOV to spuriously transfer to an undesired
position requiring operator manual re-alignment
– Seal-in feature in the time OC protective circuits for the control of medium
voltage breakers can trip off its electrical loads during a seismic relay chatter
event
• Feeder breakers for medium voltage pumps are generally provided with time OC
protective function, which usually will only trip open the affected breakers but
would not block them from re-closing
– The seal-in design of the time OC relays in the control circuits for the supply
breakers of essential buses is typically devised to cause a lockout relay to
activate, tripping open these breakers and blocking from re-closing until
manually reset, which in many cases are performed locally at the switchgear
– For the control of EDG output breakers, many of the protective conditions are
designed with a lockout signal which will not only trip open the EDG output
breakers but also block it from re-closing until manually reset.
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Thanks for your attention
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