Protection Flashcards
How do arc fault circuit interruptors work?
An arc fault circuit interruptor protects residential circuits by de-energizing the branch circuit when a dangerous level of arcing is detected
What happens to the trip characteristics of an adjustable circuit breaker after the time delay setting has been changed to be more inverse compared to the previous setting (assume no changes have been made to the pickup setting)?
The circuit breaker will take more time to operate for the same level of fault current
The relationship between transformer inrush current and differential protection
Transformer inrush current occurs in the primary winding only and can potential cause a differential protection relay to operate due to mismatch
Interrupting capacity of a circuit breaker
The maximum current that the circuit breaker can safely interrupt or stop
AIC = ampere interrupting capacity
(Typically above 5000A)
Are fuses single phase devices
Yes
You have to have a fuse for each phase
One cannot be used to detect multiple phases
Is a fuse a sensing, interrupting, and/or switching device?
A fuse is a sensing and interrupting device, but not switching
Fuses have a thermal element that can sense increased heat from high currents or over-currents
Fuses interrupt the circuit by melting and thus opening the circuit once melted
Is a circuit breaker a sensing, interrupting, and/or a switching device?
A circuit breaker is a switching and interrupting device, but not sensing
Circuit breakers automatically trip and interrupt the circuit when a current higher than the breakers continuous current rating persists in the circuit
Circuit breakers can be manually switched on and off to make or break the circuit
Are protective relay devices sensing, interrupting, and/or switching devices?
Protective relays are sensing only devices
Protective relays use instrument transformers such as CTs and PTs to monitor current and voltage in a line or circuit
Protective relays send trio signals to OCPDS, but protective relays themselves are not interrupting or switching devices
Protection scheme consists of 3 parts
- Measurement
- Relay devices
- Tripping devices
Recloser
A type of circuit breaker that can automatically close the circuit after the circuit breaker has been opened due to a fault
Used on medium and high voltage transmission/distribution lines and typically not used in buildings low voltage distribution systems 
The faults in transmission lines are typically cleared by themselves so the circuit can reclose and continue working
Sectionalizer
Is a switch that isolates a faulted circuit
It works in conjunction with a re-closer or circuit breaker to isolate a faulted circuit
The sectionalizer counts the number of times the re-closer has operated and once a certain count is reached, the sectionalizer will isolate the faulted circuit
OCPD (overcurrent protection device)
Must be sized to meet
- Rated current
- Rated voltage
- Ampere interrupting current (AIC)
MVA Method
Transformer = MVA / Z%
Generator = MVA / X’’
Transmission line = kV ^2 / Z
Add normal in parallel
Add 1 over in series
I sc = MVA total / V fault location * root 3
Arc fault circuit interruptor (AFCI)
These circuit breakers trip when a dangerous level of arching is detected
All 120V, 1ph, 15-20A branch circuits supplying outlets or devices installed in dwelling unit kitchens, family rooms, dining rooms, living rooms, parlors, libraries, dens, bed rooms, recreation rooms, closets, hallways, laundry areas, or similar rooms or areas
Prevents residential fires
Ground fault current interruptor (GFCI)
Measures the difference between the hot and neural wires and if that difference is greater than 4 mA then the device trips
4mA because shock can occur at currents higher than this level
equipment-grounding conductor (EGC)
The conductor that connects noncurrent-carrying parts of equipment and raceways to ground at the service equipment or the source of a separately derived system
The purpose of an equipment-grounding conductors is to provide what?
Low impedance to ground to limit voltage to ground on exposed conducting surfaces and help ensure ground fault clearing
An industrial plant is served by a 12,460V delta/480V grounded-wye transformer. The high voltage side of the transformer is served from a 12,470V wye connected distribution line that has a grounded neutral. The best protection of the transformer against lightning strikes on the 12,470V line will result from surge connection of:
Phase to ground on the 12,470V side of the transformer
A grounded industrial power system would be LEAST likely to improve
A. THE POWER FACTOR
B. Service reliability
C. Personnel safety
D. Lightning protection
Overcurrent/undercurrent relays
A CT measures the current through a conductor
A relay will take the measured value and tell the circuit breaker to open if needed
Overvoltage/undervoltage relay
A potential (aka voltage) transformer measures the voltage at a certain spot at a conductor
A relay will take the measures value and tell the circuit breaker to open if needed
Directional relays
Used on distribution and transmission lines to distinguish the direction of the fault
By determining the direction of the fault current, the protection devices will know which devices the fault falls in between and the appropriate overcurrent protection devices can be tripped to isolate the fault
Differential relays
Consists of multiple CTs on either side of a bus or on withers side of a transformer
The CTs measure the current going into the bus or transformer and they also measure the CTs leaving the bus or transformer
If the current leaving and entering are not equal, then the relay will trip the circuit breakers
Distance relay
Typically set to trip a circuit when a fault occurs within a certain distance from the relay
The rally can cover a specific range of a transmission line
Pilot relays
System that is typically used on high voltage systems
Low voltage, low current systems that transmit a signal to activate a circuit breaker based on the input of a relay scheme like overcurrent, current differential, directional, etc
The pilot relay will control the high current and high voltage system with only a small current and small voltage
Instantaneous overcurrent protection (50)
Operates instantaneously (less than 0.1 seconds)
Relay pick up value
The amount of current that will cause the relay to trip
Definitely time overcurrent protection
The current must reach a certain value but it must maintain that amperage for a certain number of seconds
This type of relay can be used as a backup to instantaneous overcurrent protection
Inverse time overcurrent protection
An inverse time overcurrent protection relay has a curve on the TCC graph
It is downward sloping, so at higher currents, the time to trip is much smaller than it is at lower currents
The difference in time to trip between smaller and larger currents is much greater for the extremely inverse curve
For the moderately inverse curves, the difference in time is much smaller
