Protection

Question 1

How do arc fault circuit interruptors work?

Answer 1

An arc fault circuit interruptor protects residential circuits by de-energizing the branch circuit when a dangerous level of arcing is detected

Question 2

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)?

Answer 2

The circuit breaker will take more time to operate for the same level of fault current

Question 3

The relationship between transformer inrush current and differential protection

Answer 3

Transformer inrush current occurs in the primary winding only and can potential cause a differential protection relay to operate due to mismatch

Question 4

Interrupting capacity of a circuit breaker

Answer 4

The maximum current that the circuit breaker can safely interrupt or stop

AIC = ampere interrupting capacity

(Typically above 5000A)

Question 5

Are fuses single phase devices

Answer 5

Yes

You have to have a fuse for each phase

One cannot be used to detect multiple phases

Question 6

Is a fuse a sensing, interrupting, and/or switching device?

Answer 6

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

Question 7

Is a circuit breaker a sensing, interrupting, and/or a switching device?

Answer 7

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

Question 8

Are protective relay devices sensing, interrupting, and/or switching devices?

Answer 8

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

Question 9

Protection scheme consists of 3 parts

Answer 9

1. Measurement
2. Relay devices
3. Tripping devices

Question 10

Recloser

Answer 10

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

Question 11

Sectionalizer

Answer 11

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

Question 12

OCPD (overcurrent protection device)

Answer 12

Must be sized to meet

1. Rated current
2. Rated voltage
3. Ampere interrupting current (AIC)

Question 13

MVA Method

Answer 13

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

Question 14

Arc fault circuit interruptor (AFCI)

Answer 14

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

Question 15

Ground fault current interruptor (GFCI)

Answer 15

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

Question 16

equipment-grounding conductor (EGC)

Answer 16

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

Question 17

The purpose of an equipment-grounding conductors is to provide what?

Answer 17

Low impedance to ground to limit voltage to ground on exposed conducting surfaces and help ensure ground fault clearing

Question 18

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:

Answer 18

Phase to ground on the 12,470V side of the transformer

Question 19

A grounded industrial power system would be LEAST likely to improve

Answer 19

A. THE POWER FACTOR
B. Service reliability
C. Personnel safety
D. Lightning protection

Question 20

Overcurrent/undercurrent relays

Answer 20

A CT measures the current through a conductor

A relay will take the measured value and tell the circuit breaker to open if needed

Question 21

Overvoltage/undervoltage relay

Answer 21

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

Question 22

Directional relays

Answer 22

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

Question 23

Differential relays

Answer 23

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

Question 24

Distance relay

Answer 24

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

Question 25

Pilot relays

Answer 25

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

Question 26

Instantaneous overcurrent protection (50)

Answer 26

Operates instantaneously (less than 0.1 seconds)

Question 27

Relay pick up value

Answer 27

The amount of current that will cause the relay to trip

Question 28

Definitely time overcurrent protection

Answer 28

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

Question 29

Inverse time overcurrent protection

Answer 29

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