1. Circuit Breaker Ratings – A Primer for Protection Engineers
Bogdan Kasztenny Schweitzer Engineering Laboratories
and
Joe Rostron Southern States, LLC

2. Typical Fault Clearing Operation
Relay Time
CB Mechanical Time
Arc Time

3. Typical Fault Clearing Operation
Contact Parting Time
Arc Time

4. Fault Clearing Timing Diagram

5. Asymmetrical Breaker Rating
Higher current → more plasma → more difficult current interruption
DC component increases the current
Breakers need extra margin to break asymmetrical currents
Today, breaker standards ask for asymmetrical rating under reference dc conditions
SEL DMC asset 1545

6. Combining AC and DC Components

7. Combining AC and DC Components

8. Combining AC and DC Components
RMS current defines asymmetrical CB rating
RMS current combines ac RMS with dc at contact parting time
S= DC%

9. S-Factor Extra Margin for Asymmetrical Currents

10. DC Component Decays With Time
DC%(t)=100%∙ e − t T DC
DC depends on t and TDC
tPART = tRELAY + tMECH
Standard dc reference condition
TDC = 2.71 cycle (X/R = 17)
Relay operating time (tRELAY = 0.5 cycle)

11. Required S-Factor Depends on Relay Time and CB Mechanical Time

12. Relay Operating Times Are Improving
Elimination of interposing and lockout relays
Naturally fast operating principles
Switch-onto-fault and stub-bus
Bus differential
Unrestrained transformer differential
New line protection principles

13. New Line Protection Principles
Based on fast incremental quantities
TD32 (directional, 1.5 ms)
TD21 (distance, 25 ms)
Based on traveling waves
TW32 (directional, 0.1 ms)
TW87 (differential, 13 ms)
SEL DMC asset

14. Ultra-High-Speed Line Protective Relays
Large processing power
1 MHz sampling
0.1 ms processing
Low-latency fiber channels (0.9 ms / 100 mi)
Solid-state outputs (10 ms)

15. Field Case 1: TW87 Operates in 0.9 ms

16. Field Case 2: TD21 Operates in 1.8 ms

17. Field Case 3: POTT Operates in 2.2 ms

18. CB Derating for Arbitrary Relay Time
Required margin for asymmetrical rating for a given contact parting time
I RATED =S∙ I SYM = I SYM e − t PART T DC 2
Contact parting time includes relay operating time and breaker mechanical time
I RATED = I SYM e − t REL + t MECH T DC 2

19. CB Derating for Arbitrary Relay Time
For the standard relay time of 0.5 cycle
I RATED(0.5cycle = I SYM e − t t MECH T DC 2
For an arbitrary relay time
I RATED( t REL = I SYM e − t REL + t MECH T DC 2

20. CB Derating for Arbitrary Relay Time
We introduce the R ratio
R= I RATED( t REL I RATED(0.5cycle = e − t REL + t MECH T DC e − t t MECH T DC 2
R < 1 – loss of rating compared with 0.5 cycle
R > 1 – gain in rating compared with 0.5 cycle

21. CB Derating for Fast Tripping Standard X/R of 17 (45 ms)

22. CB Derating for Fast Tripping High X/R of 37.7 (100 ms)

23. CB Derating for Fast Tripping Low X/R of 9.4 (25 ms)

24. “Slow” and “Fast” Tripping Gains and Losses in Margin

25. Other Cases of “Fast Tripping”
Relay misoperation
Can actuate CB at any time
Evolving faults
Fault current may start after CB actuation
Can lead to “negative trip times”
SEL DML asset 1545

26. Evolving Fault Field Case
Trip
Fault Inception
Relay Operating Time?
12 ms after fault current in the B phase
8 ms before fault current in the C phase

27. Circuit Breaker Application Worst-Case Scenario
Caused by evolving faults or relay misoperations
DC = 100% → S = 1.7 (70% margin required)
Low-probability case

28. Series-Compensated Lines Exponential DC Decay Does Not Apply

29. Conclusions
Ultra-high-speed tripping calls for small increase in asymmetrical CB rating over the 0.5 cycle reference
Fast breakers, low X/R 7%
Slower breaker, large X/R 3%
Breakers are applied with customary 20% margins
CB practitioners routinely derate breakers