Dave Bradley, PE, LEED AP
Understanding Time Current
Curves and Coordination Studies
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usa.siemens.com
In This Elective
• Time Current Curve Basics
• Uses of Time Current Curves
• Coordination Study Basics
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Page 2
How will this help me next week?
• Better understanding circuit breakers
• Understand what TCCs can be used for,
and for what they shouldn’t be used
• Understand general differences between
coordination and Selective Coordination
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Page 3
Where did it all start?
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Page 4
In the beginning...
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Page 11
Inverse Time Element
Current
Time
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Page 12
How It Works
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Page 13
How it Works – Thermal Magnetic
Tripping
“foot”
Contacts
Load
Connection
Line
Connection
Electromagnet
Bi-Metallic Strip
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Page 14
How it Works – Solid State
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Page 15
Time Current Curves
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0
TIME
How to Show a Wide Range on a Small Paper
0
CURRENT
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Page 17
Logarithmic Time
1,000s
100s
10s
1s
100ms
10ms
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Page 18
Logarithmic Current
10A
100A
1,000A 10,000A 100,000A
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Page 19
Time Current Curves in the Real World
14
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Page 20
Time Current Curves in the Real World
20
14
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Page 21
Time Current Curves in the Real World
20
M
14
10HP
14
Inrush Currents
• 20x for ½ cycle
• 4-8x for several seconds
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Page 22
Time Current Curves in the Real World
20
30
M
14
10HP
14
Inrush Currents
• 20x for ½ cycle
• 4-8x for several seconds
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Page 23
Time Current Curves in the Real World
36
30kVA
30
20
M
14
10HP
14
Inrush Currents
• Worst case: 14x +
• Average: 5x for a couple of cycles
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Page 24
Time Current Curves in the Real World
90
36
30kVA
30
20
M
14
10HP
14
Inrush Currents
• Worst case: 14x +
• Average: 5x for a couple of cycles
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Page 25
Time Current Curves in the Real World
250
90
100
30kVA
20
36
30
M
14
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Page 26
14
10HP
Time Current Curves – Thermal Magnetic
1985
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Page 27
2006
Time Current Curve Details
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Page 28
Changes to New TCCs
OLD
NEW
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Page 29
Molded Case Switches
Motor Circuit Protectors
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Page 30
Thermal Magnetic vs Electronic
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Page 31
Time Current Curves – Electronic
Long Time
• Handle Rating
• Overloads
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Page 32
Time Current Curves – Electronic
Long Time Delay
• Normal Inrush
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Page 33
Time Current Curves – Electronic
Short Time
• Coordination
• Curve Matching
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Page 34
Time Current Curves – Electronic
Short Time Delay
• Coordination
• Curve Matching
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Page 35
Time Current Curves – Electronic
Instantaneous
• Faults
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Page 36
Uses of TCCs
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Page 37
Time Current Curve Software – EasyTCC
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Page 38
Time Current Curve Software – SKM
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Page 39
Coordination Studies
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Page 40
Uses of TCCs - Coordination
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Page 41
Selectivity
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Page 43
Coordination
0.1s
800
250
100
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Page 44
Selective Coordination
0.01s
800
260
100
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Page 45
Behavior in the Field
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Page 46
Uses of TCCs
For application and coordination purposes only. Based on 40°C
ambient cold start. Connected with 4 feet of rated wire (75°C) per terminal. Tested in open air with
current in all poles.
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Page 47
Uses of TCCs - Operation and Testing
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Page 48
Uses of TCCs - Operation and Testing
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Page 49
40ºC
25ºC
NEMA
AB-4
(104ºF)
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Page 50
Same Device, Different Settings
The purple time current curve (TCC)
reflects how the breaker is set when
it leaves the plant
The blue TCC shows one possibility
for how the breaker could be set.
This leave a fairly wide range of
possible settings
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Page 55
Effect of Incorrect Settings
Add in the feed to the air conditioner
and you can see why the main
breaker tripped out
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Page 56
Summary
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Page 58
Summary
• You know how loads and circuit breakers
interact, and how the time current curve
shows that
• You understand what TCCs can be used for,
and for what they shouldn’t be used
• You can now better address initial questions
about breaker performance in the field
• You know the general differences between
Coordination and Selective Coordination
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Page 59
Questions?
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Page 60