dave bradley, pe, leed ap understanding time … · understanding time current curves and...
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Understanding Time Current Curves and Coordination Studies
Dave Bradley, PE, LEED AP
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In This Elective
• Time Current Curve Basics
• Uses of Time Current Curves
• Coordination Study Basics
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• 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
How will this help me next week?
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Where did it all start?
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In the beginning...
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Inverse Time Element
Current Time
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How It Works
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How it Works – Thermal Magnetic
LineConnection
LoadConnection
Electromagnet
Bi-Metallic Strip
Tripping “foot”
Contacts
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How it Works – Solid State
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Time Current Curves
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How to Show a Wide Range on a Small PaperTI
ME
CURRENT
0
0
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Logarithmic Time
1,000s
100s
10s
1s
100ms
10ms
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Logarithmic Current
10A 100A 1,000A 100,000A10,000A
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Time Current Curves in the Real World
14
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Time Current Curves in the Real World
20
14
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Time Current Curves in the Real World
Inrush Currents• 20x for ½ cycle• 4-8x for several seconds
20
14 14
M 10HP
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Time Current Curves in the Real World
Inrush Currents• 20x for ½ cycle• 4-8x for several seconds
20
14 14
M 10HP
30
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Time Current Curves in the Real World
20
Inrush Currents• Worst case: 14x +
14 14
M 10HP
• Average: 5x for a couple of cycles
36
30
30kVA
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Time Current Curves in the Real World
20
Inrush Currents• Worst case: 14x +
14 14
M 10HP
• Average: 5x for a couple of cycles
36
30
30kVA
90
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30
100
250
Time Current Curves in the Real World
20
14 14
M 10HP
36
90
30kVA
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Time Current Curves – Thermal Magnetic
1985 2006
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Time Current Curve Details
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Changes to New TCCs
OLD
NEW
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Molded Case SwitchesMotor Circuit Protectors
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Thermal Magnetic vs Electronic
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Time Current Curves – Electronic
Long Time• Handle Rating• Overloads
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Time Current Curves – Electronic
Long Time Delay• Normal Inrush
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Time Current Curves – Electronic
Short Time• Coordination• Curve Matching
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Time Current Curves – Electronic
Short Time Delay• Coordination• Curve Matching
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Time Current Curves – Electronic
Instantaneous• Faults
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Uses of TCCs
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Time Current Curve Software – EasyTCC
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Time Current Curve Software – SKM
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Coordination Studies
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Uses of TCCs - Coordination
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Selectivity
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Coordination
100
250
800
0.1s
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Selective Coordination
100
260
800
0.01s
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Behavior in the Field
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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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Uses of TCCs - Operation and Testing
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Uses of TCCs - Operation and Testing
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40ºC(104ºF)25ºC
NEMA AB-4
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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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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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Summary
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• 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
Summary
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Questions?