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

Restricted © Siemens Industry, Inc. 2015 All rights reserved.

In This Elective

• Time Current Curve Basics

• Uses of Time Current Curves

• Coordination Study Basics


• 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?


Where did it all start?

Restricted © Siemens Industry, Inc. 2015 All rights reserved.Page 11

In the beginning...


Inverse Time Element

Current Time


How It Works


How it Works – Thermal Magnetic

LineConnection

LoadConnection

Electromagnet

Bi-Metallic Strip

Tripping “foot”

Contacts


How it Works – Solid State


Time Current Curves


How to Show a Wide Range on a Small PaperTI

ME

CURRENT

0

0


Logarithmic Time

1,000s

100s

10s

1s

100ms

10ms


Logarithmic Current

10A 100A 1,000A 100,000A10,000A


Time Current Curves in the Real World

14



20

14



Inrush Currents• 20x for ½ cycle• 4-8x for several seconds

20

14 14

M 10HP



Inrush Currents• 20x for ½ cycle• 4-8x for several seconds

20

14 14

M 10HP

30



20

Inrush Currents• Worst case: 14x +

14 14

M 10HP

• Average: 5x for a couple of cycles

36

30

30kVA



20

Inrush Currents• Worst case: 14x +

14 14

M 10HP

• Average: 5x for a couple of cycles

36

30

30kVA

90


30

100

250


20

14 14

M 10HP

36

90

30kVA


Time Current Curves – Thermal Magnetic

1985 2006


Time Current Curve Details


Changes to New TCCs

OLD

NEW


Molded Case SwitchesMotor Circuit Protectors


Thermal Magnetic vs Electronic


Time Current Curves – Electronic

Long Time• Handle Rating• Overloads



Long Time Delay• Normal Inrush



Short Time• Coordination• Curve Matching



Short Time Delay• Coordination• Curve Matching



Instantaneous• Faults


Uses of TCCs


Time Current Curve Software – EasyTCC


Time Current Curve Software – SKM


Coordination Studies


Uses of TCCs - Coordination


Selectivity


Coordination

100

250

800

0.1s


Selective Coordination

100

260

800

0.01s


Behavior in the Field


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.


Uses of TCCs - Operation and Testing


40ºC(104ºF)25ºC

NEMA AB-4


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


Effect of Incorrect Settings

Add in the feed to the air conditioner and you can see why the main breaker tripped out


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

Summary


Questions?

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