Types of Overcurrents & Calculations - NEC
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Transcript of Types of Overcurrents & Calculations - NEC
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© 2001 Cooper Bussmann, Inc.
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Types of Overcurrent&
Calculations
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© 2001 Cooper Bussmann, Inc.
NEC® Article 100 – Definitions
Overcurrent Any current in excess of the rated current of
equipment or the ampacity of a conductor. It may result from overload, short circuit, or ground fault.
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© 2001 Cooper Bussmann, Inc.
NEC® Article 100 – Definitions
Overload Operation of equipment in excess of normal, full-load rating,
or of a conductor in excess of rated ampacity that, when it persists for a sufficient length of time, would cause damage or dangerous overheating. A fault, such as a short circuit or ground fault, is not an overload.
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© 2001 Cooper Bussmann, Inc.
Normal LoadSo
urce
R R
RR
R
R
Load
R
IL= VS / RT
VS
IL
IL
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© 2001 Cooper Bussmann, Inc.
OverloadSo
urce
Within the normal path
R R
RR
R
R
LoadR
VS
IOL= VS / RT
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© 2001 Cooper Bussmann, Inc.
Short CircuitSo
urce
Outside the normal path
R R
RR
R
R
ISC= VS / RT
VSLoadR
Out of the circuit
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© 2001 Cooper Bussmann, Inc.
Types of Faults
• Bolted Faults• Arcing Faults
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BoltedShort Circuit
A B
Arcing Fault
A B
Current Thru Air
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© 2001 Cooper Bussmann, Inc.
Bolted Faults –Various Types
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
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© 2001 Cooper Bussmann, Inc.
Bolted Faults – Three Phase
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
Typically considered the “worst case” or highest magnitude
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© 2001 Cooper Bussmann, Inc.
Bolted Faults – Line to Line
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
87 % of the three phase bolted fault
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© 2001 Cooper Bussmann, Inc.
Bolted Faults – Line to Ground
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
Typically much lower than 3 fault, but can be > 3 fault near Xfmr terminals
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© 2001 Cooper Bussmann, Inc.
A B
Arcing Faults – Many Variables Effect Current & Whether Sustainable
System VoltageGap spacingAvailable 3 Short Circuit AmpsAmount of Copper VaporizedDegree of ContainmentConfiguration of Equipment
Typically does not sustain on 208Y/120V
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© 2001 Cooper Bussmann, Inc.
Arcing Faults – Progression
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
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© 2001 Cooper Bussmann, Inc.
Arcing Faults – Three Phase
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
Can vary widely possibly up to 89% of 3 bolted fault
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© 2001 Cooper Bussmann, Inc.
Arcing Faults – Line to Line
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
Can vary widely possibly up to 74% of 3 bolted fault
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© 2001 Cooper Bussmann, Inc.
Arcing Faults – Line to Ground
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
Can vary widely
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© 2001 Cooper Bussmann, Inc.
Arcing Faults – Sustainability
ABC
GroundN
480Y/277 V3 / 4WSolid Grd
Rule of thumb:Arcing faults will typically not sustain
at less than 38% of 3 bolted fault
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© 2001 Cooper Bussmann, Inc.
How do you know what the short-circuit current is throughout a system?
andWhat are some typical values
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© 2001 Cooper Bussmann, Inc.
MM
MSB
Short Circuit Currents Vary Depending on Many
Factors
Transformer Size & % ZVoltageConductor Size & Length
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© 2001 Cooper Bussmann, Inc.
MM
MSB
Short Circuit Currents Vary Depending on Many
Factors
60,000 A
27,000 A40,000 A
60,000 A
18,000 A
9,000 A
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© 2001 Cooper Bussmann, Inc.
500 KVA
480/277V
1
5 % Z
500 KVA
480/277V
1A
2 % Z
1500 KVA
480/277V
2
5% Z
1500 KVA
480/277V
2A
2 % Z
Short Circuit Current Examples #1
1500 KVA
208/120V
3
5% Z
1500 KVA
208/120V
3A
2 % Z
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© 2001 Cooper Bussmann, Inc.
How to Calculate Transformer Secondary (assuming infinite primary)
Isca = (Xfmr FLA) x 100 / %Z
(increase result by 10% due to UL
tolerance for transformer impedances)
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© 2001 Cooper Bussmann, Inc.
500 KVA
480/277V
1
5 % Z
500 KVA
480/277V
1A
2 % Z
13,222 A
33,055 A
1500 KVA
480/277V
2
5% Z
1500 KVA
480/277V
2A
2 % Z
39,666 A
99,165 A
Short Circuit Current Example #1 Answers
1500 KVA
208/120V
3
5% Z
1500 KVA
208/120V
3A
2 % Z
91,608 A
229,020 A
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© 2001 Cooper Bussmann, Inc.
208/120V
4
4A
6
6A
480/277V
5
5A
480/277V
40,000 A. 40,000 A.40,000 A.
50 ft # 1 50 ft 250 kcm
Short Circuit Current Example #2
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© 2001 Cooper Bussmann, Inc.
Calculating Short Circuit Currents
Utilize Point-to-Point MethodSteps 4, 5 & 6 for 3 Faults
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© 2001 Cooper Bussmann, Inc.
Short Circuit Current Example #2 Answers
208/120V
4
4A
40,000 A.
50 ft # 1
12,367 A.
480/277V
5
5A
40,000 A.
20,322 A.
6
6A
480/277V
40,000 A.
50 ft 250 kcm
28,818 A.
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© 2001 Cooper Bussmann, Inc.
Summary / Questions / To come• Overcurrent – overloads, bolted faults, & arcing
faults• Fault currents can be determined through out
distribution system• Short circuit currents needed to assess I.R.,
Component Pro, Coordination, Arc Flash Hazards & OCPD Selection