Short Circuit Current Examples #1 1500 KVA

Suggestion on How to Use
• Industry Trainers are encouraged to use this
material in their sessions
• Download the presentation file
• Print the Notes pages and read them as you
view the presentation in the “Slide Show”
view. In this way you see the slides in large
format and have animation (when available)
© 2015 Eaton. All Rights Reserved..
1
Types of Overcurrent
&
Calculations
© 2015 Eaton. All Rights Reserved..
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.
© 2015 Eaton. All Rights Reserved..
3
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.
© 2015 Eaton. All Rights Reserved..
4
Normal Load
IL= VS / RT
IL
VS
R
Source
R
R
Load
IL
R
R
R
R
© 2015 Eaton. All Rights Reserved..
5
Overload
IOL= VS / RT
VS
R
R
Source
R
Load
R
R
R
R
Within the normal path
© 2015 Eaton. All Rights Reserved..
6
Short Circuit
ISC= VS / RT
VS
R
R
Source
R
Load
R
R
R
R
Out of the circuit
Outside the normal path
© 2015 Eaton. All Rights Reserved..
7
Types of Faults
• Bolted Faults
• Arcing Faults
© 2015 Eaton. All Rights Reserved..
8
Bolted
Short Circuit
Arcing
Fault
Current Thru
Air
A
B
© 2015 Eaton. All Rights Reserved..
A
B
9
Bolted Faults –Various Types
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
© 2015 Eaton. All Rights Reserved..
10
Bolted Faults – Three Phase
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Typically considered the “worst case” or highest
magnitude
© 2015 Eaton. All Rights Reserved..
11
Bolted Faults – Line to Line
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
87 % of the three phase bolted fault
© 2015 Eaton. All Rights Reserved..
12
Bolted Faults – Line to Ground
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Typically much lower than 3
 fault, but can be > 3
fault near Xfmr terminals
© 2015 Eaton. All Rights Reserved..
13
Arcing Faults – Many Variables Affect Current
& Whether It Is Sustainable
System Voltage
Gap spacing
Available 3
 Short Circuit Amps
Amount of Copper Vaporized
Degree of Containment
Configuration of Equipment
Typically does not sustain on 208Y/120V
A
© 2015 Eaton. All Rights Reserved..
B
14
Arcing Faults – Progression
A
480Y/277 V
B
3 / 4W
Solid Grd
C
N
Ground
© 2015 Eaton. All Rights Reserved..
15
Arcing Faults – Three Phase
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Can vary widely possibly up to 89% of 3

bolted fault
© 2015 Eaton. All Rights Reserved..
16
Arcing Faults – Line to Line
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Can vary widely possibly up to 74% of 3

bolted fault
© 2015 Eaton. All Rights Reserved..
17
Arcing Faults – Line to Ground
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Can vary widely
© 2015 Eaton. All Rights Reserved..
18
Arcing Faults – Sustainability
A
480Y/277 V
3
 / 4W
B
C
Solid Grd
N
Ground
Rule of thumb:
Arcing faults will typically not sustain at less than
38% of 3
© 2015 Eaton. All Rights Reserved..
 bolted fault
19
How do you know what the short-circuit current
is throughout a system?
and
What are some typical values
© 2015 Eaton. All Rights Reserved..
20
Short Circuit Currents Vary
Depending on Many Factors
Transformer Size & % Z
Voltage
Conductor Size & Length
MSB
M
M
© 2015 Eaton. All Rights Reserved..
21
Short Circuit Currents Vary
Depending on Many Factors
60,000 A
60,000 A
MSB
40,000 A
18,000 A
27,000 A
9,000 A
M
M
© 2015 Eaton. All Rights Reserved..
22
Short Circuit Current Examples #1
500 KVA
1500 KVA
5%Z
5% Z
480/277V
1
1500 KVA
5% Z
208/120V
480/277V
2
3
500 KVA
1500 KVA
1500 KVA
2%Z
2%Z
2%Z
480/277V
1A
480/277V
2A
© 2015 Eaton. All Rights Reserved..
208/120V
3A
23
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)
© 2015 Eaton. All Rights Reserved..
24
Short Circuit Current Example #1 Answers
500 KVA
1500 KVA
5%Z
5% Z
480/277V
5% Z
208/120V
480/277V
13,222 A
1
1500 KVA
39,666 A
2
91,608 A
3
500 KVA
1500 KVA
1500 KVA
2%Z
2%Z
2%Z
480/277V
480/277V
33,055 A
1A
208/120V
99,165 A
2A
© 2015 Eaton. All Rights Reserved..
3A
229,020 A
25
Short Circuit Current Example #2
208/120V
4
480/277V
40,000 A.
5
480/277V
40,000 A.
6
50 ft # 1
4A
40,000 A.
50 ft 250 kcm
5A
© 2015 Eaton. All Rights Reserved..
6A
26
Calculating Short Circuit Currents
Utilize Point-to-Point Method
Steps 4, 5 & 6 for 3 Faults
© 2015 Eaton. All Rights Reserved..
27
Short Circuit Current Example #2 Answers
208/120V
4
480/277V
40,000 A.
5
480/277V
40,000 A.
6
50 ft # 1
4A
50 ft 250 kcm
5A
12,367 A.
40,000 A.
6A
20,322 A.
© 2015 Eaton. All Rights Reserved..
28,818 A.
28
Summary / Questions / To come
• Overcurrents – overloads, bolted faults, & arcing
faults
• Fault currents can be determined throughout
distribution system
• Available short circuit currents are needed to
assess I.R., short-circuit current ratings, selective
coordination, arc flash hazards & OCPD selection
© 2015 Eaton. All Rights Reserved..
29
© 2015 Eaton. All Rights Reserved..
30