Short Circuit and Arc Flash

Question 1

What is arc flash?

Answer 1

Arc flash can be defined as short circuit in the air.

Question 2

What are the four categories of arc flash PPE and their minimum arc rating?

Answer 2

PPE Category 1: Minimum Arc Rating 4cal/cm2
PPE Category 2: Minimum Arc Rating 8 cal/cm2
PPE Category 3: Minimum Arc Rating 25 cal/cm2
PPE Category 4: Minimum Arc Rating 40 cal/cm2

Question 3

What are the required layers of PPE for Arc Flash PPE Category 1?

Answer 3

PPE Category 1: Minimum Arc Rating 4cal/cm2
Required Clothing: Long sleeve shirt and pants or AR coverall with minimum arc rating of 4 cal/cm2
Required Face and Head Protection: Face shield or Arc Flash Suit Hood
Required Hand Protection: Heavy-Duty Leather Gloves
Additional PPE: Hard Hat, Eye Protection, Hearing Protection
Footwear: Leather Footwear (as needed)

Question 4

What are the required layers of PPE needed for Arc Flash PPE Category 2?

Answer 4

PPE Category 2: Minimum Arc Rating 8 cal/cm2
Required Clothing: Arc Rated Long sleeve shirt and pants or Arc Rated Coverall with minimum arc rating of 8 cal/cm2
Required Face and Head Protection: Arc Rated Arc Flash Suit Hood or AR Face shield
Required Hand Protection: Heavy-Duty Leather Gloves
Additional PPE: Hard Hat, Eye Protection, Hearing Protection
Footwear: Leather Footwear (as needed)

Question 5

What are the required layers of PPE for Arc Flash PPE Category 3?

Answer 5

PPE Category 3: Minimum Arc Rating 25 cal/cm2
Required Clothing: Arc Rated Flash Suit Jacket and AR pant or AR coverall with minimum arc of 25 cal/cm2
Required Face and Head Protection: Arc Rated Arc Flash Suit Hood with minimum arc rating of 25 cal/cm2
Required Hand Protection: Rubber insulating gloves and leather protectors or arc rated gloves
Additional PPE: Hard Hat, Eye Protection, Hearing Protection
Footwear: Leather Footwear

Question 6

What are the required layers of PPE for Arc Flash PPE Category 4?

Answer 6

PPE Category 4: Minimum Arc Rating 40 cal/cm2
Required Clothing: Arc Rated Flash Suit Jacket and AR pant or AR coverall with minimum arc of 40 cal/cm2
Required Face and Head Protection: Arc Rated Arc Flash Suit Hood with minimum arc rating of 40 cal/cm2
Required Hand Protection: Rubber insulating gloves and leather protectors or arc rated gloves
Additional PPE: Hard Hat, Eye Protection, Hearing Protection
Footwear: Leather Footwear

Question 7

Why is selective coordination important in an electrical system?

Answer 7

Selective coordination can help insure continuity of service. A coordinated system is one where only the faulted circuit is isolated without disturbing any other part of the system.

Question 8

Sources of short-circuit current that are normally taken into consideration include:

Answer 8

• Utility Generation
• Local Generation
• Synchronous Motors
• Induction Motors
• Alternate Power Sources

Question 9

Short circuit calculations should be done at all critical points in the system. These would include:

Answer 9

• Service Entrance
• Panel Boards
• Motor Control Centers
• Motor Starters
• Transfer Switches
• Load Centers
• Disconnects
• Motor Starters

Question 10

What is a bolted 3-phase bolt condition?

Answer 10

This can be categorized as all 3-phases “bolted” together to create a zero impedance connection. This establishes a “worst case” (highest current) condition that results in the maximum three phase thermal and mechanical stress in the system. Typically used for short circuit studies.

Question 11

True or False

Short circuit calculations are performed with current-limiting devices in the system.

Answer 11

False.
Short circuit calculations are performed without current-limiting devices in the system. Calculations are done as though these devices are replaced with copper bars, to determine maximum “available” short-circuit current.

Question 12

What are the steps for a basic point-to-point calculation used to determine short circuit calculations?

Answer 12

1. Determine transformer FLA
2. Find the transformer multiplier (Multiplier = 100/%Z) (%Z is transformer impedance)
3. Determine transformer let-through short-circuit current.
4. Calculate “f” factor
5. Calculate “M” multiplier (M = 1/(1+f))
6. Calculate available short circuit symmetrical RMS current at the point of fault. Add motor contribution, if applicable
   Isc,rms = Isc x M

6a. Motor short circuit contribution, if significant, may be added at all fault locations throughout the system. A practical estimate of motor short circuit contribution is to multiply the total motor current in amps by 4. Values of 4 to 6 are commonly accepted.

Question 13

In point-to-point short circuit calculations, how do you determine the transformer let through short circuit current?

Answer 13

Isc = FLA x multiplier

Multiplier = 100/%Z 
(Z = transformer impedance)

For total short-circuit at this point, motor contributions should be added, an estimate can be 4 x the total motor current in amps.

Question 14

In point-to-point short circuit calculation, how do you calculate the “f” factor?

Answer 14

For 3phase faults
f = [1.732x L x (Available 3-phase short circuit current)] / [C x n x E]

For 1phase line to line faults
f = [2 x L x (available line to line short circuit current)] / [C x n x E]

For 1phase line to neutral faults
f = [2 x L x (available line to neutral short circuit current)] / [C x n x E]

L = length
C = constant value "C" for conductors and busways. (This is basically the one over the impedance per foot value based upon resistance and reactance values found in the IEEE Gray Book)
n = number of conductors per phase
E = voltage of circuit

Question 15

What is the difference between a short circuit calculation using a utility source vs a generator?

Answer 15

At the utility source you might not know the total available fault current so an infinite bus condition is typically used for the calculation, in which case the available fault being calculated is based on the available fault current that is let through the secondary side of the utility transformer which is impacted by the transformer impedance. With a generator, if you have the generator specifications, you’re able to determine the available fault that the generator provides which can be impacted by the generator sub transient reactance rating (Xd).

Question 16

For the kVA method of short circuit calculations, how would you calculate short circuit ratings for generators, motors and transformers?

Answer 16

The kVA method determines short circuit current in terms of kVA.
Motors short circuit is equal to its starting inrush current.
Generators short circuit is equal to its kVA divided by the nameplate subtransient reactance (Xd).

For transformers with an infinite bus flowing through the primary side:
short circuit = (Transformer kVA) / (transformer impedance)

For transformers with a known value flowing throw the primary side:
short circuit = 1 / [ (1/known value) + (1 / ((transformer kva) / (transformer impendance)) ) ]

Question 17

How would you reduce available fault current?

Answer 17

• Use reactors (which add inductive reactance to the system impendence which will reduce the fault currents)
• Use isolation transformers
• Increase cable lengths between source
• Use current limiters

Question 18

How would you reduce arc flash incident energy?

Answer 18

• Trying to clear the fault as quickly as possible without sacrificing coordination, since the longer the clear time, the larger the incident energy. This can be done via current-limiting fuses or other breaker settings.
• Can use zone selective interlocking to try to clear the fault quicker while maintaining coordination. Its so that breakers don’t wait to clear the fault due to coordination, and just trip if the signals are present.
• Employing differential protection. Its where zones of protection are set up using differential relays and faults. If a fault occurs within the zone, the relay trips at extremely low pickups and time delays.
• Switchgear with arc flash venting

Final note, arc flash incident energy might be lowered via work procedures or design that:
1) lower the fault current
2) increase the working distance (such as using chicken switches to switch gear, or remote racking devices)
3) Reduce device fault clear times
Also, IR windows can be helpful since electricians can scan important locations for hot spots without having to open the equipment covers or doors.

Question 19

What is short circuit current?

Answer 19

Short circuit current is what happens when two or more conductors of different phases come into contact with each other and shunts out a part of the impedance, and a large portion of current ends up flowing through the un-faulted phases. You end up having a large flow of current which might either overheat equipment or destroy and damage equipment due to the force of electrodynmics interaction in the flow of short circuit current.

Question 20

What is differential protection?

Answer 20

Differential protection is a unit-type protection for a specified zone or piece of equipment. It is based on the fact that is only in the case of faults internal to the zone that the differential current (difference between input and output currents) will be high.