# Short Circuit and Arc Flash Flashcards

What is arc flash?

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

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

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

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

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)

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

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)

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

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

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

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

Why is selective coordination important in an electrical system?

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.

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

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

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

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

What is a bolted 3-phase bolt condition?

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.

True or False

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

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.

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

- Determine transformer FLA
- Find the transformer multiplier (Multiplier = 100/%Z) (%Z is transformer impedance)
- Determine transformer let-through short-circuit current.
- Calculate “f” factor
- Calculate “M” multiplier (M = 1/(1+f))
- 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.

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

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.

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

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

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

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).