LLL (Line-Line-Line)
A balanced three-phase fault where all three phases are shorted together, typically producing the maximum available symmetrical fault current.
Services
Short Circuit Analysis
Short Circuit
A short circuit study determines the available current that the power system can deliver into faults, and then compares those duties to equipment ratings. This determines if breakers, switchgear, switchboards, panelboards, transfer switches, and other equipment are being operated within their rated capabilities.
What It Is
What is it?
A short circuit can occur between phases, from phase to ground, from phase to neutral, or in multi-phase combinations. The system impedance from the source to the fault location drives the magnitude and X/R ratio of the fault current.
Faults can be caused by insulation failure, loose or damaged conductors, contamination, water intrusion, equipment deterioration, failed components, incorrect installation, or foreign conductive material.
Fault Types
Fault types
LL (Line-Line)
A fault between two phase conductors, without involvement of ground.
SLG (Single-Line-to-Ground)
A fault where one phase conductor is connected to ground or grounded equipment.
LLG (Line-Line-Ground)
A fault where two phase conductors are shorted together and connected to ground.
Equipment Duty
Equipment ratings and available fault current
NEC 110.9 and 110.10 state that interrupting devices and equipment short-circuit current ratings must be adequate for the available fault current. NEC 110.24 also requires available fault current marking at service equipment in many non-dwelling applications.
If the available fault current exceeds an equipment rating, the result can include breaker failure, switchgear or switchboard damage, bus bracing failure, enclosure rupture, fire, personnel exposure, or even more severe cascades of events. The study identifies these overduties such that it can be addressed to lessen the possibility of such event occurring.
Commonly evaluated equipment ratings
- Interrupting or AIC rating
- Momentary, close-and-latch, and making duty where applicable
- Short-time withstand for low-voltage power circuit breakers and switchgear
- UL 1558 and UL 891 withstand time durations of 30 cycles and 3 cycles respectively (evaluated in coordination study)
- Overall withstand or UL listed rating
- X/R and asymmetrical current effects
Standards
Calculation methods and standards
ANSI-based short-circuit software evaluates fault current using ANSI/IEEE C37 procedures, including calculation of the fault-point X/R ratio. The X/R ratio is commonly determined using separate resistance and reactance network reductions and is used to account for DC offset when evaluating asymmetrical, peak, momentary, and interrupting duties against applicable equipment ratings.
ANSI/IEEE C37.010
Used for application of AC high-voltage circuit breakers above 1000 V rated on a symmetrical-current basis.
IEEE C37.13
Covers low-voltage AC power circuit breakers used in enclosures, including rating structures used to evaluate interrupting, withstand, and short-time capability.
ANSI/IEEE C37.5
A withdrawn legacy guide for AC high-voltage breakers rated on a total-current basis. It is relevant when older breakers were not rated under modern symmetrical-current methods.
Study timing basis
Low-voltage short circuit duties are typically evaluated at 1/2 cycle from fault inception, while medium and high voltage breaker duties normally include both a 1/2 cycle momentary calculation and an interrupting calculation tied to the breaker's contact parting time.
X/R Ratio
X/R Ratio
The X/R ratio determines the amount of DC offset in the fault-current waveform and directly affects the resulting asymmetrical and peak currents. Circuit breakers are tested at specified X/R ratios in accordance with applicable UL standards. If the calculated system X/R ratio exceeds the breaker's tested X/R ratio, the resulting asymmetrical or peak fault current may exceed the breaker's tested capability, even when the calculated symmetrical fault current is below the breaker's symmetrical interrupting rating.
In these cases, the calculated interrupting duty must be adjusted to verify that the equipment's tested asymmetrical capability is not exceeded.
Cases that need extra attention.
Generator contributions based on subtransient reactance, X"d may look acceptable from a maximum symmetrical fault-current perspective while producing a high X/R ratio near the generator. The elevated X/R ratio can result in large multiplying factors and create overduties if the scenario is not modeled. Equipment near large transformers or areas with significant motor contributions can similarly have elevated X/R ratios.
Study Inputs
What goes into a short circuit calculation
Source data
Collect utility maximum and minimum fault current, X/R ratio when available, service transformer data, generator data, and source operating assumptions.
Impedance modeling
Model transformer impedance, cable and busway impedance, reactors, grounding, motors, generators, and other meaningful system contributors.
Equipment ratings
Record breaker AIC, switchgear and panel SCCR, short-time ratings, close-and-latch ratings, fuse ratings, voltage ratings, and legacy breaker information.
Operating scenarios
Evaluate practical lineups, alternate sources, generators paralleled with utility, tie positions, closed-transition transfers, and other cases that can create worst-case duty.
Sequence
Study sequence
Protective device coordination and arc flash calculations rely on a valid and accurate short circuit model. The equipment must first be shown to be properly rated for the available fault current before TCCs and arc flash hazard calculations can be relied on.
Supporting Deliverables
- Available fault current reports
- Equipment Evaluation/Equipment duty reports
- Summary of overdutied equipment in report write-up
- Recommendations to address overdutied equipment if requested as an add-on
References
Standards and code sections commonly considered
- IEEE C37.010 - high-voltage AC circuit breaker application on a symmetrical-current basis.
- IEEE C37.13 - low-voltage AC power circuit breakers used in enclosures.
- ANSI/IEEE C37.5 - withdrawn total-current-basis guide for legacy high-voltage circuit breakers.
- NEC 110.9, 110.10, and 110.24 - interrupting rating, equipment short-circuit current rating, and available fault current marking requirements, subject to the edition adopted by the authority having jurisdiction.
- Equipment rating references may include applicable UL standards such as UL 67, UL 98, UL 489, UL 857, UL 891, UL 1008, UL 1558, UL 1778, UL 845, UL 924, UL 508A, and UL 508C/UL 61800-5-1 (as applicable for drives by equipment listing and vintage).