Maximum Available Fault Current: What is it?
What is maximum available short circuit current?
What is a short circuit fault current?
How do you calculate fault current?
What effects do fault currents have on arc flash?
Below I will answer these questions about fault currents and more!
Let’s jump right in!
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What is maximum available short circuit current?
Maximum available fault current, or maximum available short circuit current, is the maximum current available should there be a short circuit, such as a ground fault, which could cause an arc flash. Fault currents are common design constraints for any generator or load, including your own home! To protect critical circuitry in a system, breakers are commonly used. You might recall that time you tried to turn on too many devices in the same socket with an extension cable and suddenly nothing is working. This is because when a combined load draws too much current and the circuit breaker in your panel box will trip to protect the internal wires in your house for that branch. Although this scenario is not a fault, the principle is similar that the flowing current is different from normal circumstances, causing the breaker to act to prevent further damage.
What is kAIC rating
The maximum current a breaker can interrupt without damaging itself is called the Kilo Ampere Interrupting capacity or kAIC for short. This rating is key in selecting breakers, as its paramount to provide adequate current breaking power to protect critical system circuitry. NEC section 409.22 defines that panels cannot be installed where the possible short circuit exceeds the short circuit rating. Breakers should be selected with a proper kAIC rating to account for the possibility of a max short circuit current.
Short circuit fault currents
There are many types of fault currents, but short circuit fault current is the most common and, simultaneously, the most destructive current if left unchecked as current. NEC defines short circuit fault current as: ‘The largest amount of current capable of being delivered at a point on the system during a short-circuit condition’. Short circuits can be caused in many cases with the typical cases being:
- Line to ground (aka 1 phase to ground)
- 3 phase to ground (line to line to line to ground)
- Line to line
- Line to line to ground
Line to ground being the most common occurrence of all faults (approximately 85%) but are the least severe fault compared to the others. Nevertheless, the potential energy and associated arc released by these types of faults are still life threatening and should always be calculated and accounted for protection wise, with proper equipment selected. Fault currents that flow through equipment can be caused by all sorts of internal and external factor with examples below as:
- Water leaking into an electrical box
- Lose connections inside a junction box
- Insulation degradation between phases
- Tree falling onto live power lines.
In order to minimize short circuit occurrences, damage or injuries, the following preventative steps can be taken:
- Install relays with current transformers (CT) to detect fault currents and coordinate in tandem with circuit breakers to protect critical equipment, particularly:
- 50/51 which are instantaneous and overcurrent protection respectively
- 87 which is a distance relay which is utilized to communicate with:
- primary and secondary of a transformer
- ends of a transmission line (or in between depending on how critical the transmission system is) to detect downstream faults.
- Perform annual circuit breaker maintenance to ensure breakers are properly acting.
- Updating electrical receptacles every 15 years.
- Perform external inspection of equipment housing/boxes/connections on regular interval.
- Have an up to date one line with an accompanying short circuit study to verify potential. maximum fault currents for safety labels.
- Perform an arc flash study to obtain exact PPE requirements and safety labels.
How to calculate fault current
Prior to explaining the mathematics related to fault currents, it is important to note that although the short circuit current magnitude is important, the time it takes to clear such a fault is also very important, particularly with incident energy and arc flash. The simplest way of calculating fault current in a system is highlighted in IEEE 551-2006 (Violet Book) as:
Where I_f is the fault current and E is the voltage and Z is the impedance. Theoretically, this implies that a current traveling through a solid line with barely any resistance would have a current close to infinity. In reality, there is some inherent resistance or impedance which causes the current to not be infinite, but in the thousands of amps and many times greater than the normal rated current of the system.
Transmission Line example
Considering AC sources with components like a transmission line which has its own Impedances and ratings, the fault calculation becomes (figure with source and impedance source):
Where I_fs is the steady state component as:
And I_ft is the transient component as:
With Z and θ calculated as:
Below is an example of a fault calculation for a transmission line fault with an ideal source
Figure 1:Sample TL short circuit calculation
Since the impedance is relatively high in this example, the current transient component of the current will not be very high and die out pretty quickly. In this example, when the time reaches 0.3 seconds, the transient component would be:
Which is significantly less than the rated current of 56.19A.
Transformers are key components of electrical systems and their respective fault currents pose a significant risk to an electrical branch, as well as potential arc flashes. IEEE 551-2006 states that the fault current through transformers is:
Where MVA_base is the transformer MVA rating, kV is the secondary base kV rating and Z is the combined impedance of primary bus and the transformer impedance. Example shows a typical example for a tap changing transformer where the tap does not match the base kV and the secondary does not match the base kV rating:
Figure 2:Sample transformer short circuit calculation
Fault Currents in Arc Flashes
Up to now we have established that fault currents can be severe and detrimental to the physical system. However, a more hazardous situation also arises from potential fault currents and that is the arcing hazards they may pose to workers that are working on these potentially live systems. In order to keep these workers safe certain protective measures must be taken. Personal Protective Equipment (PPE) is vital to worker safety but to know which category to use it must be determined. One method is using the table method. The table method is employed in NFPA 70E (US) as:
- Table 130.7(C)(15)(a) for AC systems
- Table 130.7(C)(15)(b) for DC systems
- appendix H
- tables 6A through 6C
Available fault current labelling Proper labelling will be instrumental in providing key information to personnel and operators working in and around electrical equipment. Always ensure that your equipment is properly labelled with proper ratings listed. Equipment labels for transformers should comply with NEMA, ANSI Z535 and NESC Section 110A standards and include the following labels:
- MVA rating
- Maximum current rating
- Rated voltages on primary and secondary
- Basic Insulation Level (BIC)
- Tap settings (if applicable)
- If cooled:
- Fan type and configuration
- Temperature settings
- If liquid filled, the:
- fill type
- temperature ratings
- Pressure rating
- Phasor diagram indicating the configuration of primary and secondary (delta-wye, delta-delta, etc.) with grounding scheme clearly labelled
- Weight of transformer components
- Reference standard
Circuit breakers should also meet NEC compliance and have the following labelled if applicable:
- Rated voltage
- Rated current
- Surge Voltage rating
- Short circuit current
- Short circuit breaking current (kAIC)
- Operating sequence
- Circuit breaker weight
- Reference standard
- If gas operated:
- Gas pressure
- Gas mass
Annex Q CSA Z462 discussed and highlights procedures for labeling arc flash hazards and shock protection. The minimum arc flash label requirement per CSA 22.1-18 is:
Figure 3:CSA Arc Flash label template for CSA 22.1 Requirements
Whereas the CSA Z462 recommends that the label look something more like:
Figure 4:CSA Z462 Recommended ARC Flash label structures
Which indicates important Arc flash and shock hard information on the left- and right-hand side respectively.
I hope this article has helped to better explain available fault current.
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