Fault Current Calculations
Fault current is electrical current that is released during a system fault within the electrical system. There are many different causes of these types of faults, but with any fault there is unintentional, large magnitudes of current that is released. This release of electrical current can cause equipment damage, production loss, and personnel injury.
- The National Electric Code (NEC) 110.24(A) states:
"Service equipment in other than dwelling units shall be legibly marked in the field with the maximum available fault current. The field marking(s) shall include the date the fault current calculation was performed and be of sufficient durability to withstand the environment involved."
- Consequently NEC 110.24(B) covers any change to the power system that might have an adverse affect on the fault current:
"When modifications to the electrical installation occur that affect the maximum available fault current at the service, the maximum available fault current shall be verified or recalculated as necessary to ensure the service equipment ratings are sufficient for the maximum available fault current at the line terminals of the equipment. The required field marking(s) in 110.24(A) shall be adjusted to reflect the new level of maximum available fault current."
Fault current calculations are based simply on the theory of Ohm's Law. Ohm's Law is a simple formula relationship of Voltage, Current, and Resistance. The formula is V=IR and can be shuffled in any any mathematical arrangement such as I=V/R, or R=V/I. The formula is used to calculate any of the three elements, Voltage, Current, of Resistance based upon knowing the other two elements. During a system fault the resistance at the fault becomes very small causing the current to grow exponentially. A typical 200A circuit could carry 10kA to 20kA or even higher base upon the system conditions.
In order to utilize Ohm's Law, a system must be minimized to a basic circuit composed of a Voltage source, Resistance, and a Current. The process for minimizing a circuit to this basic form is known as network reduction. Network reduction is the process of combining, and minimizing complex circuit components to their most basic form. In the end this network reduction process leaves a circuit known as a Thevenin Equivalent circuit.
There are several types of faults that can take place in an electrical power system:
- 1. Three-Phase Fault - Otherwise known as Line-to-Line-to-Line faults are the most rare types of faults to take place, but are also the most widely calculation used for choosing protective devices and settings, as well as choosing electrical equipment. Three-phase faults typically provide a user with the worst case fault value.
- 2. Ground Fault - Otherwise known as Line-to-Ground faults are probably the most typical types of faults. This is when there is a fault between a single-phase conductor and to ground.
- 3. Double Line - Otherwise known as Line-to-Line faults are faults that take place between two separate phase conductors.
- 4. Double Line Ground Fault - Otherwise known as Line-to-Line-to-Ground faults take place between two-phase conductors and ground.
Three-phase faults typically provide the most conservative fault results, and are used in the selection and application of electrical equipment. Electrical equipment must be selected and applied in a manner minimizes equipment reliability, production loss, and chance of personnel injury.
When calculating fault current for any type of fault or in any type of electrical system all sources of current contribution must be identified and included in the analysis process. Below is a list of equipment and current sources that must be considered:
- Electric Utility Service Provider - The main source of fault current in any facility comes from the local utility provider. The magnitude of the fault takes into account the location of the facility on the power grid as well as the size of the utility transformer supplying power to your facility.
- Onsite Generation - Many facilities have backup power generation for critical loads. There are also facilities that have onsite generation as part of their process in order to cut utility costs, or just recoup heat generated within the facility.
- Motors - There are two types of motors that generally contribute to a fault; synchronous, and induction type. Motors are not typically known to produce electrical current, but during a faulted situation the motors act as a generator for several electrical cycles.
Fault current calculation are performed on a worst case scenario. These types of faults only take into account the characteristics of the power system and consider the fault itself to be a zero impedance fault. These are typically referred to as bolted faults. The though is essentially that you are taking the fault type such as a three phase and are literally bolting the three phases together and measuring the current flow. Bolted faults can and do occur in power systems but more common fault types are arcing faults. The principles aforementioned still apply though the fault current is flowing through air which has a specific impedance.
Some arcing fault currents can be as much as 50% the bolted fault value due to the added impedance into the circuit.