Fault Current Contribution

Sinexcel has received inquiries as to the relationship of fault current contributed and kilo-Amperes Interrupting Capacity (kAIC) ratings.

With regard to the fault current contribution of Sinexcel inverters, the contribution from the inverters is far less than the kAIC rating of any circuit breaker.

Only circuit breakers, fuses, and some load-rated switches have AIC ratings. As a current source, a utility interactive inverter does not, by itself, require an AIC rating.

Some utility Electrical Services Requirements state that the Short-Circuit current and Over-Current protection is to be based on the type of service that is provided to the system (PCC). For instance, for residences, duplexes, and mobile homes supplied at 120/240 volts, one phase, three wire:

• 0-225A Service Ampacity the Short-Circuit Current rating (AIC) required is 10kA.
• 226-400A Service Ampacity, the Short-Circuit Current rating (AIC) required is 22kA.

Utilities are concerned with the kAIC rating of the main breaker for two reasons:

• First, will it interrupt the current when a fault is present on the residential (load) side of the breaker?
• Secondly, will it interrupt the current supplied by the Distributed Generator (DG) when the short is located on the grid side? Is it possible for the DG to deliver more current than the circuit breaker can interrupt?

In Understanding Fault Characteristics of Inverter-Based Distributed Energy Resources, a National Renewable Energy Laboratory study published in January 2010, Keller & Kroposki address fault current contributions of Distributed Energy Resources (DERs) and inverters in particular:

“Inverters do not dynamically behave the same as synchronous or induction machines. Inverters do not have a rotating mass component; therefore, they do not develop inertia to carry fault current based on an electro-magnetic characteristic. Power electronic inverters have a much faster decaying envelope for fault currents because the devices lack predominately inductive characteristics that are associated with rotating machines. These characteristics dictate the time constants involved with the circuit. Inverters also can be controlled in a manner unlike rotating machines because they can be programmed to vary the length of time it takes them to respond to fault conditions. This will also impact the fault current characteristics of the inverter.”