National Electrical Code

Abstract: Arc faults can produce very high temperatures and can easily ignite combustible materials; thus, they represent one of the most important causes of electrical fires. The application of arc fault detection, as an emerging early fire detection technology, is required by the National Electrical Code to reduce the occurrence of electrical fires. However, the concealment, randomness and diversity of arc faults make them difficult to detect. To improve the accuracy of arc fault detection, a novel arc fault detector (AFD) is developed in this study. First, an experimental arc fault platform is built to study electrical fires. A high-frequency transducer and a current transducer are used to measure typical load signals of arc faults and normal states. After the common features of these signals are studied, high-frequency energy and current variations are extracted as an input eigenvector for use by an arc fault detection algorithm. Then, the detection algorithm based on a weighted least squares support vector machine is designed and successfully applied in a microprocessor. Finally, an AFD is developed. The test results show that the AFD can detect arc faults in a timely manner and interrupt the circuit power supply before electrical fires can occur. The AFD is not influenced by cross talk or transient processes, and the detection accuracy is very high. Hence, the AFD can be installed in low-voltage circuits to monitor circuit states in real-time to facilitate the early detection of electrical fires.

Abstract: This suggested practices manual examines the requirements of the National Electrical Code (NEC) as they apply to photovoltaic (PV) power systems. The design requirements for the balance of systems components in a PV system are addressed, including conductor selection and sizing, overcurrent protection ratings and location, and disconnect ratings and location. PV array, battery, charge controller, and inverter sizing and selection are not covered, as these items are the responsibility of the system designer, and they in turn determine the items in this manual. Stand-alone, hybrid, and utility-interactive PV systems are all covered.

Abstract: National Fire Protection Association 70E, standard for electrical safety in the workplace is a consensus standard approved as an American national standard. Whereas the NFPA 70 National Electrical Code is primarily an installation standard for premise electrical systems, focuses on electrical safety requirements to protect individuals working on or near electrical equipment. This article will discuss some of the key aspects of the 2004 Edition of NFPA 70E, with a focus on safety-related work practices. In addition, this article also presents some of the potential directions for requirements in future editions of NFPA 70E.

Abstract: Photovoltaic (PV) systems have caused residential and commercial building fires when an electrical arc fault initiates in the conduction path. Article 690.11 in the United States 2011 National Electrical Code requires new photovoltaic systems on or penetrating a building to include a listed arc fault protection device to prevent additional fires. In response, manufacturers are creating arc fault circuit interrupters (AFCIs) using electrical frequencies for detection, but their operation is not fully characterized. Sandia National Labs has undergone a major effort to identify detection difficulties and establish tests for PV AFCI manufacturers to ensure their product can robustly detect arcing conditions while avoiding false trips from noise sources. In previous studies, arc fault signatures have been compared to string noise and frequency-dependant attenuation through PV modules has been quantified. In this paper, a frequency response analyzer was used to measure radio frequency (RF) propagation through arrays of varying irradiance and size. Irradiance did not affect module frequency response, but the length of unshielded wiring significantly affected the frequency response of the system above 100 kHz due to RF effects. Based on the RF affects in PV systems, it is recommended that arc fault circuit interrupter manufacturers select detection frequencies below 100 kHz.