The unique nature of photovoltaic (PV) system power generation necessitates the need for new and effective electrical protection products for overcurrent, overvoltage and isolation events. With a protected electrical system, renewable power generation capacity can be optimized and enhance safety and equipment protection.
Solar disconnects have an essential safety role for PV systems, providing a way to isolate equipment for essential maintenance or in the case of catastrophic events. In a grid-tied PV system, the National Electrical Code (NEC) 690.15 requires disconnects to be used on both the direct current (DC) and alternating current (AC) side of the inverter. The Underwriters Laboratories (UL) 98 rated DC disconnect switches can be installed inside the inverter enclosure, attached to the inverter enclosure, or (more typically) in the line of sight of the inverter in order to disconnect the power connection to the modules. The UL98 rated AC disconnect is used to disconnect the inverter from the power coming into the home or building and the electrical grid.
Select the right device: refer to the appropriate codes
In order to select the most appropriate disconnect devices, understanding the relevant codes and standards is crucial. Unlike typical grid-connected AC systems, the available short-circuit current within PV systems is limited, and the overcurrent protective devices (OCPDs) need to operate effectively on relatively low changes in current levels.
For this reason, PV fuses, circuit breakers and disconnects should be specifically designed and tested to protect PV systems with high DC voltages and low fault currents.
NEC 2014 Article 690 addresses PV systems in order to enhance safety and protect property. Article 690 addresses “Solar Photovoltaic Systems” and provides definitions, installation guidelines, circuit guidelines and more. It also mandates that permanent labels effectively convey the hazard and are designed to be permanent and appropriate for the environment where they are installed.
690.7(C) sets the maximum PV system voltages for one and two-family homes to 600 Vdc. Systems over 1000 volts are covered by Part IX of Article 690.
Per 690.15(A), for utility-interactive inverters that are not mounted in readily accessible locations, both the AC and DC disconnects must either be within sight of or in the inverter.
One of the critical areas of the NEC Article 690 details the requirements for the rapid shutdown of PV systems (Section 690.12). First responders, such as fire fighters, need to contend with the elements of a PV system that remain energized after the AC disconnect is opened. The rapid shutdown requirement provides a zone within which the potential for electrical shock has been mitigated. Conductors more than 5 feet inside a building or more than 10 feet from an array will be limited to a maximum of 30 volts (V) and 240 volt-ampere (VA) within 10 seconds of activation shutdown. Ten seconds allows time for any DC capacitor banks to discharge.
There is more than one way to meet NEC 690.12 rapid shutdown requirements; designers and engineers have the flexibility to select the most effective method for a given system. The devices typically used to meet this requirement include shunt trip safety switch products and separate stand-alone components. Designers and engineers also have the ability to install the inverter on the roof within 10 feet of the PV array with a fireman switch to shut down the inverter.
It should be remembered that PV module output changes with the operating temperature and the amount of sunlight to which it is exposed. The amount of exposure depends on irradiance level, angle of incidence and the shading effect from trees, buildings and clouds. In operation, PV fuses and circuit breakers are influenced by ambient temperature. The PV OCPD’s ampacity should be derated according to the manufacturers’ published curves and NEC 690 requirements.
It is also important to understand whether the PV system is grounded or ungrounded. A large number of the PV systems in North America to date are grounded systems, where only the positive or negative side will need to be switched on or off. In a positive grounded system, the negative conductors will be required to be switched and the contrary for negatively grounded system. In ungrounded or floating PV systems using transformerless inverters, both the positive and negative conductors will need DC switching capability. Depending if the system is grounded or ungrounded, it will require the engineer to specify the correct DC disconnect in order to build and operate a safe PV system.
Enhance safety and protect property
The main purpose of the Code is to support safer PV systems for building occupants and owners, maintenance personnel and first responders. In the case of either an unplanned event or for regular maintenance, there must be a means to disconnect the system and de-energize the power being generated.
A disconnect is required before the inverter (on the DC side) and after it (on the AC side), isolating the inverter. With disconnects in place on both sides of the inverter, equipment and system maintenance can be performed on a de-energized system by disconnecting the power on the DC side; and first responders can shut down the power on the AC side, in the case of a fire.
System design that meets the code requirements
Selecting the appropriate disconnect device depends on the system voltage and whether the system is grounded. There are a variety of both fusible and non-fusible options to select from, key considerations include:
- Ampacity before and after the inverter
- Voltage before and after the inverter
- Available fault current
- Required circuit protection
- Environmental exposure
- Conductor sizes, type and ratings
- The number of strings to disconnect
- Grounded or ungrounded system
Typically, NEMA 3R enclosures are specified for outdoor applications and both fused and non-fused disconnect switches can be enclosed in these. However, there are NEMA 4X rated, thermoplastic enclosures available that are UL 94-5VA flammability rated and can be molded with additives to resist damage from prolonged exposure to UV.
Additional information on the latest code requirements can be found on the Nation Fire Protection Association (NFPA) and UL websites. Additionally, specific information on solar circuit protection can be found on Eaton’s application guide.
Written by Nick Stone, OEM sales engineer at Eaton