Disconnect
A disconnect enables a circuit to be disconnected or isolated from the rest of an electrical system. Disconnects are vital for safety and security as they allow different components in the system to be isolated in order to perform maintenance or service. Traditional disconnects, like in the image, come in countless different sizes and configurations, including custom-built configurations. Although they are not often used in small-scale offgrid installations as most disconnecting needs can be met with breakers, fuse holders, or module connectors which are cheaper and simpler. A traditional disconnect must be rated for the proper current (AC or DC), maximum voltage and maximum current of the circuit. Disconnects can also be found that incorporate fuses for suitable applications.
The appropriate type of disconnect for a circuit depends upon the type of circuit - there are two different types of disconnects that are required in an offgrid PV system.
Power source disconnects
Power source disconnects enable the isolation of any potential power source, including while under load (while current is flowing) in case of emergency. In an offgrid PV system there are two power sources - the PV source and the energy storage system - that meet this requirement. If a system incorporates a generator then it will also require a seperate disconnect. For offgrid PV systems, breakers are typically used as power source disconnects as they can fulfill the role of an OCPD at the same time, although a properly sized (suitable for DC, correct voltage, correct current) traditional disconnect would work just fine. Any power source disconnect must be rated to disconnect a circuit with current flowing, therefore fuse holders or any type of connector cannot qualify as a power source disconnect as they cannot be opened under load.
Equipment disconnects
All major components in a PV system should have a disconnect that enables them to be isolated from these two potential power sources or any others, although for equipment that is not a power source, it is not necessary that an equipment disconnect be able to function under load. Therefore, touch safe fuse holders and connector - like MC4 connectors on PV modules - can qualify as equipment disconnects. It is necessary to have a power source disconnect that makes it possible to stop any current from flowing if using an equipment disconnect that is not rated for use under load.
