AE 868
Commercial Solar Electric Systems

Protection Devices and Disconnects

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Overcurrent Protection Devices (OCPD)

Overcurrent protection devices are classified by how quickly they activate. Overcurrent protection devices can include fuses and circuit breakers.

A non-current-limiting device operates slowly, allowing damaging short-circuit currents to build up to full values before opening. An example is a fuse, which is a link that melts when heated by current greater than its rating to provide overcurrent protection due to extra loading.

A current-limiting device opens the circuit in less than one-quarter cycle of short-circuit current, before the current reaches its highest value, limiting the amount of destructive energy allowed into the circuit.

Overcurrent protection devices must be listed and specifically rated for their intended use, such as DC. For example, automotive fuses may not be used in PV systems.

Sizing OCPD

OCPD are highly recommended for PV systems and are sized not to be less than the highest current.

  • DC array side
    • Since the derate factor is 156% of the short circuit current of the PV module, the OCDP is sized not to be less than that value since the 125% factor is included already as part of the calculation.
  • AC side
    • At the output of the inverter, OCPD are usually sized to be at least 125% of the highest inverter current possible provided from the manufacturer’s datasheet.

Disconnects

A disconnect must be provided to open all current-carrying conductors of a PV power source from all other conductors in a building or structure. This disconnect is also known as the DC disconnect or PV disconnect. Disconnects used in the PV output circuit must be rated for DC and identified as such. Equipment such as PV source-circuit isolating switches, overcurrent protection devices, and blocking diodes are permitted on the array side of the array disconnect.

Disconnects Sizing

  • DC array disconnect

    • Since the derate factor is 156% of the short circuit current of the PV module, the DC is sized not to be less than that value since the 125% factor is included already as part of the calculation.

  • AC disconnect

    • At the output of the inverter, disconnects are usually sized to be at least 125% of the highest inverter current possible provided from the manufacturer’s datasheet. In an interactive system, the PV system’s AC disconnect should be located near the main utility disconnect. This facilitates the quick removal of all power to a building or structure in an emergency.

      In stand-alone systems, the AC disconnect is typically located near the array disconnect or the AC power distribution panel. Disconnects must be provided to open all ungrounded conductors to every additional power source and each piece of PV system equipment. Other equipment that requires disconnecting means can be inverters, charge controllers, and other major components. If equipment is connected to more than one power source, each power source must have a designated disconnecting means. It is particularly important for battery bank circuits to include a specific DC disconnect.