BESS — DC side: overcurrent protection and cable sizing for battery energy storage systems

The guide on home batteries & energy storage systems covers the installation and registration requirements of a battery energy storage system (BESS) in general, and the guide on thermal runaway covers the underlying fire-safety risk. This article covers a third, preceding aspect: the electrical design choices on the DC side of a BESS itself — the connection between the battery modules and the inverter — where both overcurrent protection and cable sizing require fundamentally different considerations than a regular AC installation.

Why a DC arc is fundamentally harder to extinguish

In an AC fault, the current naturally passes through zero 50 times per second — each zero-crossing offers a moment at which an arc naturally extinguishes and a switch or fuse can complete the interruption with relatively little energy. With direct current this natural zero-crossing is entirely absent: once a DC arc has formed, it keeps burning until the device itself actively introduces enough impedance into the circuit to force the current to zero. This makes interrupting a DC fault inherently more energy-intensive than AC, and is the reason why an ordinary AC fuse or switch is not automatically suitable for DC applications, even at a comparable current rating.

DC fuses and switches: a separate classification

Both fuses and switches applied on the DC side of a BESS must be explicitly tested and classified for DC interruption at the actual system voltage — not just for the current rating. Relevant product standards include IEC 60269 (low-voltage fuses, with specific DC classifications) and IEC 60947-2 or IEC 60898-2 for DC-suitable circuit breakers/miniature circuit breakers. An important difference from AC: the breaking capacity of a DC fuse or switch depends on voltage in a way that does not scale linearly with current — a component approved for 60 V DC is not automatically suitable for the same current at 600 V DC, even though the physical arc gap inside the device is identical.

String fuses: protection against circulating current between parallel strings

In a BESS with multiple battery strings connected in parallel, an internal fault or a difference in state of charge between strings can cause one string to inject current into another — a circulating current that does not flow through the main connection to the inverter and is therefore not detected by a single main fuse. To limit this, a dedicated fuse is typically applied per string (or sometimes per module), similar in principle to string fusing in a PV installation, but sized specifically for the characteristic discharge current of the battery chemistry involved.

Cable sizing: peak current of both charging and discharging

Unlike a PV string, where the current is inherently limited by irradiance and the panel characteristic, the DC cable between a battery module and the inverter can carry a substantial, actively inverter-controlled current during both charging and discharging. The cable cross-section must therefore be sized on the higher of the two peak currents (not an average), with the usual correction factors for ambient temperature and bundling of multiple cables (see the guide on grouping factor). In addition, voltage drop across the DC cable is relevant to system efficiency: a DC cable that is too long or undersized between battery and inverter noticeably reduces the effective charge/discharge efficiency, in addition to the usual safety considerations.

Insulation monitoring and earthing configuration

A BESS DC bus, like a PV string, is typically not functionally earthed (an ungrounded, IT-like DC system), with continuous insulation monitoring (IMD) that signals a first insulation fault without immediately disconnecting — see the guide on IT-system first fault detection for the underlying principle. This prevents a single earth fault in the battery pack from immediately disconnecting the entire system, but it does require that the insulation monitoring device itself is functional and periodically checked.

Note: the exact topology (earthed versus ungrounded DC system, mid-point earthing) follows from the inverter and battery manufacturer's specification and the applicable product standard (including IEC 62619 for the battery cells themselves); this article covers the principle, not a ready-made design for every manufacturer's equipment.

Practical relevance

When assessing a BESS installation, it is important to verify that the DC fuses and switches applied are explicitly classified for the actual DC system voltage (not just the current rating), that the DC cable cross-section is based on the higher peak current of charging or discharging, and that any insulation monitoring device is functional and has not been bypassed or taken out of service.

Common mistakes

Applying an ordinary AC fuse or switch on the DC side without verifying that the breaking capacity has been explicitly tested for the actual DC system voltage — the absence of a natural zero-crossing creates a real risk of a sustained arc on fault.
Basing cable cross-section on average power rather than the higher peak current of charging or discharging — this can lead to overload during brief but significant charge or discharge peaks.
Not applying separate string fuses in a BESS with multiple parallel strings — an internal fault in one string can then receive undetected circulating current from another string.
Not periodically checking the insulation monitoring device (IMD) of an ungrounded DC system — a faulty IMD no longer warns of an actual first fault in the battery pack.