Simultaneous Disconnection of Positive And Negative Poles in A Photovoltaic DC System Is A Necessary Design

In a photovoltaic DC system, it is necessary to simultaneously cut off both the positive and negative lines. The core reason is determined by the characteristics, safety regulations, and fault protection requirements of the DC system, as follows:

1. The "no natural zero crossing" characteristic of the DC system means that a single pole cut-off cannot completely cut off the power

AC power has a natural zero crossing point (voltage and current periodically return to zero). After cutting off a single pole (such as a live wire), the circuit current will naturally interrupt with the zero crossing point; The voltage and current of direct current are constant and there is no zero crossing point.

-If only the positive (or negative) pole is cut off, the other pole may still form a weak circuit through parasitic capacitance, insulation leakage, etc. inside the device, causing the device to remain charged (especially in high-voltage systems, such as string photovoltaic DC side voltage can reach hundreds of volts), posing a risk of electric shock.

-Simultaneously cutting off the positive and negative poles can completely disconnect the DC circuit, ensuring that the system truly loses power and avoiding safety hazards caused by residual voltage.

2. Prevent the expansion of faults caused by "single pole grounding"

In a photovoltaic DC system, if only one pole is cut off, the other pole may form a "single pole grounding" with the grounding body (such as metal brackets, ground) due to insulation damage (such as cable aging, moisture):

-When the positive pole is grounded, the voltage between the negative pole and the ground is equal to the total system voltage (such as 600V), which may cause insulation breakdown of the equipment;

-When the negative electrode is grounded, the voltage between the positive electrode and the ground is also equal to the total voltage, resulting in the continuous existence of grounding faults and even causing arcs and fires.

-Simultaneously cutting off the positive and negative poles can completely isolate the fault point, prevent the single pole grounding from further developing into a "two pole short circuit" (such as the other pole accidentally touching the grounding body after grounding), and avoid larger scale equipment damage.

3. Comply with safety regulations and ensure the safety of operation and maintenance personnel

The voltage of photovoltaic DC systems is usually high (about 300-600V for household systems and up to 1500V for large power stations), far exceeding the safe voltage for human health (below 36V).

-During operation or maintenance, if only one pole is cut off, personnel may accidentally touch the other pole that has not been cut off, or contact both poles simultaneously through tools or body (forming a circuit), resulting in electric shock accidents.

-International and domestic safety standards (such as IEC 62109, GB 50797) explicitly require that the disconnecting devices on the DC side of photovoltaics (such as DC circuit breakers, isolating switches) must have the ability to "disconnect at both poles" to ensure that both poles are disconnected simultaneously during operation, physically cutting off all conductive paths.

4. Avoid the risk of reverse discharge or energy feedback

In a photovoltaic system with energy storage, if only one pole is cut off, the battery may discharge in the opposite direction to the photovoltaic module through the unconnected line, or the residual electrical energy of the photovoltaic module may be released through the single pole circuit, causing overheating of the components and cables, affecting equipment life and even causing failures. Simultaneously cutting off both poles can completely block the flow of energy and avoid such risks.

In summary, the simultaneous disconnection of positive and negative poles in a photovoltaic DC system is a necessary design determined by the DC characteristics and safety regulations, in order to completely disconnect the DC circuit, prevent residual voltage and fault expansion, and ensure the safety of personnel and equipment.