Rafael Gallo

A common question among photovoltaic system integrators is related to the installation of the string box. We are alarmingly encountering faulty installations where cable polarities are reversed, a situation that could be avoided through simple electrical tests that professionals in the field should perform during system commissioning. When acquiring the various options of pre-assembled string boxes available on the market, integrators often encounter great difficulty due to the variety of standards adopted by manufacturers. It is common for string box manufacturers to have their own specific features and to provide manuals and explanatory diagrams for the use of their equipment. However, negligence by some installers may cause the system to not boot. Faced with a system initialization failure, the integrator usually tries to attribute the problem to other components, such as the inverter, without giving due attention to all aspects of the project. Reverse polarity can occur due to a simple mix-up in making the connections that begin at the modules, pass through the string box, and end at the inverter. Some brands of inverters available on the market even warn about the fault through their displays. However, many models use the connection to the solar panels to power their electronics. When reversed, they do not work and generate dissatisfaction for the installer and waste of time during installation due to the search for a solution to the problem. In addition to these situations, it is possible to run the risk of inverting the polarities between the photovoltaic arrays, breaking the voltage limits of the modules, the string box or even the inverters. Figure 1 - Left: reading with inverted polarity on the voltmeter. Right: Correct reading of the photovoltaic string voltage. To avoid this type of occurrence, it is important to carefully study the diagrams and manuals that accompany the string box and identify the particularities between the connections of the terminals, surge protection devices (SPDs), disconnect switches, or fuse bases. In addition, measurements should be taken with a multimeter at each connection point to confirm that the voltage and polarity values are correct at the component inputs and outputs. Figure 2 - String box assembled with inverted polarities Figure 3 - Orientation diagram for string box assembly A practical example of the situation mentioned above consists of the construction of a string where the modules present an open circuit voltage of 45,84 V. With a string of 10 modules we will have a total voltage of 458,4 V. Small variations are expected due to external factors such as temperature, orientation and tilt. It is also important after this step that the professional pays attention to the correct values presented in the multimeter reading and the polarity when connecting the string box. A reading of -447 V indicates an acceptable voltage drop, but reversed polarity with respect to the multimeter probes. Another recurring situation occurs when the connection pattern in a string box changes in relation to the models used by the installer, familiar with another supplier. Some manufacturers use equipment such as bipolar disconnector switches with direct passage, while others use tetrapolar switches with crossed outputs, which, when falling into the hands of an inattentive professional, can generate an inversion in the polarities at the output of the string box, which can be avoided by means of a new reading of potentials and polarities. After leaving the string box and passing the cables, a final reading of the corresponding parameters is interesting. Figure 4 - The multimeter is a powerful ally for the installer to verify if the polarities of the string terminations and the string box outputs are correct. Building monitoring and measurement processes throughout the installation of photovoltaic systems proves essential in order to avoid risks and inconveniences for integrators and their clients. Using appropriate materials and equipment, such as a multimeter with a full clamp meter capable of performing measurements in both direct and alternating current within the project's scales, will be essential for a successful installation. The distributors have a technical team prepared to assist integrators and ready to answer any questions the installation professional may have.

Rafael Gallo

Published articles

The risks of polarity inversion in photovoltaic systems