The basics of surge protection | Areas of application As part of the electrical installation, • Over a specific nominal value of the corresponding legal or regulatory requirements are to be fulfilled for the connection and overcurrent protection of surge protective devices. These predominantly aim to guarantee the operational reliability of the system. Furthermore, specific conditions regarding connection and fuse protection are to be taken into account for correct surge protection function. upstream overcurrent protection, the minimum cross sections are determined by the connecting cables' need for short-circuit withstand capability • If the SPD connecting cables carry operating current, then the continuous current load can be used to determine the minimum cross section as of a certain current value The requirements are based on various parts of IEC 60364 for creating low-voltage systems: on the one hand, Part 5, Section 53, Main Section 534 [11], regarding the selection and setup of surge protective devices, and on the other, Part 4, Section 43 [12], regarding protective measures against overcurrent, as well as the product standard for surge protective devices, IEC 61643-11 [6]. Connection cross sections If these requirements are combined, this results in the following conditions for dimensioning the connecting cables of SPDs (based on PVC-insulated copper cables): • The minimum cross sections for the SPD connecting cables initially result from the requirements for installing surge protective devices, depending on the active conductor connection or the main grounding busbar/ protective conductor (PE(N)) as well as the type of the SPD: – Connection cross section of the active conductor for type 1 SPDs: min. 6 mm2 – Connection cross section of the active conductor for type 2 SPDs: min. 2.5 mm2 – Connection cross section for the main grounding busbar or the protective conductor for type 1 SPDs: min. 16 mm2 – Connection cross section for the main grounding busbar or the protective conductor for type 2 SPDs: min. 6 mm2 36 PHOENIX CONTACT Overcurrent protection When designing the overcurrent protection of SPDs, the following elements must first be prioritized: • Priority of the system supply: Branch wiring with separate F2 overcurrent protective device in the branch • Priority of the system surge protection: V-wiring or branch wiring without separate F2 overcurrent protective device In the first case, the F2 separate overcurrent protective device ensures that this is triggered in the event of an SPD failure, e.g. due to a short circuit. The F1 upstream overcurrent protective device is not triggered so that the supply of the equipment to be protected is not interrupted. In this case, however, the equipment is no longer protected from subsequent overvoltage events. In the second case, the F1 upstream overcurrent protective device takes on the overcurrent protection in the event that the SPD fails. In this process, the failure of the supply is accepted so that no damage can arise from subsequent overvoltage events. When dimensioning the overcurrent protection, the following points should be kept in mind: • Selectivity of the respective overcurrent protective device to upstream overcurrent protective devices. • The final overcurrent protective device before the SPD must not exceed the maximum nominal value of the upstream overcurrent protective device as specified by the SPD manufacturer. • The upstream overcurrent protective device is intended to be able to carry the amplitudes of lightning and surge current required by the lightning protection class when possible. Especially with regard to high-energy lightning currents, under-dimensioned fuses can pose a risk, as they can be destroyed in a very short time due to high-energy inputs. Adhering to the selectivity is therefore the top priority. In the simple case that the two overcurrent protective devices to be taken into account are gG fuses, then a nominal value of 1250 A applies, which must be F2 × 1.6 = F1. If one or both of the overcurrent protective devices is a circuit breaker or miniature circuit breaker, then their tripping characteristics must be compared with each other or with the fuse characteristics and, if applicable, tailored to each other. This is the case if the curves do not touch or overlap (Fig. 55 and 56). Furthermore, they must have a sufficient time interval in areas with short-circuit currents so that the respective downstream overcurrent protective device can address the other two devices and switch them off. A similar scenario applies in the event that a miniature circuit breaker or circuit breaker is intended to provide the overcurrent protection for the SPD as F1, without a separate F2 overcurrent protective device. Then, the switching-off characteristics of the switch must be compared with the characteristics of the maximum overcurrent protection specified for the SPD by the manufacturer. This must not be exceeded in the range for short- circuit currents. It is difficult to make general statements on this point. Statements can only be made regarding comparatively low nominal currents of switches