Figure 2: Unaffected overvoltage (left) vs. applied overvoltage with active protection by means of SPD (right).

© Schukat/Citel

Transient overvoltages affect electronic devices in various ways and with varying degrees of intensity. In the absence of hardware damage, they can lead to a temporary malfunction or undefined operation of the system due to data or transmission errors, as well as causing program crashes or the deletion of memory contents. In the worst case, the hardware may be destroyed due to a voltage breakdown of semiconductor junctions, destruction of the bonding wire connections of components or the conductor tracks. In the event of a hardware defect, additional service costs are incurred and the system downtime lasts longer. In both cases, however, restrictions or downtimes are to be expected.

If only minor overvoltages occur as a result of the switching processes described, they may not directly damage the connected devices. However, they stress the electronic components installed in the devices and result in premature ageing and therefore shorter operating times.

Surge protective devices (SPDs) offer effective protection. The term generally applies to all devices that protect against voltage peaks. In order to achieve adequate protection, the components must be selected according to the risk and installed in compliance with applicable standards. The requirements and tests for SPDs for low-voltage systems are set out in national and international standards such as EN 61643-11 and UL 1449.

How does a surge protective device work?

As a safety element of the system, the SPD protects the system against transient overvoltages without any faults. Surge protective devices are generally based on varistors or gas-filled spark gaps or a combination of both. The latter represents the best possible compromise between the most important features for efficient surge protection, namely a fast response time of less than 25 ns and the highest possible leakage current. The maximum leakage surge current for surge protective devices is the maximum surge current with 8/20 µs pulse. A surge protective device can withstand this without being destroyed. The rated leakage surge current is the value of the surge current that a surge protective device can withstand several times without being destroyed. In the event of an applied overvoltage, the SPD discharges the energy and reduces the maximum residual voltage to a defined protection level (Figure 2).

The protection level of the surge protective device must be selected so that it is matched to the dielectric strength of the devices to be protected. A basic distinction must be made between coarse, medium and fine protection. The lower the protection level, the better the surge protection. The IEC 60364 standard specifies a maximum protection level of 2.5 kV for surge protection devices used at the input of 230 V or 400 V networks. This value corresponds to the dielectric strength of robust electromechanical devices and should be reduced as far as possible by means of fine protection.

Figure 3: Surge protection devices for different connection and mounting types: MLPCA1 series from CITEL for IP67 flash connection (left) and DACN10 from CITEL with spring-cage terminals for DIN rail mounting (right).

© Schukat/Citel

In addition to the protection for the ₂₃₀ VAC or 400 VAC _supply lines, the protection of the data and control lines must also be taken into account in networked systems. Damaging overvoltages can also couple onto data lines and the dielectric strength of the interface modules can be relatively low. Coarse or medium protection installed at the feed-in point, which is also prescribed for new industrial systems, is often not sufficient on its own. For cable lengths of approx. 10 m or more to the end device, for systems in industrial environments and especially for outdoor operation, a type 2 or type 3 arrester or a combination of both directly on the end device or in the associated switch cabinet is recommended. SPDs are available for mounting on the top-hat rail as well as for installation in the end system with stranded wires, plug-in or screw connection (Fig. 3).

The SPD protects the device regardless of the level of overvoltage. If the pulse energy exceeds the discharge capacity of the protective element, it may be overloaded, but the end system is still protected in this case.

In accordance with the standards, surge protective devices must also be equipped with an internal and an external disconnecting device, which provide the best possible protection for the connected electrical applications in the event of a fault.

As an indispensable safety element, the internal thermal disconnection device disconnects the surge protective device from the mains in the event of a fault or overload. The operator is then notified via the fault signaling of the arrester to replace the corresponding protection module. The user must provide protection against a short-circuit current in each SPD feeder. The manufacturer specifies the tripping current of this fuse in the product data sheet and the installation instructions.

The author: Frank Stocker is Field Application Engineer Power Supplies at Schukat electronic.

© Schukat/Citel

The external electrical disconnecting device, usually a fuse or a circuit breaker, safely disconnects the surge protection device from the mains in the event of a short circuit. Some surge protection devices are available in a version with remote signaling. The potential-free changeover contact is used for status monitoring and can be integrated as a binary output in modern networked industrial systems, for example, in order to monitor the operating status and proper function.