Erich H. Reuss, of DEHN (UK), discusses the potential problems associated with MOV combined arresters, and urges engineers to be aware of the limitations of these devices
According to European Standard EN 61643-11, low-voltage surge protection devices are classified as types 1, 2, and 3. Nowhere are these discrepancies more obvious than with so-called combined arresters, where a multitude of names such as combined arrester set, type 1/type 2 combination, B-C arrester, T1+T2+T3 arrester or even BCD arrester are a recipe for confusion, leading to the situation where different products are available with parameters, protective effects and even different wave shapes which deviate from one another.
The EN 62 305 standard for lightning protection (now adopted in the UK as BS EN 62 305) requires that co-ordinated surge protective devices are installed at the entry (of the low-voltage supply system). It describes precisely the use of surge protective devices for internal lightning protection. In particular, it specifies that the lightning protection zone concept requires the installation of surge protective devices whenever an electrical conductor crosses the boundary between two lightning protection zones. These surge protective devices have to be energy co-ordinated so that the total loading of the protective devices is subdivided according to their power carrying capability, so that the original lightning hazard will be reduced to values below the immunity of the devices to be protected. It is this energy co-ordination between the various surge protective devices and the devices to be protected that determines the requirements which define a combined arrester.
So, what is a combined arrester? A combined lightning current and surge arrester has the following features:
- High discharge capacity for lightning currents with the 10/350µs waveform for lightning equipotential bonding purposes
- A voltage protection level of below 1.5kV for safe insulation co-ordination with terminal devices and the operating equipment of the electrical installation
- Energy co-ordination with downstream surge protection devices in the installation and with the consumers to be protected.
For larger electrical systems, it is best to install surge protective devices at the locations specified by the lightning protection zones concept, for example, lightning equipotential bonding at the building entry and surge protection at the distribution box and/or in the vicinity of the terminal devices.
Even then, according to the standards, this arrester is a type 1 (according to EN 61643-11) and class 1 device (IEC 61643-1).
Interest in combined lightning current and surge arresters has been stimulated by the trend towards more compact electrical systems, where they can replace lightning current arresters to provide both lightning and surge protection for the electrical system and the sensitive consumer and control devices.
Spark-gap and varistor arresters
For many years, lightning current and combined arresters based on creepage discharge spark-gap technology have been used for the integration of the low-voltage supply into the lightning equipotential bonding system, and spark-gap devices now safely protect hundreds of thousands of electrical systems throughout the world.
Varistor based arresters, however, almost exclusively use metal oxide varistors (MOVs). The MOV acts as a voltage-controlled resistor, and can reduce its resistance value considerably as the current load increases. The response time is in the region of tens of nanoseconds. This behaviour makes the MOV appear to be an almost ideal surge protective device for applications where the arrester is situated near the terminal device.
Many data sheets for varistor based products state that MOVs have a high discharge capacity for high-energy impulse currents. This is only partially correct, however. It is certainly applicable when you compare the energy absorption of MOV devices with other components such as diodes. It does not, however, take into account the fact that lightning equipotential bonding impulse currents with the 10/350µs waveform have to be assessed according to the standards and directives mentioned earlier. In fact, the loading on a typical installation can be many times higher than the discharge capacity of a typical varistor type arrester.
Another reason that varistor-based lightning current and surge arresters are not suitable as combined arresters is the fact they lack co-ordination capability with other protective devices and with the equipment and devices to be protected. The continuous effectiveness and the fixed current/voltage characteristic of the MOV are the reason for these application restrictions. It is difficult, therefore, to co-ordinate varistor arresters with 10/350µs impulses, or use them in situations with downstream inductances or installation lines.
This is a different situation from that which occurs with a combined lightning current and surge arrester based on a spark-gap, where the voltage across the arrester collapses to the so-called arc voltage immediately after its response, which means that the subsequent protective elements are no longer loaded.
The conclusion is that lightning current and combined arresters based on MOVs are not a viable alternative to creepage discharge spark-gap arresters because of their inherent operating characteristics. There is, therefore, an urgent need for planners, engineers and installers at all levels to be aware of the limitations of these devices, and to promote safety awareness by offering competent advice and execution of the specialised engineering tasks involved.