Shielding and surge voltage protection

In modern industrial plants, each part of the installation is subjected to a number of influences that may adversely affect the functioning and reliability of the entire facility.

Electromagnetic interferences originating from other parts of the system or from external surge voltages, such as lightning, can result in the breakdown of the entire facility.

It is therefore imperative to plan effective protection from the outset.

For an electrical or electronic installation with constant and long-term availability, an effective protection concept that takes the whole range of possible interferences into account has to be developed during planning.

These interferences include SEMP (switching electromagnetic pulse), which is caused by switching inductive equipment.

In addition, the list includes ESD (electrostatic discharge) and LEMP (lightning electromagnetic pulse).

The transient power surges generated by these interferences not only affect power lines but also extend to all other signal lines entering the facility.

Hence it is not sufficient to protect only the power lines with effective lightning and surge voltage protectors - all signal paths in the facility require protection.

Three-stage surge voltage protection scheme

Practical experience has shown the value of three-stage surge voltage protection.

The first stage consists of lightning protectors and needs to satisfy the requirements of Type B/Class I.

The second stage comprises surge voltage protectors and the third stage the actual device protection.

The second stage should comply with Type C/Class II and the third stage should meet Type D/Class III.

The tripartite implementation of surge voltage protection is necessary because it is not at present possible, using a single component, to safely discharge high currents in a short time while simultaneously restricting surge voltages to a level permissible within the facility.

Instead, in the three-stage concept, a voltage exceeding the normal level reaches the device protection.

This third stage protects devices by conducting lower surge currents to earth. To prevent overloading, the energy passes through a second stage - the upstream surge voltage protector. This is designed to discharge higher currents than the device protector, so it protects both the device and the device protector.

If even greater energies need to be discharged, the lightning protector comes into action and protects all downstream devices.

The three-stage surge protection concept essentially protects downstream devices and protection components using three controlled short circuits.

The stages require precise co-ordination with one another.

For instance, specified line lengths must be maintained between the three stages to ensure that each upstream stage is supplied with the necessary response voltage.

If it is not possible to meet these line lengths or if two stages are to be integrated within one control cabinet, an inductive decoupling element has to be used.

The first stage is generally installed in the facility feed while the second level is integrated into a sub-distribution panel at a different location.

Alternatively, the second level is installed in the same feed but with an additional inductor. Phoenix Contact's triggered spark gap technology, also called AEC (active energy control), eliminates the need to install the first and second stages at different locations.

Even the additional inductive decoupling element traditionally needed for immediate proximity installation for lightning and surge voltage protection is no longer required.

Triggering the spark gap in the lightning protector causes it to respond at much lower voltages. This results in space and cost savings in the feed.

Protecting signal and data lines

Data interfaces and signal inputs are much more sensitive to surge voltages than components in the power supply.

Admittedly, the energies expected in these circuits are also much lower.

The surge voltage protectors employed here usually consist of two-stage solutions combining fast acting suppressor diodes and gas tube arresters with a high discharge capacity. This is, in effect, a miniaturised version of the second and third stage of the power supply protection concept.

The Phoenix Contact Plugtrab protector family has solutions for this kind of protection.

Here the protection elements are in a plug that fits into in a base terminal which is part of the installation.

This allows protection elements to be replaced if necessary, without interrupting or disturbing the signal path.

For protecting signal lines there is the Termitrab product family.

The components of this consist of simple terminal blocks with built-in protective functions either as gas tube arresters or suppressor diodes.

These terminals are now also available with spring cage connection and their matching contours mean they can be combined with rail-mounted terminal blocks.

Because even the bridge shafts are arranged all in one line, it is easy to switch and protect signals. The advantage is the flexibility that results.

Shielding

Electromagnetic compatibility is always a critical factor when planning facilities. Signal and power transmission must not interfere with one another.

Due to increasing data transmission rates, the devices concerned are becoming increasingly sensitive to capacitive or inductive interferences from neighbouring devices.

This calls for improved shielding of components and wires. When planning and implementing the shielding scheme, the components used should be selected to ensure that commissioning and maintenance are kept quick and cost-effective.

Shielding connection

The shielding of a signal cable should be grounded immediately it enters the control cabinet. However, it may still be necessary to connect the individual wires in the signal cable to a large number of terminal blocks at a switching level.

Depending on the available space in the control cabinet, this wiring can turn into a time-consuming and tedious task if the cable's shielding has already been fixed with a shielding terminal.

In this case it is better if the terminal concept allows for connecting the shielding after wiring the signal wires.

The so-called pigtail method involves twisting the shielding braid and connecting it to a grounded PE terminal just like a signal wire.

This connection method easily reduces the shield's attenuation by a factor of five.

This method is hardly satisfactory and can at the best be used for low frequency interference of up to 10 kHz.

A much better result is achieved by connecting the shield to a large area using suitable shield terminals.

Shielded plug connectors

Full-area shielding connection is also an important criterion when selecting suitable plug connectors.

At the same time it has to be easy to implement the connection when assembling the plug connectors. Yet little is gained from having a perfect connection between cable shield and plug connector if the shield housing of the plug connector only provides a one-point link to its counterpart in the device wall.

This problem exists with some M12 plug connectors, where the shield is transmitted via the fifth pin only.

Instead, every transition point should provide a maximum shield contact. Also, special care should be taken with the choice of material for the connector housing.

The material must combine maximum conductivity with the physical robustness its use requires.

Although metallisation for plastic housings has undergone significant improvement, problems such as brittleness due to ageing still persist.

Zinc die cast housings with a copper and nickel surface are still the first choice for use in harsh industrial situations.

Shielding in decentralised installations

Due to the steady miniaturisation and improvement of controllers, decentralisation is becoming increasingly common in plant engineering. At the same time, there is a constantly increasing volume of sensor and actuator signals passing to and from these controllers.

The sensor and actuator lines within the production area are exposed to all the various interferences that are produced.

For example, sensor cables for an industrial welding robot are required to transmit interference-free signals, even though in the immediate vicinity there are high welding currents, which to compound matters are being switched at regular intervals.

The shielding requirements apply correspondingly to the sensor and actuator boxes that are used for collecting and distributing the signals.

This is precisely where only a shield with full-area contact continuing across connectors can provide good protection.

As well as the pure functionality of devices and installations, an increasing number of interference variables that affect this functionality have to be taken into account in development and planning.

However, careful selection of protection components enables implementation of the required protection mechanisms without great expense.

When it comes to surge voltage and electromagnetic interference protection, as with all protection mechanisms, even the best protection is useless if incorrectly applied - whether through lack of expertise or because it is too complicated to use properly.

So it is vitally important to select these components carefully to ensure constant availability of installations.

Matthias Tieben is the Head of Sales Promotion Connection Technology and Trabtech Phoenix Contact GmbH, Germany.