PROTECTION AGAINST INDIRECT LIGHTNING STRIKES

Indirect lightning strikes and run-off Quality of the installation's wiring, especially the earthing system (continued)

Uab = [Za.Ia + Zc(Ia+Ib)] - [Zb.Ib + Zc(Ia+Ib)]

It can be seen that the factors Zc (Ia + Ib )] cancel each other out and that consequently the voltage between earths A and B does not depend on the quality of the installation's earths.

If Ia=10kA surge current, Ib=100A residual current and impedances Za and Zb at the same value of 10Ω: Uab = (10 x 10 000) . (10 x 100 ) = 100 000 V

If the two ground networks A and B are close together, this potential difference may be sufficient to cause an ignition between these two networks.

The reduction of local potential rises in ground networks is achieved by interconnecting as much as possible everything that can serve as a return conductor to ground currents. This reduces the impedances of ground networks and allows the ground currents to be distributed between several conductors.

The essential and fundamental condition for the quality of a protection against the electromagnetic effects of lightning is to limit the potential differences between adjacent masses, so as to eliminate the causes of flashovers and current flows. It is then understandable why meshed ground networks are indispensable.

A "mass network" or "local earth" is the term used to describe all the networks used to ensure the reference potential of a set of installations.

A network of meshed grounds does not suppress common-mode potential rises, but the grounds of all the devices connected to it rise in potential at the same time.

As a result, there is no more parasitic current between the devices and if all the elements of this network have the same potential reference, there is no more overvoltage between the grounds.

From the above, one could imagine that the quality of the land is of no importance and could even be dispensed with. This would be true if the facility was a perfect Faraday cage, but it is not: the quality of the grounding must be ensured for the following reasons: -The grounding of the installations is a legal obligation: the grounding system must conform to standards relating to the safety of people. -Also to ensure the safety of persons, it is necessary to limit longitudinal potential differences and in particular step voltages, especially in the vicinity of the circuits and earthing points. -A good earth reduces the consequences of a bad earth network by limiting the value of transverse potential differences.

 Separate Lands

Some computer equipment suppliers recommend a separate land in order to obtain a "clean land" for their equipment. This idea of separate land is absolutely to be avoided. This idea is opposed to the intangible principle of equipotentiality. Separate earths are only acceptable on installations that have absolutely no physical link between them, of any kind whatsoever: data bus, power source, cable tray, fluid pipe etc. Very distant installations that share a data bus will be treated like installations connected to the telephone network.

Wiring quality In order not to cancel out the beneficial effects of lightning protection, the wiring must reinforce the equipotentiality of the system and tend to reduce the coupling effects between polluting and sensitive wires. For this purpose, the following rules must be observed:

- The trunking and cable trays must be metallic, each component must be bolted to the previous and the next. The whole thing must be carefully connected to the various cabinets served. This is particularly important, as a 2 m long wire connection is enough to eliminate the reducing effect of the cable tray. A connection of only 10 cm in length already halves the efficiency. And in general: Any connection to the equipotential network must be short and straight

- An earth conductor that runs over a metal cable tray must be bare and in open contact with it. -Power cables and low current cables are installed in separate trunking or, failing that, placed on either side of the trunking. A distance of 20cm between these cables is necessary to avoid the risk of coupling on lightning strike. A metal cover, placed on a trunking where a disturbing cable passes, reinforces the coupling.

- The most polluting cables should be located in the corners of the trunking and as far as possible in a buried trough or at least as close to the ground as possible in order to reduce the surface of the emission loop. This loop is made up of the conductor and the connections to the earth or ground plane.

Shielded cables are placed in the middle of the trunking.Sensitive cables that cannot be separated from polluting cables must be shielded. Shielding provides protection against electric or electromagnetic fields. A shield must be continuous from one end of the conductor to be protected to the other. Any discontinuity in the shielding allows radiated interference to enter. The shielding of cables must be connected to the ground network. Long cables, which connect equipment located on sites where equipotentiality is not ensured, must be equipped with protection (galvanic separation, lightning arrester) and unless absolutely necessary, the shielding must be connected at one end only.

- In the general case where the wanted signals are of a sufficiently high level compared to the interfering signals, the cable shields shall be connected to earth at both ends and regularly to adjacent earths along the route of the link.

However, this rule must never be an obstacle to the instructions of the manufacturers of the equipment used. - In special cases where there is a disturbing coupling between the disturbing current flowing in the shield and the useful signal flowing in the active conductor, then and only for short connections, the shield should only be connected on the sensitive device side. This means that potential rises between conductor, screen and ground must also be controlled at the end where the screen is not connected to ground.

In thecase of long links, one solution is to use a double shield, with the first shield being connected to ground on both sides and the internal shield being connected to ground only on the side of the equipment to be protected.

- A single shielded cable, connected at one end only, is ineffective in H F - Shielded cables should be entered into electricalenclosures by connecting the shield to thedevice ground at 360°. Avoid twisted braids stuck in a terminal block.

- A shielded shielded cable that is not connected correctly is totally ineffective.

Immunity of electronic equipment

The entire installation of the grounding network and of the meshed masses being carried out, the separation of high and low currents being carefully respected, the shields connected correctly, the quality of the wiring is ensured. The immunity of the equipment to be protected must now be considered.

When the concern for the protection of an electronic system is taken into account as soon as it is defined, the characteristics of the sensitive equipment's resistance to disturbance are defined in the specifications for this equipment. The requirements of the specifications will be all the more stringent the greater the exposure to risk and/or if a malfunction is considered to be a major problem. However, whether the equipment is already on site or has yet to be defined, it is essential to know its characteristics in terms of resistance to disturbances. - In the first case, this knowledge will make it possible to determine the importance of the protective measures to be implemented, - in the second case, the comparison of the performances of the different equipment considered will offer the possibility of an informed choice.

Susceptibility During these tests the equipment is subjected to different types of disturbances and according to the operating characteristics set by the manufacturer it is possible to determine where the limit between susceptibility and immunity lies. In order to comply with the requirements of the CE marking, the equipment tested must be subjected without prejudice to disturbances of a level determined by the applicable standards. If we refer to the definitions given by the relevant IEC standards, we can say that the level of immunity required ensures trouble-free operation in industrial office applications where the wiring is carried out by separating the various voltages. For other operating conditions the user's requirements must be specified.

Rapid transients. This involves checking the behaviour of equipment subjected to randomly generated, steep-front parasites in regular bursts. Static electricity. This test determines the ability of the product to withstand electrical discharges produced by contact with or proximity to a high potential source. These very low-energy discharges simulate, for example, contact with a person who has accumulated an electrostatic charge by rubbing against plastics. The voltages can reach 15000V and can be destructive to accessible parts of the electronics, such as displays or LEDs.

-Radiated susceptibility. The product is subjected by radiation in the air, in vertical and then horizontal polarity, to a radioelectric source, varying from a frequency of 80 Mega hertz to 2.7 Giga hertz. Throughout the test the behaviour of the inputs/outputs is observed and must remain in conformity with the characteristics described by the manufacturer. Displays, LEDs and all other visible parts shall be monitored, the values of the inputs/outputs shall remain within the defined limits, the recordings if the device contains them and in general all the information presented or reproduced by the device under test shall comply with the manufacturer's specifications. -Driving susceptibility. The product is subject to parasitic pulse trains (0,15 to 80MHz) brought by the wires connected to it.

Lightning shock. The lightning shock resistance test involves the application of standardised 8/20 hybrid wave shocks in common mode on all the product accesses. The discharges are caused when the generator load has the voltage specified for the CE marking, depending on the channel tested: i.e. 2000V on the low voltage supply channel and 200V on the signal and telecommunication channels. The current generated must not cause permanent malfunction of the equipment under test. In practice, it is often useful to ask the manufacturer to specify the value of the voltage supported before destruction. These figures must be known for each individual channel.

Emission As the name suggests, emission tests are intended to determine the level of radio frequencies generated by the equipment under test! Radiated emission: measurement of the level of disturbances that the product sends into its environment, which may cause malfunctions in other equipment used in its vicinity. The measurements carried out must comply with one of the templates defined by the standard. Conducted emission. For lower frequencies, from a few tens of Kilo hertz up to 80 Mega hertz, measurements are carried out on the conductors of the product, in order to determine the level of disturbance injected by the equipment on the wired networks. Here too, the gauges must be respected.

Chapter 1: LIGHTNING PROTECTION: Lightning Reminders

Chapter 2: PROTECTION AGAINST DIRECT SHOCKS  

Chapter 3 PROTECTION AGAINST INDIRECT LIGHTNING STRIKES

Chapter 4 : PROTECTION AGAINST INDIRECT FOUNDATIONS (continued)

Chapter 5: PROTECTION PERFORMANCES OF FOUNDATIONS