Scaling systems basically consist of powerful electronic components. And that`s the very reason why these systems are prone to damage through bolts and excessive tension voltage.

A LIGHTNING STROKE

Air movements are caused by warm, rising air (thermals) and by cold, falling air.While the air is rising, it is cooling down.A cloud consisting of water drops is emerging.Higher up this cloud is then consisting of ice crystals.A cloud in a thunderstorm travelling at high speeds is such a system.

Inside this particular cloud the air is rising and falling, thus causing an electrical charge. Whereas the water drops in the upper part of the cloud are positively charged, the ice crystals in the lower part get a negative charge.So electrical charges become seperated. Warm updrafts sweep positive charges aloft, leaving the bottom of the cloud negatively charged.The attraction between the ground and the negative charges in the bottom of the cloud creates the lightning stroke, a brief current of negative charge that travels from cloud to cloud or from cloud to ground (15 %).

Lightning seems to strike tall objects preferably.Trees, towers and masts are favorite targets.

Since the ground beneath the cloud has far fewer negative charges than the bottom of the cloud, there is an attraction between both.Therefore, any electrons liberated near the ground are pulled down toward the ground.An electrical connection is completed between the cloud and the ground.The ionized air becomes the path of the main bolt of lightning.

This bolt may induct excessive tension voltage into underground power lines.It has been shown that a bolt of light hitting a railway scale system within an area of 300 m in diameter definitely has a negative impact on that system.On the other hand charges travelling from cloud to cloud do cause a profound elctromagnetic impulse,that can cause damage as well.

DAMAGE TROUGH LIGHTNING STROKES

Flexible variable resistors are the core element of modern scaling systems.These resistors are made by coating an epoxyd-or polyamid band.A scaling line is usually consisting of four or a multiple of four flexible variable resistors forming a Wheatstone-Bridge-Configuration.

The Wheatstone-Bridge injection lines are synchronized with several resistors.These resistors compensate changes in temperature and calibrate the output signal of the scaling line.

Flexible variable resistors can only resist low voltages of 15 to 20 volts.The resistance of the foil carrier is approx. 400 to 500 volts.

Excessive tension voltage destroys the scaling line by destroying its resistors or by damaging the foil carrier that is connecting the bridge with the casing.In both cases the component may entirely burn out.Maybe only part of the scaling line wiring or the resistor`s adhesion is damaged.In this case the scaling system doesn`t work properly or doesn`t return to the zero point.Sometimes these problems only occur weeks after the lightning stroke.In addition to that the measuring device, printers or card reader may be damaged.But again, only a minor part of the electronics may be affected, like some condensers, which are sometimes overlooked. That`s why it is immportant to check the circuit board for damages or burnt remnants.

LIGHTNING PROTECTION

Total protection can only be achieved by protecting the entire system.While deciding whether you protect a system enirely or only partially you should consider the following points:

-place (inner city,open country)
-frequency and density of lightning strokes
-costs through time of nonuse
-costs for spare parts and employees
-demands of the insurance companies

There is internal and external lightning protection.External protection means that there is a connection between the steel roof of the control room and another tall lightning rod.But this external protection is harshly questioned.On the one hand a bridge scaling system is not a favorite target for bolts, but the lightning rod does provide an attraction radius of 81 metres.

A bolt that hits a building, a mast or a tree in a distance of 60 metres generates a surge in the very near surrounding of the scaling system.Without proper protection such a surge causes an electromagnetic impulse that damages the scale severely.

high of the construction (m)	attraction radius (m)
5	30
10	50
15	67
20	81
25	95
30	108
40	132
50	155

In the table above you can find the attraction radius of a construction of a certain height.Masts to protect bridge scaling systems protect the scale from being directly hit by the bolt, but on the other hand they actually raise the risk for the scale to be hit. Internal protection is based on the creation of an equipotential bonding inside the whole system through shrouds and by building the scale in the shape of a Faraday cage.There is another Faraday cage around the control center and the measuring booth.A relatively short distance between the two cages demands an earth link, that means the potential of the whole system is always stable and balanced. An equipotential bonding is automatically included in any scaling system made by Paari GmbH & Co. KG. All external connections like alternating current lines,communication plug ins and signal injection lines are a potential source of surges and excessive tension voltage.The installation of an overvoltage protector at the entrance of the Faraday cage for any incoming and outgoing line is fundamentally necessary.