Monitoring a monolith

The taller a building, the more susceptible it is to damage from lightning strikes, which can cause devastating damage to both the structure itself and to plant equipment.

Depending on the strength of the lightning strike’s current, large-scale damage or destruction can occur, which often leads to further adverse effects. Since cities are becoming larger and buildings taller — in keeping with the trends toward urbanisation and megacities — lightning strike measurement is becoming increasingly important. Phoenix Contact developed the LM-S lightning monitoring system to detect and evaluate lightning strikes and it is now being used to measure values at the top of the tallest building in the world, the Burj Khalifa in Dubai.

Lightning sensor on the Burj Khalifa.

Magneto-optic Faraday effect

Construction of the 828-metre Burj Khalifa was completed in 2010. With a total of 526,760 m² of floor space, the building includes a hotel, restaurants, offices, private residences and two floors of underground parking. Fifty-seven elevators connect the 160 habitable floors and the building also features the world’s highest elevator stop, at a height of 638 m. Under windy conditions, the deflection in the top floors of the building is approximately 1.5 m.

The LM-S lightning monitoring system consists of an analyser, a connecting cable and up to three sensors. If lightning strikes a lightning rod such as the one installed on the top of the Burj Khalifa, a magnetic field is created in an arrester that carries the lightning surge current. LM-S uses the magneto-optic Faraday effect to measure these surge currents in lightning arresters. Moreover, the light is polarised in the measuring section located inside the sensor. If the sensor’s measuring section detects a magnetic field that has been caused by a lightning strike, the previously polarised light is bent in a measurable way. The lightning event’s characteristic values — amplitude, maximum slope, specific energy, charge — are determined and stored together with the date and time of the lightning strike. The sensor transmits the light signal to the analyser via fibre-optic cable.

Magnetic field B is created by the lightning current flowing in the conductor and affects the measuring section — polarisers set at 45° to one another determine the impact of lightning current in the measuring section.

Learning from wind power

The lightning monitoring system in Dubai was installed under extreme conditions, and it is also expected to make record-breaking measurements in operation. For example, 10 m cables suffice to connect the sensor in the blade base with the analyser in the hub in wind turbines, whereas the cable used in the Burj Khalifa measures 200 m in length. LM-S was originally developed to prevent lightning strikes from damaging wind turbines, since unscheduled servicing and repairs at these sites repeatedly incur high costs.

In wind turbines, the lightning monitoring system supplements status monitoring — drawing on typical strike characteristics experienced at wind farms and allocating damage symptoms allow for better preventive maintenance planning. Meanwhile, it is becoming increasingly clear that comprehensive lightning protection concepts in buildings can also benefit from this practice — skyscrapers in megacities even more so than single-family homes. Because of this, smart monitoring systems are also used in buildings — these permanently monitor operational states and report the results directly to a central control unit.

Extreme environmental conditions

A lightning rod is placed on the highest point of a building, as is the case with the Burj Khalifa. The lightning sensor was attached to the rod with metal cable ties. Usually, the sensor is black, but in this case a special white design was selected because of the extreme exposure to direct sunlight at this location; a white sensor absorbs less heat from sunlight. Due to the extreme installation conditions, every single work step needed to be tested multiple times.

Installing the lightning monitoring system in the Burj Khalifa — the lightning sensor at the very top of the building is connected to the analyser in the server cabinet via fibre-optic cable.

The analyser in the Burj Khalifa operates in an air-conditioned server cabinet at a height of 680 m — this spot in the building provided the required space as well as the necessary power supply and data interface. The over-200-metre-long fibre-optic cable between the sensor and the analyser was specially tested to ensure that it could withstand these environmental conditions. It has a temperature resilience of over 70°C. When the cable was being laid, it had to be fixed in a particularly secure way; at every 10 m in altitude, a small segment of the cable is laid horizontally in order to prevent the cable from stretching and drooping under its own weight.

Control cabinet at 680 m — the analyser detects the signals from the lightning sensor and forwards them to the network via the data interface.

Maximum values of over 50 kA

The lightning monitoring system installed on the world’s tallest building has already recorded several lightning strikes that hit the very top of the building, all of which had maximum current values of over 50 kA. The building operator uses this information purely for statistical evaluation, since the building already features a comprehensive interior and exterior lightning protection system. Potential irregularities in the Burj Khalifa’s electronic and building facilities systems are checked against current readings from the lightning monitoring system.

When lightning strikes are measured in buildings or wind turbines, this enables conclusions to be drawn at all times about how the lightning parameters relate to the damage they cause. Moreover, evaluating specific events makes it possible to draw conclusions about the effectiveness of the lightning protection system. Claim settlements also draw on information gathered on lightning strikes with the help of lighting information systems, which can locate a lightning strike with an accuracy of 200 m. After all, one can only determine whether and where a lightning strike hit a building by using a lightning current measuring system — such as the Phoenix Contact LM-S.

**************************************************

Integrating measurement readings

The analyser features an Ethernet interface that integrates it into a standard network, which allows the LM-S operator to easily access data for permanent monitoring. If a mobile network is within range, communication can be handled independently of the existing network structure. The system can be configured and data can be called up using an internal web server. This allows the system to be accessed remotely at all times, eg, by using a smartphone.

The analyser also contains a switching relay with a remote contact fed out of it. This NC contact produces a short pulse for every event that can be evaluated by a counter. This allows the number of lightning strikes to be recorded. The relay contact does not move into its rest position until the system has started up. The relay de-energises in the event of system malfunctions. This means that system readiness can also be queried using the remote contact. When the signal is connected to a controller, a follow-up action can be initiated once lightning has struck.