Basics of a protection concept for mobile phone systems

Electronic equipment and systems, especially communication technology, are often installed in environments exposed to electromagnetic disturbances. Transmitting and receiving systems in mobile phone networks are primarily installed at open spaces and hence are subjected to atmospheric discharges.

A complex protection concept comprising grounding, equipotential bonding, external and internal lightning protection is therefore required to ensure protection against surge voltages due to lightning discharges and switching operations.

Years of experience in the protection of base stations of leading national and international providers of mobile phone services has shown that only a multi-stage protection system can fulfill the requirements for the availability and safety of assets. Multi-stage surge protection comprises the following components:

Lightning arrester based on spark gaps,
type I (class I)
Surge arrester based on varistor (MOV),
type II (class II)
Device protection based on varistor,
type III (class III).
These are often included in the input circuits of DC power supply units and system technology.

Lightning arrester based on spark gaps,
type I (class I)
Surge arrester based on varistor (MOV),
type II (class II)
Device protection based on varistor,
type III (class III).
These are often included in the input circuits of DC power supply units and system technology.

Protection concept

The surge protection concept can be implemented uniformly at all locations. It is based on the latest technical standards and also takes economic aspects into account.
The principle structure of base stations comprises the following modules:

Electrical subdistribution AC 230/400 V (UV)
48 V DC power supply unit
Radio link
Base transmitter station system technology (BTS or node B)
Feeder cable

Principles of planning

The following principles must be followed when planning the location and implementing the surge protection concept:

Structure and planning of external lightning protection system
Planning of the surge protection system
Grounding and equipotential bonding
Classification of lightning protection zones (BSZ)
Installation as per DIN EN 62305 parts 1-4
Installation as per DIN VDE 0855 part 300

The following measures must be taken into account as well:

Verifiability of surge protection devices as per DIN EN 62305 part 3
EMC compliant control cabinet design of the electrical subdistribution with a final inspection in a suitable laboratory
Installation of individual modules in the lightning protection zone 0B or higher

Protection of base stations

Outdoor base stations are normally located on buildings. The building and BTS can thus be considered as two adjacent buildings to which power is supplied through a common source. Since the protection zone concept is used, both are assigned to BSZ 1. Interfaces must therefore be connected with suitable surge protection devices.

In practice, protecting the power feed of the building with a high-performance type I arrester has proven to be adequate. On the other hand, cost-intensive and sensitive system technology is effectively protected by an arrester combination (type I + type II) with an active energy control.

Individual modules of base stations are often located in separate buildings. Ideally, cabinets should be arranged in one row. If this is not possible due to structural conditions, cabinets are installed individually. Inputs and outputs of supply cables form a transition from BSZ 0B to BSZ 1 and must be connected accordingly.

Alternatively, cabinets can be connected with each other using a closed and galvanically connected cable route (steel conduit, steel sheet cable troughs). Owing to this "protuberance", the interiors of cabinets are integrated into a BSZ.

Principle of the protective circuit

The principle of the protective circuit must be used to satisfy the requirements of a complex protection concept. An imaginary circle defines the volume to be protected. All cables intersecting this circle must be connected to suitable surge protection devices.

In the case of a base station, the 230/400 V AC power supply, the fixed network connection (2MB) and existing alarm and signal cables must be protected. Coaxial feeder cables [image 3], electrical connections of radio link modules and peripheral devices (e.g. air conditioner) also belong to the protective circuit.

Future applications with the so-called remote radio heads primarily use a DC power supply that needs to be protected as well.

Requirements for surge protection devices

Using lightning current and surge arresters in the systems of providers of mobile phone services has become a standard. IEC 60364-5-53 demands for a lightning current test impulse of at least 12.5 kA(10/350 µs) per channel when lightning arresters (type I) are used in three-phase power supply systems. With a lightning current arresting capacity of approximately 3 kA(10/350 µs), varistor arresters that are primarily used as type II arresters do not fulfill these requirements.

The requirement for the lightning current arresting capacity of the entire arrester combination has been directly derived from lightning current classification as per IEC 61024-1. Unless a risk analysis is carried out, it is assumed that 50% of the lightning current is distributed in the supply cables connected to the system.

DIN VDE 0855 part 300 stipulates the lightning protection level III with 100 kA (10/350 µs) for lightning protection measures in radio/transmitter-receiver systems. As a result, there is a requirement of at least 50 kA (10/350 µs) for the entire arrester combination for lightning protection.

Circuits up to the lightning protection level I (200 kA (10/350 µs)) are available with Phoenix Contact. They are equipped with integrated floating PDT contacts to indicate both optically and remotely that the arrester combination is functioning correctly.

Instable networks

Strongly fluctuating networks with voltage values greater than the rated surge arrester voltage UC indicate undue high load for surge protection devices. It has been observed that surge protection devices used in the field of mobile phones, where long power supply lines are often used, get damaged despite selecting the correct lightning current arresting capacity. The reason can often be attributed to short circuits in the power supply unit that could not be switched off on-time (within 5 s) by a safety device owing to long cables and, sometimes, due to extremely small cross-sections.

In the case of a three-phase system, paths of two surge protection devices are exposed to an undue high voltage during this period. Interrupting the neutral conductor may have the same consequences. It has been often observed that only varistor arresters are visibly damaged. These devices have been designed for protection against transients and surge voltages in the µs range, but not for the stationary surge voltages that prevail over a longer period.

The thermal disconnect device requested as per IEC 61643-1 has only been designed for a gradual aging process and hence it cannot disconnect the arrester from mains in time. A risk of fire cannot be therefore ruled out. IEC 61643-1 describes TOV tests (TOV: Temporary Over Voltages). According to them, arresters can be operated with an increased voltage for 5 seconds:

UTOV = 1.45 x U0
= 1.45 x 230 V
UTOV = 333.5 V

Devices must pass this test with either TOV-failure (test passed, arrester is no longer functional and no risk of touching live parts) or TOV-withstand (test passed, arrester is fully functional). Using the arresters that pass this test with TOV-withstand is recommended in the IEC 60364-5-53 installation directive.

Only the arresters with UC > UTOV are both fully functional after this test and also suitable for working with a higher voltage over a longer period without getting damaged. A rated surge arrester voltage of UC = 350 V was recommended for a standardized global usage.

Verifiability of surge protection devices

The continuous availability of the entire protection system requires periodic inspection. The lightning protection system and all its components must be especially checked as per DIN EN 62305 part 3.

If automated test devices are used (image 4), all surge protection devices can be checked comfortably and conclusively. Universal pluggability of all surge protection devices increases the availability and also contributes towards reducing maintenance costs.