GAS TUBE SURGE ARRESTORS FOR REMOTE RADIO HEAD (RRH)

OVERVIEW

The Insurance Institute for Business & Home Safety found that an estimated $26B dollars annually is lost due to non-lightning power surges. Additionally, the are an estimated 25 million lighting strikes in the US each year that cause between $650M to $1B in losses according to the Insurance Institute, State Farm . Critical data communications infrastructure such as 5G must be protected from threats due to lighting, power surges and power induction.

How Gas Tubes Operate

Gas Discharge Tube (GDT) Surge Arrestors operate on the principle of arc discharge. Operating as a voltage-dependent switch, an arc is formed within nano-seconds inside the hermetically sealed discharge chamber once a voltage exceeds the GDTs spark-over voltage. During its on-state, the gas tube essentially forms a short circuit allowing the entire surge current to flow and instantaneously eliminating the overvoltage transient. Upon dissipation of the overvoltage event, the GDT device extinguishes and the internal resistance returns to its high impedance off-state. GDT devices reliably limit over voltages to permissible levels, can handle large surge currents and are invisible to the system being protected due to low capacitance and very high insulation resistance.

Remote Radio Head

A remote radio head (RRH), also called a remote radio unit (RRU) in wireless networks, is a remote radio transceiver that connects to an operator radio control panel via electrical or wireless interface. RRHs can be installed in a low-profile arrangement along a rooftop, or can involve a much higher tower arrangement. When installed at the highest point on a structure (whether a building or a dedicated cell tower), they will be more vulnerable to receiving a direct lightning strike and higher induced lightning levels, compared with those installed in a lower profile manner below the upper edges of the building. RRH and Antennas are typically powered by 48V DC that runs from the BBU to the RRH and tower mounted antenna making the it particularly suspectable to surges caused by direct lighting or induction. To mitigate threats from surge events a Gas Discharge Tube is typically integated into the RRH and antenna designs.

Remote Radio Head (RRH) System

Selecting the appropriate Gas Discharge Tube

Using a GDT device on a 48V DC power system does require some special consideration. IF the DC power supply powering the RRH is capable of suppling continuous current into the virtural short circuit created when the GDT turns on, care must be taken to assure the GDT device will reset. Citel's approach to using GDT on an RRH 48V DC power supply is to implement a multi-chambered single GDT surge protection device solution. In the case of the BFX3, 3 GDT devices are integrated into a series configuration and packaged into a single surge protection device (SPD). see figure below. The design of the BFX3-60DC-M8 is specific to 48V DC power supplies used in powering active antenna systems. During a surge event, the BFX3 provides a total on-state (arc) voltage that is greater than the 48V DC power supply voltage guaranteeing a reset of the GDT after a surge event subsides. In addition to the 3 integrated GDT device, the BFX3 also incorporates capacitors that aide in turning on all 3 GDT during a surge event which helps to limit the peak voltage. The BFX3 provides a single event surge rating of 60kA based on an 8/20µs current waveform making it very capable of handling the heavy surges experienced by tower mounted RRH and antennas.

BFX3 Multi-GDT Design

Recommended GDT

Part Number	Circuit	DC System Max	In	Picture
BFX3-60DC-M8	48Vdc	60Vdc	60kA

References