GAS TUBE SURGE ARRESTORS FOR RS-485

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 acquisition and communications infrastructure 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.

RS-485 Protection

RS-485 interface links deployed into Industrial and Instrumentation applications must function in harsh electromagnetic environments. Voltage transients caused by lightning strikes, ESD, and power induction can damage communication ports. Many RS-485 applications require data transmission between multiple systems, often over long distances making them particularly susceptible to electromagnetic events. The RS-485 electrical standard is the most widely used physical layer specification for Industrial and Instrumentation applications. RS-485 is used in industrial automation, process controls, motor controls, HVAC, security systems and renewable energy just to name a few. Surge immunity standards such as IEC 61000-4-5 call out levels of immunity required for equipment to function satisfactorily and reliably in its intended electromagnetic environment. The diagram below provides the basic protection circuitry for a an RS-485 port. In this circuit, a Gas Tube provides primary protection due to its high current carrying capability. Typically, an overcurrent series device provides coordination between the primary protection Gas Tube and a secondary protection device which is typically a TVS. In this coordinated protection scheme, the TVS device clamps the leading edge of the voltage surge until the Gas Tube turns on creating a low impedance current diversion path to ground and eliminating the overvoltage.

RS-485 Protection using GDT and TVS

Selecting the appropriate Gas Tube

Key considerations for selection of a GDT for RS-485 interfaces include system voltages, package type and surge ratings. In the circuit protection example above, a 3-electrode gas tube is selected to provide common mode primary protection. Selecting a GDT with a nominal surge rating of 5kA per line to ground or 10kA total, provides a high level of surge immunity. RS-485 pcb space is a consideration so a surface mountable and compact 5mm diameter size GDT is desirable.

Recommended GDT

Part Number	Circuit	DC Sparkover	In	Link
BM CMS(T&R) 150/20	RS-485	150V +/-20%	10kA

References