Lightning-Induced Voltage Spikes on Audio Cables

Outdoor venues can subject audio equipment to overvoltage failure caused by high voltage spikes being coupled into the cables connecting equipment.  Lightning strikes do not have to directly hit the cables.  They can magnetically induce high voltage spikes in the cables without directly hitting them.  The peak current of the lightning discharge can induce very large magnetic fields which couple into the lines.  The principle is the same that transformers operate on: a varying magnetic field coupling its energy into a conductor. 

How Gas Discharge Tubes Provide Protection

Gas discharge tubes (GDT) can help protect the equipment.  They are sealed tubes filled with an inert gas with electrodes inside.  When the voltage across the electrodes reaches a high enough voltage to ionize the gas, the gas rapidly establishes a very low impedance path between the electrodes.  This mechanism can be used to create a path to ground to direct the energy safely away.  It happens quickly.  Before ionization starts, it does not load the signal.  The electrodes are spaced apart so that even the capacitance between electrodes is nil for audio purposes.

Proper Grounding Requirements

When designing the system, care must be exercised to specify a GDT that can take the anticipated peak current that it must deal with.  Breakover voltage is engineered into these tubes at differing levels.  Take this into consideration when specifying them.  

The GDT system cannot do its job unless a good path to earth ground is established.  Typically, they connect to the chassis (frame) grounds of the protected systems.   If each chassis is not solidly connected to earth ground, there is nowhere for the discharge to go.  If the power mains system grounding is suspect, drive additional grounding rods into the soil.  Use heavy cables to connect them to the system ground.  If the soil is totally dry, it cannot complete the ground.  In that case, one possible solution is connecting to metal water pipes that are buried in the ground along their length connecting to the main supply of water. 

Specifying and Installing GDTs Correctly

Never place a GDT directly across a power source.  When there is a fault, it will continue conducting even if the fault condition has passed if the power source voltage is higher than the conducting voltage of the GDT.   There are other good solutions for protecting power sources.