We study the role of dust-dust collisional charging in protoplanetary discs .
We show that dust-dust collisional charging becomes an important process in determining the charge state of dust and gas , if there is dust enhancement and/or dust is fluffy , so that dust surface area per disc volume is locally increased .

We solve the charge equilibrium equations for various disc environments and dust number density \eta , using general purpose graphic processors ( GPGPU ) and cuda programming language .
We found that as dust number density \eta increases , the charge distribution experience four phases .
In one of these phases the electrostatic field E caused by dust motion increases as E \propto \eta ^ { 4 } .
As a result , macroscopic electric discharge takes place , for example at \eta = 70 ( in units of minimum-mass solar nebula ( MMSN ) values , considering two groups of fluffy dust with radii 10 ^ { -2 } \mathrm { cm } , 10 ^ { 2 } \mathrm { cm } ) .
We present a model that describes the charge exchange processes in the discs as an electric circuit .
We derive analytical formulae of critical dust number density for lightning , as functions of dust parameters .

We estimate the total energy , intensity and event ratio of such discharges ( ‘ lightning ’ ) .
We discuss the possibility of observing lightning and sprite discharges in protoplanetary discs by Astronomically Low Frequency ( ALF ) waves , IR images , UV lines , and high energy gamma rays .
We also discuss the effects of lightning on chondrule heating , planetesimal growth and magnetorotational instability of the disc .