  We consider tunneling from the spin-polarized tip into the Luttinger liquid
edge state of quantum spin Hall system. This problem arose in context of the
spin and charge fractionalization of an injected electron. Renormalization of
the dc conductances of the system is calculated in the fermionic approach and
scattering states formalism. In lowest order of tunneling amplitude we confirm
previous results for the scaling dependence of conductances. Going beyond the
lowest order we show that interaction affects not only the total tunneling
rate, but also the asymmetry of the injected current. The helical edge state
forbids the backscattering, which leads to the possibility of two stable fixed
points in renormalization group sense, in contrast to the Y-junction between
the usual quantum wires.
