electron levels in a high voltage conductor what is the electron energy level in a 300,000 volt power line? This voltage is way above the ionisation potential but electrons are not emitted from the wire. 
 A: The electron energy in the line is $300kV$ * the electron charge $q$.  Whether or not electrons are emitted depends on the gradient of the voltage, i.e the electric field, at the surface of the wire, which determines the force on the electron.  See the wikipedia article on field electron emission.  
A well-designed high-voltage line keeps the peak electric field low enough that emitted power (power that is wasted) is small enough to be economical.    
A: (As far as I know, 300kV is the exclusive domain of HVDC so I will not mention AC here.)
Ionization potential is the energy difference between a free electron at rest outside an atom, and a particular electron energy state inside the atom. If you zap a bound electron with enough energy, it can assume a new, unbound wavefunction and travel away.
But, tunneling effects aside, that wavefunction is still localized to the same general place, which is inside the insulator (or air) surrounding the wire. If the electric potential at that point in space is little different from inside the metal, ionization is not energetically favorable.
Consider the instant the power line is energized. Suppose it immediately jumps to –150kV (relative to its surroundings, in equilibrium with the earth). Electrons will be torn from its surface, and migrate outward. Upon encountering air molecules, they form negative ions which migrate outward as a wind. The wind encounters physical resistance, which sets up a strong electric field. Over a little while the system reaches a dynamic equilibrium, because the air must support an electric field to keep the electrons inside the wire. (Or, for the positive line, to keep electrons out.)
If the wire is insulated, the insulator will contain some of the electric field as well, and provide an unfavorable environment for free electrons (raising the ionization potential).
In fact, this effect is called corona discharge and it is a major source of loss for HVDC transmission.
A: Thanks for the interest.  On thinking about the matter the electron is still in the conduction band of the metal and is only moving as part of the electron "gas" in response to the voltage in the line. We are told in solid state theory that in a metal the elecrons are delocalised and can move through the crystal lattice in response to a potential difference. Its energy level relative to a metal atom is not affected. (A special case is in superconductivity, the electrons are quantum linked and feel no resistance). I was thinking about atomic energy levels and Fermi levels in relation to the large pd in the line when I posed the question. 
