ve2pid wrote:
I am modeling a square loop. The feed point is at the center of the
base segment. But I want to have a gap at the middle of a vertical
wire, in order to place a relay there to switch to this feed point
instead of the base point, so it would change polarization.

Seems to me, that relay should always be closed during operation
from the base feedpoint so there will essentially be zero volts
across the contacts. Since the relay should not be switched
with the key down, your question seems irrelevant.

My question: does EZNEC provides a way to get the voltage at the
vertical (open point) mesured at the gap when the base source is
'active'? Just to be sure that arc won't be created there.

You don't want the gap to even exist when the base source is
active. The relay contact should always be closed when the
base source is active. The base source relay should always
be closed when the side source is active.
--
73, Cecil http://www.w5dxp.com