In single phase case; 220 v is potential difference between phase and neutral, whereas in three phase case potential difference between any two phase terminals will be around 415 v. All three lines show a potential difference of aprox 220 v with respect to neutral but are but are apart from each other by an angle of 120 degree (phase difference) so will exhibit almost double potential difference between any two line terminals. So for all practical purposes potential difference between any two phase terminals will be around 415v. However for power calculations etc we multiply 415 with squareroot of 3 to get actual total line voltage which is a sine function of phase angle between them.

In three phase power line, each phase is 120 degree from the other phase.therefore, when we draw a graph of phases then two phases have positive values then at the same time the third one has negative value so we can calculate and use only two phases at a time.all three phases changes their position every time so the two phase may be phases 1 &2/phases 1&3 or phases 2&3 gives the values and at the same times phase 3 or 2 or 1 will be at negative interval respectively so cannot be calculated or contribute the value. that's the reason we use 440v instead of 660v.

The question is incorrect, the line voltage is not 440v, it
will be 400v if your single phase is at 230v.
Reason why its 400v is because if you put a voltmeter across
both live phase voltages, it show 400v. But you will need a
picture showing waveforms to explain it properly.

that is where frequency does matter, because with frequency one of the three phase become neutral after a timeperiod. so for that reason at a moment two phases are showing 440v in pakistan).

Voltage is nothing but potential difference, so we can take
only 2 objects if we want to measure the difference between
them. So we always measure this difference of potential
between 2 not 3.

is KVAR (not reverse power) relay necessary to protect from
excitor field failure on a brushless, synchroneus generator?
Some people with previos experince tell me no but the
manufacturers tell me it is designed for that purpose.
Is it that it is only necessary for brushless generators
and not the older style with the slip rings and brushes?