3 phase faults

[Bestboy]

My question is very basic.
We had a big discussion last night with my work mates about 3 phase faults. One engineer said that ALL 3 PHASE FAULTS ARE SYMETRICAL. He even explained that a fault in 3 phases would cause all currents to increase symmetrically and the reverse for voltages. I feel saying all 3phase faults are symmetrical is allitle bit radical. My reasoning is that if the fault occurs in all the phases, at the same point, and at the same instant then the fault will be symmetrical….may I call that ‘a perfect fault’? An example to this would be dropping a metal piece at a perfect position on busbars or cutting all the phases at the same point. So I want to believe that it is possible to have 3 phase faults that are not symmetrical because a perfect fault is almost an impossible happening. I would think that the origin of a 3-phase fault is actually between 2 phases. Then the 3rd phase get involved (to make it a 3 phase fault) due to the aftermath of the initial fault i.e. ionisation of air, flash, insulation breakdown, heat, rupture etc. Any input or correction about this?

 

[521AB]

Just some comments here, as this topic is quite popular in the relay protection field:

I generally agree with the fact that a "pure 3-phase fault with relative amount of fault resistance" does not exist. It is the evolution of a 2-phase fault into the third phase, asnd from relay point of view we have to pick.up already the 2-phase fault.
So usually the fault resistance in an impedance protection relay is calculated for the 2-phase faults and not for the three-phase ones.
A three phase fault is usually the load. That's why the fault resistance should be limited by the minimum load impedance (today's distance relas usually have the phase-phase loops responsible for "both" 3-phase and 2-phase faults).

Anyway I saw some of them, reorded by the relays:

1) fire below the overhead line. It looks th eionisation managed to be homogeneous and it was recorded as "pure" 3-phase fault with a quite symmetrical fault resistance.

2) 2-phase fault. Trip. Autorecloser. The other line-side did not trip and the fault evolves . When A/R recloses, it recloses on a "pure 3-phase fault with symmetrical fault resistance". Nice job for switch onto fault.

my regards