Reasons That Make It Fard To Break Any Salat

1 - Salat must be broken in order to save a person who screams for help, to save a blind person who is about to fall down into a well, to save a person who is about to burn or drown, or to put out a fire.
2 - When your mother, father, grandmother or grandfather calls you, it is not wajib [1] for you to break a fard[2] salat, but it is permissible; yet, you must not break it if it is not necessary. But supererogatory salat [even the sunnats [3]] must be broken. If they call for help, it is necessary to break the fard salat, too. If they call you knowing that you are performing salat, you may not break even the supererogatory salat. But if they call you and do not know that you are performing salat, you have to break it.

[1] wajib: (act, thing) never omitted by the Prophet, so almost as compulsory as fard and not to be omitted. Wajib al-wujud: Being whose existence is indispensable and nonexistence is impossible.
[2] fard: an act or thing that is commanded by Allahu ta'ala in the Qur'an al-karim. Fard (or fard) means (any behaviour or thought or belief which is) obligatory. Islam's open commandments are called fard (pl. faraid).
[3] sunnat: i) act, thing that was, though not commanded by Allahu ta'ala, done and liked by the Prophet ('alaihi 's-salam) as an 'ibada (there is thawab if done, but no sin if omitted, yet it causes sin if continually omitted and disbelief if disliked; the Sunna; i) (with fard) all the sunnas as a whole; ii) (with the Book or Qur'an al-karim) the Hadith ash-sharif; iii) (alone) fiqh, Islam.