As we all knows Sun is very hot and fiery planet and we consider the same fiery nature of sun in Vedic astrology too -
However energy of sun is main source of life in this earth, and sun is considered as giver of life, however due to its fiery nature Sun is considered mild malefic planet in Vedic astrology, Because fire has no friendship with anyone its just act as per its nature and burn everything which comes closer to it.
Same thing happen with the Sun, If any planets which comes closer to the Sun gets burned and which is destroying the significance of the burning planets -
However in Vedic astrology distance between two planets is been measured by the degree's hence in ancient text there are certain degree has been described for combustion for specific planet -

Moon: Moon is under 12 degrees from Sun considered as Combusted.
Mars: Mars is under 17 degrees from Sun considered as Combusted.
Mercury: Mercury is in 14 degrees considered as Combusted. (12 degrees If in retrograde Motion)
Jupiter: Jupiter is under 11 degrees from the Sun considered as Combusted.
Venus: Venus is under 10 degrees considered as Combusted. (8 degrees if in retrograde Motion)
Saturn: Saturn is under 15 degrees from the Sun considered as Combusted.

However in modern world everyone including me is denying above parameters for combustion of any planet, because in modern time old parameters are not working 100 accurately and even all practical experiences and live feedback's from peoples are allowing all modern astrologers, not to trust on ancient written text books blindly.

So as per modern time and based on practical researches any planet gets start burning when its under 10 degree, and if planet is more closer to the sun like under 5 degree to 1 or less degree planet will be considered as burned or combusted.
However there is one debate i have seen almost everywhere, which is about combustion of Mercury and Venus, because they both are the planets which are always too closed to the sun and quite used to with Sun heat, So can we put Mercury and Venus under combustion concept ???
So answer is "Yes"

Mercury and Venus also gets burned if they goes too close to the Sun, however as they are quite used to with the Sun heat hence they become combusted when they are under 1 to 4 degree, Mercury is always either would be one house previous or next to the Sun or could be found in conjunction with Sun in horoscope and mercury cant break this boundary, Same happen with Venus also as Venus could be either one or max two houses ahead or previous to the Sun or could be in conjunction with Sun in horoscope, but as i said fire has no friends hence Venus and Mercury also gets affected when they are too closed.

However combustion of Venus and Mercury is really a matter of debate, as when we see horoscope of successful peoples or celebrities where Venus and Mercury conjunct Sun then we easily saying that Mercury and Venus are combustion free, however trust me if you refer chart of lower class peoples then you will actually realize that they both planets are also not free from combustion and really fire has no friends.
Another important point about combustion i would like to tell you, is that, while checking combustion of the planets, must need to check as which planet holds lowest degree, because its also matters a lot.
Suppose There is a conjunction of Mars, Sun and Venus in horoscope and Sun is in 06:00 degree, mars is in 10 degree, and Venus is in 02:00 Degree, so actually whats happening here as Sun is in 06:00 degree so as per other planets degree Mars is going away from the sun as hold higher degree from the sun, but Venus hold lowest degree from the sun and coming closer to the Sun, As an impact on human life Mars will do lot better after the age of 42 even being under combustion (As its going away from the Sun heat, so later with the time suffering coming due to combustion will also be less), However Venus will give more struggle and will create issues as going near to the Sun, Hence please keep this concept in mind while calculating combustion of the planets in horoscope.

However there are few parameters which are known as saving factors for the planet and even being in combustion position, planet can perform very well in horoscope and these are as below -
1 - If combusted planet is occupying its own or exaltation sign in horoscope.
2 - If Combusted planet is getting strong benefic aspect.
3 - If Combusted planet is sitting in the house where it gets Digbal (Directional Strength).
4 - If Combusted planet is exalted and well placed in divisional charts (Specially in D-9 & D-60).
5 - If Combusted planet is retrograde in horoscope.