To get a sufficient Tesla coil output voltage (hundreds of thousands of volts) we need a starting voltage that is somewhat higher than your normal household mains. The first step therefore is to boost the mains voltage up initially by using a conventional HV (high voltage) transformer.
A popular choice is the NST, or Neon Sign Transformer. These typically give output voltages of between 7,500 volts to 10,000 volts in Europe, or in the USA and other parts of the world, a considerably healthier 15,000 volts with an average current of 60 m/amps.
I have used the word 'healthy' from a performance point of view, rather than what happens if you come into contact with it.
An accidental brush against a live NST terminal with that level of current (60m/a) and 15KV will be extremely painful causing burns, or may even prove fatal.


From a safety point of view I use a switchable power feed to my coil. This is simply a box housing the 20amp variac and output ammeter and a key operated double pole switch. This is wired via a 16 amp plug although it often draws 21 amps with no ill effects.
The key can only be withdrawn when the power is switched off. So I make a point of ONLY ever going near, or making any contact with the NST or coil, if I have the key about my person. I normally have it in one hand as it has a nice big key fob. You soon remember to mentally check its there.

Another way if you have a small domestic plug fitted is to unplug and have the plug in your hand or pocket if a suitably long lead is used. Simple measures could save my, or your, life.
Given the right circumstances a NST power source may kill, as quick as I can type this comma, more likely though is that it will give you a long lasting memory of how painful a high voltage burn can be. The situation with MOT's from a Microwave oven and OBIT's (both lower voltage than an NST but much higher current) is even worse, as these stand a very good chance of killing you. While pole distribution transformers (pigs) and my own homemade transformer definitely will.








A Single 10Kv 48 m/amp unit)


NST's are not the most robust things as they were not designed to run at a high output voltage continually. Normally they would only be open circuit, and therefore at max voltage, for half a second or so untill the neon tube fires. The tube then presents a fairly low impedance short to the NST and its output voltage drops to 300 or so volts at its max current rating.
For this reason it can be a good idea to use alternative sources for a high voltage power supply, such as oil burning furnace's ignition transformers (OBITS) and Microwave Oven transformers (MOT).


High Voltage distribution transformers (often called 'Pigs') can be used if wired in reverse. By that I mean the output terminals are used for the input, and vice-versa. Old X-Ray machine transformers are usually too high a voltage, but the cores can be used to make either a ballast or to wind your own transformer.See link below

The availability of NST's still make them a popular choice though despite their fragility. Because of this a safety circuit to protect the NST is needed. See Safety Gap  and Terry Filter     







A Microwave Transformer (MOT)


With most transformers you will need to have a 'ballast' added to stop excessive current being drawn through the transformer's windings when the secondary is short-circuited. This is not needed with NST's (all NST's except some very old ones apparently). Because NST's are actually designed to operate with virtually a short circuit across their output at times, they are wound onto a core that is designed to self limit the maximum amount of current that can flow (so are most OBIT's).
If you use other types of transformers you will need to add some ballast (usually in the form of an inductor) in series with the primary (or secondary) winding of the transformer to limit the current that can flow.
MOT's do have some in built ballast in the form of shunts like the NST does, but they are pretty ineffectual, and you will most likely need some additional external ballast with these as well.