I have heard of CW and CWI ballasts, how do they compare to regular CWA? Also, I want to know more about those 3-coil ones.

CWI stands for an isolated output, so it allows the primary to be connected across two phase wires (e.g. using 240V), but keep the lamp socket shell grounded. This is requirement of e.g. Canadian code, whichat teh end mandates the use of these ballast for any supply above standard grounded Neutral 120V...
Compare to a CWA it costs an extra winding wire, so more mass and higher cost and/or more losses.

The triple coil is a "mag-reg" ballast, where the first and second coils form a voltage stabilizer, the second and 3'rd then the main lamp ballast, usually of a HX type.
Compare to CWA it means way better regulation, maintaining the shape of the load characteristic more tilted (higher arc voltage causes lower curent), so the power does not increase that much as the lamp ages, so improves the thermal stability. All that without compromising the robustness against the mains fluctuation. In fact it combines the HX load characteristic (essential foe many lamp types, mainly those of saturated vapor design) with a CWA line variation compensation ability.
The price to pay is even higher cost, weight and losses, even higher than the CWI.

I believe here in Canada, standard CWA ballasts can be used on 347V supplies because it is phase-to-neutral. However, as Medved states, 208V and 240V have no neutral and require a CWI ballast in Canada. I don't believe there are any special electronic ballasts for Canada, I think any electronic ballast, be it fluorescent, HID or other, can be connected to 240V with no neutral, provided said ballast is designed to run on that voltage.

I believe here in Canada, standard CWA ballasts can be used on 347V supplies because it is phase-to-neutral. However, as Medved states, 208V and 240V have no neutral and require a CWI ballast in Canada. I don't believe there are any special electronic ballasts for Canada, I think any electronic ballast, be it fluorescent, HID or other, can be connected to 240V with no neutral, provided said ballast is designed to run on that voltage.

They can be driven without any issues, it's just required to meet code the socket shell can't ever be live so you need a CWI or a SCWI so you can actually ground the shell, this is done so that if for some reason if only one live is switched there is never a risk to the person re-lamping being exposed to a live shell in what they think is an un-powered fixture, but short of code a 240 volt ballast of any type will work just fine for testing and playing around with on 240 but not for permanent installation, and please use a dual pole switch and kill both lives before working on it, and while it's not super critical on 240 we use 600 volts in Canada phase to phase in some locations and 347 so it's not something to mess around with which is why the code addresses ballasts when used without a neutral, you don't normally get second chances with 600 volts..

And depending on where you are located in Canada you might actually need a permit to work on or install a ballast with an input voltage greater then 150 VAC to ground... 

And expect to see more SCWI and SCWA in the USA they are for the most part EISA 2007 complaint and they also have a much better input to output regulation allowing up to a 45% voltage variation before affecting the bulbs output or drop out..




   

To be exact, the Code does not require isolated ballast per se, but requires the lamp shell to never be connected to any live vltage when it could be touched (= when the lamp is not yet fully in).
Having an isolated ballast is one way how to meet that, using power system with one input with no voltage towards ground (so Neutral on e.g. 120V) another one.
Yet another one is the use of a socket, which connect the shell only when the lamp is fully in (= same as required for domestic sockets in Europe regardless of the connection).
The last one is practically the only option with any electronic ballasts, as these all have both outputs live with respect to any input terminal. Therefore the popularity of all the different two-pin sockets instead of the old fashioned screw base with the modern lamps, as that is way cheaper way to meet the code requirement than redesigning the standard screw sockets (mainly Mogul are problematic, because of the large gaps they need around the lamp; the Medium are way easier - all European E27 are already designed that way, so ready made components and tooling are present).

In Europe these requirements apply only for installations accessible for untrained people (households,...), the E40 are on installations, where you need some form (exact form depends on a country) of electrician license anyway and there it is expected the knowledge of the workers (which is supposed to be "tested" with each license renewal) is strong enough to ensure the safety (the ability to safely work on really live things)

Medved, while I will agree with you, common scene doesn't apply to the code, and they are talking about the Canadian Code, and some how that got added to the topic but it's pretty clear when it reads "lamp socket shell must be grounded" and the only way to do this is either use a CWI or SCWI ballast, or do like I simply do, use a 120 or 347 volt ballast and call it a day, so you can have the shell connected to the neutral line, I have almost never seen an application where you can't avoid the use of a CWI with a little planning or rewiring, and in places where you get really stuck and have to order a CWI are places like this, the light is miles from a home on a farm and they only ran 240 to power a well and the yard light, and if it's only one or two it's not an issue, CWI ballast don't cost that much more, and it's way cheaper then running another wire, and while most people will just use the 240 volt tap of a quad tap ballast I like things to code if I'm getting paid, because if they fail or catch on fire I know I can say it was done to code and not a hack, and if the light is miles away I normally use a SCWI because it will survive much greater voltage variations, and the lamp doesn't drop out when the well pump kicks in, which does happen sometimes with a CWI or other ballast.. 

Is there any advantage of CWI over CWA when it's used on 120 volts?

The Canadian electrical code does not make much sense indeed....

 - It assumes that Neutral is safe to touch because it is Neutral (in the L-N connected loads, without the CWI). It takes exactly one bad connection to make the Neutral and socket shell connected to it live, giving somebody replacing a lamp a nice Pikachu, maybe at ballast Voc and maybe at HID ignitor output voltage, depending where the bad connection is

 - It would be far more safe and energy efficient to just demand disconnecting both supply leads for switching of L-L connected loads, than any isolated ballasts. Even including a contactor in every luminaire (!), that breaks both connections to the lamp holder when the input power is switched off on one Phase only (!!!) would be cheaper and more energy efficient than the CWI vilekulla. Nevermind the much simpler option of a 2-pole microswitch disconnecting the inputs when the luminaire is opened

 - When somebody does get a pikachu from the output of a CWI (from touching the other terminal, for example lamp ruptured in hand) the RCD won't do anything even if there is an RCD in the circuit

 - No possibility of using screw base lamps with electronic ballasts ?

The Canadian electrical code does not make much sense indeed....

 - It assumes that Neutral is safe to touch because it is Neutral (in the L-N connected loads, without the CWI). It takes exactly one bad connection to make the Neutral and socket shell connected to it live, giving somebody replacing a lamp a nice Pikachu, maybe at ballast Voc and maybe at HID ignitor output voltage, depending where the bad connection is

 - It would be far more safe and energy efficient to just demand disconnecting both supply leads for switching of L-L connected loads, than any isolated ballasts. Even including a contactor in every luminaire (!), that breaks both connections to the lamp holder when the input power is switched off on one Phase only (!!!) would be cheaper and more energy efficient than the CWI vilekulla. Nevermind the much simpler option of a 2-pole microswitch disconnecting the inputs when the luminaire is opened

 - When somebody does get a pikachu from the output of a CWI (from touching the other terminal, for example lamp ruptured in hand) the RCD won't do anything even if there is an RCD in the circuit

 - No possibility of using screw base lamps with electronic ballasts ?

We are allowed to use electronic ballasts, they are basically a switched mode power supply and are isolated by there very design they have an inductor and opto-isolators so there is no physical connection to the mains supply.
 
The trick is not to challenge the code or the inspector but to use your knowledge of the code to do what you want to do, and I've never seen an inspector fail a light using 347 or 120 with any ballast you can get and you don't have to worry about grounding the shell.. 

Plus isn't it good practice to only bond neutral and ground at one point to prevent back feeding of power through unwanted paths if something shorts to ground?

Plus isn't it good practice to only bond neutral and ground at one point to prevent back feeding of power through unwanted paths if something shorts to ground?

The Code has prohibited re-grounding the neutral after the service since the 1923 edition, it's bad...

But if you have an isolation transformer or ballast there is no ground return so you can ground either side of the transformer or ballast.. 

We are allowed to use electronic ballasts, they are basically a switched mode power supply and are isolated by there very design they have an inductor and opto-isolators so there is no physical connection to the mains supply.
 
The trick is not to challenge the code or the inspector but to use your knowledge of the code to do what you want to do, and I've never seen an inspector fail a light using 347 or 120 with any ballast you can get and you don't have to worry about grounding the shell.. 

So your electronic ballasts too follow the isolated designs..

There is no trick - you have no choice but to follow the code, but this does not contradict the fact that the code is idiotic in the first place