In an emergency how much and what kind of protection would a spacesuit provide protecting against an explosive decompression event in the habitat the wearer is in?
The first four Space Shuttle missions had ejection seats, which is basically an intentional explosive decompression. The astronauts wore Launch Entry Suits designed to survive the decompression and ejection process.
The current NASA suit is the Advanced Crew Escape Suit. Like the LES it derived from, it is designed for decompression and escape, although with a built-in parachute. But you have to be wearing it:
- Each astronaut wears an ACES during launch. This means that there are enough suits on-board for every crewmember.
- The suit connects to the vehicle's life support systems and will use it if available.
- Should the vehicle's air supply fail, the suit has 10 minutes of its own oxygen. This is really intended to allow the astronaut to exit the craft and parachute to Earth, so it's not all that useful in space.
- The built-in oxygen has a pressure of only 3.5 psi (0.24 atm), and prolonged exposure (15 minutes) to that pressure can cause the bends.
- The suit cannot maintain pressure unless the visor is down and locked, and the gloves are connected. The Columbia accident investigation board determined that all of the suits failed on that mission because no one had their visor down, and several astronauts were not wearing their gloves.
The Russian launch suit is the Sokol. It was designed after the loss of the Soyuz 11 crew due to depressurization, so the suit's ability to maintain pressure is a primary goal.
- All Cosmonauts wear the suit during launch, so again there is enough for every crew member.
- The suit normally circulates cabin air.
- If cabin air pressure is lost (0.59 atm), the suit has its own oxygen supply (0.39 atm). However, the suit tends to balloon at this pressure. If more mobility is needed, the suit pressure can be reduced to 0.26 atm for up to 15 minutes.
- It can be worn up to 30 hours if the cabin is pressurized. In a vacuum, it can be used for only 125 minutes, because it will overheat.
- Like the ACES suit, it has a visor that can be opened and gloves that can be removed. The visor must be closed and locked and the gloves worn for the suit to maintain pressure.
The Extravehicular Mobility Unit is the suit used for spacewalks. It's not designed for emergencies, although it should retain integrity during a decompression, as it is tough enough to withstand abrasion and micrometeorites.
- There is no guarantee that there are enough suits for all crewmembers.
- The astronauts must go through a long "pre-breathing" process to adjust to the lower pressure of the suit. On ISS, this takes 4 hours. On the Shuttle, the Shuttle's cabin pressure was reduced for 24 hours, and then the EVA astronauts spent another 45 minutes decompressing in the airlock. Without the pre-breathing process, the bends are inevitable.
- The suit can use vehicle life support, if available.
- The suit has its own life-support system, with an 8-hour oxygen supply and 30 minutes reserve.
The Hollywood scenario that a person could quickly put on any of the above suits in an emergency is extremely doubtful.
I'm posting this as a second answer because this refers to a spacesuit that only exists in prototypes. The other answer refers to "real" spacesuits.
If the habitat uses suitports, an astronaut could theoretically get into a suit fast enough while holding their breath, without passing out. In other words, it's a way to make the dramatic Hollywood scenario work. Here is the procedure:
Run to the suitport. There are two layers of hatches, one on the habitat side and one on the suit side. Open both layers of hatches.
Normally, one would put on a Maximum Absorbancy Garment (diaper). Skip this step and hope you don't have to urinate/defecate.
Normally, one would pull on a Liquid Cooling and Ventilation Garment. Skip this step (it takes a lot of time). You will eventually overheat in the suit, before its air supply runs out.
Normally, one would put on a Communications Carrier (snoopy cap). Skip this step. You will have no way to talk to anyone else.
Optionally for longer EVAs, one puts a drink bag and granola bar into the suit. Skip this step.
Jump feet-first into the suit.
Put only one of your arms into the suit arm. Let's call this the outside arm.
Using the other arm (the inside arm), pull the hatch on the suit closed and lock it. (Hopefully the hatch is designed to do this from inside the suit.) At this point, you have an airtight seal inside the suit.
Using the outside hand, turn on the suit air supply using the controls on the suit chest.
Breathe. You are safe for the moment.
You now can try wriggling your inside hand into the other suit arm. It may or may not work.
What happens next depends on whether the habitat hatch was also closed. This can only happen if a mechanical linkage was designed between the two hatches, or if another crew member stayed behind to close the hatch (sacrificing themself).
If both hatches were closed, you can undock the suit from the habitat.
Walk to another nearby, intact vehicle. Go inside and you're safe!