My power steering belt just shredded the inside of itself. Had a mechanic reset my timing belt not long ago (maybe 1000km) and everything seemed fine after that, but this afternoon my belt just shredded bits everywhere. Power steering stopped working, just got a loud squeel. Thankfully it happened close to home and I just swapped on a new one. However because the belt was shredded I figured that the 'push and look' method wouldn't be worth diddly squat since its lost much of its structural strength, so I just cut it off with a scalpel so I could drive it to my work area and not have more bits of belt flying all over the engine bay

I set the drive belt back to the perceived tension, which was done by pushing the belt in as hard as I could, and it lined up with a particular mark on my oil filter. When twisting the belt sideways I can get it to twist a bit past half-way (about 100 degrees?). This was how the mechanic set it, so I set it back to the same tension
The power steering belt i'm not sure. I did the twist method and it can go further, about 130 degrees around, but it also has a further distance between pulleys so I figure that's probably about right. Started engine and no squeel so it's not loose, but I did have to turn the tensioner quite a lot to achieve that sort of tension. It seemed a bit excessive actually for a brand new belt. Want to make sure I havn't done it overly tight

Additionally, I presume the belt must have been hitting something in order for only the inside to shred apart? From what I can see it looked like it may have been hitting the timing belt covers as there's some roughed up edges on the bottom side right where the belt wraps around the crank. Is it probably to assume that he put the belt on 1 tooth out and it was riding against the cover?

Edit: Should mention that if I have the car parked, A/C and fans off so I can hear clearly, then turn the wheel side to side I can hear a slight whirring sound as the pump is under load, not there when the wheel is stationary. If I rev the engine higher the noise actually increases. It's not something i'd normally hear with the A/C and fans on so I can't tell if its normally been that way. Is this something to do with the belt tension? The tensioner and pump spun nice and smooth when the belt was off

I'm not sure what you mean by : "Is it probably to assume that he put the belt on 1 tooth out and it was riding against the cover?" The power steering and alternator/AC belts are not toothed (teethed? toothy? whatevs..) and the PS belt is a V-belt (at least it is on my car) which can only ride one way. If there was an alignment issue between the pulleys it could cause the belts to fail prematurely, but when belts fail (the flat belt in particular) they often 'de-laminate' or shred longitudinally, so this isn't necessarily a sign of any alignment issue. How old are the belts? They generally go about 100,000 I think (mine have), so it might have just been its time to die. My usual method of tensioning (right or wrong) is to tighten until I can get only about 1cm of deflection when shoving the longest part of the belt in with my thumb while lying under the car in an awkward position where getting any sort of leverage is practically impossible. This means it will usually be a bit under-done, if it squeals on full lock (PS) or with AC and high beams on (alt/AC) I give the tensioners another turn or so to shut them up.

Been through this before, but anyhow . . .
The rear belt has 4 ribs on it the front belt has 6. If either belt isn't sitting on the pulleys correctly, so one rib is actually running on the edge of a pulley - ie displaced by one rib - the belt will shred in no time flat. The mechanic obviously got it wrong if the belt only lasted 1,000 km.
You really need to get up close with a torch on all of the pulley/belt interfaces to check they are seated correctly. Both from above and underneath the car.

Tension: A press of 10 KG force should deflect the belt on its longest run by 1 cm on both belts.
10 KG is as hard as I can push with one thumb - test your thumb on a bathroom scale. So basically you are lying on your back pushing upwards on the lowest runs of the two belts.
A lot of old time mechanics do the ribbed flat belts up like they used to with "V" shaped belts - this is way too loose!

These belts do go on really tight, guitar string like tightness - check the belts on a new Mitsu at your local dealer if you want to see.

In fact the manual describes a method using a microphone and oscilloscope. You "twang" the belt like a guitar string and the frequency of the twang on the oscilloscope tells you if the belt is tight enough. lol

My mistake, it's been 100,000 km since I did the PS belt, didn't realise it was ribbed. The OP doesn't actually state the age of the belts, just 1000 km since the timing belt was done.

Don't know how old it is, though my perception were that all the belts were in excellent condition and probably all done when the timing belt was done (about 20,000km ago)
The mechanic confirmed this, and thus why he reused the belts. No cracks, no bits missing, no obvious signs of a worn belt

Sounds like the belt was ok to use but not installed correctly.

Big danger with a shredded belt is that bits can wrap themselves around the crank between the plastic cover and pulley, and damage the crank angle sensor. Your car then stops. You can guess the rest.

If this is the same mechanic who had to redo the cam belt tension, check if he wears glasses. If so, suggest politely it is time for another pair?

http://www.pbase.com/mainecruising/image/143089427.jpg

Nice picture I just found in case anyone is wondering how you can get the belt on wrong . . .
left side pulley picture - notice the two edges - both should be visible when the belt is on correctly. If the belt is displaced one rib to the left, the first valley of the left side of the belt belt will ride on the edge of the pulley. This will take all the load, and shred the belt quickly.

http://www.mazda3club.com/mazda3-mazdaspeed3-98/how-mazda-3-2-3-belt-install-55704/

Properly installed belt - both edges of the pulley visible. (You might need to scroll down to the bottom. lol)
Worth double and triple checking every ribbed pulley after the belts have been off. Plain pulleys don't matter, the back of the belt rides on those.