From the tank books I have (that's why I can't give a link) it's said that the Merkava is one of the best protected tanks in the world, and only the Challenger is heavier.
Not only is the motor in the front and the front side of the turret is as tiny as possible, they also placed the crew as deep and as far at the back as possible. The armour of the tank is actually not known, but it is assumed it is a composite armour, similar to the modern western tanks.
So I would guess if anything, the Merkava (and the Challenger) are one of the best protected tanks at all. Doesn't mean they are impenetrable.
Edit:
German wiki also says the following things:
- Merkava IS the heaviest tank in the world
- even the bottom of the tank has a several centimeters thick armour and is slanted to the sides, to deflect the blast of mines
- the seats of the crew also have no connection to the hull bottom
- the AFEDSS is a system which detects a piercing hollow charge and immediately extinguishes it with halon.
- MK I and II had only steel armour, MK III finally composite armour
- The tank has hollows at the sides of the turret for a better protection
- Mk IV finally receives a new hull with improved armour, instead of the ordinary steel armour
- the exterior, removable armour parts are at least 0,5m thick with the first 30cm apparently being laminate armour with different metals and some white layers, probably glass reinforced plastic
- the roof is at least 15cm thick
- the Trophy HV system detects projectiles (missiles?) fired against the tank and tries to stop them with an explosively formed penetrator a few meters before they hit. Then the system reloads itself to stop the next rocket. It is recorded that the system works, and an RPG has been stopped that way. It also calculates where the shot came from, so the tank can engage immediately
- the Merkava also has a laser detection system
- due to the more evenly distributed armour the Merkava does not reach the maximum armour values of the other western tanks, although it has even more weight
Some armour values:
Wannenfront = hull front
Turmfront = turret front
KE = APFSDS
Those values are RHA equivalents, which means it shows the thickness of the needed steel armour like in WW2 to reach the same extend of protection. It's a system to compare the different modern composite armours.
For comparison:
Abrams:
Challenger1:
unten = bottom
oben = top