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Shaped charges pose a dangerous threat to armor on the battlefield. They are very lightweight, are not dependent on their velocity upon impact for penetration, and are cheap and easy to manufacture.

Typically a shaped charge has a penetration six times its diameter.  Thus the 66mm M72A2 LAW rocket would be expected to penetrate up to  almost 400mm of Rolled Homogenous Armor.

The US M1 tank uses composite “Cobham” armor, in which a secret matrix of  various materials disrupt the explosive jet and prevent penetration.  But Russian tanks by and large don’t have composite armor, and so for them, Explosive Reactive Armor is a very attractive option.

ERA is simply two thin sheets of steel with a thin bit of explosives in between. The incoming jet of a shaped charge causes a sympathetic detonation of the embedded explosives, which in turn disrupts the jet, and prevent penetration.

ERA isn’t without its drawbacks, however. First, if a box successfully prevents a penetration, that area of the tank is now vulnerable to a second hit. Also, the blast of the tile can be quite hazardous to any nearby friendly exposed personnel, as the outer steel sheet is transformed into shrapnel.