## 2 Cage Armour Working Principle
### PG-7(M) Detonation Chain  
The PG-7(M) round consists of two parts, as shown in Figure 2. The first part contains the warhead and a fixed rocket engine. The second part, which is screwed onto the base of the first part just before using the round, contains the folding fins to stabilize the rocket on its flight towards the target, next to a booster charge, which serves to propel the rocket to a safe distance from the launcher before ignition of the rocket engine. The latter method of firing prevents the shooter from being injured by the hot exhaust of the rocket engine upon firing.  
Figure 1: In order to counter the threat associated with the RPG-7 weapon system (**a**) many countries have equipped their vehicles with a cage armour system (**b**). The working principle is based on creating a short-circuit in the electrical circuit of the detonation chain and not, as often wrongly assumed, on increasing the standoff distance like the 'bazooka plates’ as used in World War II (**c**). Source pictures: Wikipedia.org, all pictures released under GFDL or CC-BY 2.5.  
Figure 2: Schematic view of the PG-7 antitank round, consisting of a first part with the warhead and a fixed rocket engine and a second part containing the folding fins and the booster charge, which is screwed onto the base of the first part. The complete round, i.e., with the second part attached containing the folding fins and the booster, is actually designated as PG-7V (and PG-7 VM for the complete round with the PG-7M warhead).  
The detonation chain of the PG-7(M) warhead is based on a specific design for the electrical circuit used to trigger the detonator. This circuit runs from the piezoelectric element in the nose of the projectile (which will produce a voltage difference and hence a current upon impact on a hard surface used to initiate the detonator) to the detonator in the back of the warhead. The produced current passes over the external, conical metal skin of the projectile (which also serves as the nose part of the complete projectile and hence determines largely the aerodynamic profile of the projectile) and is closed using a metal connector from the detonator to the liner, over the liner and finally a second, inner metallic trumpet-shaped cone. Isolating rings between the inner and outer cone prevent the current from passing from the outer to the inner cone without passing through the piezoelectric element and the detonator. Both the inner and outer metallic cones are typically made out of a thin aluminium sheet, yet rounds can be found that use mild steel for the outer cone as well (it is not clear if the steel has been used for the inner cone as well).  
When the projectile is fired and hits a hard target, the pressure developed in the piezoelectric element will cause a voltage difference to occur over the piezoelectric element as shown in Figure 3. This voltage will cause a current to flow in the circuit formed by the piezoelectric element, the outer cone, the detonator, the liner and the inner cone back to the piezoelectric element. This current will initiate the detonator, which, in turn, will initiate the explosive of the shaped charge. The specific geometry of the nose of the projectile will also ensure that the shaped charge is triggered at an optimum standoff distance from the target.