This light armour also detonates explosive warheads prematurely. High Explosive Anti Tank-type warheads (HEAT) use a focused hypervelocity jet of copper or steel to penetrate armour. To be effective, HEAT warheads must detonate at a specific distance from the target's primary armour to ensure maximum penetration. Thus early detonation greatly reduces the penetration of HEAT ammunition. This requires a distance of 1.2 meters even for an early 100mm projectile, thus conventional skirts are effective against HEAT only at very low angle of incidence.