An outer armour belt that triggers shells before the main belt

How do outer decapping armour belts work?

The explanation centres on an outer plate that stops smaller shells and triggers the fuzes of larger ones early enough for detonation outside the main belt.

The explanation here concerns an outer plate that initiates an incoming shell's fuze. Sufficient separation from the main belt would then allow the shell to explode before penetrating the principal protection. The outer plate can also be thick enough to stop smaller shells outright. Larger shells are described as passing that plate with their fuzes already triggered.