a downhole explosive detonation comprises a high voltage electro - explosive initiator comprising an input high voltage power supply with a low impedance shunting fuse , a flexible electrical link and a capacitor discharge unit . explosive is initiated in a direction approximately parallel , or in another version perpendicular to the capacitor discharge unit . a unique configuration and construction of the assembly allows installation through a small service port in the gun housing structure for more efficient gun arming . a real time downhole voltage monitoring is described that transmits voltage readings to the surface during a firing sequence .