What is the triaxial cable and when would I use it instead of standard coaxial?

A triaxial cable (triax) has three concentric conductors: a center conductor, an inner shield (guard), and an outer shield, separated by insulating dielectrics. This is different from standard coaxial cable, which has only two conductors (center conductor and one shield). The key advantage of triaxial cable is improved shielding and the ability to implement a guard circuit: (1) Structure: center conductor (signal), first insulating layer, inner shield (guard), second insulating layer, and outer shield (ground/drain). The inner shield (guard) can be driven to the same potential as the center conductor, virtually eliminating leakage current between the signal and the outer ground shield. (2) Shielding effectiveness: standard coaxial cable: shielding effectiveness of 60-100 dB (depending on braid coverage and frequency). Triaxial cable: the double shield provides 20-40 dB additional shielding over standard coax (total: 80-120+ dB). The two shield layers are isolated from each other, so currents induced on the outer shield by external EMI do not couple to the inner shield (and therefore do not reach the signal conductor). (3) Guard technique: in precision measurement applications (semiconductor testing, picoammeter measurements): the guard (inner shield) is driven by a unity-gain buffer amplifier connected to the center conductor. Guard voltage = signal voltage (zero potential difference between signal and guard). This eliminates leakage current through the inner dielectric (because there is no voltage across it). The result: the effective insulation resistance from signal to ground increases from gigaohms (typical cable dielectric) to teraohms (limited only by the outer dielectric). This enables measurement of currents below 1 picoampere. (4) When to use triaxial instead of standard coaxial: low-level signal measurement (below 1 nA or 1 mV), semiconductor device characterization (I-V curves, leakage current), electrochemistry (cell current measurement), high-EMI environments (where the additional shielding is needed), and any application where guard-driven measurement is required. Triaxial is NOT typically used for standard RF signal transmission (the guard adds unnecessary complexity and cost for normal 50-ohm signal paths).