{ "background": "Power distribution losses in sub-Saharan networks are a critical engineering challenge, with technical losses exacerbated by ageing infrastructure and limited diagnostic capabilities. Existing efficiency studies often rely on modelled or aggregated data, lacking rigorous field-based causal evidence.", "purpose and objectives": "This study aimed to conduct a randomised field trial to empirically evaluate the efficacy of a systematic diagnostic protocol for identifying and rectifying inefficiencies in medium-voltage distribution equipment.", "methodology": "A cluster-randomised controlled trial was implemented across 47 primary substations. Treatment clusters received the diagnostic intervention—a sequenced protocol of thermographic surveys, load profiling, and power-quality analysis—followed by targeted remedial work. Control clusters continued with routine operations. The primary outcome was the percentage reduction in technical losses, analysed using a mixed-effects model: $Y{ij} = \\beta0 + \\beta1 T{ij} + uj + \\epsilon{ij}$, where $Y{ij}$ is the loss reduction at substation *i* in cluster *j*, $T{ij}$ is the treatment indicator, $uj$ is the cluster random effect, and $\\epsilon{ij}$ is the error term. Robust standard errors were calculated.", "findings": "The intervention group demonstrated a mean reduction in technical losses of 4.7 percentage points (95% CI: 3.1 to 6.3) compared to the control group. Thermographic diagnostics identified faulty connections as the most prevalent issue, constituting 61% of all actionable faults.", "conclusion": "The structured diagnostic protocol proved effective for delivering statistically significant and practically meaningful efficiency gains in a real-world grid context.", "recommendations": "Utilities should integrate systematic diagnostic regimens into routine maintenance schedules. Further research should investigate the cost-benefit analysis of scaling the protocol and its integration with smart grid technologies.", "key words": "distribution losses, field trial, diagnostic protocol, grid efficiency, randomised evaluation, power systems", "contribution statement": "This paper