Unitech Power Systems has performed a root cause assessment related to a rock blasting incident in Norway, which was suspected to be related to the use of electrical excavators on the site. Although we typically perform studies on electric power systems of varying size and complexity, investigation of incidents is another specialty of our company.

Excavation work for a new water and sewage trench running parallel to a 20-year-old existing trench, shifted about 3 m, somehow triggered detonation of an unexploded ordnance (UXO) directly beneath an electrical excavator. The excavator in question was powered from a nearby 400 V (TN) transformer kiosk via a provisional cable and was situated directly above the old trench when the incident occurred. Fortunately, no personnel suffered physical injury. A battery-operated excavator was also working in the vicinity. Missing records of any similar incident initiated suspicion that these electric machines might somehow have triggered the detonation.

Unitech Power Systems was contracted to perform a high-level root cause assessment of whether one of these excavators might indeed have triggered the UXO’s electric detonator. About six months had then passed since the incident occurred, making it virtually impossible to obtain verified information on the detailed circumstances at the site at that time. Although some factual details remain unclear, it was found that yet another near-parallel trench, about 10 m away, could explain how excavation work could proceed for some time before the incident occurred. This third trench contained local utility medium voltage cables and, most importantly, included a bare earth conductor that led back to the same transformer kiosk that was feeding the excavator.

A hypothesis in which earth fault combined with PE conductor discontinuity, most likely in a damaged plug-type connector at the excavator, was established. In such a case, an earth fault current would have to flow through electrically poorly conductive soil/bedrock to return to the transformer's neutral point. The nearby trench with the bare earth conductor would likely attract a significant portion of the earth fault current as the excavator moved further from the transformer kiosk. As the excavator approached the UXO, presumably having vertically oriented lead wires of some meter’s length, the poorly conductive soil/blast rock became shunted by said wires. The 50-Hz earth fault current would then shift into these lead wires as they provided an easier path towards the nearby bare earth conductor. Dissipated energy in the electric detonator then eventually caused it to trigger the UXO once the cabled excavator had reached a position practically directly above it, with the detonator wires collecting maximum earth fault current.

Although the described scenario has not been verified by real-life testing, it does highlight the risk of uncontrolled earth fault paths in case of cable or connector damage. Consequently, electrical excavators powered via on-ground cable should never be used to work at sites with a history of previous rock blasting.

Purely battery operated excavators are not prone to the same concern, as the on-board energy source cannot cause any significant electric current outside its own chassis. In principle, an on-board battery-powered excavator is electrically equivalent to any conventional excavator that would also have (smaller) on-board battery.

The above recommendations have been presented by our client at relevant conferences in early 2026. The conclusions might be adopted by a future working group on defining national guidelines and/or regulations for the use of electrical excavators.