Case study · Commercial Property
Complex LV Switchgear Fault Investigation
Central London Commercial Building
Specialist investigation of phase loss within a commercial LV distribution system, combining measured testing, safe isolation and detailed inspection to localise the fault evidence.
Commercial Electrical Fault-FindingLV DistributionSwitchgear InvestigationElectrical Testing
Project context
- Site
- Central London Commercial Building
- Sector
- Commercial Property
- Location
- Central London
The challenge
Partial power loss after the main supply was restored.
Following a major supply failure affecting part of the landlord's commercial distribution system, the main supply was reinstated but several circuits remained without power. Testing at the reception distribution board confirmed that phases L1 and L3 were absent.
The fault therefore had to be traced upstream through the LV distribution arrangement, with the affected equipment isolated and the condition of the downstream installation established before remedial decisions were made.
Initial diagnosis
Incoming supply present. Two outgoing phases absent.
Testing at the upstream 250 A switch-disconnector confirmed all three phases on the incoming side, but only one phase on the outgoing side. Internal damage, arcing evidence and debris were identified within the device.
The switch-disconnector was treated as unsafe and isolated. A controlled temporary alternative supply arrangement was established to maintain reception services while the investigation and remedial process continued, with the existing protective arrangements retained.
Measured testing
Test the downstream circuit before assuming the failed device was the cause.
With the damaged equipment removed and the downstream sub-main confirmed dead, insulation-resistance testing returned readings greater than 999 MΩ. The satisfactory result helped narrow the investigation away from the downstream cable and connected distribution board.
This distinction was important: the visibly failed switch-disconnector was consequentially damaged equipment, but it did not by itself establish where the fault evidence originated.
Root-cause localisation
The investigation continued beyond the component that had failed.
Inspection of the switchboard and connected equipment found arcing and burnt-metal residue around the fire-alarm-panel isolator connected to the incomer busbar. The isolator was removed and the remaining busbar assembly returned insulation-resistance readings greater than 100 MΩ.
The measured evidence localised the fault to the fire-alarm isolator and incomer-busbar area. The precise initiating object or mechanism could not be verified because the intensity of the arc had destroyed or disintegrated the relevant evidence, so no unsupported cause was assigned.
Investigation evidence
Inspection and verification within the existing switchboard.
Engineering capability
Evidence-led investigation across LV distribution and switchgear.
Commercial distribution systems
250 A switch-disconnector investigation
Phase-loss diagnosis
Sub-main insulation-resistance testing
Busbar inspection and verification
Arc-fault evidence localisation
Safe isolation and continuity planning
Evidence-led root-cause investigation
Photographic technical reporting
Outcome
From phase loss to a defensible fault location.
The investigation separated the failed switch-disconnector from the area in which the fault evidence originated. Systematic testing verified the downstream sub-main, while progressive inspection and further insulation-resistance testing localised the evidence to the fire-alarm isolator and incomer-busbar area.
This provided a technically supported basis for the next remedial steps without presenting an unverified initiating cause as fact.
Fault-finding is not simply replacing the component that failed. It is understanding why it failed.