Damage to a hydro-electric generator under test

Hydroelectric generator, during maintenance


  • A hydro-electric generator, under test


  • During pre-delivery balancing and testing, a balance-weight retaining nut fell into the air gap, damaging both the rotor and stator
  • It was uncertain as to whether this occurred whilst the generator was insured


  • To determine the probability that the damage occurred during the period of insurance cover, and thus establish the insurer’s liability


  • Review of witness statements, damage reports and electrical test results
  • Construction of an event tree, with probabilities assigned to all branches


  • Demonstrated, without doubt, that the nut which caused the damage had not come loose, as suggested by the plant owner, but had been inadvertently left in the machine during balancing trials
  • Probability of damage occurring during the period of insurance cover was demonstrated to be less than 2%


  • Insurer’s liability limited to no more than 2% of the total loss

Plant Integrity