Introduction: It had been noted that during de-energisation of certain grid transformers on the ENWL Network, unfamiliar audible phenomena was occurring that gave rise to concern about the safety of personnel and the potential effect on asset life. A project was commissioned with the University of Manchester and Physical Acoustics to investigate further.

Objectives: The stated project objectives are:

• To ensure the transformers have not suffered any damage and will meet their expected design life of 40 years

• To gain better knowledge to manage the de-energisation of grid transformers

• To consider the findings and to implement any design policy changes regarding cable/transformer configurations.