Fire at a generator of a power plant

There was a massive explosion at one of the generator-turbine units of a coal-based power station.
Undetected hydrogen leak from the generator was identified as a major trigger.
According to the news, some workers, unaware of the leak, were trying to fix some dirt blocks and loose nuts and bearings in the generator. Just below was an oil tank. Fire sparked spread in minutes up to 16 metres, setting off the blast “with a heavy noise”.
It took hours to control the fire. Four persons were injured.

The INITATING CAUSE of the leak is unknown.

The released hydrogen ignited possibly because of the activities of workers using non spark-free tools. Escalation may have occurred due to the proximity of an oil tank. There are also doubts on the proper or effective actions of the fire extinguisher.
The ROOT CAUSE could be attributed in general to lack of safety measures, procedures and/or practices. Why were the workers not equipped with detectors?

turbine, Colling system

According to the news, some workers, unaware of the leak, were trying to fix some dirt blocks and loose nuts and bearings in the generator.

DESCRIPTION OF THE FACILITY
The turbine-generator affected was a 500 megawatt and was commissioned in 2011. Hydrogen was used as coolant.

Undetected releases of hydrogen used as cooling agent of a turbine or generator are often the cause or contributor to sever damage of the unit (se for example HIAD event 1000).
A procedure of the plant affected by this incident foresaw a shut-down in case of hydrogen leakage, but the plant was not shut-down. In this particular case it is unclear if the hydrogen leak had been checked and detected. Anyhow, the unit was not shut down before escalation took place.