An energy expert has described Wednesday's fatal explosion at the Eskom Lethabo power station in the Vaal as a failure on the part of the state utility. One person was killed and another was critically injured in the blast.

"The power station has six units that produce electricity. One had a technical problem which resulted in a pipe rupturing - some call that an explosion," Eskom spokesperson Khulu Phasiwe told News24 on Thursday.

"One of our employees lost their life, while another is left critically injured," he said.

READ: One killed in explosion at Eskom power station

The critically injured man was transported to the hospital for medical care after the incident.

'Failure of epic proportions'

Energy expert Chris Yelland told News24 the explosion was a "failure of epic proportions" and that repairs and loss of revenue would run into billions of rand.

According to Yelland, there could be a number of causes for an explosion of such a nature.

"It could be equipment failure, it could be that protection devices failed, it could be poor maintenance, lack of skills, neglect - there are a number of issues we don't know."

Yelland told News24 that the "explosion" was preceded by the tripping of a generator.

"Energy from coal is being converted into high-temperature steam which drives a turbine, which in turn is rotating and driving a generator.

"These generators put out a lot of power - 600MW being fed into the grid.

Thorough, independent investigation recommended

"When you shut off a generator, the steam would continue to build up and the turbine would go faster and faster until it flies apart."

This, says Yelland, causes "massive destructive damage".

"If you were to shut off the steam from the turbine, the pressure would just build up until a pipe or the boiler ruptured or exploded."

Yelland says there are contingency measures such as valves and other automatic protection devices to avoid these situations.

He could not say why these devices did not prevent the explosion in this case.

"Protection devices can get bypassed for maintenance purposes - there are a lot of possible explanations. It is conceivable that the protection systems may not have been working properly, but I can't say what was the cause."

According to Yelland, a proper and thorough independent investigation would need to take place to determine the exact cause.

'This should not happen'

"This is not something that should happen. It causes loss of life, injury, as well as massive damage to equipment that can run into billions of rand."

Yelland said that repairs at the Duvha power station, 15km east of Emalahleni in Mpumalanga, which suffered a similar incident in 2015, hadn't even started and may never start because of the cost.

In June last year, the South Gauteng High Court interdicted Eskom from continuing with a controversial R4bn tender to replace Duvha's damaged boiler, Fin24 reported.

Eskom awarded the contract to Chinese firm Dongfang in March 2017, even though it quoted R1bn more than its rivals.

National Union of Metalworkers of South Africa spokesperson Phakamile Hlubi told News24 the incident was "very sad".

She said she did not have enough information to give a more extensive comment and would do so at a later stage.

Disgraced former Eskom acting CEO Matshela Koko tweeted: "I wish my former colleagues well and a speedy normalisation of the situation. Remember [our] drill. 'Safety first and be your brother's keeper'. My family and I will keep you in our prayers."