The prosecution has argued that a 300-kilogram metal casting came out of the bucket of a skid steer loader and landed on the operator Peter Watson, 42, on July 22, 2014, starting a fire that killed him at the Wodonga foundry.

It had been suggested that the casting could have been squeezed out of the jaws of the bucket.

But engineer Roger Kahler said that scenario was "impossible".

Holding the dry bar of soap between his finger and thumb in Wangaratta Magistrates Court on Wednesday, he said there was not enough friction to pinch the soap out of his hand.

It was only when he licked his fingers that the soap became slippery enough to pinch out - he argued the same rules applied to the skid steer loader holding a metal casting.

Bradken's defence barrister Robert Taylor asked Judge Julie Condon if he should officially tender the bar of soap as evidence in the trial.

"I think not, the jury understands the analogy," she said.

Bradken Resources is fighting a charge of failing to provide a safe workplace.

Mr Kahler conducted his own tests on a similar skid steer loader, but one that was about seven years older than Mr Watson was operating.

The defence team had Bradken create lumps of sand like those on the Wodonga foundry floor and Mr Kahler hired a professional driver to run the machine, carrying a metal casting, over them at 10km/h.

"I knew at that time we were never going to throw it out of the bucket," he said.

He claimed the rules of engineering meant it was impossible for the casting to jump 70 centimetres and out of the bucket.

"It's not going to end up in the cabin, it's going to roll about the bucket," Mr Kahler said.

But under cross-examination from Crown prosecutor Andrew Palmer, Mr Kahler changed his story and said there was one "extreme" scenario where the metal casting could fall back onto the operator - if the machine's arms were raised more than 1.8 metres in the air and it immediately jolted from reverse to forwards.

He said he had not mentioned this before because any worker who lifted the castings that high would be "asking for trouble".

"I thought no one is going to do that," he said.

Mr Palmer accused him of avoiding the questions put to him in court by giving long answers instead of a simple "yes" or "no".

"You're not giving your evidence in a way that's honouring your agreement to be honest and impartial," he said.

Mr Kahler did not attempt to throw the casting out of the bucket at any higher than 1.8 metres during his testing because he said it would have been too dangerous.

He said the use of an excavator, which the prosecution has suggested should have occurred at the foundry, would not have been safer than a skid steer loader.

The trial will continue on Thursday.

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