A moment of madness has cost Jim Cassidy a month on the sidelines with the veteran jockey outed for using an elbow to push apprentice Lester Grace's horse out of the way in a race at Randwick.

Stewards found Cassidy guilty of improper riding in the 1400-metre race after he admitted losing his cool when Grace, on Sensualism, laid in on Tears In Heaven.

When asked to plead, Cassidy answered: "Crazy."

Cassidy told stewards he became fed up with Sensualism laying in on his mount.

"He was all over me and I had had a gutful," he said.

"I called three or four times and then I elbowed him out.

"My priority was to look after the rider inside me (Kathy O'Hara) and he was leaning into me.

"It was out of character for me to do that. I have never done anything like that before."

Stewards agreed it was an uncharacteristic reaction from the champion jockey.

"Normally on a charge of this nature, the penalty starts at two or three months," presiding steward Marc Van Gestel said.

"We acknowledge your frustration and that is was an out-of-character reaction so the penalty is one calendar month."

Grace received a penalty of half that for his actions on the Godolphin-owned Sensualism which Van Gestel said bordered on improper.

"It was close to improper and it was a futile attempt to correct an earlier mistake and it was never going to come off," he said.

"It was in the high range of careless riding.

"You are riding for a high-profile stable and there is evidence of some degree of desperation but two wrongs are never going to make a right."

Cassidy's ban begins on December 30 while Grace will be out from New Year's Day.