Former Department of Enterprise Trade and Investment (DETI) Permanent Secretary Dr Andrew McCormick has denied he was approached by a DUP Special Advisor over claims he was bullied by former DETI Minister Jonathan Bell.
DUP Spad Timothy Cairns alleged that Mr Bell acted in "an intimidating on threatening way" towards him on several occasions, a claim Mr Bell denies.
Mr Cairns says he approached Dr McCormick with the bullying concerns but Dr McCormick told him he could only act if the matter involved civil servants.
However Dr McCormick told the inquiry that he thought he would remember a report of bullying.
The inquiry heard how Mr Bell had a "sharp disagreement" with Spad Cairns over RHI during a breakfast ahead of a meeting with then Secretary of State for Energy and Climate Change Amber Rudd in June 2015.
Dr McCormick said that Mr Bell was advised to discuss the scheme with First Minister Arlene Foster but "Jonathan Bell said I'm the Minister I'll take the decision".
"Tim Cairns intervention was a sharper rebuke to a Minister from a special adviser than I'd seen before, the tone the language was terse and pointed and right at the borderline of normal conversation if not a bit over it," Dr McCormick said.
"It wasn't long before we realized this was not for us to be involved in and we were either asked to leave or got up and left,"
Dr McCormick said he did not see anything he determined threatening or intimidating from either man and it did not trigger a duty of care from him.
Following the meeting Mr Bell said Mr Cairns was dismissed for the day while Mr Cairns thought he was dismissed permanently.
After a leave of absence Mr Cairns returned to work in DETI.
Dr McCormick denied that Mr Cairns approached him in relation to "bullying" by Mr Bell and said that there were no complaints from members of the civil service about Mr Bell's behaviour during his time as Minister.
He said that he thought any disagreement between the men "was a matter for the party".
Dr McCormick also told the inquiry that he did not recall any attempts from DUP spads to stop the discussion of the RHI scheme in meetings during summer 2015 shortly after Jonathan Bell became DETI Minister.
Mr Bell has claimed that that DUP advisers did not want the scheme to be discussed.
However, Dr McCormick said the issue wasn't discussed because a submission on RHI with recommendations was being prepared by the department.
"I don't see any basis or why anyone would withhold that fundamental information," he said.
In the morning sitting Dr McCormick told the inquiry he could not explain why warnings from a whistleblower about key flaws in the scheme were not acted upon.
Businesswoman Janette O'Hagan contacted the department in 2015 to tell them the scheme was being abused and that businesses were burning fuel to make a profit.
He told the inquiry he only found about about Ms O'Hagan's emails after the scheme was closed.
He said: "Someone saying there is a grave weakness in the scheme and at that point in time the team also becoming aware they have a budget problem. I cannot understand why that did not take us back to a fundamental question- what is going wrong."
However, Dr McCormick said he accepted that during his time as DETI minister he did not manage risks regarding the RHI scheme "effectively".
Former DETI Minister Jonathan Bell is set to appear before the inquiry on Thursday and Friday.
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Here's how the day unfolded: