It's been a long day in the hotseat for Mr Cooper and he'll have to come back to answer many more questions about the casework committee's work.

That might have to wait for a while, though, as the committee moves on to its second phase next week, looking at how the scheme operated after opened.

Image source, AFP

Later in the week, we'll hear from the woman who tried to draw the then DETI minister Arlene Foster's attention to how some people were taking advantage of the scheme's overgenerosity.

But first, when the inquiry resumes on Tuesday, we'll get a broad overview of what's to come in phase two.

Join us then!

BBC News Northern Ireland

An official who scrutinised and approved the RHI scheme mistakenly believed there was a "big, red button" to stop it if it threatened to run out of control, the inquiry was told.

Senior DETI official Trevor Cooper chaired a key departmental scrutiny committee in March 2012 that assessed the arrangements for the project.

Image source, Press Eye

He was told by the team setting-up the initiative that it could be stopped "in-year" if too many people applied and the budget was threatened.

But Mr Cooper did not ask for details of the stop mechanism, which in effect did not exist.

Dr MacLean challenges the casework committee's challenge function, saying it took "assurance after assurance after assurance".

And he's not at all happy with the minuting of the approval meeting either.

Image source, RHI Inquiry

The minutes report that Mr Cooper asked how often meters on biomass boilers would be read, yet one fundamental question, which the witness claimed to have asked - "should we be doing this at all?" - isn't there.

Dr MacLean says the committee was accepting what was laid in front of it and signing it off without making fundamental challenges.

DETI's energy team had a crucial responsibility to ensure that an emergency stop function actually existed whenever it told the department's casework committee that it did, says Mr Cooper.

He suggests that from the evidence the team members have given to the inquiry they "at least appear to have known that they couldn't close it", and there's a "big difference" between what they told the committee and what was actually possible.

The emergency stop would've had go to public consultation and be scrutinised by the Northern Ireland Assembly before it could be implemented.

Image source, RHI Inquiry

If the committee had known that, Mr Cooper says it would've said: "That isn't good enough - it doesn't work."

Given the length of time that process would've taken, inquiry panellist Dr Keith MacLean says that instead of the scheme being closed "in-year" it would've been "in a year".

Mr Aiken suggests that the witness is making the point that he was "sold a pup" in relation to the ability to close the scheme.

The "novel", demand-led nature of the RHI scheme meant it was "absolutely crucial" to have an emergency power to stop the scheme, says inquiry chair Sir Patrick Coghlin.

"Correct," says Mr Cooper, who adds that "the scheme that we approved had a big, red button to stop it" on the basis that the DETI team behind the initiative had assured the panel that it existed.

Image source, Getty Images

Sir Patrick says it "must've been obvious, surely" to the committee to find out more from the energy team about the "big, red button".

Mr Cooper says he was "absolutely attuned" to the risk of overspending and he asked "lots of questions" but he can't say if he probed deeper about the emergency stop measure.

One of the big risks in the RHI scheme that Mr Cooper identified when he was scrutinising it was that the subsidy levels on offer could be set either too high or too low, creating a danger of overspending.

DETI's Peter Hutchinson, one of the key people who drew up the scheme, gave the assurance that the "scheme could be closed to new applicants mid-year if... budgets risked being overspent".

Image source, RHI Inquiry

But that emergency stop measure was not actually possible and wasn't mentioned in the risk register for the scheme, which laid out how various issues could be prevented or mitigated.

Mr Aiken says the committee didn't spot that, even though the risk register was provided for them to review.

Mr Aiken quotes from the 16-page synopsis that forms part of the RHI scheme papers submitted to the casework committee, which gives the assurance that "there are going to be built-in reviews".

So, what did Mr Cooper take that to mean?

"What I understood was there were going to be regular, detailed reviews of this scheme," he says.

Sir Patrick appears unimpressed and probes the strength and effectiveness of the committee's challenge function, given that there was no legislative or detailed written requirement regarding reviews.

"All you had really to reassure yourself was something you were told by a member of the department who was selling the scheme," says the inquiry chair.

Mr Cooper says he took the assurances at face value and he "firmly" believes that was a reasonable position to take.

It would've been helpful for DETI's casework committee to have had some important papers related to the RHI scheme when it was considering whether or not to approve it, says Mr Cooper.

While the committee assessed hundreds of pages of material, some - including the regulations for the scheme - were not submitted to it.

Image source, Getty Images

Mr Aiken points out that time was not set aside for committee members to look at the material in their normal working day, and Mr Cooper tells the inquiry: "You would've had to do it at home."

He would've generally read casework papers "at night, at the weekend".

There was no mechanism for DETI's casework committee to check if assurances given to it about how the RHI scheme would run would actually be put into practice, says Mr Cooper.

Even if there was, he says, it would have been "impossible" for the committee members to have done so, given the sizable amount of cases that some members might assess in a given year.

Image source, Getty Images

That wasn't the committee's responsibility, he goes on, and it wouldn't have been "physically practical" for it to do so.

Mr Aiken says that what effectively happened was that the committee was "taking on trust" that the assurances DETI's energy team gave it would be implemented.

Back up and running again after lunch and on to the meat of today's session - DETI's casework committee meeting of March 2012, when the RHI scheme was assessed and approved.

Mr Cooper, as we've already heard, chaired this vital discussion, and a previous witness - Philip Angus, who also sat on the panel - told the inquiry that Mr Cooper wasn't an easy touch on the committees.

Image source, RHI Inquiry

Mr Aiken goes through the role of the committee in some detail.

It was essentially a peer review with a challenge role, requiring the policy team behind the scheme to justify their approach to the project.

The inquiry will be back for more after a quick bite to eat - join us again from 14:00.

DETI was warned by the Treasury's Mr Parker that it would face a financial penalty of about 5% in the case of an overspend on the RHI scheme.

DETI finance officials - including Mr Cooper - worked on a submission for their minister in late-November 2011 that contained details about the funding arrangement for the initiative, but it didn't outline the unusual nature of it or the prospect of that penalty coming into play.

Mr Cooper "totally accepts" that the document Mrs Foster received "wasn't accurate".

Image source, PA

Asked why those details weren't reflected in the submission, he says it was a case of "word blindness".

Explaining that, he says "people do become used to [using] standard terminology".

"Did it spark with me? No - it's either that or it wasn't in my consciousness at that time."

Attention turns to the famous, or possibly infamous, email from Treasury official Jon Parker in April 2011, a key document in the inquiry so far.

In reply to a query from DETI, he outlined the arrangement the Treasury had with the Department of Energy & Climate Change (DECC) - the department operating the GB RHI scheme - that involved an unconventional "risk-sharing" arrangement to cover overspends.

Mr Parker copied Stormont's Department of Finance and Personnel (DFP) into his email, and subsequently Mr Cooper was hauled over the coals - or should that be burning wood pellets? - by a DFP official and given what Mr Aiken describes as "a roasting".

DFP was angry that DETI had contacted the Treasury directly.

As Sir Patrick puts it: "Look here, you're in breach of protocol again!"

Rewinding back to 2010, the inquiry looks at the early stages of DETI's work in establishing the RHI scheme, with Ms Hepper making a submission to the department's then minister Arlene Foster about commissioning consultants to work on an economic appraisal for the initiative.

Inquiry panel member Dame Una O'Brien asks whether it was common practice for policy staff to put a submission with financial implications to the minister without the explicit approval of the department's finance division.

"Meanwhile," she adds, "people were copying you in just in general, perhaps to imply that you'd seen it?"

Image source, RHI Inquiry

Mr Cooper says that's why he would've had "a degree of discomfort about being copied into everything".

Sir Patrick says that means that "if things go wrong you are going to be the first person to whom a finger is pointed", and describes the system as "a defensive, rather unpleasant" one.

The witness says changes were later implemented, meaning submissions should've got the "imprimatur" from finance officials, but that didn't always happen because often staff "wanted to steamroll on with things, to be perfectly frank".

Mr Aiken says there doesn't appear to have been any consideration of whether the scheme should have been sent back to Stormont's Department of Finance and Personnel due to a breach of financial conditions.

Mr Cooper is asked whether it should have been.

"I hesitate, because clearly I thought [the cost increase] was material," he replies.

Image source, RHI Inquiry

Inquiry chair Sir Patrick Coghlin asks what would have happened if it had been referred back, given that the scheme legislation was already in place and it was open to the public for applications.

"At the very least there would be a very engaged discussion around how have you got to this figure now - what's the justification for it," Mr Cooper says.

It appears that Mr Cooper was "overruled" in his insistence that the increased admin costs of the RHI scheme be referred back to DETI's casework committee, says Mr Aiken.

Image source, Getty Images

Mr Cooper, who chaired the committee, says the admin costs were the "one of the big issues" in the approval of the scheme, and he was "very unhappy" about the increase.

He says it went against the "normal process" of how money was allocated within the department.

A deal on the administration costs for the RHI scheme had yet to be struck a month after the initiative was opened for applications in November 2012.

In December that year, DETI was told by administrator Ofgem that the costs would increase from an earlier estimate of about £1.1m over the first four years to £1.8m.

That was raised with DETI's top management, including the permanent secretary David Sterling, who wondered whether the cost increase was a "material change" to the scheme, which would mean the matter would have to go back to the casework committee for examination.

Image source, Getty Images

DETI's energy team boss Ms Hepper said it wasn't a material change, just a "refinement" of Ofgem's original estimate and therefore it didn't require review.

But Mr Cooper's view was different to hers - he tells the inquiry that he felt it was "a material change" and there was "no doubt" that it should've gone back to the committee for consideration.

In an email to another DETI official, the permanent secretary said he wanted an agreement to be reached between Mr Cooper and Ms Hepper.

Mr Cooper evidently had a highly pressurised job, dealing with projects involving many millions of pounds.

He attended many casework committee meetings within DETI to assess those projects and determine whether they would be approved or rejected.

Image source, Getty Images

Mr Aiken asks him about the time required to read casework documents, and Mr Cooper says the papers for the RHI scheme would've taken a day or a day-and-a-half to get through.

But was it possible for him to read all of the submissions for all the projects he worked on?

"It's physically impossible to do that," he replies.

Mr Cooper claims that he asked at the DETI casework committee stage of the RHI scheme's development why Northern Ireland needed its own heat initiative.

He claims that he believed that a scheme for the region could've been run and managed within the existing Great Britain RHI initiative.

Mr Aiken says that point was not raised by Mr Cooper at the time when the scheme was being given internal approval at DETI, according to the casework committee minutes.

"The minutes are the minutes - I believe I did ask that question; in fact, I know I asked that question," says the witness, who chaired the committee that assessed the initiative.

He was given a "lot of legal jargon" by Fiona Hepper, who was then the head of DETI's energy team, about why that wasn't possible.

Invest NI would have been a "natural home" for running the RHI scheme, says Mr Cooper, because it was an "expert delivery body" and would've been better placed than DETI to run it.

There was a question-mark over whether the RHI scheme should ever have been done in Northern Ireland, Mr Cooper told PwC.

He queried whether DETI was the right place to run such a complex initiative.

Image source, RHI Inquiry

When questioned as part of the PwC investigation, he said: "Were the overall resources... applied to it appropriate?

"Was the technical understanding in the frontline unit engaging on it appropriate?"

Elaborating on that, he now tells the inquiry: "I don't think anyone was trying to mislead - I think they actually believed what they were saying."