Mr Sterling has previously told the inquiry that DETI's energy division was stretched and had resource pressures.

The inquiry counsel asks if that's simply a polished civil service way of saying "energy division had too much on its plate".

Image source, DETI

Mr Sterling says DETI tried to provide extra resource to meet the division's requests, adding: "Nobody came to me and said: 'I think it's reckless to proceed with this project because we're under-resourced.'

"If anybody had, I would have taken cognisance of that, and either stopped the project or done something else."

The RHI scheme is a project that DETI should have looked at and decided to "pause and say: 'Let's do something different'", says Mr Sterling.

There was "considerable pressure to move forward" with it at the time but he accepts that it "was a project too far for us".

Image source, Getty Images

A number of witnesses to the inquiry have suggested that running a complex energy incentive scheme was beyond DETI's capabilities.

Asked if DETI had the expertise to set up the scheme, Mr Sterling says: "If I'd had a magic wand we wouldn't have done this."

He points to last month's National Audit Office report on the similar scheme in Great Britain, external - it said the initiative didn't represent value for money, and Mr Sterling says it illustrates that there are "fairly significant issues" with the RHI model, regardless of who is putting it into place.

Brexit is posing major challenges for the policy-makers in the Northern Ireland Civil Service, says Mr Sterling.

As discussion strays away from the RHI scheme for a moment, and on to how Stormont departments work with their Whitehall counterparts, he says there is an "imbalance" between the two in terms of resources.

Image source, Getty Images

Picking out the example of new agriculture and environment laws which will be needed once the UK leaves the EU, he explains that the relevant UK government department dealing with that has 1,200 people working on policy.

"Here we're barely able to muster 30 people to do similar work," he says.

It's not just in agricultural policy that Stormont is hamstrung, he adds, saying that that is a position replicated that across other areas, including economy and criminal justice.

The civil service must address the problem, but it is compounded by the fact that Stormont is without ministers and an executive at the moment.

Northern Ireland's RHI scheme was based closely on a similar initiative in Great Britain, which was run by the Department for Energy and Climate Change (DECC).

DECC wrote to DETI in 2013 and 2014 to keep it informed about issues it was looking at in its scheme, such as subsidy reviews and cost controls.

So, should DECC's communications not have spurred DETI to take action on those matters in its initiative, asks Mr Scoffield.

Image source, Getty Images

"Clearly, knowing what we know now, yes," replies Mr Sterling, but he says there was a perception at the time of a low uptake of the scheme and that DETI was having to return unspent money to the Treasury.

Sir Patrick says it could be seen from the correspondence that DECC was introducing cost controls even though the uptake in GB was lower than in Northern Ireland.

"I don't see any evidence that that was thought about," he says.

"I have accepted that I should have been more inquisitive," Mr Sterling replies.

Ofgem, the administrator for the RHI scheme, warned DETI in mid-2012 not to go ahead with the scheme without adding cost controls similar to those adopted in the Great Britain RHI initiative.

DETI's energy boss Fona Hepper says she met Mrs Foster after the Ofgem warning and relayed it to her, but there's no written record of that happening.

Image source, Getty Images

The minister has told the inquiry that she has no recollection of being clearly informed of the risks of proceeding without cost controls, adding that Ofgem's warning "must have been significantly downplayed" if they were even raised at all.

Mr Sterling says that is the type of issue that should have been the subject of a written submission to the minister, which he would also have seen.

But he adds that if Ofgem did have "major concerns" then it should have "picked up the phone... and say: 'Look, you're proceeding here on a basis that we think is unwise, to say the least.'"

Legal advisers warned that the regulations for the RHI scheme were deficient in some aspects, and DETI was told that it could mitigate risks that might arise from them by amending them before opening the initiative.

Asked if that decision should've gone to the then minister Arlene Foster, Mr Sterling says senior civil servants exercise judgement on issues like that "on a regular basis" and there is a case that she didn't need to be informed.

He says DETI's energy team at the time were not "excessive risk-takers" and "would normally have been reasonably prudent and cautious in their approach".

But he says it should've at least gone to his second-in-line David Thomson for a decision to be made on it.

It was agreed at an internal DETI scrutiny committee that the RHI scheme business case to be sent to Stormont's Department of Finance and the then DETI minister Arlene Foster for approval should explicitly address the reasons why the subsidy model was favoured over the grants fund.

Mr Sterling was copied into the subsequent submission sent to the minister in March 2016.

Image source, RHI Inquiry

Mr Scoffield reads from the submission, and it does not appear that the reasons for preferring one scheme over the other were put before the minister.

"The minster should have been given a very clear explanation of the differences and the pros and cons between the main options that have been considered, so I would accept that," says Mr Sterling.

Asked why that didn't happen, he pauses says: "I'm not sure I can easily answer that."

Inquiry chair Sir Patrick Coghlin says that a decision about what model to adopt for the RHI scheme "should be properly informed" and there was "no debate" within DETI on the issue of admin costs.

"What we might be concerned about is the quality of the decision-making and why it wasn't fully informed."

Image source, RHI Inquiry

Inquiry panellist Dr Keith MacLean says it appears that DETI had already made "if not a full decision, a preliminary decision" to run with the subsidy scheme, regardless of the extra long-term cost.

Mr Sterling accepts that there was a "direction of travel" that "suggested a strong preference for the incentive scheme" that was ultimately adopted by DETI.

One of the key questions for the inquiry is why DETI chose to push ahead with a ongoing subsidy model for the RHI scheme, rather than an up-front grants initiative that would've cost £329m less.

Effectively, DETI effectively said it couldn't do the cheaper option because the initial £5m admin costs for it weren't affordable.

Instead it chose the much more expensive one because the £1.5m admin costs could be covered at the time.

Image source, Getty Images

This was in spite of the fact that the much cheaper option having been projected to result in the production of the most renewable heat at the lowest cost.

Quizzed about why the cheaper model was overlooked, Mr Sterling says the interpretation of DETI's energy division at the time would've been "that level of admin cost would not have been available just given the budgetary pressures we were under".

But he acknowledges that DETI "should have paused" to consider whether it was possible to see if the admin costs for the ultimately cheaper grants model could've been found in the budget.

Image source, RHI Inquiry

Having taken the affirmation last week, Mr Sterling doesn't need to do so again, but he's reminded that he remains under obligation to give truthful answers.

Posing the questions today is the inquiry's senior counsel David Scoffield QC.

BBC News Northern Ireland

Last week when he was appearing before the RHI Inquiry, Northern Ireland's top civil servant admitted that meetings between Stormont ministers and their staff were sometimes not minuted in order to frustrate freedom of information requests.

Mr Sterling said the practice of taking minutes had "lapsed".

Image source, Press Eye

He said the DUP and Sinn Féin were sensitive to criticism when they were in government and civil servants had therefore "got into the habit" of not recording all meetings.

His comments prompted significant debate throughout last week, with politicians and commentators expressing concerns about a lack of transparency at Stormont.

BBC News Northern Ireland

The head of the Northern Ireland Civil Service since his appointment last summer, David Sterling has a lot on his plate, especially in the absence of ministers at Stormont.

He joined the civil service in 1978, rising up the ranks and eventually heading some Stormont departments, including the Department of Finance.

Image source, Press Eye

At the time of the set-up of the RHI scheme, he was the permanent secretary - the top civil servant - at the Department of Enterprise, Trade and Investment (DETI).

In November 2016, he appeared before the Northern Ireland Assembly inquiry to answer questions about the cash-for-ash debacle and denied trying to "duck responsibility" for his role in it.

BBC News NI's business correspondent Julian O'Neil profiled Mr Sterling last June, and you can read that here.

BBC News Northern Ireland

An independent inquiry into the RHI scandal was established in January last year by the then finance minister Máirtín Ó Muilleoir.

He ordered it in the wake of the huge public concern and what was then a developing political crisis surrounding the scheme.

The RHI Inquiry began in November and Sir Patrick Coghlin (below), a retired Court of Appeal judge, is its chair and has been given full control over how it will operate.

Image source, Pacemaker

It will look at:

• the design and introduction of the RHI scheme
• the scheme's initial operation, administration, promotion and supervision
• the introduction of revised subsidies and a usage cap for new scheme claimants in 2015
• the scheme's closure

For more information on the RHI Inquiry, you can read our handy Q&A.

When the scale of the overspend emerged, public and political concern rocketed.

As the minister in charge of the Stormont department that set up the RHI scheme, the Democratic Unionist Party (DUP) leader Arlene Foster faced calls to resign from her role as Northern Ireland's first minister in December 2016.

Image source, PA

She resisted, and Sinn Féin's Martin McGuinness then quit as deputy first minister in protest at the DUP's handling of what had by then become a full-blown political crisis.

That move brought about the collapse of the Northern Ireland Executive. Now, more than a year on from that, Northern Ireland remains without a devolved administration.

You can find much more detail on the RHI scheme in our need-to-know guide.

The budget of the RHI scheme ran out of control because of critical flaws in the way it was set up.

Claimants could effectively earn more money the more fuel they burned because the subsidies on offer for renewable fuels were far greater than the cost of the fuels themselves.

The most recent estimate for the overspend was set at £700m, if permanent cost controls aren't introduced.

The massive overspend bill will have to be picked up by the Northern Ireland taxpayer.

The Renewable Heat Incentive scheme - or RHI for short - came to the fore of the Northern Ireland public's knowledge in late-2016... and the fallout from the scandal attached to it is still being felt in the region's politics today.

Image source, Getty Images

The scheme was set up by the Northern Ireland Executive in 2012, as a way of encouraging businesses to switch from using fossil fuels to renewable sources for generating their heat.

Those who signed up were offered financial incentives to buy new heating systems and the fuel to run them.

We're back up on Stormont hill on this nice, bright morning for another week at the Renewable Heat Incentive Inquiry - thanks for joining us.

Image source, AFP

Also returning is David Sterling, the head of the Northern Ireland Civil Service, to answer more questions after his eyebrow-raising evidence last week.

Proceedings begin at earlier than usual today, at 09:30, so stick with us for a full live stream and text commentary throughout the day.