Ms Morelli explains the limitations of finance department approval of business cases.

"We are not saying we approve a business case, that this business should go, is fully funded" she says.

Responsibility remains with the departmental accounting officer, which in the case of the RHI scheme was the DETI permanent secretary.

Image source, RHI Inquiry

The inquiry chair Sir Patrick Coghlin intervenes to say the arrangement cannot exclude responsibility on the part of the finance department to give advice if it sees something that represents a threat to finances.

He says that if anyone had compared the two schemes "it would have been abundantly clear there was no budgetary control, there was no tiering of subsidies and they were both coming out of the same pot."

After the RHI scheme - which was for non-domestic claimants - was up and running, DETI followed it up in 2014 with a scheme for domestic users.

It would come out of the same pot of money as the non-domestic scheme and the finance department was asked to approve the spending for the new initiative.

It did so but the approval for the domestic scheme was not give a time limit, like the original, non-domestic scheme had been subject to.

Image source, Getty Images

And nor was there any mention of the non-domestic scheme in the finance department's approval letter for the new scheme.

Ms Morelli tells the inquiry that it shouldn't have been done in that way - the civil servant who dealt with the approval case should have considered the original scheme in coming to the decision.

Inquiry chair Sir Patrick Coghlin says it was a "positive requirement" to refer to the original approval.

Image source, Getty Images

Ms Morelli explains that the primary function of the Stormont finance department's analysis of business cases for projects - like the RHI scheme - is to assess whether they represent value for money.

She describes it as a "second round approval check" after the department presenting the business case has already assessed its value for money credentials.

If the finance department officials and economists are not happy with what they've been presented with they can choose not to approve it.

Ms Morelli took over as head of the finance department's supply division after a major restructuring exercise.

Poor resourcing of civil service departments has been a regular feature to emerge during the inquiry, particularly with reference to the Department of Enterprise, Trade and Investment (DETI) team that was working on developing the RHI scheme.

The inquiry's junior counsel Joseph Aiken (below) wants to know if there was a similar problem when the two finance department supply divisions were merged.

Image source, RHI Inquiry

In a witness statement Ms Morelli said there was a reduction in staff numbers at the time and she expressed concern at this.

But she tells Mr Aiken no problems emerged from that in relation to the RHI scheme.

"The restructuring only added consistency across the supply role and function," says the witness.

Emer Morelli is the head of supply at the Department of Finance.

She took the role in October 2014 and her involvement with the RHI scheme began in the summer of 2015.

Image source, RHI Inquiry

In the most basic terms, her involvement in the scheme has been in approving spending decisions and working on managing the finances whenever it ran out of control.

You'll find her witness statement on the inquiry's website, external.

Ms Morelli has also written some of the Department of Finance's corporate statements to the inquiry.

BBC News NI

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, Pacemaker

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, a year-and-a-half 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.

What a lovely morning it is in Belfast and up at Stormont's Parliament Buildings the temperature is rising.

Unfortunately for the Renewable Heat Incentive (RHI) Inquiry team, the numerous lawyers, today's witness and for us, we'll be inside for the latest chapter of the inquiry.

Image source, AFP

We'll be hearing from Emer Morelli, a senior Department of Finance official, on why the RHI scheme was given financial approval in 2015.

Proceedings begin shortly so stick with us for updates throughout the day.