A number of witnesses have told the inquiry that the DETI team dealing with renewable heat was under-resourced - there was simply not enough staff.

This was particularly clear on Tuesday, when we heard from civil servant Peter Hutchinson, who did much of the early spade work on the RHI scheme.

Ms Hepper says the staffing was "adequate but not optimum" and that she recognised the problem and acted on it, bringing in "a bespoke principal" to head a separate renewable heat branch.

Image source, RHI Inquiry

Inquiry chair Sir Patrick Coghlin doesn't seem impressed, pointing out that that principal, Joanne McCutcheon, only worked part-time.

"She was the principal in charge and she was part-time, and you were asking for resources and Mr Hutchinson was doing most of the work?" he says.

"He was doing a lot of the work," Ms Hepper replies.

Taking a look at the Northern Ireland's civil service, Mr Scoffield says it is viewed as "generalist".

As the public and those in business see it, he says, it appears that civil servants who have connections and experience in a specialist area are moved to another part of the service.

Image source, Getty Images

Ms Hepper says DETI's energy division was different: "People did tend to stay in post for a longer period."

That's because it was a "fascinating area to work in" and staff had a "genuine interest" in it, but also because it did take "some time to build up the knowledge and experience" as it was such a "complex" field.

DETI needed powers to be devolved from Westminster if it was to set up a Northern Ireland heat incentive scheme.

So why didn't it use the opportunity to join the Greta Britain RHI scheme that was set up by the Department of Energy and Climate Change (DECC), as the Welsh and Scottish devolved administrations had done?

Image source, RHI Inquiry

Ms Hepper says that "by good luck" DECC was putting an energy bill through Westminster and DETI could piggy-back on it.

She says that by that stage the GB scheme was far advanced and DECC wouldn't have wanted a Northern Ireland RHI "to derail their progress".

"The feeling was we need to plough our own furrow here " she says, to find a solution appropriate for Northern Ireland.

Some major manufacturers in Northern Ireland were keen in 2010 for a renewable heat scheme to be put in place, says Ms Hepper.

They were facing increasing gas and electricity prices and were "finding things difficult", she adds.

Image source, Getty Images

Those within industry and commerce brought those issues to the then enterprise minister Arlene Foster.

And they were keen to know when a heat initiative, like the RHI scheme that existed in Great Britain, would be implemented in Northern Ireland, says Ms Hepper.

Senior counsel to the inquiry David Scoffield will be doing the questioning today - he introduces Ms Hepper and she is sworn in.

Today's session will cover matters up until the the making of the scheme's regulations in October 2012.

Image source, RHI Inquiry

Ms Hepper explains her role in the instigation of the RHI scheme and it quickly becomes clear that she was a central protagonist.

She was the head of the renewable energy team at DETI when the scheme was being set up.

The list of tasks she outlines runs from procurement of energy expertise from consultants, all the stages of the scrutiny and approval process, ministerial approval, the state aid approval process and the passage of the necessary regulations in the assembly.

BBC News Northern Ireland

Stormont finance specialists did not fully understand the budget plan for the RHI scheme before its costs spiralled, a civil servant told the inquiry.

Senior finance department official Mike Brennan said various things had gone wrong in the scheme's planning stages.

Image source, RHI Inquiry

When those thing combined they created "the horror that we've seen".

There was no grasp the scheme's potential to "critically undermine" Northern Ireland's budget, he said.

BBC News Northern Ireland

An independent inquiry into the scandal was established in January 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.

Image source, Pacemaker

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

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.

Public and political anger erupted when the scale of the overspend emerged.

The Democratic Unionist Party (DUP) leader Arlene Foster (below) had been the minister in charge of the Stormont department that set up the RHI scheme in 2012, and she faced calls in to stand down as Northern Ireland's first minister in December last year.

Image source, PA

She refused, 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.

By doing so, he brought about the collapse of the Northern Ireland Executive. Now, nearing a full 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 RHI scheme's budget ran out of control because of critical flaws in the way it was set up - the most recent estimate for the overspend was set at £700m if cost controls are not introduced.

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

Before we begin, here's a quick refresher on what the RHI scandal is all about...

The Renewable Heat Incentive scheme - or RHI for short - came to the fore of the Northern Ireland public's minds in autumn last year... 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, biomass boilers, for example, and the fuel to run them.

A chilly, blustery day up here at Stormont's Parliament Buildings but the heat is on inside the Senate chamber for the RHI Inquiry.

Image source, Pacemaker

We're back here to bring you live video and text coverage of the inquiry's proceedings, and today former senior Department of Enterprise, Trade and Investment (DETI) official Fiona Hepper will sit in the witness chair.

She's due to face a full day of questioning from the inquiry's counsel, and we'll bring you the key details.