A so-called emergency stop measure that was suggested for the RHI scheme if it was in danger of going overbudget would've taken "a number of months" to take effect.

It was outlined to the casework committee that the RHI scheme could be closed to new applicants if "budgets risked being overspent", according to the minutes from the meeting.

Image source, Getty Images

But that power wasn't in place from the outset.

Ms Hepper explains that would've required a change to the scheme's original regulations, and the lengthy process to do that would've involved a public consultation as well as scrutiny by Stormont's Enterprise Committee.

The RHI scheme had to go before DETI's casework committee in 2012 before its approval.

Ms Hepper explains that the committee is made up of senior management within the department, and it scrutinises a project before it goes on to be rubber-stamped... or goes back to the drawing board.

Image source, Getty Images

In a document presenting the RHI scheme to the committee, senior DETI official David Thompson said the scheme should be introduced as a "matter of urgency", and he urged that he was "content" that proposals had been "thoroughly researched, analysed and appraised".

Inquiry panel member Dame Una O'Brien notes that by the time the committee came to scrutinise the plans the scheme was already well in motion.

"A lot of money's been spent, a lot of commitments have been made... for the kicking of the tyres to come at really quite and advanced stage," she adds.

He set up the RHI Inquiry in January and he's keeping a close eye on it - Sinn Féin's Máirtín Ó Muilleoir is in the Senate chamber.

Mr Scoffield asks whether DETI or Stormont's agriculture department realised that there were going to be some uses by which claimants would have to run their heating installations "for a lot more than 17% of the time".

Two such examples are poultry farming or mushroom farming.

Image source, AFP

Ms Hepper says he is not sure but they would have assumed that such users would have been utilising larger boilers "and therefore the tariffs would be lower".

It's important to note that smaller tariffs were offered for bigger boiler installatons.

Mr Scoffield turns to the question of the amount of time that RHI scheme boilers were in use, known as the load factor.

He displays a CEPA table showing how the tariff for boilers was calculated with an assumption that they would be used for 17% of the time in a given year.

"We now know that to be very wide of the mark in terms of boiler usage when the scheme was up and running," he says, and he puts it to Ms Hepper that no-one noticed that at the time.

Image source, RHI Inquiry

She says they would have accepted that the consultants had based the figures on industry standards and what was used in the Great Britain RHI scheme.

Asked if there would have been any level of challenge to the figures, she says there would have been discussion with the consultants and "we would have made sure we understood it".

One of the issues that emerged later was the sensitivity of the load factor, she adds, saying: "I don't think at that early stage we would have appreciated that."

Matters move to the issue of the lack of tiering of the subsidy tariffs on offer through the RHI scheme.

Tiering is a key way of controlling the cost of the scheme and works by dropping the tariff on offer once a certain limit of usage has been reached - that prevents claimants from abusing the scheme by overusing their heating system to harvest more cash, effectively burning to earn.

It wasn't included in the scheme, however, even though it was in the similar initiative in Great Britain, and its absence was one of the main reasons why the budget spiralled out of control.

Ms Hepper says that if CEPA had recommended tiering in 2012 it would've been included in the scheme from the outset.

But former DETI official Peter Hutchinson said yesterday that the department decided not to include cost controls because it would've needed a public consultation and time pressure wouldn't have allowed that.

Ms Hepper says her view is that it would've just been a "refinement" and wouldn't have required a consultation.

CEPA has told the inquiry that the money it received for carrying out additional work for DETI in 2012 was not enough to produce a detailed analysis on key parts of the RHI scheme.

But Ms Hepper says that's not the case and argues that the "door was open" to CEPA to ask for more money.

Image source, Getty Images

She explains: "It's not as if we said: 'That's the amount and not a penny more' - I would've preferred to have a piece of work that cost a bit more and was right."

Sir Patrick asks if there "was a degree of happiness" between DETI and CEPA about the amount of money that was on offer.

"There was absolutely no tension," replies Ms Hepper. "In fact, a very, very good and positive working relationship."

The then DETI minister Arlene Foster was not told of a cost difference of more than £200m between the two main options that were being considered for the RHI scheme.

As we heard yesterday, the projected changed dramatically over the course of a month in June 2011.

Image source, Getty Images

An ongoing subsidy scheme - which was eventually adopted - was estimated to cost significantly more than the other option, the up-front grant scheme.

Ms Hepper is asked if she drew the minister's attention to that when she received that information from CEPA and she replies: "Not specifically, no."

Inquiry chair Sir Patrick Coghlin refers Ms Hepper to those two increases between May and June 2011, and June 2011 and February 2012.

He asks whether the minister's attention was drawn specifically to those increases in the business case for the scheme.

Ms Hepper says she doesn't believe that the increases were in the business case.

Image source, RHI Inquiry

Pressed again by Sir Patrick to confirm that the minister probably was not told, Ms Hepper says: "I couldn't confirm that she actually was - it definitely didn't go in the submission to her."

Sir Patrick says "it seems quite important".

He notes that the consultancy Cambridge Economic Policy Associates (CEPA), which produced an economic report about the scheme for DETI, was "relied on almost entirely" and yet the significant cost increases that it estimated were not flagged to the minister.

Fiona Hepper, the former boss of DETI's energy division, which set up the RHI scheme, settles into the Senate chamber for her second appearance before the inquiry and proceedings begin.

The inquiry's senior counsel David Scoffield returns to a question that was put to Ms Hepper's previous evidence session because he wants to make a clarification.

Image source, RHI Inquiry

Dr Keith MacLean, the technical assessor who sits on the inquiry panel, asked why Ms Hepper didn't inform the minister about a £111m increase in the projected cost of the RHI scheme between May and June 2011.

Mr Scoffield says the rise was, in fact, £107m - from £227m to £334m.

The £111m increase came later, when the costs were revised up again in February 2012.

BBC News Northern Ireland

A civil servant who set up the RHI scheme said it would have been "prudent" to advise Arlene Foster that the projected cost had risen significantly after she made her decision to proceed with it.

Image source, Reuters

Mrs Foster, the then DETI minister, gave the go-ahead in June 2011 based on the draft findings of an economic appraisal.

But when DETI received the final report a month later, the projected cost had risen by over £100m and civil servants did not inform her of the change.

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 concernand what was then a developing political crisis surrounding the scheme.

The RHI Inquiry began this week 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.

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

The Democratic Unionist Party (DUP) leader Arlene Foster 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 budget of the RHI scheme 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.

Need a quick refresher on what the RHI scandal is all about? Here goes...

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 and the fuel to run them.

It's a nice morning up here on Stormont hill and we're here to bring you what should be another enlightening day of the RHI Inquiry.

Former Department of Enterprise, Trade and Investment (DETI) energy boss Fiona Hepper is back to give more evidence and could be in the witness chair for much of the day.

Proceedings begin shortly, and we'll have a live video stream and text commentary throughout the day.