Dr MacLean asks how such a "major decision" to drop 16 of the planned changes to the RHI scheme could have been taken without going to the DETI minister.

And Mr Aiken asks whether it was ever drawn to the attention of the then minister Jonathan Bell (below) and his adviser that the department had a "big problem" in terms of the budget for the scheme.

Image source, Pacemaker

He points out that there was a period of a month in which the minister could've been "properly briefed... through the proper process".

Mr Wightman says he was never part of any discussion with the minister but adds that it could've been raised at by someone at a higher level within that department than his.

He adds that civil servants usually wouldn't "go cold to the minister" and they had questions they had to answer themselves before raising the issue with Mr Bell.

DETI finance officials warned the team running the RHI scheme to stop spending money on it in May 2015 as the extent of the budget issues became clear.

Inquiry panel member Dr Keith MacLean (below) wants to know why the response to the warning to stop spending was to propose the introduction of a cost control - subsidy tiering - rather than to simply stop spending.

Image source, RHI Inquiry

Mr Wightman says DETI still thought it could get additional funding for the scheme from the Treasury and "still thought the scheme was a worthwhile venture in terms of the European targets".

He isn't aware that DETI were at any time considering closing the scheme, adding: "We wanted to reduce demand for the scheme if we could and that's where the tiering came from."

In May 2015, DETI civil servant Seamus Hughes - who worked under Mr Wightman - was working on policy changes for the RHI scheme - there were 18 planned amendments, which had been publicly consulted on in 2013.

But that month, the department's finance officials informed the team working on the scheme that the scheme was spending too much money and there could be repercussions for the department's overall budget.

Image source, PA

As a result, 16 of the 18 policy changes to the scheme were dropped, leaving just two - including the addition of cost controls to the scheme - to take forward.

Mr Aiken says there's no record of the decision-making and Mr Wightman is asked who made the call to strip out the vast majority of the changes.

He explains that he had a discussion with his superior, DETI's energy boss John Mills, who decided that all of the other issues were to be parked and the focus would be on two policy changes, including the addition of tiering of subsidies.

Stuart Wightman hangs his jacket on his chair and settles into his seat for another all-day session.

The witness headed DETI's energy efficiency branch from June 2014, meaning he had oversight of the non-domestic RHI scheme and also worked on developing and introducing the domestic RHI scheme.

Image source, RHI Inquiry

This is his fourth appearance before the inquiry - his first back in March and his most recent this week.

You can have a look at Mr Wightman's written statement on the inquiry's website, external.

The inquiry's junior counsel Joseph Aiken starts with the questions, taking us straight back to the spring and early summer of 2015 and some complex policy issues relating to the scheme.

BBC News NI

The inquiry heard a claim that three biomass boilers had been bought through the RHI scheme for poultry houses that had not, at that point, been built.

In the meantime, the boilers had been approved, commissioned and were heating empty space.

The issue was raised with the scheme administrator Ofgem's counter-fraud team.

They said that just because the shed they were in was empty on the day of inspection it did not necessarily mean it was empty on other days and no further action was taken.

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.

The weekend beckons but we're putting it on hold to bring you day 74 of the Renewable Heat Incentive (RHI) Inquiry live from Stormont's Parliament Buildings.

There's a familiar face in the hot seat today - the Department of Enterprise, Trade and Investment's (DETI) ETI's former RHI scheme manager Stuart Wightman, who was last here the day before yesterday.

The session kicks off shortly so stick with us for live video and commentary throughout the day.