A Radio Teleswitching signal (RTS) controls when storage heaters receive ‘charge’ and when the hot water tank is topped up with hot water.
The specific times for the off-peak rate depend on the weather forecast and THTC works by storing up heat during this time.
It was a government decision that the RTS and associated tariffs such as THTC would come to an end, with all heritage meters replaced with smart meters.
OVO has been criticised for failing to update customers, many of whom are elderly, about the timings of this but has said it is likely to be next year.
Some of the meters are between 20- 25 years old, exceeding their shelf life according to experts and customers have reported faults including dangerous surges and pre-payment meter credit being “swallowed up” leading to supplies being cut off.
A report by Local Energy Scotland found that THTC was considerably more expensive than other tariffs and customers say Ovo makes it “extremely difficult” to switch to smart meters.
Kirsty Bailey said she complained to SSE and OVO for around three years about faults with her pre-payment meter before it caught fire last Thursday.
The mother-of-three, who lives in Kingussie, says it was fortunate the incident happened at 6.30pm and that the cupboard door was locked, containing the fire.
She was checked by paramedics for smoke inhalation and the family has been put up in a hotel by landlord Highland Council.
The Herald was shown images of the burnt-out fuse box.
Investigations into the cause of the fire are still ongoing but Ms Bailey claims both SSE and OVO failed to send out engineers after she raised concerns.
She said: “After about four months of complaint after complaint, I went to the Ombudsman, who carried out an investigation. SSE was advised to run tests on the meter and take it out, which they didn’t do.
“Then I went over to OVO and stated my case with them. They sent out the wrong engineer, who wouldn’t touch it. He put it through as an emergency but they never showed.”
She says the last call she made was to report a fault that was resulting in the meter rapidly deducting money and cutting off her electricity supply.
She claims the engineer told her he couldn’t fix it “because the system was outdated”.
She said: “When the house fire started, all my electrics were flickering, then it went off.
“I spoke to a lady who lives in Kincraig and the same night of my fire her heaters were going extremely hot and the lights were flickering and there was a surge of energy which blew her receivers.
“OVO went to her property the next day and removed her meter.”
Katie Miller is a trained energy advisor and has been acting as an advocate for THTC customers.
She said many had reported the same fault as the family in Kingussie and there is some concern that others may have meters that are a fire risk.
She said: “Many of the meters are out of date.
“It seems like we have to wait for something really bad to happen for them to make a decision.”
Kate Forbes, SNP MSP for Skye, Badenoch and Lochaber said she was “deeply concerned” by the house fire and said uncertainty over the timings of the upgrade had created a lot of anxiety for customers.
She said: “I have written to companies like OVO several times on behalf of my constituents, and to date, I am no further forward on what they intend to do.”
A spokeswoman for OVO said: “Our team has been in touch with Ms Bailey to offer our support while we’re working with the relevant authorities to understand the cause.”
The company said smart meters will be installed this year in homes within the Smart Metering Wide Area Network (SMWAN) signal and it is working on a solution for other households. It said no customers would be liable for the cost.