Four fire engines are leased by the Fire Authority at a cost of £34,000 but are only used when there is a strike.
The move has contributed to a £72,000 overspend on part of the authority's budget and is only deemed necessary because of fears striking firemen would refuse to drive engines which had been used by contingency staff during a strike.
Firemen have been involved in a prolonged dispute with the government over pensions.
Authority chairman Coun Jim Andrews said: "We will not use fire engines that are used by people who are on strike.
"I do not wish to escalate this strike. It's about not stirring up more trouble with the FBU than is necessary."
Graham Wilkinson, Fire Brigades Union chairman, said fire engines used by contingency staff who are not as highly trained as full time firefighters would need thorough safety checks after they had been used and said it was 'far simpler' to use other vehicles.
During the authority meeting last Monday, Coun Andrews said the strike action had cost the authority a total of £345,000 - which it was hoping to claw back from the government.
The additional costs comes as the authority agreed to cut the number of staff who work at Barnsley fire station at night to reduce costs and avoid fire stations closing or engines being removed altogether.
In future at Barnsley, just one full time crew will work overnight, while the station's second fire engine will be manned on a part time basis.
The authority's director of finance, Beverley Sandy, said the overspends in this year's budget had been cancelled out by underspends in other areas.