Big gas and power users are experiencing the Big Chill this winter, as Wasatch Front utilities were forced to punch the interrupt switch.

Both Mountain Fuel Supply Co. and Utah Power & Light Co. say they experienced record demand last week.In order to provide natural gas to its residential customers, Mountain Fuel curtailed service to about 100 industrial customers last week, for the first time since 1983. Company spokeswoman Louise Jacobsen said those customers were notified that their supplies would be cut again on noon Saturday for at least three days.

UP&L also cut back electricity supplies to its interruptible customers last week, said spokesman John Serfustini.

During peak user periods, "what we do is we go shop around the power market in the West and look for additional electricity," Serfustini said. "(Large) customers then have the option of taking the higher-priced electricity or taking a cut in power usage."

Large industrial customers such as Geneva Steel and Kennecott pay cheaper rates all year round. At peak times, utilities have the option of asking customers with interruptible contracts to supply their own natural gas and power.

Mountain Fuel set a 24-hour rec-ord for delivery to residential and commercial customers last weekend, Jacobsen said. Demand is running 50 percent above normal. Between noon on Dec. 22 and noon on Dec. 23, the company delivered 680 million cubic feet of fuel. The previous 24-hour record was 610 million cubic feet of fuel.

UP&L also hit a new record on Dec. 20 and 21. "That means that we had more demand on our power plants on one instant last Thursday and Friday than we have ever recorded," Serfustini said.

Geneva Steel was without natural gas from Friday, Dec. 21, until Thursday morning, Dec. 27. Spokeswoman Mary Kay Lazarus said the company switched to its own gas supply, fuel oil No. 6, which is similar to heating oil used in the northeast portion of the country.

Geneva experienced a short period of downtime while the company switched fuels but then was able to operate normally. "It is an effort, some complication in doing it, but the plant is prepared to proceed efficiently and smoothly in the event it does happen," Lazarus said.

The switch to an alternate fuel source caused problems at General Refractories Co., which manufactures fire bricks. The company is using fuel oil No. 2 to heat its kilns but had a difficult time getting its alternate system on line because it hasn't been used for a time.

The company reduced production 10 percent to 15 percent, according to Larry Morton, general foreman. "If it went on very long, our production would be cut . . . right off," Morton said.

Provo City Power used kerosene heaters to keep the plant warm and 100 percent diesel fuel to operate the diesel engines that produce power, said Glen White, operations supervisor. The plant normally operates its engines using 95 percent natural gas and 5 percent diesel fuel. Operating the engines on diesel fuel is much more expensive, particularly in light of recent price increases; diesel fuel now costs $1.17 a gallon, compared to last summer's price of 50 cents per gallon.