TO BEGIN WITH, a fashion tip: Dark, expendable outfits are just perfect for those times when you're touring one of Potomac Electric Power Co.'s coal-burning plants.

This is because most visitors emerge from these facilities wearing hundreds of tiny black flakes. It's also because, midway through the tour, your guide will ask you to kneel on a sooty grate and peer through a hole into a cavern of fire -- a sort of scale-model hell -- and you'll be too intrigued to say no.

Your guide, a man perspiring under a Pepco hardhat, will crouch beside you and explain what's happening on the other side of the hole, which is actually a vent on the side of a boiler. As you peer in at white-orange flames, sweat beads clustering on your forehead, you'll hear the guide say that the coal-fed fire, 950 degrees hot, boils water that courses through pipes embedded in the structure's top and sides, and the resulting steam drives turbines that generate electricity.

When you stand up to walk away from the vent, you'll feel sore-legged and a bit woozy, and one or both knees of your trousers will be stamped with a dark smudge. But in all likelihood you'll finish the tour charged with a certain exhilaration, a sense of widened wisdom.

After all, relatively few individuals have witnessed the creation of electricity.

Not that there's anything secret about the process. Pepco offers free guided tours of all but one of its plants. Strictly public-relations endeavors, the tours are little-promoted and not meant to be profitable. Not surprisingly, school kids and teachers are the only type of visitors the guides regularly see.

At a few other places around town -- the Government Printing Office, Intelsat headquarters, and the D.C. central post office -- it's the same story: Free PR tours, few takers.

Do these tours deserve heavier patronage?

Accommodating a hunch, I recently searched for an answer to that question. What I've come up with is a qualified yes: The four aforementioned tours will appeal to inquisitive persons who enjoy offbeat distractions and who value raw knowledge over ephemeral thrills. (Peeking into the Pepco boiler was the only thrill I got in any of the places.)

In other words, while it's nice that these tours are available to the public, it's quite appropriate that they're free. Herewith, some capsule reviews. PEPCO

Pepco has six generating stations -- one in Virginia, two in Washington and three in Maryland. I toured the 41-year-old Potomac River plant in Alexandria, the oldest of the lot.

The first stop is a cool, wood-paneled conference room. Laid out neatly on a table are orange hard hats, safety glasses and ear plugs. You'll want to don this gear right away and start roaming the plant, but the Pepco people first oblige you to sit through a promotional video. Then you're encouraged to ask questions. After the Q&A, the tour begins.

If you've never been inside a coal-burning power plant, you'll be awed by the look and feel of the place. Eight stories high, noisy, hot, it's a miscellany of monstrosities. You wonder how humans could have dreamed up such structures and machines, let alone built them. You also wonder how 15 people (the average strength per shift) could run the whole plant, which sits within a 28-acre site.

Stopping in a lab, you're shown a sample of the coal dust that fuels the boiler. It's as fine as baby powder, thanks to the pulverizer that grinds it up after the coal is transferred on a conveyor belt into the plant from a nearby storage area. The guide also shows a sample of the ash residue that falls to the bottom of the boiler after combustion. In its little jar, this toxic waste looks as benign as cinnamon.

Up in the control room, you feel as if you've entered a time warp. Clunky fluorescent light fixtures illuminate a space filled with industrial artifacts seemingly from the '50s. One wall is mostly covered by rows of black boxes fronted by white gauges, each with a little pointer. Hardly a thing in sight is sleek or digital. Even the workers sitting at desks studying the gauges look old-fashioned with their long sideburns and flannel workshirts.

Soon, the tour's highlight: boiler inspection. As you kneel and the guide shouts his explanation over the boiler's roar, he holds up a fiber glass shield for you to look through. You can make out the motion of coal powder being blown in from one side. The boiler -- nearly 1,000 degrees inside -- has a negative draft, meaning air is sucked in from outside, rather than blown out from within. This, the guide observes, makes it possible to kneel right next to the thing without fear of cremation.

Thankfully, you get to cool off right away. Leaving the boiler area, you step into a rattling elevator and ascend to the plant's top level. You go up some stairs and through a door. Outside, a catwalk leads to a corner of the plant. The catwalk's floor is a grate, so avoid looking down as you walk.

A fresh breeze comes off the Potomac and your view is superb. You see Old Town, National Airport, downtown Washington. Across the river there's the U.S. Naval Research Lab and the Blue Plains Water Treatment Center. Looking down, you see the Pepco railyard, coal cars, ash silos, a huge coal pile next to the plant. Inspecting your bare arms, you see they're flecked with coal morsels. Time to go.

PEPCO'S ALEXANDRIA plant is at the corner of Bashford Lane and N. Royal Street, less than a mile north of Old Town. Parking is ample. The closest Metro stop is Braddock Road, from which the plant is a $4 cab ride (including tip). Groups of 10 to 30 can be accommodated on the two-hour tour. Not every plant is open for tours every day of the week. To arrange a tour of a Pepco plant, call Patricia Whorl at 872-3571, a month or more before your preferred date. GOVERNMENT PRINTING OFFICE

Washington has been called, among other things, a city of paper shufflers. So it's probably fitting that one of the city's few industrial tours involves paper. The Government Printing Office, located a few blocks from Union Station, churns out periodicals (including the Congressional Record and the Federal Register), brochures, forms, Congressional stationery, and other printed stuff by and for the federal government.

Its mammoth presses are fun to see, even if you're not a printing buff. Unfortunately, the presses aren't always switched on. And if your tour passes through the press rooms during a lull in the action, you'll have to let your imagination provide the sound and visual effects.

At GPO you learn the difference between web presses (which use one continuous sheet of paper that unwinds off a roll) and sheet-fed presses. In the web-press room, ask to examine a printing plate; the description of how it's made is more intelligible if you have one in your hands as you listen. Also take note of the 1,400-pound rolls of newsprint stacked in pairs. (Newspaper junkies, inhale deeply and savor the aroma.)

The library binding section, where hardbound books are produced, may be the tour's strong point. Over here, a bookbinder affixes a leather surface to beveled cardboard covers. Over there, a craftsman stamps gold lettering on books' spines. Across the way, a colleague shows how colorful marbling is applied to page edges. (Ask for a marbled souvenir notepad here.) Most books made in this section are copies of the U.S. Code produced for members of Congress or the Cabinet.

The most impressive job performed at GPO is the overnight production of the Congressional Record. After 6 p.m., when Congress is in session, transcripts start flowing in from Capitol Hill. The copy is keyboarded, typeset, proofread, printed and bound. The first bundles of Records are delivered to the Hill by 7 the next morning.

Guides say the place's pace turns frenetic at night (my tour was shortly after noon, when GPO is comparatively quiet). So when you set up a tour, ask the PR people if you can start at 9 a.m., when the Congressional Record presses are still running with the previous night's work.

THE GOVERNMENT PRINTING OFFICE is at the corner of H and North Capitol streets NW. Metro: Union Station. Hour-long tours for up to 20 are offered Monday through Friday between 9 and 1. For tour information and reservations, call Barbara Shaw at 275-3541. WASHINGTON, D.C. POST OFFICE

The District's big post office on Brentwood Road NE processes up to 5 million pieces of mail each day, including all classes of letters and packages entering and leaving the city. On display is a river of paper moving through a cavernous room, from one work area to another, mostly via conveyors. Numerous tributaries flow into and out of the river.

Even with a guide explaining what's going on, you'll likely be bewildered by many aspects of the operation. This is partly because the guides use a lot of postal jargon (mine did, anyway), and partly because their voices are often drowned out by machine-made din. You have to ask lots of questions -- shout them -- or risk remaining confused.

After collected mail arrives at the facility's truck docks, it gets transferred to robot-driven bins, which roll slowly toward the spot where the mail river begins. Step in front of a robot, and it advances to within an inch of you before a sensor makes it halt.

At the river's source you see "rough-cull" machines, which mechanically separate the packages and thick envelopes from the "flats" -- flat envelopes of any size. The floor about this area is littered with letters that have fallen off the belt. Workers periodically go around picking these up and tossing them back into the river.

Packages go off in one direction and the tour goes in another, following the flats through the system. Ask to see where packages go, which I neglected to do. The Post Office tour guides are willing to let visitors see just about any part of the operation that interests them.

Even though you've always taken for granted that machines perform most of the Postal Service's processing work, you'll still be awed by the variety, accuracy, speed and flexibility of the gadgets in view. Flats, which start out lying haphazardly on a slow-moving conveyor, travel through a series of turns, drops, funnels and slots, as the river narrows and narrows. The eventual result is a single-file line of envelopes standing lengthwise and speeding toward the canceling and bar-code machines.

The canceling machine prints the postmark on a passing envelope. It also checks whether an envelope already has a bar code (i.e., is precanceled); if so, it plucks it off the line and drops it into a special bin.

The next set of machines, optical character readers, or OCRs, decipher the address on each envelope. In approximately one-ninth of a second, the OCR computer verifies that the Zip code matches the city, and commands the machine to spray an appropriate bar code on the envelope's face. The bar code enables speedy processing of the mail in the destination city.

A technician was near the OCRs when I was there, and he kindly removed the top of one to show me the camera-like lens that each piece of mail passes. The speed of the operation dazzled me -- nine pieces a second! A mere decade ago, humans performed this task, scanning each envelope and tossing it into an appropriate bin.

There are many processing chores still performed by humans, most involving hand-addressed and Zip-codeless envelopes. These are all processed, at roughly one envelope per second, by men and women who have fast fingers, sharp eyes and an uncommon ability to appear simultaneously alert and bored out of their skulls. The tour doesn't linger long in the humans' domain.

THE POST OFFICE is on Brentwood Road NE between Rhode Island and New York avenues. Metro: Rhode Island Avenue. Hour tours of up to 25 are offered Tuesday through Friday between 9:30 and 7. For tour information, call 636-1208. INTELSAT

Intelsat -- the International Telecommunications Satellite Organization -- owns and operates satellites. It's not a corporation, but an international cooperative made up of 119 members and about 60 other countries that are "nonmember users." Intelsat's 14 working satellites carry half the world's telephony services -- telephone, telex, fax -- plus 99 percent of all live international TV signals entering or leaving the United States.

In 1985, the co-op moved from L'Enfant Plaza to a futuristic, oddly shaped, glassy building in Cleveland Park. Intelsat's PR people receive many requests for tours of the building, but they allow the public to see only the visitors center. (Reason: The offices have glass walls, and workers don't like sightseers peering in at them.)

The visitors center consists of four areas, the first of which is a spacious lobby hung with prototypes and scale models of satellites. Moving your eye clockwise around the ceiling, you see the seven satellites used by the co-op since its inception in 1965. Intelsat I is small and stubby, and each successive model is a little bigger and sleeker than the previous one. The latest model, Intelsat VII, is dominated by long, thin appendages that are covered with solar panels.

The first of five Intelsat VII satellites will be going up in 1992. Three VIs have been sent skyward, but only two work. Due to a launch snafu, the other one never ascended to its proper orbiting position, which is 22,000 miles above the equator. It's now circling safely but uselessly a few hundred miles up, awaiting a NASA rescue mission scheduled for 1992.

Off the lobby, you enter a small balcony overlooking the launch control center. The ultra-modern center is dominated by a semicircular array of work stations, each with a computer screen or two. Launches aren't actually controlled here, only monitored. Intelsat's technicians take charge of a satellite after the rocket releases it; from this room they send commands to the bird via earth stations in other parts of the world.

The center gets used only during a launch and immediately before and after. At those times, the room bustles. (The last launch was last June; the next will be sometime next spring.) Unfortunately, the balcony is off-limits to visitors at those active times -- you can see the hive only when the bees are away.

Crossing the lobby, you enter an overlook atop the operations center -- another 21st-century-type setup. Half a dozen people sit at desks, looking at computer screens or talking on phones.

They are "watching the traffic that goes over the system," my guide explained. They don't monitor the content of the satellite transmissions, just the quality. "Also they are the last word as to who comes on and off the system," the guide said. "That's why this room is sometimes called 'the conscience of Intelsat.' "

The room's dominant feature is a floor-to-ceiling, 34-foot-long world map painted on the wall across from the overlook. The map is studded with tiny lights -- red, yellow, green, white -- each representing an Intelsat earth station or tracking station. At the equator line you see 14 boxes with numbers -- the Intelsat birds. The number on each box is the satellite's longitudinal position.

The tour's last stop is a small theater where you're asked to watch a short promotional film. I give this film a solid 10 rating (on a scale of 1 to 1,000). All in all, the look of the place is by itself arresting enough to warrant a visit. To make the Intelsat tour really worthwhile, read an encyclopedia entry on satellites beforehand.

INTELSAT is at the corner of Connecticut Avenue and Van Ness Street NW. Visitor parking is limited. Metro: Van Ness/UDC. Hour-long tours for up to 35 are offered Tuesdays at 10 and Thursdays at 3, by appointment. For tour information, call Isabella Kyser at 944-7841.

Kevin McManus last wrote for Weekend about the National Aquarium.