Here's what comes to my mind. There's my favorite switch - the thermostat. There's the switch portion of the relay (more on relays in an upcoming column). And there are all of those switches called limits - high limit switches, low limit switches, aquastats, low water cut-off switches, flame rollout switches and more.

Think of switches in series as a highway with a number of drawbridges. If you're going to travel the entire road, all of the drawbridges have to be closed. If even one is open (same as a switch in open or “off” position), you can't complete the journey. (Note: For electrical circuits, the highway is a circle, not a straight line. Electricity's destination is to get back to where it started by completing the circle.)

I also think of switches in series as a “safety committee.” The committee is constantly voting on the issue of whether or not to bring on the burner. All committee members must vote “yes” before action is taken - it's not a majority vote.

As in real life, each member of the switch safety committee represents a special interest. Only one is interested in whether there's a need for heat in the living space. That's the thermostat. It will close when there's a call for heat. If all the other committee members are closed, the burner will come on.

The high limit is concerned only about boiler temperature. As long as the water temperature is below the limit's set point, then the high limit switch will stay closed. The high limit is a normally closed switch; if conditions are normal, it will stay closed and let the boiler function. If the water temperature gets too high, the high limit switch opens.

Its action can be described as “opens on temperature rise.” When open, it keeps the boiler burner from firing to prevent the water from getting any hotter than it already is.

Another limit switch is a flame rollout switch. It, too, is normally closed. So long as there's no flame in the wrong place, the switch stays closed. The switch opens if it senses flame where it shouldn't be. This shuts down the burner, even if all the other switches remain closed.

A low water cut-off is normally closed unless it senses that the water level is too low. If the water level gets too low, the switch opens and prevents the burner from firing.

You get the idea - when everything is normal the safety switches remain closed. Then the only switch that actually turns the burner on and off is the thermostat. The thermostat is in series with the limit switches. When the thermostat has a call for heat, it closes. When it, in addition to all of the other switches, is closed, the burner fires.

Fun In Class

Well, have you ever role-played switches? No, never tried it? Here's how it might go in a class of mine.

Let's say I've been teaching from the front of the room for about an hour. Fascinating as the subject of wiring controls is, perhaps a few students are drifting a bit. It's time to stir things up.

As I take a few steps toward the back of the room, the Back Row Boys sit up straight. And they do it fast, really fast. Some of them sit up for the first time since the class started.

As I meander to the back of the back of the room, the rest of the class wakes up too. They sit up straight and turn in their seats, eyes following me to the back of the room. They're curious as all heck: “What's going to happen to those schmucks in the back?”

They're thinking both, “Oooh, do they ever have my sympathy,” and “Cool, are they ever gonna get it now!”

Here's what happens next.

I say, “I need some volunteers. We're going to have a safety committee meeting!” The Back Row Boys have nowhere to hide, nowhere.

“C'mon. No volunteers?” Back Row Boys are rarely volunteer material, but they know what's coming next.

“OK then, you three,” I say, as I point to the middle of the back row. “Please stand up. This won't hurt.”

They stand up. Awkwardly. Stiffly. Looking very tall, and knobby, and sheepish.

“Just stand in a row, shoulder to shoulder.”

A student voice behind me, toward what recently was the front of the room, pipes up.

“Oh, man, she's gonna make you hold hands!”

The three standing Back Row Boys start shuffling.

I say, “No, no, it's nothing as bad as that. You're going to be switches.” To the one closest to me I ask, “What's your favorite switch?”

“Whaaat?!” you can see him wondering. “My favorite what?”

“Switch. What's your favorite switch?” I answer, knowing he's never considered that possibility before.

“Here, I'll help,” I offer. “Do you want to be a thermostat?” I could ask if he wants to be an earthworm, and he'd agree just to move on with this and sit down as soon as possible.

I motion toward the second guy. “What switch do you want to be?” He's been standing there wishing he'd paid more attention to the switch portion of the class so that he'd know the name of one.

I'm going to rescue this one as well.

“Would you like to be a high limit switch?”

“Sure,” he's relieved to answer. Everything about him is saying, “Anything you say, ma'am, just get me outta here.”

I continue. “And you?” motioning to the third Back Row Boy.

He's no better off than the first two except that he has a better idea of what the topic is.

“Switch,” I repeat. “Would you like to be a flame rollout switch?”

“OK. Flame rollout switch,” he agrees, shrugging his shoulders.

Ah, now we have the safety committee. Time to make it work.

“That's right, stand side by side, face me and the class. Put your hands right up next to your shoulders, like a goal post. There. You're all open switches. In fact you're open switches in series. Cross your arms in front of you. Now you're closed switches in series.

“Thermostat - if we don't need heat, are you open or closed?”

“Open.”

I put my arms in goal post position, and he does also.

“High limit switch, are you normally open or closed?”

“Closed.”

He's onto the game, and keeps his arms in closed position.

“Flame rollout switch, are you normally open or closed?”

“Closed.” He too keeps his arms in closed position.

“What's it going to take to get the burner to fire?” I ask the class.

“Close the thermostat,” several respond.

“What's it take to close the thermostat?” I ask.

“Call for heat,” someone says.

The thermostat guy closes his arms.

“Uh-oh,” I exclaim, “now the temperature is too high in the boiler. What happens?”

The high limit guy opens his arms.

“And then what happens?” I ask.

“Burner shuts down,” someone answers.

“Until when?” I ask.

“Until the temperature drops and the high limit closes.”

“Right! And what normally causes the burner to turn off?

“End of the call for heat and the thermostat opens.”

“Absolutely. Safety committee, thanks for your help.”

The Back Row Boys lunge for their seats, relieved.

Safety committee dismissed. Time for a break.

Next month: Parallel circuits.