A single-pole switch has only two conductor terminals. To one terminal, you connect the supply (hot) wire, and to the other you connect the load wire. All this switch does is interrupt or not interrupt the path from supply to load. This switch gives us on and off. Power comes in one terminal, and whether it leaves on the other depends on whether the switch is in the on or off position.
A two-pole (or double-pole) switch is two one-pole switches operated by the same lever (or similar activator); that is, if it’s a single-throw switch. You “throw” this lever one way, and you interrupt the path from supply to load in two separate circuits. Throw it the other way, and you complete the path. This switch has four terminals, just as two separate single-pole switches do in total.
You can see that all single-pole switches are necessarily single-throw. But that is not true of double-pole switches. A double-pole, double-throw switch has three terminals. A double-throw switch connects an input (hot, also called the common) terminal to one of two output terminals (carrier wires). This arrangement allows us to have three-way switching. With another terminal, it permits four-way switching.
A three-way or four-way switch can’t be used as a power disconnect for two reasons. First, it allows power to flow past it whereas a single-pole switch does not. Second, the only wiring you can connect to its input or output terminals are ungrounded circuit conductors (meaning that in, say, lighting applications you don’t run the neutral to it) [Sec. 404.2]. These have a ground terminal, so you do run the ground wire to such a switch. Note that ground wire doesn’t mean earth ground [NEC Art. 100], it means circuit ground. And the circuit ground is actually a bonding conductor.
Can you use a switch or breaker to disconnect the grounded conductor of a circuit? Yes, but only where all circuit conductors are disconnected simultaneously or the device is arranged such that the ungrounded conductors are disconnected first [Sec. 400.2(B), Exception note].
Switch actuators may be rotary, push-button, or lever/handle arrangements. A common mistake with lever-operated ones that operate vertically is mounting them such that the lever is up when the switch is off. This violates the requirements of Sec. 404.7, which requires that the up position be the closed position.
This requirement is based on the principle that the fail-safe position should not be opposed to gravity. If the circuit is off because the lever is in the down position, what happens if a mechanical failure prevents the lever from staying in the up position? Nothing. But if installed the other way around, the switch would change from on to off with no operator intention for it to do so.
But what about three-way and four-way switches? Simply turning on the lights with three-way switch Alpha and then using three-way switch Bravo to turn them off will result in one of those flip handles being up when the lights are off. So how can Sec. 404.7 or this gravity principle be complied with?
The trick here is a three-way switch isn’t exactly an on/off switch. Think of it as an on/on switch. Many models of one-pole light switches actually are marked “ON” and “OFF” but you do not see this marking on three-way or four-way switches.
The three-way switch has two carrier wires, and it just switches between them. One or the other of these two carrier wires carries the power provided via the “common” (hot) wire, at all times. The switch isn’t stopping the power from getting past it (as in a one-pole), it’s just switching the power from one carrier wire to the other. If the other switch doesn’t also select this “hot” carrier, then there’s no power to the lights. But there is still power on a carrier wire at each switch; you just need each switch to select the same carrier at the same time for the lights to be on. A similar concept applies to four-way switches.
It’s good to keep this understanding of switches in mind when working on industrial or commercial systems and equipment. Just because you flip a control switch off doesn’t mean the circuit is dead or an enclosure is de-energized. Lockout/tagout procedures for utilization equipment always take you back to the branch or feeder supply, which is a fuse or breaker. A similar concept applies to service equipment.
