Archive for September, 2011

GFCI Receptacle

Wednesday, September 28th, 2011

You know those outlets in your bathroom with the red and black buttons in the middle that sometimes need to be reset? Those receptacles are designed to keep you from electrocuting yourself while making toast and taking a bath at the same time.

They are called Ground Fault Circuit Interrupters or “GFCI” for short. The GFCI limits that shock you can receive from the receptacle to 4 milliamps or less, which is not lethal. It might tingle real bad, but it won’t kill you.

Our ninja learns about GFCI receptacles the hard way. He thinks he can electrocute the red ninja by forcing his knife blade into the receptacle, but since it is a GFCI receptacle, it only stuns his foe. And once he shakes off the stun, he proceeds to complete the lesson.

Where in this picture are the red and black buttons on the receptacle? The answer is that they are in another receptacle upstream of this receptacle. One GFCI receptacle can protect multiple standard receptacles on a single circuit. That is why our MEP ninja did not realize that his plan was destined to fail.

Mark Robison, PE

Power Lines

Wednesday, September 21st, 2011

Everyone knows it is okay for birds to land on power lines. But you would never think of touching a power line, right? Well, if you could do it like a bird does, you would be fine. In this case the ninja thinks he is every bit a bird.

As long as he is touching just the power lines, there is no way for the current to complete a path. He probably feels a little tingle from the static charge, but nothing more. Unfortunately, he forgets to release before touching the fire escape railing. Once he touches the railing, there is a way for the potential on the power line to make its way to ground through the ninja and the fire escape.

The voltage on these lines is probably 7200 volts from wire to ground, which is a pretty common voltage for urban power lines. You can estimate the voltage based upon the size of the insulators on the pole. Higher voltages require bigger insulators. These insulators are pretty small, so the voltage is fairly low, at least in the world of high voltage. In the world of “will-it-kill-me”, it is more than high enough.

Mark Robison, PE


Wednesday, September 14th, 2011

In movies and video games, ductwork seems to exist for the sole purpose of moving people through a building. In reality, it is used to move air. When a system is running, air is moving through it. If you are going to actually travel through a duct, you want to take this into account.

Engineers think of air velocity in ductwork in terms of feet per minute (FPM). A normal speed is 1000 FPM, but high velocity systems can run at 5000 FPM or higher. Converting to miles per hour, that is 11.3 MPH in most ductwork and 56 MPH in high velocity systems. These speeds would make travel through the ductwork difficult, but probably are not high enough to actually uncontrollably pull you. The ninja must have stumbled into a veryhigh velocity system.

When fan blades are exposed, there is often a guard screen to protect against large objects from entering the fan. Unfortunately for the ninja, this fan does not have that safety feature.

For the engineers in the audience, this fan lacks some critical details, such as bearing supports, but we doubt the ninja was thinking about that as he went through it.

Mark Robison, PE

Turning Vanes

Wednesday, September 7th, 2011

There are lots of obstructions inside actual ductwork that would make it difficult to travel more than a few feet inside of them. In this example, the ninja runs into turning vanes.

Turning vanes are installed in the elbows of square ductwork. They help the air to turn a corner smoothly. Without them, a more powerful fan is needed to move the air through the ductwork, and the air movement is much noisier.

As you can see in the comic, the turning vanes look like a set of bars across the ductwork. Air can move through them, but they would completely block anyone trying to crawl past.

Mark Robison, PE