Archive for the ‘Comic’ Category

Changing Lighting Bulbs

Wednesday, November 30th, 2011

This comic attempts to answer the age-old question, “How many ninjas does it take to change a light bulb?” Credit for this idea goes to Dharmesh Patel of Phoenix, Arizona, who wrote in several suggestions for MEP Ninja scripts. This one especially caught my fancy. More ideas are welcome, and as you can see, we publish the good ones.

P.S. One of our avid MEP Ninja followers commented that the unrealistic part of this cartoon is not that the ninjas would use their swords to change the light bulb, but that ninjas are creatures of the night and would not need light to study by! My bad.

Mark Robison, PE

Beer Conduit

Wednesday, November 16th, 2011

You recall the famous Pee-wee Herman bar scene from Pee-wee’s Big Adventure, right? That’s my favorite scene from my favorite movie. (You can watch it on Youtube if you’ve never seen it or want a refresher).

In this comic, we find the ninja in the basement of the US Green Building Council headquarters below the bar. The ninja sees a 4″ electrical conduit which he assumes must be the main power feed for the building. Instead, it turns out it is the conduit that carries the beer lines from the beer cooler in the basement to the beer taps in the bar!

Every electrical engineer knows that one of their duties as the engineer is to provide the 4″ electrical conduit for running the beer lines. It really is a conduit, and it really is installed by the electrician! But as the ninja learns, there are no wires in the conduit, just beer lines. And nothing angers a mob of LEED accredited drinkers more than cutting off their beer!

For once, the ninja is able to dance his way out of his situation, rather than being killed or captured. Don’t expect this to happen regularly!

Mark Robison, PE

Centrifugal Fan

Wednesday, November 2nd, 2011

The ninja almost, but not completely, understood what he was trying to do here.

The exhaust fan has a three phase motor. Three phase motors run in one direction or another depending upon the phase rotation of the power. The phase rotation is easily changed by reversing two of the three wires connected to it, which the ninja did.

Unfortunately, he assumed that reversing the direction of the motor would reverse the direction of airflow when the fan was on. For propeller or turbine fans, this assumption is true. But centrifugal fans blow air in the same direction no matter which direction they turn.

Instead of the gas being sucked into the building and killing the occupants, it was blown back at the ninja and he dies instead. Victory to our heroes at the Green Building Council!

Side note: what if the fan had a single phase motor? In that case, reversing the wires would have made no difference. The motor always turns in the same directions. But for three phase motors, the connection sequence of wires is critical to the performance of the fan. I can hardly count the number of times I have “fixed” a design problem by reversing the direction of the fan. It is one of my favorite fixes.

Mark Robison, PE

Swimming Pool Drain

Wednesday, October 26th, 2011

Of all the cartoons we have done so far, this one gives me the shudders every time I think about it. Maybe it is because this one could actually happen to any one of us. So what’s the big deal about a pool drain?

Well, let me describe what happens in terms we can all understand. First, you need to know that a pool drain is often a pipe 12 inches in diameter. That is not your bathroom sink drain!

Now let’s think of the water in the pipe as a little freight train. That freight train is really long, and it is going really fast. You have the misfortune of jumping right in the middle of the train as it is rushing past. Instead of just bashing you to a pulp like a real train would do, it sucks really hard on whatever is sitting on the drain. It squeezes you though the drain like a martini through a drink strainer. For just a moment you look down and see your skeleton sitting on the drain, completely cleaned of all your flesh. I told you this would make your shudder.

In real life, pools always have two drains far enough apart that you cannot sit on both of them at once. But as you see in this cartoon, the ninja accidentally covers one of the drains with his stolen goods, and all the force of the water is transferred to the other drain where he sits.

At least this cartoon is not bloody like all our other ones. Oh, wait, the water is about to be recirculated. Only, its red all of a sudden! Sleep well, and stay out of the pool’s deep end.

Mark Robison, PE

Solar Panels

Wednesday, October 19th, 2011

Watch out for the bull when cow tipping–but a bull is nothing compared to 400 volts!  The public (and ninjas) have a lot to learn about the new solar power technologies coming our way.

In this example, the ninja learns a crucial lesson about photovoltaic panels (PV panels): they can generate a lethal shock even when it is almost dark. The ninja knows that moonlight can produce a voltage on the PV panels, so he waits for the moon to go behind the clouds. What he did not count on was the reflection of the city lights from off the clouds. Even street lights can generate a high voltage in a solar panel.

So if the moon and street lights can generate a lethal voltage, why do PV panels require bright sunlight to generate electricity? The voltage on the PV panels can be high even if they are not producing power. Recall (or learn if you don’t have an electrical engineering degree) that power is voltage times current. No matter how big the voltage is, if there is no current, there is no power.

The next question is then, why did the ninja die? The answer is that it only takes a small shock at a high voltage to stop the heart. And large solar arrays can kick harder than a bull even at night.

Mark Robison, PE

System Coordination

Wednesday, October 12th, 2011

Inside the elevator, the ninja sees a camera that he wants to turn off. He shorts out the circuit to the camera, intending to trip the breaker and killing power to the camera. But what happens is that he trips the breaker for the entire building. The elevator comes to a halt and the ninja is trapped.

The name for this scenario is “system coordination.” When an electrical system is coordinated, a short on a small circuit will trip the breaker for that circuit, but not the breakers for the larger circuits. This coordination contains the power outage to a small area in the building.

In this building, the breakers for the elevator were not coordinated. Shorting the small circuit also tripped the main building breaker. As any good electrical engineer knows, the National Electrical Code requires coordinated circuit breakers for elevators in order to avoid the situation the ninja encountered. One can only imagine the desperate fate of the electrical engineer who designed this building when the ninja escapes his new found captivity!

Mark Robison, PE


Wednesday, October 5th, 2011

We have all seen the movie sequence where the hero is trapped in a building and sets the sprinkler system off to create a distraction while he escapes. In the movie, the entire building is doused in a torrential downpour with wet occupants scattering in all directions.

But what happens in the real world? Mostly, very little. If you put heat under a sprinkler head, only that sprinkler head goes off. This makes sense because the fire is where the heat is! None of the other sprinkler heads are needed until they become hot. If the sprinkler system is doing its job, the fire is extinguished locally and other areas of the building remain dry.

The big distraction that our ninja was expecting did not materialize; the police simply arrest him for being a public nuisance. The ninja will also have to pay civil damages for the water clean-up. I wonder if he carries insurance?

Mark Robison, PE

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