Archive for October, 2011

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

Sprinklers

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