Most of us have computer workstations, and some of us have lots of peripherals and ancillary equipment, such as a printer, a scanner, a modem, a router, a cordless phone, a desk lamp or two, a charging bay (for devices such as tablet computers and cell phones), and so on. All of these things get their power, either directly or indirectly, from the 117-V utility system. As a result, anyone with a substantial computer workstation will end up with a mess of wires behind and underneath the work desk: a “tanglewire garden”!
“Tanglewire gardens” can look dangerous, as if they would inevitably present a high fire risk, but they don’t have to pose a hazard. If you know how to connect and arrange the wires properly, it doesn’t matter from a safety standpoint how much you tangle them up, although you might want to affix labels on the cords near their end connectors (on each end) so that you don’t get them confused with each other when the inevitable malfunction occurs and you have to pull out and replace one of the components of your system.
Figure 3-11 shows the “tanglewire garden” underneath my home electronics workbench. In addition to a computer, this system includes an amateur (“ham”) radio transceiver, two displays, an interface between the radio and the computer, a microcomputer-controlled power-measuring meter, an audio amplifier for the computer and radio, a wireless headset, a desk lamp, and an external hard drive that needs its own “power brick.” That’s 10 devices or cords in total, all deriving their power from a single outlet in the wall underneath the table!
FIGURE 3-11 “Tanglewire garden” beneath the author’s electronics workbench. A heavy-duty UPS (out of the picture to the right) serves two power strips mounted on a metal baking sheet that rests on detached plastic shelves.
In order to ensure smooth operation of the system in case of a power failure, all of the devices are connected to the wall outlet through a commercially manufactured uninterruptible power supply
(UPS). The UPS has a battery that charges from the AC utility under normal conditions, but provides a few minutes of emergency AC (with the help of some sophisticated electronic circuits) if the utility power fails. That few minutes gives me time to deploy my backup generator without having to shut any of the devices down. The UPS has four outlets in the back, two of which go to power strips with six outlets each, and the other two of which remain empty. There are 12 available outlets in the power strips, 10 of which are engaged, as shown in Fig. 3-11. The UPS also has a transient voltage suppressor built-in. Figure 3-12 is a block diagram of the general arrangement.
FIGURE 3-12 Block diagram of the “tanglewire garden” beneath the author’s workbench. The power strips include breakers but not transient suppressors; the UPS contains a transient suppressor that serves the whole system. Gray arrows represent unused outlets.
Did You Know?
You should not use power strips with transient suppressors in conjunction with any other component, such as a UPS, that also has a transient suppressor. When you connect two or more transient suppressors in series (cascade them one after another), they’ll
probably conflict. Therefore, in the system shown in Figs. 3-11 and 3-12, the power strips do not include transient suppressors (or, as ill-informed people call them, “surge protectors”).
Quick Question, Quick Answer
• Doesn’t the presence of 10 devices, all plugged into a single wall outlet, as shown in Figs. 3-11 and 3-12, create a danger by overloading the outlet?
• Not in this case! All of the devices, taken together, consume less than 10 A (2/3 of the breaker rating for the outlet), even if they all run at once. The radio interface, the cordless headset, the audio amplifier, the power-measuring meter, and the desk lamp draw less than 1 A combined. The rest of the devices, taken together, draw about 7 A.
I’ve taken three extra precautions, aside from making sure that I don’t overload the wall outlet, to
ensure that my “tanglewire garden” remains safe. You should do the same with your pride and joy!
1. First, if you look carefully at Fig. 3-11, you’ll notice that I’ve mounted the power strips on a metal sheet. It’s a solid aluminum baking sheet. I glued the strips down there with epoxy resin. This precaution keeps the power strips from setting anything (other than themselves) on fire if they start sparking, a stunt that these things have been known to perform, occasionally with disastrous results.
2. Second, I have not allowed any cord splices or other sensitive electrical points to lie directly on the floor. The baking sheets, as well as all points in the cords where splices exist, are set up on thick plastic shelves. My basement floor will get wet if a huge, sudden rainstorm occurs. (Of course, in that event I won’t use the workstation until the floor dries out!)
3. Third, I’ve connected a dedicated ground wire from the chassis of the UPS to a known electrical ground. I tested the wall outlet underneath the workbench to ensure that the “third prong” actually goes to the electrical ground for the entire house. You can test the “third prong” of any three-wire outlet by following the procedure I describe in “Grounded, or Not?” a little later in this chapter.