Nuts and Volts - June 2016

Unknown Danger

My first job out of college was as a development engineer for a company that made large industrial motor drives up to 1,000 HP. I worked in a laboratory that had many prototypes and breadboards out in the open on top of work benches. Most of the circuits were powered from 240V or 480V AC power lines. One day I was working on a drive that was acting erratic, so I decided to look at the gate drive waveform with an oscilloscope. As I connected the clip lead of the scope probe to the gate drive circuit board, I was temporarily blinded by a flash of light. When I could see again, I found that the palm of my hand was black and slightly burned. The probe clip lead was gone as were most of the copper foil traces on the printed circuit board. I was very lucky in that my injuries were minor and short lasting.

As I went about trying to understand what had happened, I learned about a subject that was not taught in engineering school but is well known to every real electrician: GROUNDING. In many parts of the world, what we in the US call ground or grounding is called earth or earthing. Grounding systems and practices are intended to make things powered by AC outlets safer. When someone is working on the guts of an AC powered device, the grounding system must be fully understood in order to work safely.

My accident occurred because the metal enclosure of the oscilloscope and the probe clip lead were connected through the ground pin on the AC line plug to the building ground system. One side of the 480V power line was also connected to ground through the neutral conductor, so basically, I had shorted out a 480V circuit with the oscilloscope clip lead. I was very lucky that the foil traces on the circuit board acted as a fuse and quickly interrupted the current before really serious injury occurred. Modern oscilloscopes are in plastic cases, but if you measure the resistance between the probe clip lead and the ground pin on the AC power cord you will find that they are connected.

Later in this article, we will explain how to use an oscilloscope safely, but first read on so that you totally understand grounding as it applies to the AC power line in your house.

Technical Language

Technical language is meant to be very precise when used properly. Unfortunately, it is often misused by both non-technical and technical people alike. We tend to throw around terms like heat and temperature or force and pressure as though they are the same. We often say “Turn up the heat” or “Apply pressure to the pushbutton,” etc.

Does your car have a negative ground system? If your

In general, the projects you see in electronics magazines are powered by batteries, wall mount adapters, or from a computer USB port. At some point, you might work on equipment that is powered directly from the AC power line; things like antique radios and amateur radio transmitters. The purpose of this article is to convey some little known or often overlooked information about the AC power line so that you can be aware of the possible danger as you work.