Electrical Safety

High voltage is dangerous, not high current, unless of course the current is running though the human body. For example, if one grabs the two live ends of a SiC heating element which is drawing 20 A at an applied 40V, what determines the current though the grabber's body is the 40 volts and the resistance of the current path through the body. The 20A in the parallel circuit is immaterial. High voltages are especially accessible in our furnace room. Technically, touching a live connection (that is a connection which has a potential relative to ground), will not cause electrocution unless a portion of the body also touches ground. This is easy to do since all metal casings for electrical devices must be, by law, connected to ground. For this reason, when working/debugging malfunctioning equipment, it is good idea to avoid working with two hands; if one hand touches a live location while the other hand touches grounded metal, the current path goes through the human heart. Shoes are essential in this environment since naturally moist feet in contact with the floor make an excellent ground connection. The fluid in the body acts as an excellent electrical conductor, and the dominant resistance to current flow is the skin. Therefore, avoid working with wet hands, and take precautions against electrical components piercing the skin. Voltages on the order of flashlight batteries may be fatal if the skin is pierced.

When trouble-shooting equipment, make sure that all power has been removed from the equipment. It is good practice to always know where the upstream breaker box for an given power source is. Also, be aware that removal of power from equipment does not mean that high voltages will not persist, particularly if the equipment contains sizable capacitances which are charged by high voltage. An ``off'' switch does not guarantee that all power to an instrument box is off. To work on instrument electronics, it is better to unplug it and/or flip the upstream breaker. Before touching a malfunctioning instrument, it is a good idea to test separately for ac and then dc voltages using a multimeter: touch one end to the location of interest, and the other to ground. All metal enclosures are, by law, grounded, so touching one probe to a screw (avoid painted surfaces) on the box will be a ground connection.

One of the most important safety precautions when building or repairing equipment is to make sure that it is adequately grounded. Electrical work has strict color conventions, the most important of which is that the green wire is ground. This wire should connect at one end to the ground connection from the power supply (e.g. the ground plug on a 110V wall receptacle) and at the other end, intimately to the metal housing. The purpose of this is so that if a live wire makes contact to the chassis, extreme current will pass though the metal which will immediately blow a fuse. Fuses and breakers thus serve a very important function and should never be bypassed. Note that if a high frequency (e.g., rf) ground is required, the shape of the conductor may differ from that used for a D.C. ground. When designing high power apparati for a moist environment, ground fault circuit interruption should be added to the design.