On Thu, 21 Jan 2010 14:02:58 -0500, Joe Subich, W4TV wrote: >While the Neutral (the white wire in the US three wire power >cable) must be bonded to ground at the service entrance and >nowhere else, by code the Neutral must ** NOT ** be bonded to >the case/chassis of any equipment (e.g., white should never >be connected to the case) so the neutral is not at issue here.
>However, the SAFETY ground (green wire/round pin) is also >bonded to ground at the service entrance - and in most >residential installations ONLY at that point. That's tradition, and common practice, but there is nothing in NEC to discourage additional earth connections to the ground (the green wire). >It is this situation, where the safety ground (chassis) and >the shield of the coaxial cables (also connected to chassis) >are at different potentials that is potentially unsafe. No, it is bonding of neutral to the EQUIPMENT GROUND (in that equipment) that is the problem, because it allows return current to divide between neutral and equipment ground and any other conductors that the equipment ground is bonded to. For example, the return current might flow on steel building structure, or in low voltage wiring (shields of audio or video cables, etc.), or on metallic plumbing pipes. It makes it less likely that a fuse will blow in the case of a fault, AND it spreads out the magnetic field rather keeping it contained between the phase (hot) and neutral conductors. There is a tutorial drawing of this in the Ham Interfacing tutorial, and also in the Power and Grounding tutorial. The connection of power system ground wiring (the equipment ground, also called the green wire) to earth at multiple points is highly desirable. As you noted, it helps all the wiring "rise and fall together" in the event of a lightning event, thus minimizing the voltage between interconnected equipment. That is, of course, a double edged sword, since currents will be induced in loops by magnetic coupling, but authorities agree that having the building rise and fall together is far more of a factor than induced current. 73, Jim Brown K9YC ______________________________________________________________ Elecraft mailing list Home: http://mailman.qth.net/mailman/listinfo/elecraft Help: http://mailman.qth.net/mmfaq.htm Post: mailto:Elecraft@mailman.qth.net This list hosted by: http://www.qsl.net Please help support this email list: http://www.qsl.net/donate.html
