General Asylum General audio topics that don't fit into specific categories.

---

[ Follow Ups ] Thread: [ Display  All  Email ] [ General Asylum ]

---

[ Alert Moderator ]

---

Re: ? about a Cheater plug to eliminate hum---might this configuration cause "death"?

Posted by pande (A) on April 6, 2007 at 22:28:46

In Reply to: ? about a Cheater plug to eliminate hum---might this configuration cause "death"? posted by jjk12 on April 6, 2007 at 15:19:26:

Hello,

In a 120V country, like the U.S., one of the leads is hot, the neutral lead is actually attached to ground at the fuse box, and, in recent years, outlets incorporate a 3rd ground pin. This means that every 120V appliance, whether it has three wires or two, actually has one ground attachment as is. But this is not a safety ground, because if the hot lead in a metal box appliance somehow breaks loose, or shunts through a defective component to ground, it can elevate the chassis to "hot", or 120V, unless there is a safety ground, which is the purpose of the "ground" conductor.

However, this is extremely unlikely to happen in an appliance. Consider plug-in lights, ie., table lamps, and floor lamps. In the U.S., these are all two wire devices, typically with exposed metal work. If the zipcord in these lights somehow becomes frayed, and the hot conductor touches the metalwork of the lamp, the lamp will become hot. If someone touches the lamp, and a good ground, he could be electrocuted. But, the NEC, obviously based upon a very careful risk assessment, does not require the metalwork of the lamp to be grounded by a safety ground. The NEC authors do not think it represents a significant shock hazard in the enviroment in which the lamps are typically used. However, in kitchens, and bathrooms, where water and plumbing are at close hand, the NEC has more stringent requirements.

For many years, audio equipment was manufactured with two wire plugs as well, up to the early 90's, in some cases. The manner of construction of most of this equipment is identical to equipment specified for three wire safety ground operation. Internally, there is no difference at all in the safety considerations of the transformers for these devices. Since the early 90's, all this two-wire equipment has had polarized plugs. There is a reason for this. One is that it is desirable that the on-off switch must break the hot side of the circuit. The other is to take advantage of the fact that one of the two prongs on the power cord is actually at ground potential. When a transformer is wound, one terminus of the winding is at the iron core. The other is on the outside. When the inner winding termination is connected to neutral, this provides protection against a transformer short. Even if the inside of winding shorts to the frame, which is extremely unlikely, it will not elevate the potential of the amplifier chassis above neutral, which is actually at ground potential.

The only other way a 2 wire amplifier, hooked to a 120V circuit, could present a hazard is if the hot side of the line cord actually breaks away and contacts the chassis. But there is no more risk of that than with a plug in lamp, for which the NEC approves 2 wire operation. So, if the polarized plug is plugged into a properly polarized circuit, a two wire stereo, OR a three wire stereo with the ground lifted by a cheater plug, is no more dangerous than a residential plug in light. Eventually, the NEC moved to a higher standard, "double insulated." Modern two wire equipment requires two separately identifiable insulating layers. However, if the line cord breaks off internally, a chassis short could occur. In a 120V environment, there is not much difference between so-called double-insulated and single-insulated equipment.

This is not true in a 220 volt country. In this case, neither lead is at ground potential. A transformer short in a two wire device will result in elevation of the chassis above ground. The "double insulated standard" is meaningful for two wire devices. Combined with the higher voltage, one should take a little more care.

Hope that helps,

Pande

This post is made possible by the generous support of people like you and our sponsors:

Sonic Craft

---

[ Top of Page ] [ Contact Us ] [ Support/Wish List ] [ Copyright © 2024, Audio Asylum, all rights reserved ]

---

[ General ] [ Speakers ] [ Tubes ] [ Vinyl ] [ Digital ] [ Hi-Rez ] [ Video Asylum ] [ Cables ] [ Tweaks/DIY ] [ Music ] [ Films ]

---