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General Asylum General audio topics that don't fit into specific categories. |
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General Asylum General audio topics that don't fit into specific categories. |
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In Reply to: Finally! posted by Rod M on April 6, 2007 at 18:13:29:
Your use of cheater plugs is dangerous.Let me provide a more simple explanation than the correct ones given so far.
Take a typical appliance or audio component with a metal case. The AC goes in to a power transformer or other device, with both the hot and neutral wires insulated from the case.
If something goes wrong inside and connects the hot wire to the case, the neutral will not be affected because it is insulated from the case. However, the case will be at the hot wire potential. If you touch the case while also touching something connected to earth, you will be killed.
The AC "ground" is a separate conductor that goes back to the entrance panel and is tied to the neutral there. If the ground is present and the case is connected to it, the fault that connects the case to the hot wire will cause large fault current to flow through the ground conductor, which will trip the circuit breaker. This will happen before the case can reach the hot wire potential and harm anybody.
Some appliances and audio components are designed with what is called "double-insulation" and do not need the "ground" wire. Ayre is one maker that uses this approach. The insulation is such that a fault that would connect the case to the hot wire is considered unlikely by the Underwriters Laboratories.
Cheater plugs defeat the AC "ground" connection and create the possibility of lethal voltage appearing on the case. Hum from ground loops is a problem in pro audio as well as home audio systems. There is a safe solution sold to the pro audio market: the Hum X from ebtechaudio.com. See the link.
Thanks to Tuckers for finding this device and calling it to my attention. It contains a pair of rectifier diodes wired anti-parallel, and shunted by a 1000-ohm resistor. Diodes have the property of not conducting much below their "forward" voltage of about 0.5 volts, so the pair presents high impedance to small ground loop hum voltage, but low impedance to fault current. The 1000-ohm resistor is there so that devices such as battery backup UPS that look for a ground will see a ground and continue to operate. Audio components don't need the resistor.
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Follow Ups
- **IMPORTANT** The "ground" is there to protect your life. - Al Sekela 09:09:44 04/07/07 (2)
- A very common exception - Sean 10:21:53 04/08/07 (0)
- Seems like this subject comes up every couple of months. - powermatic 11:07:50 04/07/07 (1)
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