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Old 13th January 2015, 05:08 PM   #1
ridikas is offline ridikas  United States
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Default Terminating AC Power Cord's Ground Via Resistor, Thoughts?

Let's say we have a three conductor power cord, neutral, hot, and ground. Now let's say instead of attaching the ground wire directly to the AC plug's lug, we terminate it via a 1 mega ohm resistor. The ground basically goes through a resistor.

Are there any benefits to such a design? Drawbacks? Dangers?
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Old 13th January 2015, 05:13 PM   #2
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Do NOT do this!!

If you have to ask the question, you do not understand power electricity enough to play around with safety issues!

Seriously, the incoming earth wire should go directly to a chassis ground point with a screw and star washer and nowhere else.
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Old 13th January 2015, 05:42 PM   #3
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From what I recall UL/CSA specifies the ground lead to be tested from the wall-plug to the chassis at 30A, voltage drop not to exceed 4 volt. So: NO! E

Last edited by mickeymoose; 13th January 2015 at 05:44 PM.
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Old 13th January 2015, 06:11 PM   #4
Hikari1 is offline Hikari1  United States
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ridikas:

Are you trying to isolate ground loops? It is typical to include some sort of ground loop isolator between the circuit ground and case...this can be a resistor, a thermistor, lot's of different ways. But it is unusual to put a resistor between the ground wire of the power cord and the case...and potentially dangerous. If that resistor fails you have a "hot" chassis.

If you are trying to isolate the circuit from ground loops do a little googling or looking at power supply schematics...lot's of ways of doing this.
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Old 14th January 2015, 02:28 PM   #5
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In the US 120V type system, under fault conditions the Safety Ground (Protective Earth) conductor may carry from 150 to 1000 Amps for a short period of time until the building's circuit breaker clears. So it's important to have a robust low impedance path back to the circuit breaker box.
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Old 14th January 2015, 02:42 PM   #6
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You can do that to isolate your circuit, BUT the ground from the wall must be securly connected to the chassis of the device, direct connection with nothing in between, usually the ground line is terminated to an eyelet which is bolted directly to the chassis. No ifs or buts.

From that anchor point you can attach whatever resistor/Thermistor/inductor you like to connect to your circuit/signal grounds, just don't mess with the wall ground to chassis connection.
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Old 14th January 2015, 03:28 PM   #7
ridikas is offline ridikas  United States
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So I understand that a design like this does not meet the proper UL requirements for ground. But what would be a benefit of a design like this? What else would the resistor accomplish? A filter? I know of at least one huge audio company (shall remain nameless) who is selling a power cord as such. To be more precise, their power cord is a star quad with a shield. Two conductors are connected to neutral and two to hot. The shield is directly connected to the ground pin inside the IEC, but through a 1 mega ohm resistor in the AC wall plug.

I'm just trying to wrap my mind on why such a design?
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Old 14th January 2015, 03:32 PM   #8
SY is offline SY  United States
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Quote:
Originally Posted by ridikas View Post

I'm just trying to wrap my mind on why such a design?
Because they are wildly incompetent.
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Old 14th January 2015, 04:09 PM   #9
Hikari1 is offline Hikari1  United States
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Because they dont want the house ground to be part of the shield. Maybe they are worried that the house ground is dirty or that it will act as an antenna for rfi or that the other amps and whatnot plugged in to the same outlet close by will mess with the shield. I am not sure but they are definately trying to isolate the cord shield from the ac outlet ground. Is it dangerous? Well, technically. Only if that resistor fails and only for the millisecond of time it takes for the resistor to pass current. But if you really think about it using a ground loop breaker between the chassis and circuit is also dangerous. Same rules apply. Some countries like Japan dont even have a third pin ground so there you go...
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Old 14th January 2015, 04:13 PM   #10
SY is offline SY  United States
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Quote:
Originally Posted by Hikari1 View Post
Is it dangerous?
Dangerous, period. Thoroughly irresponsible.

Class 1 vs Class 2. These are the standards to be aware of.
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