GFCI Receptacle For Appliance With 3 - Prong Cord.

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Let throw this out there to the 'A GFCI takes the place of an EGC' crowd.

Do you think the CMP for grounding will be OK with letting people get shocks?

The reason I ask is many cases the GFCI device will not prevent an electrical shock to the user. It only limits the duration of the shock. Until the GFCI opens the circuit the victim is getting a full voltage / current shock. This can lead to other injuries such as falls etc.
 
I can think of many scenarios where somebody would get a full fledged shock on a GFCI protected circuit. The number of cases where it has happened is pretty small I bet.

What are the numbers of someone getting a shock on a two wire circuit where a plain ole three wire plug or a hardware store cheater has been installed?
 
ActionDave said:
I can think of many scenarios where somebody would get a full fledged shock on a GFCI protected circuit. The number of cases where it has happened is pretty small I bet.
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Anytime a person who is grounded touches a live GFCI protected conductor they will get a full voltage / current shock for whatever number of cycles it takes GFCI to see the fault and open the circuit. Every time.


The only time that changes is if the faulted utilization equipment or live conductor faults to ground and trips the circuit before a person touches it.


What are the numbers of someone getting a shock on a two wire circuit where a plain ole three wire plug or a hardware store cheater has been installed?
Click to expand...

Again, I can't argue that something is better than nothing. However I doubt that the CMP for grounding will see it that way.
 
iwire said:
Let throw this out there to the 'A GFCI takes the place of an EGC' crowd.

Do you think the CMP for grounding will be OK with letting people get shocks?

The reason I ask is many cases the GFCI device will not prevent an electrical shock to the user. It only limits the duration of the shock. Until the GFCI opens the circuit the victim is getting a full voltage / current shock. This can lead to other injuries such as falls etc.
Click to expand...

Let's throw this out there to the 'A EGC prevents shocks' crowd.

Consider a 125V device fed by 75 feet of 12-2 W/G NM cable. Now consider a ground fault between the hot conductor and the grounding conductor. What is the instantaneous voltage on the metal part "protected" by the EGC? Answer: 125V/2 or 62.5V.

Electrical resistance of a human is variable. Let's take a conservative value (we are talking about life safety) of 1000+300+1000Ohms (http://van.physics.illinois.edu/qa/listing.php?id=27110). Now 62.5V / 2300Ohms yields a current of 27mA, well above the 1mA perception level for 60Hz and above the level for loss of muscle control.

The non-metallic raceway impedance for #12 CU wire is .054Ohms/kft. and the resistance is 2.0Ohms for a total of 2.054 total impedance. For circuit distance of 75ft. the total impedance is 0.3081Ohms. At 125V/0.3081Ohms we get 406A. 406A is in the magnetic part of the trip curve for a 20A breaker and should trip in about 1 cycle or 16.7mSec. If the fault is not a "bolted fault" but has a resistance of about 0.7Ohms, the current is about 200A and is in the thermal portion of the trip curve and should trip in about 4000mSec.

Now consider the response rate of a GFCI for a differential current of 27mA. The maximum trip time allowed is expressed in Sec = (20 / mAmps)**1.43 where **1.43 indicates raised to the 1.43 power. For 27mA this value is about 750mSec. In practice many GFCI operate faster, a few mSec.

Facts and arithmetic.
 
fmtjfw said:
Let's throw this out there to the 'A EGC prevents shocks' crowd.

Consider a 125V device fed by 75 feet of 12-2 W/G NM cable. Now consider a ground fault between the hot conductor and the grounding conductor. What is the instantaneous voltage on the metal part "protected" by the EGC? Answer: 125V/2 or 62.5V.

Electrical resistance of a human is variable. Let's take a conservative value (we are talking about life safety) of 1000+300+1000Ohms (http://van.physics.illinois.edu/qa/listing.php?id=27110). Now 62.5V / 2300Ohms yields a current of 27mA, well above the 1mA perception level for 60Hz and above the level for loss of muscle control.

The non-metallic raceway impedance for #12 CU wire is .054Ohms/kft. and the resistance is 2.0Ohms for a total of 2.054 total impedance. For circuit distance of 75ft. the total impedance is 0.3081Ohms. At 125V/0.3081Ohms we get 406A. 406A is in the magnetic part of the trip curve for a 20A breaker and should trip in about 1 cycle or 16.7mSec. If the fault is not a "bolted fault" but has a resistance of about 0.7Ohms, the current is about 200A and is in the thermal portion of the trip curve and should trip in about 4000mSec.

Now consider the response rate of a GFCI for a differential current of 27mA. The maximum trip time allowed is expressed in Sec = (20 / mAmps)**1.43 where **1.43 indicates raised to the 1.43 power. For 27mA this value is about 750mSec. In practice many GFCI operate faster, a few mSec.

Facts and arithmetic.
Click to expand...

More like smoke and mirrors. :D

The only thing a GFCI device can do is reduce the duration of the shock. It cannot reduce the intensity of the shock.

The circuit characteristics stay the same with or without the GFCI device.
 
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