Nature of ground fault currents

Thread Starter

Blue_Electronx

Joined Jun 10, 2019
72
Hello,

I'm just wondering about ground fault currents. Let's say we have a ground fault circuit breaker that states it trips at 30 mA. Is this RMS, peak, or some high frequency pulse?
 

dl324

Joined Mar 30, 2015
10,003
Line voltage is measured in volts RMS, so the resulting current would also be RMS.

Why do you care? A GFCI will respond quickly enough to prevent electrocution. You'll probably feel a shock, but it won't be long enough to kill you.
 

panic mode

Joined Oct 10, 2011
1,821
in an circuit, current entering load and current leaving load are the same. but if they are not, there must be another unintentional connection that causes the difference. normally that is the ground connection. ground connection is meant to act as a reference potential, not current carrying conductor.

GFCI monitors and compares current on L and N and checks if difference exceeds some threshold. threshold is set low enough so that if human is accidental part of the circuit (and path of the ground leakage current) breaker would kill the power to prevent electrocution. 30mA can be painful but if exposure is brief, it is just a sting. read the datasheet of specific product for details. AC devices normally have ratings expressed as RMS
 
The 30mA limit is from about 1970 in Germany and France, older European GFCI that were mechanical voltage-sense technology. Above 40-50mA or so is heart failure, it's really an integral depending on time.
The limit for muscles and a person holding something live is 10mA, the "let go" limit.
5mA is the tingle limit but you can still let go.
GFCI in North America UL943 came a few years later and adopted below that to 4-6mA because electronic GFCI can be more sensitive than mechanical relays.
A lot of the numbers depend on if you are wet, and if the current path is hand-hand or hand-foot for example.
Limits of 6mA for personal shock protection and 30mA for a branch circuit I think are the standard in NA.

I added a GFCI outlet (5mA) to power my entire workbench power for extra safety, I recommend it to everyone.

GFCI_currents_heart_failure.PNG
 

Reloadron

Joined Jan 15, 2015
5,118
Here in the US a GFCI will typically react to a current mis-match or imbalance of 4~6 mA and do so in about or between 1/10 to 1/15 second. If you ever take a GFCI outlet apart the disconnect is mechanical but they are pretty quick. Additionally unlike a GFCI circuit breaker a GFCI outlet is not a circuit breaker and if an overload exist less any ground fault it will not disconnect.

Ron
 

Reloadron

Joined Jan 15, 2015
5,118
I added a GFCI outlet (5mA) to power my entire workbench power for extra safety, I recommend it to everyone.
Each of my two sump pumps is on a dedicated 20 Amp circuit and each on its own GFCI. All outside outlets and my whole garage are GFCI. I have gotten fond of them. :)

Ron
 

MisterBill2

Joined Jan 23, 2018
5,188
I inadvertently tested one once and can attest to it being more than a "sting".
Felt about the same as a 120V AC shock without the GFCI, although it did trip.
I was using a power tool in wet weather and it kept tripping the GFCI and I never once felt a shock, and yet I know that the current went through my hands and arms. evidently some folks are far more sensitive than I am.
 

MaxHeadRoom

Joined Jul 18, 2013
20,000
I was using a power tool in wet weather and it kept tripping the GFCI and I never once felt a shock, and yet I know that the current went through my hands and arms. evidently some folks are far more sensitive than I am.
Many power tools intended for outdoor use are double insulated, But all exterior outlets should be GFCI, what can happen is that there was leakage from the wet/damp power tool to GND and resulted in the CFCI trip, rather than through you!
Max.
 
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