Just learned about GFCI

[jim dungar]

I have just perused UL943 that was issued back in 2006.

Ground fault circuit interrupters now actually check for ground faults.

GFCIs must trip if there is a neutral-ground on the load side of the GFCI. The UL standard for tripping on a N-G fault is the same as for a "normal" fault: T (trip time in seconds) = (20/mA)^1.43. There is no requirement that this N-G sensing be performed without some amount of current flowing and any current that may flow can be as high as 6mA.

But regardless of this new requirement the basic way a GFCI works remains the same: The current flowing out of the GFCI must not be more than 6mA different than the amount flowing back in.

edit: title change

 

[gndrod]

I have just perused UL943 that was issued back in 2006.

Ground fault circuit interrupters now actually check for ground faults.

GFCIs must trip if there is a neutral-ground on the load side of the GFCI. The UL standard for tripping on a N-G fault is the same as for a "normal" fault: T (trip time in seconds) = (20/mA)^1.43. There is no requirement that this N-G sensing be performed without some amount of current flowing and any current that may flow can be as high as 6mA.

But regardless of this new requirement the basic way a GFCI works remains the same: The current flowing out of the GFCI must not be more than 6mA different than the amount flowing back in.

edit: title change

Jim, Would you have any data on how this may affect a feeder combination AFCI sensing through the GFCI as may occur in the SA-dining room BC? I realize that depends on the appliance and fault characteristics. Any comment will help me. rbj

 

[jim dungar]

Jim, Would you have any data on how this may affect a feeder combination AFCI sensing through the GFCI as may occur in the SA-dining room BC? I realize that depends on the appliance and fault characteristics. Any comment will help me. rbj

I have no data on this at all. And i don't have any interest in investigating the internal electronics of any device. I simply noted that the current UL standard for GFCI devices includes a requirement to perform some type of N-G sensing and to trip if the N-G current exceeds 6mA.

 

[al hildenbrand]

any current that may flow can be as high as 6mA.

I'll bet the language doesn't limit the peak current to just 6 milliamps.

The generic block diagram of this, developed by our benefactor, shows what I interpret as an autotransformer with the grounded conductor connected to the centertap between the primary and the buck winding.

Primary winding impedance and the N-G contact impedance will limit the H-N-G fault current. The return path impedance in whatever ground paths happen to be present in a particular "real world" event may well be a significant influence in the circuit analysis.

The point of contact between the neutral and the ground may be dynamic (think of a bouncing contact or moving sawteeth, etc.). Given the purported 5.0 Amp AFCI trip level, I'll bet that an AFCI will (or at least should) "see" the GFCI N-G tripping event under certain conditions.

Edit -- Changed "boost" to "buck". This little transformer makes my head hurt. Anyone care to weigh in on its behavior?

 

[ELA]

We thought we had a pretty thorough discussion on this here:

http://forums.mikeholt.com/showthread.php?t=92976

 

[jim dungar]

I'll bet the language doesn't limit the peak current to just 6 milliamps.

Of course not, limiting current is not even address.
The UL standard says that if there is more than 6mA of current on a N-G fault then the GFCI needs to trip following the same time curve as if the fault involved an ungrounded conductor. The standard does not address currents below 6mA.

 

[jim dungar]

We thought we had a pretty thorough discussion on this here:

http://forums.mikeholt.com/showthread.php?t=92976

There is nothing in that thread that says what the UL standard may or may not require. That thread is primarily based on anecdotal experience, which while interesting may imply/infer that specific features in some GFCI devices are applicable to all other devices.

 

[ELA]

That thread is primarily based on anecdotal experience

Yes and as Garth said, " I am not worthy!"

I posted for al hildenbrand's benefit since his post had an edit that asked if anyone cared to weigh in with more information. Since it had been discussed elsewhere I was pointing that out.

 

[al hildenbrand]

Thanks ELA,

I missed that thread.

After reading it, and the links included, I returned to the Mike Holt ?2002 diagram I posted above and made the following interpretation of the circuit.

 

[ELA]

Hello Al,
Very interesting now that you mention your posting.
Please compare it to this one:

http://www.rhtubs.com/GFCI/GFCI.htm

I believe they conflict with one another with regard to which core is upstream of the other. As well as how they might be connected?

here is another link that agrees with rhtubs:

http://cache.national.com/ds/LM/LM1851.pdf (see page 6)

These are not autotransformers from what I can tell. The second link provides a pretty detailed explanation and a little better image of the cores.