Are Multiple GFCI's a problem?

[gar]

110323-2153 EDT

GFCIs do have a trip time curve.

See the National Semiconductor datasheet I have referenced elsewhere. This curve is probably defined by an RC time constant internal to the integrated circuit. The actual curve is below the maximum limit defined by UL, and will vary somewhat from one unit to another.

At 120 V I believe the test trip current is 8 MA, 15,000 ohms for the test resistor.

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[hurk27]

Well I know this one for fact, I have been called on to many series GFCI's to count, most common is an outside outlet fed off the garage GFCI protected outlet, HI tells the Realtor that there is an outside receptacle that is not a GFCI, so Realtor send their handy dandy man to the rescue and without checking he installs a GFCI receptacle, new owners go away on vacation for two weeks, and Joe the yard man comes over and uses this double protected receptacle to power his 800 mph leave blower to clean off the sidewalk, trips the GFCI, but doesn't know he has also tripped the one protecting the garage receptacles, home owners come home 12 days later only to find their freezer which had 2 million dollars worth of rare African buffalo meat in it has now all spoiled, all to common. (well maybe not the African buffalo meat part)

Also common on pre 80's houses, where we used to hit the bath receptacle with a GFCI breaker or receptacle, then hit the outside receptacle, again another GFCI receptacle gets put in the outside outlet because some HI flags it, and again someone trips it and can't figure out why it wont reset because the bath GFCI has tripped, good for business though:roll:

 

[sameguy]

Roger I have seen the main gfi trip on site due to a shorted cord in the field that had gfi protection. The main had not been set yet still at factory settings.

 

[pfalcon]

Pragmatically
If the fault is close to the trip threshold at a GFCI, such as when using the manual test button, then line capacitance can mask the condition enough that only one GFCI trips. This is almost always the one being tested. Sometimes due to manufacturing differences a second may trip but not often.
When the fault is far above the trip threshold then all of them can see it. The question then becomes whether power will drop soon enough to hold any given GFCI. The nearest GFCI normally trips but no where near as often as when using the test button. Line capacitance then works against you providing power to trip upstream GFCIs. Manufacturing differences become one of the most important factors as to which trip at this point.

 

[don_resqcapt19]

Pragmatically
If the fault is close to the trip threshold at a GFCI, such as when using the manual test button, then line capacitance can mask the condition enough that only one GFCI trips. This is almost always the one being tested. Sometimes due to manufacturing differences a second may trip but not often.
When the fault is far above the trip threshold then all of them can see it. The question then becomes whether power will drop soon enough to hold any given GFCI. The nearest GFCI normally trips but no where near as often as when using the test button. Line capacitance then works against you providing power to trip upstream GFCIs. Manufacturing differences become one of the most important factors as to which trip at this point.

All of the upstream GFCIs, only see line to neutral current when a downstream receptacle test button is pushed. The built-in test button should not cause any other GFCIs to trip.

A plug-in tester creates an actual ungrounded to EGC load and any upstream devices will see that ground fault. One or more of the devices will trip based on the time current curve of the devices.

 

[G._S._Ohm]

Only in the movies.

The action flicks violate the laws of physics and of probability all the time but when I was a kid I liked them.
Col. John Stapp would be the first to tell you that if there were a person in an Iron Man suit he'd be killed by the extreme G forces.