Are Multiple GFCI's a problem?

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NewEnglandRedneck

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Rhode Island
I had a portable GFCI plugged in to a permanent outlet on a remodel job. There was an extension cord plugged in to that GFCI, with a second GFCI/three way plugged in to the end of the extension cord. My Site Safety Manager maintains that the second GFCI causes this application to be un-safe. My theory is that they will both protect what ever is "downstream" from it.

Anyone have any experience with this?

Thanks for any help!!!
 

bphgravity

Senior Member
Location
Florida
Multiple GFCI's in series are not a problem, at least from a safety standpoint. A fault may trilp one or both, or however many gfci's are up the line. Purely a design consideration...
 

roger

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I agree with Bryan, there is nothing wrong and nothing prohibiting using a GFCI device downstream of another GFCI or daisy chaining multiple GFCI devices.
Your theory is correct.

Roger
 

hurk27

Senior Member
They will function as design, the only problem of having one GFCI feeding through another is that in most cases they will all trip on a fault, so a problem on one cord will stop work on the other cords, if this is what he is talking about, I don't see that it should be a danger, work lights should not be plugged in to a GFCI that is used for power tools, now that could be dangerous if the lights go out while cutting a piece of wood or other things.
 

G._S._Ohm

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DC area
My Site Safety Manager maintains that the second GFCI causes this application to be un-safe.

Run a small incand. lamp to ground from the hot line to induce a ground fault downstream of the second or third or fourth GFCI and show him that it works. But you have no way of knowing which will trip.
 

infinity

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I'm not sure where these safety guys get their education but I've heard the same thing on a job site more than once. IMO many of them don't have a clue when it comes to electrical safety. I recently asked one if they enforced NFPA-70E, he asked "what's that"?
 

renosteinke

Senior Member
Location
NE Arkansas
"Daisy chaining" is an issue with power taps. It's an issue because the UL "White book," in discussing power taps, says so. Such mention does not infer that the principle applies to anything but power taps.

GFCI's, receptacles, breakers, etc., are not power taps. There is no reason to infer any sort of 'general principle' against having these items 'daisy chained.'

However .... I am given to understand that our Canadian friends may actually have a rule prohibiting feeding one GFCI from another. I may be wrong; the gent I spoke with may have simply been confusing good design with code rules.
 

pfalcon

Senior Member
Location
Indiana
Wish I had the instruction sheet cause I thought the installation instructions made series a no-no. E'en so it would be poor design to daisy them. Consider that the monthly test should trip them all.
 

G._S._Ohm

Senior Member
Location
DC area
Consider that the monthly test should trip them all.
I've never tried this but based on the trip curves I'd think only one upstream unit would trip and very very rarely two might if you have enough GFCIs chained together.
Imagine circuit breakers daisy chained with a short at the end - the fastest one will trip and then the rest will stay closed.
 

pfalcon

Senior Member
Location
Indiana
I've never tried this but based on the trip curves I'd think only one upstream unit would trip and very very rarely two might if you have enough GFCIs chained together.
Imagine circuit breakers daisy chained with a short at the end - the fastest one will trip and then the rest will stay closed.

Breakers trip by heat generation. First one to heat up trips. GFCI is supposed to trip by current differential. The test circuit should deprive all upstream devices of a balanced current.
 

G._S._Ohm

Senior Member
Location
DC area
Breakers trip by heat generation. First one to heat up trips. GFCI is supposed to trip by current differential. The test circuit should deprive all upstream devices of a balanced current.
Yes, they're all presented with the same overload or the same current imbalance, so which one fires first?

Also, due to the trip curves and tolerances of those curves for both breakers and GFCIs you might get different results depending on how severe the overload or imbalance is.
 

hurk27

Senior Member
Yes, they're all presented with the same overload or the same current imbalance, so which one fires first?

Also, due to the trip curves and tolerances of those curves for both breakers and GFCIs you might get different results depending on how severe the overload or imbalance is.


There is no over current protection in a receptacle GFCI, if the hot and neutral is more then ,5ma out of balance they will all trip in most cases, this is no different then a bolted fault where panel coordination wasn't done or required, many times a bolted fault on a 20 amp circuit can also trip the 100 amp main upstream from the 20 amp.

If all GFCI's in series see a 5ma imbalance then they all should trip.
 

G._S._Ohm

Senior Member
Location
DC area
where panel coordination wasn't done
many times a bolted fault on a 20 amp circuit can also trip the 100 amp main upstream from the 20 amp.
>I'll have to think about this one. The trip curves in the 1/2 cycle or one cycle region should account for this occurrence.
You have any links on coordination basics so that I engage my brain before my mouth?;)

If all GFCI's in series see a 5ma imbalance then they all should trip.
>And ideally all the links in an overstressed chain should break at the same time, and chain links should be a lot more uniform than more complex things like GFCIs.

It's a probability thing. Even with a half dozen GFCIs in series I'd be surprised if more than one tripped. With 50 in series you might get two to trip at the same time.
Anybody up for running some tests, let's say at 10 mA [12K to ground] and 100 mA [1.2 K to ground] imbalance?
 
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wtucker

Senior Member
Location
Connecticut
IMO many of them don't have a clue when it comes to electrical safety. I recently asked one if they enforced NFPA-70E, he asked "what's that"?

You're right--many safety guys (including OSHA compliance officers) DON'T have a clue about electrical safety (I'm a safety guy). On the other hand, many electricians don't, either. But more and more of us, safety guys and electricians alike, are becoming more versed in 70E, partly because we're more and more aware of the horror of electrical injuries and the simple procedures and equipment that can eliminate them. How many of us knew about FR clothing or Arc Thermal Protective Value 11 years ago, before the 2000 edition of 70E came out? Hats off to NFPA for kicking this snowball down the hill.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
If all GFCI's in series see a 5ma imbalance then they all should trip.
That would presume identical sensitivities and how fast the current rises, methinks. I would expect manufacturing tolerances to allow for slightly different operation levels.

It's similar to how rare it is for more than one fuse to blow on a line-to-line overload or even a short; the first one to open drops the current through the remainder too fast.
 

LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
"many times a bolted fault on a 20 amp circuit can also trip the 100 amp main upstream from the 20 amp."
Breakers do have a mechanical inertia that could contribute to multiple units tripping, plus response curves could intersect, but I'm less than well versed in this.
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And ideally all the links in an overstressed chain should break at the same time ...
Only in the movies. :)

Anybody up for running some tests, let's say at 10 mA [12K to ground] and 100 mA [1.2 K to ground] imbalance?
You supplying the GFCI's or the bodies? :grin:


Added: I will add that they can be a pain to troubleshoot. I once found three, one in the living room, one in the hall, and one in a bedroom, all on one circuit. All had tripped, but that's because they kept plugging extension cords into the next working receptacle they could find.

The teenage daughter had her whole bedroom plugged into the ever-expanding cord jungle, and something (or maybe everything) contributed to the leakage current. They never did call me back, so they obviously decided pushing buttons was better than paying me $150 to do it.
 
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infinity

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New Jersey
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Wish I had the instruction sheet cause I thought the installation instructions made series a no-no. E'en so it would be poor design to daisy them. Consider that the monthly test should trip them all.

Are you saying that pressing the test button on one GFCI receptacle will trip other GFCI rec's on the same circuit?
 

roger

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Fl
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Retired Electrician
What about the GFI main?
I guess we all put gfi on gfi when you have a gfi main.
This differs in that you are looking at different levels of GFP, the equipment is protected by GFP verses Class A personnel protection, GFCI, see article 100 definitions.



Roger
 
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