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GFI, RCB, and dirt
GFI, RCB, and dirt
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Old 1st January 2018, 10:33 PM   #1
PRR is offline PRR  United States
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Default GFI, RCB, and dirt

Quote:
Originally Posted by basreflex View Post
adding many surge protections to your local mains circuitry could result in preliminary tripping of your GFI. ....
Quote:
Originally Posted by scottjoplin View Post
...it's not really a ground fault that's being interrupted, nevermind, ignore me!
A popular path for shock is: power wire, defective appliance, person, dirt/concrete(*), and back through ground bond to power company.
(*) Also metallic pipes which often run into the dirt.

There used to be actual ground current detectors. They did not work well. There are too many possible paths back to the service dirt-bond.

The "GFI" actually detects the -difference- of the current in the two load-power wires. Mine do not even look at the green wire; it's just there to drain the appliance cabinet.

Same as my bottle water company. The slip says to drop 3 full bottles, with a space where the driver notes how many empty bottles he picked up. If 3 and 3 all is well. If 3 and 2, they have a "leak". (3 and 4 is also a "leak", and they may wonder why.)

If my hedge trimmer pulls 5.000 Amps on the black wire, and returns 4.990 Amps on the white wire, there is a 0.010A leak of current. It could go many places. Maybe I have a 12K resistor around the GFI. Maybe the cord is cracked and 0.010A is zinging one blade of grass. But maybe the 0.010A is going through ME to dirt and back. That would be bad. The GFI assumes worst-case and cuts-out.

Note that the leak path may NOT involve dirt; but there are very few real cases which don't involve a "ground wire" at least tangentially.

And these things break the load-carrying wires, not the Ground/PE wire.

Therefore the UK's "Residual Current Breaker" name is probably more accurate. Here in the US we cling to Yankee Doodle and "GFI". (Which is what National Semi called it.)

It appears that *all* these devices are based on the same original chip design, now made many places in several variants.
GFI
http://www.idea2ic.com/GFI/LM1851.pdf
http://www.3c-test.com/de/?m=Type&a=download&id=189

My bottle water company will let me slide for one bottle short; if I keep holding-out I get the nasty letter to give them bottles or bucks. Quicker if 3-shy than 1-shy. Likewise the GFI will allow 0.1A leakage only for a very short time (<0.5 sec), and 0.005mA for a longer time (~7 secs) will trip it. I believe this time-curve is why US GFIs are nominally 5mA and UK RCBs are nominally 30mA: they picked different points on the same curve.
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Old 2nd January 2018, 02:30 PM   #2
Speedskater is offline Speedskater  United States
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Much, much more on GFI's from a pro movie guy.
Local 481 GFCI Workshop Curriculum
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Old 2nd January 2018, 05:39 PM   #3
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by PRR View Post
.................I believe this time-curve is why US GFIs are nominally 5mA and UK RCcBs are nominally 30mA: they picked different points on the same curve.
The UK has readily available RCCBs in 10mA, 30mA and 100mA rating.
Domestic generally uses 30mA and commercial/industrial tend to use 100mA. The 10mA is usually used where heath preserving equipment is being installed.
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Old 2nd January 2018, 05:51 PM   #4
Rod Coleman is offline Rod Coleman  United Kingdom
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GFI, RCB, and dirt
Quote:
Originally Posted by PRR View Post
Therefore the UK's "Residual Current Breaker" name is probably more accurate.
Yes, we call them RCDs here (Residual Current Device) - but I have never felt that « Residual » was a good way of expressing the action of the trip. It seems to me that the French language gets much closer with « Interrupteur différentiel »

Interrupteur differentiel LEGRAND, 30 mA 40 A AC | Leroy Merlin
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Old 2nd January 2018, 06:24 PM   #5
scottjoplin is offline scottjoplin  Wales
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My initial question to basreflex was whether GFI was equivalent to Earth Leakage Relay as I hadn't heard the term GFI before. ELR or ELCB seem to me the most precise acronym
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Old 5th January 2018, 06:43 AM   #6
basreflex is offline basreflex  Spain
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not all GFI/RCB circuits are identical. I always thought that these were symmetrical devices and that is does not matter what is input or output. I was WRONG.. I wired accidentally some chinese (TOMZN brand) RCB/automatic fuse combo' (called RCBO) thinking that input was bottom output was top, neat for the wiring.

upon hitting the test button I heard the relay turn off, but then smoke and a dying smell came out. I had to find the reason for this. turns out the sensor circuit is a small toroid sense transformer coupled to a low power amplifier that triggers a thyristor. the thyristor is thought to trigger the release coil to turn itself off. if the RCB is wired in reverse, the thryristor and coil stay powered, and the thryristor dies a smelly death.
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Old 6th January 2018, 11:43 PM   #7
PRR is offline PRR  United States
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Quote:
Originally Posted by basreflex View Post
....GFI/RCB circuits .... I always thought that these were symmetrical devices and that is does not matter what is input or output. I was WRONG..
Indeed WRONG. Makes a big difference! (I did not know it was smelly.)

US market brand-name receptacle GFIs come with tape over the "Load" terminals, so you won't wire to them unless you need to (to feed downstream outlets with GFI protection).

US fusebox GFI/breakers in all the common US fuseboxes only go in ONE way. The box has a bus(es). The breakers snap onto the bus. That's how they get power. (With few odd exceptions) you can't run the power "backward".

I won't argue the bus against DIN-rail. There's much in favor of DIN. I am unsettled about the need to wire the "bus" in the field, because I have not worked in such boxes. I know there are pre-made buses for jumpering all the breakers in a group, but I gather they are also wired with bits of cable.

Being able to wire a GFI/RCB "backward" sounds like a drawback to me, but maybe you only do it once.
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