Ground Fault Notes

R. Kwas KA1-RBP Jun2016

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Note:  Ground Fault Protection is NOT Safety Ground Protection!  

I use the term GFI for Ground Fault Interrupter...not GFCI... that it is in a Circuit is obvious and doesn't need to be restated! 

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Ground Fault Protection can be achieved several ways as shown below...either with a GFI Breaker which protects the entire sub-circuit, by means of a GFI Outlet which protects at that location and any Slaved Outlets downstream.  220V Circuits for spas or hot-tubs can also have GFI protection.

Typical US domestic power distribution with variations of non-GFI and GFI shown:  

Ground Faults occur when the current returns to Neutral bus, by some path other than the neutral conductor of the circuit.  Since this will be by means of the Ground conductor, and it is by definition a Fault condition, it is is called a Ground Fault.    

Ground Fault Interrupters (GFI Circuit Breaker or Outlet types) sense this by using a small Current Transformer as shown.  Both Hot and Neutral wires are routed through the transformer core, such that when load current is drawn, it results in a net induced flux of zero (balanced!) under normal, non-fault conditions.  If however current finds an alternate path to neutral, which might involve a person,  there would be a flux induced in the sensing CT because of the imbalance in currents, which upon being sensed, would cause the breaker/GFI outlet to trip, preventing electrocution. 

Power Distribution with possible Ground Fault paths indicated.  Only those which are downstream of a Breaker or Outlet with sensing are protected against Ground Faults.

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GFI sensitivity to RF.  GFI sensing circuits are generally filtered to prevent nuisance trips when subjected to RF such as what an radio amateur station might be emitting.  Occasionally however, a particular GFI outlet, or installation, or the level of RF we produce and which gets into the control circuit (maybe the sensing transformer is acting like a ferrite AM antenna, and circuit has inadequate filtering for RF!) is enough to trip the GFI function.  Mitigation suggestions:  PLACEHOLDER

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Installing a GFI outlet into an old construction Bathroom.  

After consulting with Tom M. (licensed electrician, and highly experienced electronics engineer whose judgment I respect), who pointed out that installing one of these is still a good upgrade because it adds Ground Fault Protection, even though there is no Safety Ground Protection (the two are related, but very different!), I proceeded with installing a GFI outlet into an old construction bathroom.  He also noted that since there is no Safety Ground conductor present, Test button on GFI outlet would not work, but this can be explained to any inspector questioning it (and he apparently has needed to), and is "grandfathered in" because of the old/existing construction. 

After installation of the GFI outlet was completed, I tried the Test button, and in fact it did function to trip the outlet as it is supposed to...this was not what I expected and inconsistent with Tom's statement, but after a moment of consideration, I could explain it.  This explanation is illustrated in some cocktail napkin engineering here:

Test Switch 1 and its' current path requires continuous Safety Ground to the new outlet, because it returns the test GF current to the Safety Ground, which it obviously can't if none is present...so if Test Switch 1 is connected as shown, Test Function would not work as Tom points out.  ...but Test Function does work with no Safety Ground conductor...so only one explanation is possible:  Test Switch must be connected as shown for Test Switch 2, with Test Ground Fault current being returned to Neutral wire on other side of sensing transformer, such that a valid test GF current (and imbalance in sensing transformer) is still created when pushing Test Button, even without a Safety Ground present, and this trips outlet as expected.  This discussion affects ONLY GF Test function...in both cases actual GF Protection is present and functional...so Wile E. Coyote (Caninus Nervosous) would be safe in either case!

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Discussion between brother Mike and I about retrofitting a GFI outlet into the old construction Bathroom which has no Safety Ground (house is wired in "BX" armored cable, and Polarity Tester shows open Ground).

Mike;

Tnx for link...good info, OK (only) presentation.  See my comments in Blue 

Number 1, 3rd bullet point: 
 

• Run an individual ground conductor from the green grounding terminal of a grounded receptacle to the nearest water pipe or other grounded object. This “floating ground” presents various hazards.  [Calling it a "floating ground" is not necessarily correct because it presumes it is NOT continuous to the pipe going out the cellar and indeed to a low impedance Ground (municipal water pipes)...and this is indeed likely the case in copper pipe older houses, but can and should be verified by a visual inspection, and possibly electrical test in the cellar] It is likely that this ground rod of convenience will have several ohms of ground resistance so that, in case of ground fault within a connected tool or appliance, the breaker will not trip — and exposed metal will remain energized.  [I disagree with this also...continuous copper pipe has much lower resistance to ground than a wire conductor in house wiring, and will certainly conduct a ground fault current to trip a breaker!  (Because of Neutral to water pipe bonding).  Not to mention this is another argument against plastic water supply pipes!

• Another possibility is to replace the two-prong receptacle with a GFCI.  Hook up the two wires and leave the grounding terminal unattached. [Essentially what I have done.] Included with the GFCI is a sticker that says, “No equipment ground.” This sticker must be in place so that future electricians and users are not misled. The thinking behind this strategy is that even though the tool or appliance case is not grounded, the GFCI will provide enhanced safety.[... by tripping in case of a ground fault...CORRECT] It's important to note that a GFCI functions properly without the presence of a grounding conductor. The device compares current flowing through the hot and neutral conductors and trips if a difference of more than 5 milliamps is detected.

note Number 1, 3rd bullet point
 

http://ecmweb.com/design/10-worst-grounding-mistakes-youll-ever-make

I HAVE TO ASK:  SO WHAT'S IMPROPER ABOUT IT???  What the hell is wrong with this...NOTHING!...It's a solid, high-current bond to a ground rod!...I don't think the rod or clamp much care that its a pipe clamp!  ...there's just not any ACZP on it to keep the connection free of corrosion!

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