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Ground Fault Protection and the Multi-wire Branch Circuit - A Troubled Marriage |
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We’ve been contacted by Randal Andress who, as a recently retired engineer became interested in ESD about a year and a half ago. Last August he participated in the Fire Protection Research Foundation research planning meeting on "Hazardous Voltage/Current in Marinas, Boatyards and Floating Buildings" as a call-in.
Both before and since then Randal had been thinking about the effects of marina basin background currents (sometimes called foreign or stray currents) on the measurement of AC leakage from boats by clamping their shore cord with an ammeter. He found that the effect of background current depends on whether it originates from the same or opposite leg (L1/L2) of the distribution source as the current leaking from the boat. That led to the question of how faults/leaks from different legs of a main or feeder panel would be seen by a 2-pole (L1/L2/N) ground fault protection breaker.
What he found was that the protection provided is attended with subtle if not troublesome and problematic differences from the protection provided by a single pole, GFP breaker on a 2-wire circuit (H/N). The manifestation of these differences in a marine environment is of particular interest/concern.
Randal had not seen this addressed anywhere and so he began writing an article on the subject. Following are excerpts from the article. See the entire article (4th Draft) by clicking here. He is particularly concerned to get the analysis correct and easy to understand. He has made the text very wordy so as to stand fairly well without complex illustrations. And for the moment he has included illustrations that are very busy so as to be able to support an ad-hock briefing without having the text. Randal would appreciate your comments so please post them below.
Bottom Line Effects
In a nutshell, then, what we find is that when feeding a MWBC, the 2-pole ground fault protection breaker trips when the imbalance caused by the difference in the L1 vs. L2 faults/leaks exceeds the trip limit. In other words, it trips when:
| (L1 leaks & faults) – (L2 leaks & faults) | > Trip Limit.
...
To put it another way, the common, multi-wire branch circuit cannot be protected against ground faults to the same degree and with the same precision, the same measures of protection, expectation of behavior, etc. as a branch with its own neutral. This is not all that surprising when you consider that by sharing a neutral, the return current of each branch is made anonymous as to its origin (L1 vs. L2).
Here are some of the effects of using a 2-pole ground fault protection on a MWBC:
1. Fault current on one leg in excess of the rated ground fault protection level is required to trip the breaker in the presence of the inevitable leakage/fault current difference (I.e., when L1_leakage - L2_leakage is not = 0) – thus decreasing fault detection sensitivity.
2. The manual test button will not consistently perform a valid test. - i.e., a good breaker will test bad - when L1 fault/leakage current is greater than that of L2 (plus test current margin) or when L2 fault/leakage current is greater than that of L1 (plus margin), depending on whether or not the current for the test button is drawn from L1 or L2.
3. L1 and L2 circuits may be able to sustain leakage/fault currents well in excess of the rated ground fault protection level. I.e., leakages/faults may be arbitrarily high without tripping the breaker so long as their fault/leakage difference (L1 vs. L2) is less than the rated ground fault protection level. E.g.,a faulty/leaky boat which trips a 2-pole GFCI/GFPE when connected to a 120V/30A, L1 circuit could work just fine if moved to an adjacent L2 powered receptacle.
4. The removal of an appliance or device from a circuit could cause a trip. This happens when the fault/leak in the removed device is of such value that, when removed, it increases the L1 vs. L2 difference up to the trip level. The order of appliance connection and disconnection determines the possible, trip/non-trip, circuit states.
5. The likelihood of so-called nuisance trips is increased. A very small added fault or leakage can cause the breaker to trip since lop-sided L1 vs. L2 fault/leak current increases the trip sensitivity on one leg while decreased sensitivity on the other.
What Circuits Are Affected
These effects apply to virtually every 120V multi-wire branch that consists of two ungrounded conductors from opposite legs (L1 and L2) of the 240V source. However, the implications in some cases are decidedly more significant than in others.
On the less troublesome end of the spectrum might be the dishwasher and garbage disposal split outlet connection that is fed by a 2-pole Type A GFCI breaker. In the first place the Class A GFCI trip point is low (~5 mA) … Secondly, the test button dead zone for a test current of 7 mA would be only about 3 mA wide ... Thirdly, there is only one appliance connected to either leg (L1/L2) … Lastly, but perhaps most importantly, both are subject to mandatory standards and codes.
At the other end might be a 2-pole (L1/L2/N), 100 mA GFPE multi-wire circuit at a marina that feeds 120V/240V as well as and both L1 and L2 120V shore power receptacles. … The circuit connections – the boats – vary widely both as to the mix of appliances on board and also with time (here today, gone tonight, in a different slip tomorrow) and are subject to no mandatory codes. In addition the likelihood of lethal exposure is greater since the circuit faults/leaks could far exceed a safe limit and could be flowing into the water.
…
[--------------- end excerpts ------------------]
Randal Andress is an Electrical Engineering Graduate, retired after 30 years with TRW/Northrop Grumman and is an NFPA member and a published author. Please post your comments below.
randalPandress@gmail.com
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| Copyright© Mike Holt Enterprises of Leesburg, Inc. All Rights Reserved This article is protected by United States copyright laws and may not be published without prior written permission. |
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| Comments
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I just discovered that footnote 13 in the article incorrectly states that the AFCI that I tested was powering 40 “Watt” bulbs – not so. They were small nightlights drawing about 40 mA, not 40 Watts.
Randal Andress October 16 2017, 5:21 pm EDT
Reply to this comment |
In this situation would it help to add a second neutral and make a parallel splice at the circuit breaker. Kevin OConnor August 28 2016, 4:53 pm EDT
Reply to this comment |
Reply from: Randal Andress August 28 2016, 6:19 pm EDT
Kevin, The problem is that the L1 and L2 neutral currents would be combined at the splice and would pass through the 2-pole GFP breaker's current transformer as the same neutral current that it would have traveled on a common neutral all the way from the connected appliance(s). However, with the added neutral, if no 240V appliances were being fed by the MWBC, the ground fault protection could, of course, be "upgraded" using two individual single-pole GFP breakers to replace the 2-pole device.
Reply to Randal Andress Reply from: glene77is August 28 2016, 8:40 pm EDT
Randal,
I should revise my suggestion of a single DP standard circuit breaker, combined with two single pole GFCI devices , to be for a very specific application.
******* (1) My comment should have been : I suggest two GFCI recepticals as the recepticals for the MWBC at the point of providing power to the two devices ( DishWasher and Sink-Disposal ) . These were the two devices that I was envisioning as the target of the MWBC. I do not think that my suggestion would be satisfactory for other, more generally described MWBC systems. ******* (2) Your comments about the shared current on the single returning neutral conductor are valid .. ie, if the returning currents mix on the single return neutral then the a Double-Pole GFCI circuit breaker would mis-read them.
Reply to glene77is |
Mike Holt and Randal Andress: Thanks for the great info on GFCIs and multi-wire branch circuits. I update online programs for NAED based upon the NEC and your info was helpful in understanding NEC2017 422.5. Laura Simms August 23 2016, 8:44 am EDT
Reply to this comment |
A local Real estate agent had a Home Inspected, and found some Plugs in the home were not grounded, the home inspector said just put a GFCI Plug in and that would take care of the problem. My theory would be to Bond the Neutral to the ground on the plug. What do you think? Joseph Moore August 21 2016, 9:54 pm EDT
Reply to this comment |
Reply from: Glen Ellis August 21 2016, 10:55 pm EDT
Joseph Moore ,
(1) Your suggestion would place neutral return currents on the equipment-ground-circuit. Since the EGC is bonded to the surface of all equipment, this places neutral return currents on the surface of all equipment.
(2) This neutral to EGC bonding should un-balance the sensing toroid loop and trip a GFCI , as all my tests to date have shown.
(3) NEC violation.
Be Safe !
Reply to Glen Ellis Reply from: Mike Holt August 22 2016, 8:02 am EDT
Your 'theory' will kill someone see www.youtube.com/embed/M3lX4XtlYXk. The home inspector is correct, see 406.4(D).
Reply to Mike Holt Reply from: Bob August 22 2016, 9:05 am EDT
Joseph,understand that the neutral is part of the electrical circuit to the house. It carries the algebraical sum of the current of the two hot conductors unless something or someone breaks or disconnects its from the path to the transformer. In that case it will try to carry the current to the ground. Because of the high resistance of the earth ground there will be a considerable voltage difference between the neutral and anything grounded. To put it quite simply touching the washing machine, refrigerator, etc could be deadly at worst or shocking to say the least. Conditions and people breaking or cutting neutrals are common. It can be done at the panel, at the pole or in the street. The very worst thing about a GFCI using the current difference between hot and neutral is a possible nuisance trip. It will protect you.
Reply to Bob |
What I do not understand I why we drag technology of the 19th century into the 21st century and then try to use modern technology caused by outdated technology we should not be using in the first place.
The first example is the neutral. The transformer and generator have a neutral. There is no need to wire that neutral anywhere but to an earth ground. We do not need a stinking neutral wired to our house, marina or business. Europe uses 240 volt or sometimes 220 volt domestic wiring. This may rattle some people because this voltage is a little dangerous for a home handy man to mess with. Fine, modern homes are already switching to low voltage lighting using an isolated transformer. Just continue that trend.
In the past, and I mean past, like almost 50 years ago I worked with high cycle hand tools ( Electrostoom in Rotteram) and even on a 200 HP high cycle motor (American Standard R-12 refrigerant compressor). Why are we not using 300 Hz systems? They are highly efficient and reduce material costs. Do not give me the inductive reactance excuse. When I was a kid we used to say a one farad capacitor would be the size of the Empire States Building and cost hundreds of millions. Today you can buy one for under ten bucks and carry it away in your pocket. We can easily cut impedance to resistive only. There is Romex like cable manufactured though not widely distributed that has a conductor to carry 24 control voltage. Let us put aside the class2 NEC issue. Another can of worms. Using control voltage and solid state power diodes and transistors we can make all outlets controllable. No power to them without the 24 volt signal.
Bob August 20 2016, 11:34 am EDT
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Reply from: Bob August 20 2016, 2:41 pm EDT
I should have mentioned and not have left it to understood that in this case the equipment ground is run continuously to the transformer or generator neutral. Instead of the fault current riding the neutral it would be carried by the 2 point earth grounded ground conductor only.
Reply to Bob |
Great study, can you provide a basic electrical math example
on how the GFCI won't trip on a multi-wire branch circuit
application.
I x E = GF (ground faults) +/- 5ma = L1
I x E = GF (ground faults) +/- 5ma = L2
I x E = GF (ground faults) +/- 5ma = N
Thanks elaborate if possible ...... ? Mario Valdes August 19 2016, 11:00 am EDT
Reply to this comment |
Reply from: Randal Andress August 19 2016, 11:21 am EDT
I don't think I could provide anything beyond the same equation presented in the article with the sense reversed: The 2-pole GFCI breaker will NOT trip if:
| (L1 leaks & faults) ? (L2 leaks & faults) | < Trip Limit
Reply to Randal Andress Reply from: Mario Valdes August 19 2016, 11:41 am EDT
Ok I didnt see the attached report. It all seems to be there thanks.
Reply to Mario Valdes |
My first thought in reading this report is Hot tubs. These installations have used 2 pole GFCI protection with a shared neutral installations for many years. Are we now saying the protection offered is minimal?? Dave Campbell August 19 2016, 10:41 am EDT
Reply to this comment |
Reply from: Randal Andress August 19 2016, 11:06 am EDT
Thanks for pointing out this use-case. I would not use the term "minimal", meaning "not much". However, it is certainly "less" protection, meaning not as much or not as good, for any 120V circuit(s) involved (pumps, controls) than if it were fed by an individual GFCI breaker with a separate neutral. A 2-pole GFCI feeding the heating element(s) alone is probably the best we can do.
Reply to Randal Andress |
Wow!
Great article; and comments by everyone. Really very enlighting. Thanks for sharing! Tim August 19 2016, 10:15 am EDT
Reply to this comment |
MWBC the breaker are suppose to trip when there is a fault in either line.
charles Jefferson August 19 2016, 9:56 am EDT
Reply to this comment |
Reply from: Randal Andress August 20 2016, 9:20 am EDT
That is correct. The 2-pole breaker will trip when there is a fault on either line so long as there are no other leaks or faults present. The article addresses this single fault case under "An Additional Conductor" at the bottom of page 3. (Figure 2.) An L1 leak/fault is shown but the same would hold for an L2 only leak/fault. It becomes an issue when there are leaks or faults on both L1 and L2 which offset each other. See "Ground: Where Opposite Faults Meet" on page 4.
Reply to Randal Andress Reply from: David RIfkin August 20 2016, 7:43 pm EDT
We have seen this effect in the field doing ESD research. The latest instance was in measuring a voltage gradient in the water near the propeller of a boat. Both L1 and L2 loads were running, each on it's own 30 amp shore cord. Was measuring a gradient of 125mv/ft in the water. When L1 supply shore breaker was turned off, the gradient jumped to over 200mv/ft.
While these levels are not lethal (2v/ft considered lethal for nominal person), they indicate the cancelling affects we see when there is leakage into the water from both L1 and L2 on the same branch circuit.
We have also seen this when clamping shore cords. Let's say you are clamping a single 30amp cord with loads running and measuring 100ma on the cord. Then you turn off the shore supply breaker at the source (pedestal). Normally you would expect the clamp reading to go down. Have seen it a number of times where it goes up! This has to be the effect of cancellation of L1 and L2 fault leakage into the water.
Randall has done some excellent work in this area. Perhaps there is way to prevent this with a simple modiification, similar to the addition of a second CT to a GFCI to prevent leak to ground from traveling in the ground then back up into the neutral such that it is not seen by the GFCI.
Reply to David RIfkin |
Thanks for the article and link to the full article Mike. Randal, Appreciate your efforts and work in the outstanding presentation. I'm studying the figures and rereading the article for the third time. Larry Luce August 19 2016, 9:45 am EDT
Reply to this comment |
What does this have to do with a marina? Wouldn't a MWBC do the same on land? R.Mitchell August 19 2016, 8:52 am EDT
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Reply from: Randal Andress August 20 2016, 10:22 am EDT
This is a very important question and my response will be too brief to be complete as was the treatment in the article (last paragraph on page 10). But yes, on land or sea the analysis is the same. What is different is the relationship of personnel to the environment (land or water) through which the leaking current flows, the increased risk and difficulty of personnel protection in water to swimmers, and the higher trip limits in the NEC 70 code for shore cords that provide electricity to boats. The failure of the 2-pole GFCI device to protect at the stated trip level (which is allowed to be higher at marinas) adds increased danger of Electrical Shock Drowning (ESD) to swimmers who can be killed by only a few miliampers which would not be lethal if encountered on land. I should have elaborated more on this in the article. Perhaps adding an appendix explaining this more fully would be appropriate. See excelent information at the ESD and Boat US websites (electricshockdrowning.org boatus.com/seaworthy/ESD.asp ).
Reply to Randal Andress Reply from: R. Mitchell August 20 2016, 10:31 am EDT
Thank you.
Reply to R. Mitchell |
Mike,
Thank you for presenting
this very important electrical engineering "discovery"
about Multi-Wire Branch circuits and the GFCI circuit breakers in current use. ***
One possible solution may be to have a double-pole standard double-pole circuit breaker feeding an external set of Two GFCI circuit breakers, which feed the Multi-Wire Branch Circuit. The set of Two GFCI circuit breakers might be a pair of GFCI Dead-Front devices, located adjacent to the primary load center. ***
In a Marina situation, this is a critical problem,
unless all Marina MWBC are outlawed and replaced. ***
I will be watching for comments on this topic.
Glen Ellis August 19 2016, 7:34 am EDT
Reply to this comment |
Reply from: Randal Andress August 20 2016, 1:06 pm EDT
Glen, though I am not sure exactly what you are suggesting, I will make some assumptions and give a reply. First of all if the main entrance service panel contains a 2-pole GFCI (an L1 pole and an L2 pole) then, regardless of what is connected to that 3-wire (L1/L2/N) circuit, its GFCI behavior will be governed by what is addressed in the paper (assuming, of course, that I am correct in my conclusions :-). If you feed that 2-pole GFCI circuit into a sub-panel containing two individual single pole GFCI breakers and then use them (two, 2-wire circuits: L1/N and L2/N) to feed a 3-wire (L1/L2/N) MWBC you will have to connect the neutrals on the line side of the two breakers so that they both feed the MWBC neutral. For normal load currents this would split approximately in half the normal return current from both L1 and L2 loads thereby robing the respective GFCI of half its neutral current that would be needed to offset its hot current to prevent a trip for normal currents. Forgive me if I have misunderstood your suggestion, however, others may have thought the same thing and I wanted to address it. Thanks for the interaction.
Reply to Randal Andress Reply from: Glen Ellis August 20 2016, 3:08 pm EDT
Randal Andress
Let me state my idea in terms more like yours.
(1) At the Primary panel,
use a Double-Pole Standard circuit breaker.
(2) Feed a Sub-Panel, which contains two GFCI .
(3) Have the Sub-Panel feed the MWBC.
I think You made a very good point.
I think you are correct,
and I will draw this out on paper to check our thinking.
Although mostly my work now is three-phase, with neutral,
and green ground ... this GFCI controversy is a BIG safety issue ... should be explored from every angle.
Above all, Be safe!
Reply to Glen Ellis |
Reading the article I can't say the results are surprising; the simple math bears out what must happen in a MWBC given the scenarios presented.
What I find troubling is that MWBC's continue to be installed with frequency. Neither the voltage drop or the marginal increase in wire cost justify the litany of potential and even likely long-term problems with a MWBC. This GFCI issue is just one more reason to abandon the MWBC practice - I find the MWBC as the tube and k-nob of our generation... Zapphed August 18 2016, 10:49 pm EDT
Reply to this comment |
Reply from: Phil Grabrick August 19 2016, 10:59 am EDT
These were my thoughts exactly as I read through the article. Outlaw multiwire branch circuits
Reply to Phil Grabrick |
Why would one protect a MWBC with a 2-pole GFCI? It seems the proper method would be to protect each 'branch', where required, with GFCI receptacles. KC Wireman August 18 2016, 9:57 pm EDT
Reply to this comment |
Reply from: Bob August 19 2016, 11:14 am EDT
Good point KC. Use individual GFCIs.
Reply to Bob |
I have always viewed GFCI as a last ditch effort towards protection when proper grounding cannot be assured such as allowing users to plug equipment of unknown quality into a receptacle. Many people seem to feel that GFCI is the "best" protection available. My feeling has always been that GFCI is better than nothing but certainly not a substitute for proper grounding. This research seems to agree with my feeling. Donald Haskin August 18 2016, 9:13 pm EDT
Reply to this comment |
I realize that there are most applications where the GFCI functions reliably but in many applications it should not be used. Understand that the GFCI reads current indirectly, by induction as does a clamp on amp meter.
It is calibrated for a certain frequency, that is to say you should make sure,if you work internationally, that you do not install a 50 Hz GFCI in a 60 HZ application. Also, I do not believe it is possible to convert current detected by induction in a CT or the related differential transformer to true RMS current. For example, the GFCI can not detect true RMS current if there are harmonics, as there often are in the neutral.
Large breakers 600 amp plus use a ground fault relay which can detect true RMS current and detect that current directly.
I would test for ground faults with a good RMS voltmeter. The ground test meters are great but you are going to lay out 600 bucks for a Fluke and it is over kill. To do this you are going to responsibly break the ground and take a direct voltage drop or shunt current reading. But if you do this often and have the bucks the ground test specialized meter is safer and has more options.
As for problems with shared neutrals we can write a book.
Also, be it understood there ground loops and ground neutral loops. They are not the same. But that is another book. Bob August 18 2016, 9:08 pm EDT
Reply to this comment |
Reply from: Ben Jacks August 19 2016, 3:25 pm EDT
Well said. Hopefully the next NEC code cycle can expound more on coupling anomalies in bundling and neutral loop termination of NM systems. Even though the 'troubled marriage' article is intentionally directed to GFCI/GFPE marine safety conditions, the elimination of ground fault double pole protection of MWBC must be considered. New technology for AFCI and GFCI dual function systems will be the non-use result for cost savings in residential, mid-rise and floating dwelling electrical construction.
Reply to Ben Jacks Reply from: Ben Jacks August 19 2016, 3:25 pm EDT
Well said. Hopefully the next NEC code cycle can expound more on coupling anomalies in bundling and neutral loop termination of NM systems. Even though the 'troubled marriage' article is intentionally directed to GFCI/GFPE marine safety conditions, the elimination of ground fault double pole protection of MWBC must be considered. New technology for AFCI and GFCI dual function systems will be the non-use result for cost savings in residential, mid-rise and floating dwelling electrical construction.
Reply to Ben Jacks |
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