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Ground Fault Protection and the Multi-wire Branch Circuit - A Troubled Marriage |
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Weve 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. |
Mike Holt Enterprises of Leesburg, Inc. 3604 Parkway Blvd. Suite 3 Leesburg, FL 34748 |
"... as for me and my house, we will serve the Lord." [Joshua 24:15]
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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 |
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 |
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 Reply to this comment |
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 |
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 |
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 |
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 Reply to this comment |
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 |
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 |
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 |
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 |
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