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.
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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.
