It provides an unexpectedly energized circuit which is hazardous and a sneak return path. As mentioned previously by another poster, that is why an AFCI or GFCI will trip in this situation, there is a current imbalance.
Dude, it wasn't "another poster" who brought up the current imbalance. I WAS THE ONE WHO SAID THAT! And that has NOTHING to do with a dangerous situation (or lack thereof) of using handle tied breakers!
Let's back up a little. Let's forget about shared neutrals for a moment. Let's picture a simple 120V circuit: 1 hot leg, 1 neutral, 1 ground, and 1 single-pole breaker. If you have a fault where the hot leg comes into direct contact with the neutral, there will be a ton of current going through the hot leg, and a ton of current going through the neutral. That's bad. But the circuit breaker will trip almost instantaneously and shut the power off. That's what a circuit breaker does; it's the breaker's whole reason for existing! And as soon as the breaker trips, *****!* there's no more dangerous situation! The power is off, no current is flowing, nobody cares that the hot leg is still contacting the neutral!
It is no different in a multi-wire branch circuit: One of the hot legs contacts the neutral, or a ground wire, the breaker for that particular hot leg trips. Problem solved. The breaker for the other hot leg does NOT need to get involved at all, because the other hot leg didn't come into contact with anything and it's still working fine. The neutral can keep working and continue to carry current, even if the first hot leg is still contacting it. The hot leg is de-energized, and is not connected to anything. There is nowhere for current to go down that path, so there is no "sneak return path" as you say.
Now, as for GFCI and current imbalances: It seems you don't fully understand how a GFCI works, or how a multi-wire branch circuit works, or both. But let me explain how each works, and you will see why GFCI breakers need to be 2-pole for a MWBC, and how it has nothing to do with using handle-tied single-pole breakers for non-GFCI/AFCI applications.
Even though MWBCs supply 120V, they are SIMILAR to 240V circuits. On a 240V circuit, you only need 2 hot legs to supply the 240V. You do not need a neutral wire at all. So, how does current return back to the panel? The current goes up one hot leg, and back down through the other hot leg to return to the panel. The current is constantly going back and forth, up and down, between the two hot legs.
Let’s picture an electric stove. It runs on 240V. However, it also needs a neutral because some parts also run on 120V, such as the light bulb inside the oven. Let’s say you turn the oven on, and 20A starts flowing through the heating elements. Since the heating elements are 240V, you have 20A flowing up one hot leg, and 20A flowing down the other hot leg. There is NO current flowing through the neutral; like I said, we don’t even need a neutral for 240V.
So we have:
Hot leg: 20 A
Other hot leg: 20 A
Neutral: 0 A
Then you open the oven door and the light inside turns on. Let’s say the light uses 1 A of current. So that 1A flows up one of the hot legs (doesn’t matter which one) and flows back down through the neutral. So now we have:
Hot leg: 21 A
Other hot leg: 20 A
Neutral: 1 A
The neutral is only there to carry the DIFFERENCE in current between the 2 hot legs. In the first scenario, we had no difference in current, therefore the neutral current was zero. In the 2nd scenario, we have a difference of 1A between the hot legs, so the neutral carries 1A.
Now, getting back to MWBCs: A MWBC has 2 hot legs and one neutral (see any similarities to the oven??). If both legs of the MWBC are carrying the exact same amount of current, there will be no current on the neutral. If you have 13A of current on one hot leg, and 7A of current on the other leg, the neutral will have 6A of current (13A – 7A = 6A – neutral carries the difference). If you shut off the 7A on the 2nd hot leg, (
or if...*ahem*…the breaker trips) now the neutral current jumps up to 13A (13A – 0A = 13A). Again, not a problem.
Hopefully now you see how having different amounts of current on hot legs vs the neutral doesn’t matter.
Now let’s talk about GFCIs. A GFCI is there to prevent people from getting shocked/electrocuted. It does this by comparing the current leaving vs the current coming back. In a 120V circuit, the current on the hot leg should be exactly the same as the current on the neutral. So, if you have 10A on the hot leg and 8A on the neutral…wait, where did the other 2A go? They didn’t just magically disappear. The 2A is flowing down some alternate path it’s not supposed to go, like through someone’s body, electrocuting them for example. So if it sees a difference in current like that, it will trip. In the 240V oven example, we have 20A flowing down one hot leg, and the SAME 20A flowing down the other hot leg. Again, current flowing out is the same as current flowing in. When the 1A light bulb turns on, we have 21A flowing OUT one hot leg, and 20A flowing IN the other hot leg, PLUS 1A flowing IN via the neutral. So, we still have a total of 21A flowing OUT, and a total of 21A flowing IN.
In order for a GFCI to work properly, it needs to make sure the current going out is the same as what’s coming back in. Now look at the section I wrote above about how MWBCs work again, and ask yourself how the heck can a pair of SINGLE POLE GFCI breakers work if each breaker can only see ONE hot leg and the neutral? If one breaker sees 13A on the hot leg and only 6A on the neutral, as in the above example, how will it work? Answer: It can’t.
That doesn’t mean that it’s unsafe for one hot leg to be carrying more current than the other, and the neutral carries the imbalance.
If you use a double pole GFCI breaker though, you connect both hot legs and the neutral to the breaker. Now the breaker can see that it has 13A going OUT one hot leg, and 7A plus 6A coming IN via the other hot leg and the neutral, respectively. So now the breaker sees 13A going out, and 13A coming in, so it works properly.
THAT is the ONLY reason you need to use a double pole breaker if you’re using a GFCI breaker for MWBCs.
As for AFCIs, I’m not very familiar with the inner workings of them, so I won’t talk much about that, except that they are hooked up the exact same way as a GFCI breaker, where the neutral wire needs to be connected to the breaker. So again, the breaker needs to see how much current goes out vs what comes in.
Regular breakers (breakers that are NOT GFCI or AFCI) do not have a neutral wire connected to them. They do not know nor care what the neutral current is, and they don't know or care if the neutral current is more or less than the current on the hot leg. Therefore, you CAN use two single-pole breakers that are handle-tied for MWBCs!
