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I saw some circuit breakers that have a separate neutral disconnect.

For example:

enter image description here

What is that for?

When do you need to disconnect the phases only?

Why not use a circuit breaker that always cuts all (4 in this case) lines?

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4 Answers 4

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That's a breaker for a 3-phase feed, with an added neutral disconnector that snaps on. The neutral disconnect is used for test purposes on GFCI protected gear, for extra safety during maintenance, and is even required by some codes.

Read more about that here: https://electrical-engineering-portal.com/dimensioning-disconnecting-protecting-neutral-conductor

The short takeaway is that in a 3-phase system (and, North American 2-phase 220V) the neutral carries imbalance current. It can have some cross-connected voltage on it in the case of a fault, so this disconnect is an extra precaution for maintenance (the link mentions mult-story buildings in particular.)

The three phase breakers are ganged together. If one or more trips they all trip. But, in a trip the neutral remains connected: it is a switch, not an interrupting breaker. It moves independently from the gang, but is still designed to prevent energizing the phases without a connected neutral.

How? You can also see that the neutral disconnect has an interlock tab that engages the breaker gang. The tab does two things:

  • Forces the neutral to be manually connected before the 3-pole breaker can be turned on
  • Forces the ganged breakers to be off before neutral can be turned off.

Both functions prevent the case of having the phases energized without the neutral connected. That is, neutral is connected first and disconnected last, and only by manual action at the panel for maintenance and testing.

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  • \$\begingroup\$ Thanks for answering. My question is: why would not link the neutral disconnect to all the phases to make sure they are all open, or all close. In other words: When do you need to open only the phases and leave the neutral closed?. Thanks! \$\endgroup\$ Commented Nov 12, 2023 at 20:35
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For checking and measuring for continuity, being able to independently disconnect neutral (of course only when the hot lines are off) can be useful.

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  • \$\begingroup\$ If you look at the switch, you cannot disconnect the neutral without disconnecting the phases (Thanks God! :) ) - the question is, why would you disconnect the phases without disconnecting the neutral? \$\endgroup\$ Commented Nov 12, 2023 at 19:10
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Most likely the "extra pole" (on the far right of the picture) is not intended to break the neutral of the supply. Most likely it is some accessory that is added as an option to an MCB, such as:

  1. Aux contact that indicates the state of the breaker (ON, or CLOSED).
  2. An external trip device that can cause the breaker to trip from an external signal.

https://new.abb.com/low-voltage/products/system-pro-m/miniature-circuit-breakers/accessories

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    \$\begingroup\$ This looks very similar, yet is called a "Neutral disconnect terminal block": new.abb.com/products/2CCS500900R0021/nt401-63 \$\endgroup\$
    – jms
    Commented Nov 8, 2023 at 0:48
  • \$\begingroup\$ Well, I did not know that ABB made an accessory specifically for breaking the neutral - I learned something today, thankyou. In Australia, we almost never break the neutral, in fact, it is specifically forbidden, it is only permitted in very specific situations and with special sequencing (late-break, early-make) wrt the actives. Cheers. \$\endgroup\$ Commented Nov 8, 2023 at 2:20
  • \$\begingroup\$ @FabioBarone thanks a lot for the info! (I'll give you a +1 ;) ) Do you know why is that? In Switzerland it seems that for households we always break the neutral. I think in Argentina (where I'm from) it is Illegal to not to do it, all breakers break all poles and neutral \$\endgroup\$ Commented Nov 12, 2023 at 19:08
  • \$\begingroup\$ @CarlosGarcia No worries mate! The reason we do not break the neutral in Australia is because our electrical system over here is called Multiple Earthed Neutral, or MEN. As the name suggests, the neutral is earthed (connected to earth via an earth-stake) at multiple locations: at the main transformer out in the street), and at the consumer switchboard. Even the water pipes are connected to earth! \$\endgroup\$ Commented Nov 12, 2023 at 20:46
  • \$\begingroup\$ #2: At the consumer switchboard there is a link called MEN link - it connects the neutral link bar to the earth link bar - it can be removed for fault-finding purposes but must be replaced, and only done by a licensed electrician. This MEN link reduces the resistance of any electrical fault to a protective earth (say, from the active to the equipment case, such as a knife falling into a toaster such that the toaster case becomes "live"), and means that the circuit breaker will trip much faster. \$\endgroup\$ Commented Nov 12, 2023 at 20:47
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In Austria we mainly use circuit breakers with N included. This helps as others pointed out for measuring for example.

For a private person the biggest benefit is if they have a bad appliance which has an issue with N connection to PE/ground. They can switch off this circuit and then the RCCB will not trip anymore. If you do not have this N disconnect, they have to wait till an electrician has time to fix the issue. In the meantime they have to live in the dark if they only have 1 single RCCB for the whole house which is still quite common.

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    \$\begingroup\$ Thanks for the answer! My question is: In what case you would shut down the phase, but not the neutral. Why is the neutral treated differently than the other breakers? \$\endgroup\$ Commented Nov 8, 2023 at 12:47
  • \$\begingroup\$ I guess saving costs, since its enough to shut down live to prevent a fire on this cable \$\endgroup\$
    – Fritz
    Commented Nov 9, 2023 at 13:54

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