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When an AFCI or GFCI breaker trips or turned off does the neutral on the circuit break from the bus bar?

Daniel Griscom
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Jon Stiltner
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3 Answers3

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GFCI's must break neutral

GFCIs (of all types) are required to break hot and neutral, unlike regular fuses, switches, or breakers. This is not spelled out in UL 943, but is implied by the requirement in UL 943 6.7.2.2 that a GFCI must still trip and disconnect the load circuit from power if the line-hot and line-neutral connections are interchanged.

Some AFCIs also break neutral, although they are not always required to

Because early AFCI designs were derived from GFCIs, they inherited the property of breaking the neutral (as well as being sensitive to gross ground faults). However, some more modern designs (Mod 3 THQL AFCIs, most notably) dispense with the ground-fault detection, and thus may or may not break the neutral wire. (As to the standards: UL 1699 13.3 requires receptacle-type AFCIs to break neutral, whereas circuit-breaker/loadcenter-type AFCIs have the option to not do so.)

ThreePhaseEel
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Yes, Neutral must be disconnected by RCDs/GFCIs as mentioned before in the other answers.

What is only rarely mentioned is the contact sequence in the time domain if more than 1 phase are concerned, f.e. 2 or 3 phases:

In those cases, Neutral must be disconnected only after the phases are disconnected. And Neutral must be (re-) connected before the phases.

Otherwise an imbalance could occur for some milliseconds which might destroy devices.

This is why the Neutral clamps must not be used for phases and vice-versa. At least, that is the rule in most European countries with 3 phase systems.

The English Wikipedia page does not seem to mention it, therefore here a translated version of the German Wiki page - scroll for "Contact Sequence":

https://de-m-wikipedia-org.translate.goog/wiki/Fehlerstrom-Schutzschalter?_x_tr_sl=de&_x_tr_tl=en&_x_tr_hl=en&_x_tr_pto=wapp

xeeka
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I'm wiring a house and assumed this to be true, but apparently it's not with all GFCI breakers. I'm using Siemens QPF2N GFCI breakers and they do not break the neutral connection.

enter image description here

audiobrad
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  • Are you sure that's a fully accurate diagram? I'd be double-checking that with my meter on continuity....power up the breaker in a panel, trip it with the TEST button, then pull the breaker without resetting it and check for continuity from line-neutral to load-neutral. Imagine a neutral-to-ground fault with swapped line-hot and line-neutral at the input to the GFCI, and how that'd violate the UL requirement I mentioned in my answer if the breaker doesn't break both hot and neutral... – ThreePhaseEel Aug 04 '23 at 04:06
  • The spec sheet specifically says that breaker implements UL943 (which requires GFCIs to disconnect neutral in case of being hooked up backwards upstream, etc) so I'd bet that's just a "conceptual" diagram, not to be taken too literally. – nobody Aug 04 '23 at 12:58
  • Please follow up with the comments on your answer and correct it if applicable. Also if you are not the artist that drew the picture please add an attribution like to the source of the image. – Michael Karas Aug 07 '23 at 14:55