RCD neutral broken
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rcd?
Regardless, in the general sense there is no guarantee any breaker will trip for the stated condition. Would have to know all the specifics of an actual case to say a breaker could should or would trip... and even then it would only be a knowledge/experience-based guess.
Regardless, in the general sense there is no guarantee any breaker will trip for the stated condition. Would have to know all the specifics of an actual case to say a breaker could should or would trip... and even then it would only be a knowledge/experience-based guess.
kwired
Electron manager
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- NE Nebraska
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- EC
What does rcd mean?
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- San Francisco Bay Area, CA, USA
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- Electrical Engineer
Keep in mind that you are posting in a forum that, for the most part, is geared towards the North American electrical contracting industry. RCD is an IEC term, we use the term "Ground Fault Circuit Interrupter" (GFCI) or Ground Fault Circuit Breaker (GFCB). These systems are different from what people in other countries use RCDs for. Installation codes vary and we only have devices suitable for what our codes require. In some installations the GF device must monitor the Neutral, in other installations it doesn't need to.
A broken neutral is a fault condition but not one sensed by an rcd/gfci/alci. These devices work by sensing a preset difference in the current passing through the device in both directions. A line-side broken neutral may affect load current, but the difference in current passing through the device in both directions will be unaffected (at least initially
). Yes, a broken neutral could cause a secondary fault to occur for which the device would trip, but as I said this depends on case specifics.Additionally, a broken neutral on the line side of a common neutral junction in a multiwire branch circuit may or may not change the phasor relationship of the individual branches. However the phasorial relationship of line currents is forced to sum to zero (unless there is another fault condition present).
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wirenut1980
Senior Member
- Location
- Plainfield, IN
I somewhat disagree Smart. If the neutral is broken, won't the current be searching for some other way back to the source? Possibly through a parallel path provided by a communication ground, or water pipe, etc. Depends on how good this other path is and consequently, how much current flows on these other paths is enough to trip the device.
What you suggest is a ground fault, not simply a broken (open) neutral. Even so, if the ground fault occurs on the line side of the device, the current difference through the device is not affected.
no , just think of the toroidal part, three phase passing through it will induce an unbalanced current and this will trip the device i think, if there will be no trip so why we need to pass the neutral through gfci? so just let 3 phase through the device and it will trip anyhow....
wirenut1980
Senior Member
- Location
- Plainfield, IN
I don't think the Neutral to ground bond (which is where the current would spit off) is ever on the line side of a GFI. And I think calling the situation described a ground fault is not accurate, although I do not know where a formal definition of a ground fault exists.
Edit to add: I am talking about at a main disconnect.
wirenut1980
Senior Member
- Location
- Plainfield, IN
On a wye system, an RCD works by comparing current coming in (on the phases) to current going out (on the neutral and on the other phases). If you just measured 3 phases coming in without the neutral, you aren't seeing the whole picture.
wirenut1980
Senior Member
- Location
- Plainfield, IN
I am saying it might trip if the line side neutral is broken. It depends on whether there is enough current taking parallel paths on grounding conductors to make it trip. If there is not an adequate alternate path for the neutral current to take, then it probably won't trip and you may have some very unbalanced voltages.
If the neutral is open and there is no current in the neutral, then the three phases will sum to zero; the voltages and loads will shift so that they will. Current going into a node have to sum to zero; there is nowhere else for the current to go. There should be no connection between the device neutral and ground. The neutral should be connected to ground only at the source (on the other side of the open point from the device), otherwise, ground fault devices will not work properly.
as far as İ know these devices will dedect the fault by having a leakage current going out of for example 3 phase and neutral combination, so if neutral is gone and the phases are unbalanced loaded so the three phase with an asymettry will make the current sensing toroid sense a current and trip the circuit, u say grounding conductors but it has nothing to do with grounding conductors since i am talking of a situation not having a ground fault , what i cant understand is this,i suppose a asymmetry on a three phase will create a residual current on the sensing device, excuse me i ask repeatedly because i cant get the meaning of "depends on whether there is enough current taking parallel paths on grounding conductors to make it trip" part, i guess my question has nothing to do with this sentence, because neutral is broken means the three phase currents wont go out of three conductors here in my question...
Under normal operation of a 3? 4W circuit (service, feeder, or MWBC), the three line and neutral currents sum to zero... but the three line currents by themselves may not sum to zero. This is why the neutral is included in the sensing. It is needed to cancel any normal non-zero sum of the three line currents.
Open the neutral anywhere on the line side of its first current split, if any, and you would then have an abnormally operating circuit (though no difference would be noticed if the branches were balanced under normal operation). You would then have a floating voltage node(s). As such, the circuit would be self-balancing (perhaps to the point of creating a fault). The three line currents would sum to zero, provided there are no "leakage faults" (aka ground faults).
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