Forums » Electrical Engineering

MCCB switchgear

    • 3 posts
    February 8, 2019 9:18 AM PST

    Just read this, "neutral contact closes first (before phase contacts) during closing operation and the neutral contact opens last (after phase contacts open) during opening operation"

    why to open a neutral in MCCB? 

  • February 8, 2019 12:34 PM PST

    (NEC - North American Standard)

    4 pole CBs usually mean one is dealing with Separately Derived Systems - the neutrals are not inter-connected.  An example is a transfer switch separating a generator and the utility.  If the design calls for the generator to be an SDS, then the CB is required to be 4pole and switch the neutral.  It does not have to be done this way - it is a design issue.

    Most of my clients are off-grid, generation from 500KW to 20MW.  I'm familiar with SDS generation systems, but I've never used a 4 pole CB design.  However, I have worked on some that have.  When I have asked, "Why?", the response is usually a mumbled something about ground fault currents being a problem.  I think the mumbling is because they don't know.  Hopefully the designer knew why.

    I suspect the early-make, late-break is to solidly establish the N-G connection before the phases are connected

    • 65 posts
    February 9, 2019 12:19 AM PST

    Option1: 3-Pole Switching -> Lowest Cost -> 


    1. Nuisance tripping of GF relay on deenergized source
    2. De-sensitizing the energized source GF relay.
    3. Added complexity for GF relay switching as shown in Figure 5 to prevent nuisance tripping of de-energized source.


    Option 2: 4-Pole Switching-> No circulating current, so no possibility of desensitizing energized source GF relay and no possibility of nuisance tripping a GF relay protecting a de-energized source 

    1. Higher cost 
    2. Larger footprint (size)
    3. Reported neutral transients*

    Option 3: 3-Pole Switching with Overlapping Neutral-> May be less expensive than true 4-pole since overlapping neutral typically is not rated for fault duty switching

    1. During the time when both neutrals are connected, the same disadvantages as a 3-pole switch (nuisance tripping of GF relay on de-energized source and de-sensitizing energized source GF relay) exists
    2. Added complexity and reduced reliability from an external switch controlled by levers and interlocks connecting to main switch
    3. Added complexity to add GF relay switching as shown in Figure 5 to prevent nuisance tripping of de-energized source.

    Option 4: 3-Pole Switching with Special GF Sensing Scheme -> Less expensive than 4-pole or 3-pole with overlapping neutral

    1. More complex wiring as de-energized sources have their trip circuits deenergized and their CT circuits paralleled with the CTs of active sources


    Are neutral switching transients a problem on a 4-pole system?

    This post was edited by Reynaldo Danilo at February 9, 2019 12:21 AM PST
    • 200 posts
    March 9, 2019 1:47 PM PST

    Thanks for the suggestion of this topic, Gregg.
    Sorry for late reply.

    Let's try to answer your very specific question: "why to open a neutral in MCCB?"

    Consider a system that has ONLY ONE grounded source. All Ground current (If) must flow through that single-Ground (in left Neutral point, Source 1). A sensor on that grounded connection makes it easy to capture and measure all Ground current (If). A single-point grounded system with multiple sources is called a "non-separately derived system". Regardless of which source is energized, a Ground Fault (GF - red X, in low-center) on a single-point grounded system ALWAYS returns to the grounded source.
    [insert image: Single-ground+Ground-Fault=imgsnap107.png]



    GF sensing becomes more difficult when multiple sources each have their own Ground conductor and when the Neutral conductor is NOT switched. In such a case, Ground current (If) can flow through multiple paths. This complicates the GF sensing scheme. Systems with multiple-grounded sources (in left and right Neutral points, Source 1 and Source 2) are called "separately derived systems". A Ground current sensor on any ONE source may not record the total Ground current (If) flowing into the Fault (red X, in low-center), even if that source is the only source feeding the Fault.
    [insert image: Multiple-ground+Ground-Fault=imgsnap108.png]



    One of the solutions to prevent Ground current (If) from dividing through the Neutral conductor is to switch the Neutral conductor. This insures that no Neutral current can flow through a de-energized source.

    There are concerns that such a load break switching of the Neutral can result in unacceptably high transient voltages, so a method to reduce these reported transient voltages has been the 3-pole switch with "overlapping Neutral switching".

    Overlapping: when you say "neutral contact closes first (before phase contacts) during closing operation and the neutral contact opens last (after phase contacts open)" this is one of overlapping schemes among few others available in Electrical design.

    It is advised to take a look on topic "grounding system":

    And topic "MV Transformer", for Y-Y connection, which is the most frequent installation to apply Neutral switching, since all Neutral shall be grounded:

    hope that could answer your question.


    This post was edited by Alex de Moura at March 12, 2019 10:10 AM PDT
    • 200 posts
    March 12, 2019 10:12 AM PDT

    Dear Gregg, sorry for the late post of images, but the Image Upload is recently back to work thanks to Hameed, hoping the images could of any help. Thanks.