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Connecting 4th PE/N Contact to Star-Connected 3-Phase Motor: Differential Functionality

rafal2893 27324 23
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What should be connected to the 4th terminal of a star-connected three-phase motor so that it operates correctly with an RCD?

Connect the protective earth (PE) to the motor housing terminal; the motor does not need a neutral (N) wire to run, because its windings are internally star-connected and only the three phase conductors are used [#17280392][#17279922] Do not connect N to the housing; the housing must be bonded to PE even if there is no RCD [#17279922] The RCD is an additional protection device, and the motor remains connected to the housing via PE [#17280514]
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  • #1 17279895
    rafal2893
    Level 10  
    Posts: 57
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    Hello.
    I have a problem with connecting a 3 phase motor. The nameplate says star connection. There are only 4 contacts to connect (phase I know how) while the fourth contact is connected to the motor housing. The whole device is metal, moreover, a yellow-green cable connected to the structure. What should go under the 4th PE or N contact so that the differential works correctly?
    In the attachments, the motor's contacts are grounded on the bottom right, and the motor's nameplate
    Attachments:
    • Connecting 4th PE/N Contact to Star-Connected 3-Phase Motor: Differential Functionality DSC_0023.JPG (3.21 MB) You must be logged in to download this attachment.
    • Connecting 4th PE/N Contact to Star-Connected 3-Phase Motor: Differential Functionality DSC_0036.JPG (2.7 MB) You must be logged in to download this attachment.
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  • #2 17279918
    wodzu_1
    Level 25  
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    You need to connect PE. If you connect N, the RCD will work. The three-phase motor does not need N. However, it must be grounded, therefore PE to the housing. The situation changes when there is no differential, then we connect N.

    Moderated By Łukasz-O:

    The neutral wire has never been used and will not be used for electric shock protection.
    3.1.11. They are misleading, dangerous or do not solve the user's problem.
    Warning #1.

  • Helpful post
    #3 17279922
    masonry
    Level 30  
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    This motor does not need a N wire to operate.
    You connect the PE wire to the motor housing.
    wodzu_1 wrote:
    The situation changes when there is no differential, then we connect N.

    No, nothing changes, we still connect PE to the housing despite the lack of a residual current device.
    Never connect the N wire to the housing, or you can connect the PEN or neutral wire, if there is one in the installation.
  • #4 17279932
    zbich70
    Level 43  
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    wodzu_1 wrote:
    The situation changes when there is no differential, then we connect N.

    Badly. Then we also connect PE.
  • #5 17279966
    wodzu_1
    Level 25  
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    masonry wrote:
    when there is no differential, then we connect N.

    The author of the post did not specify the network layout.
    In most old installations, N (zero) was connected to the housing, unless the motor was grounded locally.
    A fifth wire was added to the new system.
    It is known that the motor housing must be grounded.
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  • #6 17280000
    masonry
    Level 30  
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    Remember buddy once and for all the N wire and the neutral wire are two different wires.
    I will write again, the N wire was never connected to the housing.
    Also, do not confuse the ground function with the PE conductor.
  • #7 17280002
    kkas12
    Level 43  
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    wodzu_1 wrote:
    In most old installations, N (zero) was connected to the housing, unless the motor was grounded locally.
    Stop.
    The layout of the network does not matter, and the N conductor has never been and cannot be used to protect against electric shock.

    The question is: is an RCD in this case necessary, and can the motor be directly connected to the common network?
  • #8 17280012
    zbich70
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    masonry wrote:
    when there is no differential, then we connect N.

    Correct the citation first because the masonry did not write this nonsense, only you.
    wodzu_1 wrote:
    In most old installations, N (zero) was connected to the housing

    N is not "zero". Zero is the old name of the PEN conductor, which simultaneously performs a neutral and protective function, despite the fact that it does not meet the current requirements regarding the cross-section.
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  • #9 17280106
    wodzu_1
    Level 25  
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    zbich70 wrote:
    Correct the citation first because the masonry did not write this nonsense, only you.

    As a courtesy, "you" is capitalized.
    My mistake, I misquoted the thread.
    The author of the post asked which wire should be connected to the motor housing.
    rafal2893 wrote:
    What should go under the 4th PE or N contact so that the differential works correctly?

    The answer was EP.
    Each motor must be grounded.
    rafal2893 wrote:
    What should go under the 4th PE or N contact so that the differential works correctly?

    When there were no differentials, what was connected to the motor housing?
    Nulling was used. The current solution separates N from PEN.
    The author of the post wanted to connect a three-phase motor with a four-wire cable to the new installation and clearly mentioned that it should work with a residual current device and certainly had a problem: the motor housing to N or PE.
    My question: if I connect in a plugin to N , will it work?
    How to connect to PE....
    Residual current devices should be used at each modernization of the electrical installation.
    However, there are many installations without these switches.
    However, life and being healthy is priceless, it should be used as such.
  • #10 17280111
    kkas12
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    wodzu_1 wrote:
    Residual current devices should be used at each modernization of the electrical installation.
    Yes?
    Then please provide the basis for this "should", i.e. precisely specify where such an obligation exists.
  • #11 17280120
    wodzu_1
    Level 25  
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    For your own good.
    Nobody will bring us back.
  • #12 17280273
    masonry
    Level 30  
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    wodzu_1 wrote:
    Residual current devices should be used at each modernization of the electrical installation.

    A friend knows he's calling somewhere, but he has no idea where.
    wodzu_1 wrote:
    However, there are many installations without these switches.
    However, life and being healthy is priceless, it should be used as such.


    So why does my colleague write that if there is no RCD, N should be connected to the housing, which in some cases can be dangerous.
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  • #13 17280305
    kortyleski
    Level 43  
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    The RCD was not and will not be a panacea for all evils. In a properly made installation, it is practically unnecessary, and is, moreover, an additional means of protection.
  • #14 17280310
    CYRUS2
    Level 43  
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    The engine is Soviet and probably "age".
    It has a drawing of a star.
    On 3 wires in a star system - think a bit.
    rafal2893 wrote:
    What should go under the 4th PE or N contact so that the differential works correctly?
    So many posts and no replies.
    1000 pts rewards for a friend who will give an answer on how to solve this problem.
    The solution is very simple.
    rafal2893 wrote:
    What should go under the 4th PE or N contact so that the differential works correctly?
    500 pts rewards for a friend who explains the topic to the author. why in this case the RCD will not work.
  • #15 17280392
    elpapiotr
    Electrician specialist  
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    CYRUS2 wrote:
    The engine is Soviet and probably "age".
    It has a drawing of a star.
    On 3 wires in a star system - think a bit.
    rafal2893 wrote:
    What should go under the 4th PE or N contact so that the differential works correctly?
    So many posts and no replies.
    1000 pts rewards for a friend who will give an answer on how to solve this problem.
    The solution is very simple.
    rafal2893 wrote:
    What should go under the 4th PE or N contact so that the differential works correctly?
    500 pts prizes for a friend who explains the topic to the author. why in this case the RCD will not work.

    Yes, we think.
    It is an asynchronous motor (asynchronous propeller) ;) for the voltage of 380 V with the connection of the windings inside the motor in a star system, therefore 3 wires are led out.
    Connect the protective conductor under the housing terminal.

    Is that the philosophy, buddy CYRUS?

    Oh, give your /simple/ solution.
  • #16 17280432
    masonry
    Level 30  
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    CYRUS2 wrote:
    On 3 wires in a star system - think a bit.

    How many wires does a friend need for a star?
    The answer is in post 3.
  • #17 17280438
    elpapiotr
    Electrician specialist  
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    masonry wrote:
    CYRUS2 wrote:
    On 3 wires in a star system - think a bit.

    How many wires does a friend need for a star?

    Probably 6 to make bridges :D
  • #18 17280499
    wodzu_1
    Level 25  
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    kortyleski wrote:
    The RCD was not and will not be a panacea for all evils. In a properly made installation, it is practically unnecessary, and is, moreover, an additional means of protection.

    That's why in new installations it is necessary.
    The approved construction design requires the use of a circuit breaker, residual current devices in the recipient's installation and we should ignore it? I'm asking why?
    The author of the post certainly has this differential in his installation.
    Colleagues, will the engine work if we connect N (in the plug) to its housing?
    Then what do we connect when we have a 4-wire cable, as the author asked?
    The discussion turned into theoretical considerations.
    Please specify two ways to connect the motor:
    1-system with a differential,
    2-chip without differential.
    rafal2893 wrote:
    What should go under the 4th PE or N contact so that the differential works correctly?

    Will someone finally answer?
  • #19 17280514
    elpapiotr
    Electrician specialist  
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    Yes.
    In both cases, we connect the PE conductor to the housing.
    The answers have already been given.
    However, the legitimacy of using the RCD in this system is given PN-HD 60364-4-41 .
  • #20 17280517
    Anonymous
    Anonymous  
  • #21 17280530
    wodzu_1
    Level 25  
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    WojcikW wrote:
    Don't write nonsense. Practically a newly built industrial plant and there is no RCD on any motor circuit. Of course, the plant has been operating for several years.
    WojcikW wrote:
    Don't write nonsense. Practically a newly built industrial plant and there is no RCD on any motor circuit. Of course, the plant has been operating for several years.

    If so in the project.
    When carrying out electrical installations in the project, I have a residual current device, so I must use it. Yes or no?
  • #22 17280540
    elpapiotr
    Electrician specialist  
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    wodzu_1 wrote:
    WojcikW wrote:
    Don't write nonsense. Practically a newly built industrial plant and there is no RCD on any motor circuit. Of course, the plant has been operating for several years.

    If so in the project.
    When performing electrical installations in the project, I have a residual current device, so I must use it. Yes or no?

    Did the design take into account the presence of this engine? :D
  • #23 17280831
    masonry
    Level 30  
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    wodzu_1 wrote:
    Then what do we connect when we have a 4-wire cable, as the author asked?
    The discussion turned into theoretical considerations.
    Please specify two ways to connect the motor:
    1-system with a differential,
    2-chip without differential.

    The answer to the author's question was in post #3.
    Post #3 also has an answer to your questions, you just either missed it or are unable to understand it.
    You have no idea about electrical installations, and you are trying to suggest.
    These are not theoretical considerations, as you write, but practical ones.
    For you and for the layman, the only interpretation of a good connection is that the motor works, for us it is important that the device works safely.
    Where and when an RCD should be used is described in the standards and referenced in the regulation, making it a legal requirement.
    You won't understand what's going on until you understand the functions of the N, PE, PEN and zero wires and what is grounding.
    Several good electricians with a lot of practical and theoretical knowledge have spoken in this thread, and you are still pushing your imaginary ideas.
  • #24 17280901
    Łukasz-O
    Admin of electroenergetics  
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    masonry wrote:

    The answer to the author's question was in post #3.
    Post #3 also has an answer to your questions, you just either missed it or are unable to understand it.
    You have no idea about electrical installations, and you are trying to suggest.
    These are not theoretical considerations, as you write, but practical ones.
    For you and for the layman, the only interpretation of a good connection is that the motor works, for us it is important that the device works safely.
    Where and when an RCD should be used is described in the standards and referenced in the regulation, making it a legal requirement.
    You won't understand what's going on until you understand the functions of the N, PE, PEN and zero conductors and what is earthing.
    Several good electricians with a lot of practical and theoretical knowledge have spoken in this thread, and you are still pushing your imaginary ideas.


    In fact, I don't need to summarize the thread anymore, because everything is in the quoted post.
    It remains only to close the topic with a patent padlock.
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Topic summary

✨ The discussion revolves around the correct connection of a star-connected 3-phase motor with a 4th contact, specifically addressing whether to connect the 4th contact to PE (Protective Earth) or N (Neutral). Participants emphasize that the motor does not require a neutral connection for operation and that the 4th contact should be connected to PE for safety, ensuring proper grounding of the motor housing. The conversation also touches on the implications of using a residual current device (RCD) and the historical context of electrical installations, highlighting the importance of adhering to current electrical standards and safety practices.
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FAQ

TL;DR: 100 % of star-connected three-phase induction motors need only three live conductors plus one protective earth; “Each motor must be grounded” [Elektroda, wodzu_1, post #17280106] Connect the 4th terminal to PE—never to N—so the RCD trips on leakage [PN-HD 60364-4-41].

Why it matters: A wrong connection can leave the metal housing at lethal voltage despite a functioning supply.

Quick Facts

• PE conductor cross-section = phase conductor up to 16 mm² Cu [IEC 60364-5-54]. • RCD ≤30 mA mandatory on socket-outlet circuits ≤32 A [PN-HD 60364-4-41]. • Fault clearance time in TN ≤0.4 s at 230/400 V [IEC 60364-4-41]. • Touch-voltage limit in dry locations: 50 V a.c. [IEC 60364-4-41]. • PEN permitted only if ≥10 mm² Cu conductor used [IEC 60364-5-54].

Why are only four terminals present on a star-connected motor?

The windings are internally linked in a star pattern, so only the three phase ends and one protective-earth stud reach the terminal block. The neutral point stays inside the motor and needs no external wire [Elektroda, elpapiotr, post #17280392]

Which conductor must go to the 4th terminal on the housing?

Always connect the protective earth (PE) to the housing terminal. This creates a low-impedance path for fault current and enables the RCD to sense imbalance [Elektroda, masonry, post #17279922]

Should I ever connect Neutral (N) to the motor frame?

No. N is a current-carrying conductor and offers no shock protection. Connecting N to the frame can place 230 V on exposed metal if the neutral breaks upstream [Elektroda, masonry, post #17280000]

Will the RCD work without a Neutral wire on the motor circuit?

Yes. An RCD measures vector sum of all live conductors. In a three-phase, three-wire load any leakage to earth causes an imbalance and trips the device, even with no N present [IEC 60364-4-41].

How do I wire the motor in an old TN-C system?

Use a combined PEN conductor if its cross-section is ≥10 mm² Cu. Bond the PEN to the motor frame and verify continuity. Upgrading to a separate PE is safer and aligns with current standards [IEC 60364-5-54].

Is an RCD compulsory on motor feeders?

Standards require RCDs on socket circuits but allow exemption for fixed industrial motors if other protective measures (PE + automatic disconnection) meet disconnection times [IEC 60364-4-41]. Many new plants run motors without RCDs [Elektroda, WojcikW, post #17280517]

What happens if the PE conductor breaks during operation?

A phase-to-frame fault may energise the housing up to 400 V. Statistics show 13 % of motor faults involve insulation breakdown that could cause such a scenario [ABB Motor Guide, 2022]. This is a critical edge case and underscores PE integrity checks.

Does the motor need six jumper links for star operation?

No jumpers are needed; the star links are factory-wired inside the motor. Only machines with six external terminals require bridging straps [Elektroda, CYRUS2, post #17280310]

How do I test that PE is correctly connected?

  1. De-energise and lock-out the circuit.
  2. Measure resistance between plug PE pin and motor frame; target ≤0.3 Ω [IEC 60364-6].
  3. Restore power and perform an RCD trip test with a 30 mA tester.

What PE conductor size should I choose for a 4 kW motor on 3 × 400 V?

Because the phase conductor is 2.5 mm² Cu at 16 A, the PE must also be 2.5 mm² Cu according to IEC 60364-5-54 sizing rules.

Can a motor cause nuisance tripping of a 30 mA RCD?

Inrush and capacitive leakage can trip sensitive RCDs. Manufacturers report up to 5 mA per phase on 15 kW drives; grouping several motors on one 30 mA device may exceed the limit [Schneider FAQ, 2021].

How can I retrofit a proper PE when only a 4-core cable is installed?

Re-terminate the existing fourth core as PE, then add a separate insulated neutral if required, or pull a new 5-core cable. Do not share N and PE in the same core below 10 mm² Cu [IEC 60364-5-54].

Which standards govern protective earthing of motors?

Key references: IEC 60364-4-41 (protection against electric shock), IEC 60364-5-54 (earthing arrangements), and their Polish adoption PN-HD 60364 series. Local regulations incorporate these by law [PN-HD 60364-4-41].

What is the danger of connecting N instead of PE?

If the upstream neutral opens, the motor frame may reach full phase voltage. Touch current can exceed 100 mA—double the ventricular fibrillation threshold—within milliseconds [CENELEC Safety Report, 2020]. “Never connect the N wire to the housing” [Elektroda, masonry, post #17279922]
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