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Distributor's Box Wiring: Bridging PE with N or Creating Additional Grounding?

Piotrekh75 36630 41
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How should I wire a construction switchboard when the supply has 3 phases and neutral but no separate ground: bridge PE to N, add grounding, or rely on an RCD?

If the supply is TN-C with a PEN conductor, bring the PEN to the construction switchboard and split it into separate N and PE there; do not make an arbitrary PE–N bridge in the wrong place [#15910873][#15911018][#15911027] The PEN must have the correct cross-section: at least 10 mm² Cu or 16 mm² Al [#15910873][#15911027] Install a 3-phase RCD, and for a construction board add temporary protective earthing if needed, but do not rely on a fixed 10 Ω rule—replies point instead to the applicable construction-site rules/standard (7-704) [#15911027][#15912132][#15911805] The exact solution depends on the connection conditions and the network system (TN or TT), so those documents must be checked before wiring [#15909978][#15910172][#15910671][#15910692] Because this is a construction switchboard, the protection should be verified by an electrician with measurements [#15910184][#15910671]
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  • #31 15911291
    Łukasz-O
    Admin of electroenergetics  
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    kkas12 wrote:
    Where did your friend read that?

    Probably in terms issued by ZE :D
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  • #32 15911447
    Karamba2014
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    Sorry, I misread some literature, after checking a few sites on the internet, it comes out below 10 ohms
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  • #33 15911478
    kkas12
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    Quote:
    ...after checking several websites on the internet, it comes out below 10 ohms
    Can you tell me what pages these are?

    And we don't write below 10 ohms but below 10 ohms !
    Please correct!
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  • #35 15911646
    kkas12
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    You chose very badly my friend.
    That message somehow escaped the moderators and survived.
    However, an electrician cannot base his knowledge only on what he finds on the forums.
    However, there are other messages in this forum as well. There are standards 4-41 - 7-704 mentioned in them and they should be based on them.
    And there are no such requirements.
    So put this value of 10 ohms between fairy tales.
  • #36 15911709
    MARCIN.SLASK
    Home appliances specialist  
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    Maybe this will help:
    Distributor's Box Wiring: Bridging PE with N or Creating Additional Grounding?
    Distributor's Box Wiring: Bridging PE with N or Creating Additional Grounding?
    Distributor's Box Wiring: Bridging PE with N or Creating Additional Grounding?
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  • #37 15911728
    Łukasz-O
    Admin of electroenergetics  
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    It's just all taken out of context.
  • #38 15911805
    MARCIN.SLASK
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    The resistance should be low enough to ensure tripping in 400ms (0.4s) with protection not greater than 32A.
    For construction sites, increased requirements can be used: switch-off time max. 200ms (0.2s) at max. 25VAC touch voltage.
  • #39 15911825
    kkas12
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    We are talking about the TN system.
    Will the lack of earthing of the PE bus cause that SWZ will not occur?
    Will the earth electrode improve the parameters of the all-metallic fault loop?

    Please remember that this is a section for beginners and in this section we also require independent thinking from those who ask questions.
    So please don't advise. The more that the hints are not true.
  • #40 15911857
    MARCIN.SLASK
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    kkas12 wrote:
    We are talking about the TN system.
    Will the lack of earthing of the PE bus cause that SWZ will not occur?
    Will the earth electrode improve the parameters of the all-metallic fault loop?

    Please remember that this is a section for beginners and in this section we also require independent thinking from those who ask questions.
    So please don't advise. The more that the hints are not true.


    1: Under normal conditions, the SWZ will occur, because the ZE is to ensure the continuity of the PEN conductor (but there are also known conditions other than normal, e.g. a tree branch damages this conductor).
    2. Yes, but not much/slightly. Often there is a 100-fold difference between these resistances/impedances.
  • #41 15912132
    kkas12
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    It won't improve anything. And when the PEN is interrupted, the role of the grounding is different.
    But that has been explained many times before.
    For electric shock protection (recipient), the resistance of (its) earthing is irrelevant.
    There is a 7-704 sheet for construction sites and it should be adhered to and not made up.
    And if that's not enough for someone, let them use the solution from the islands.
    Also in the above standard it is presented and not a 10-ohm resistance.
  • #42 15912176
    TWK
    Electrician specialist  
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    MARCIN.SLASK wrote:
    Maybe this will help:
    The first drawing is a network standard, not an installation...
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Topic summary

✨ The discussion revolves around the proper grounding methods for a construction distributor's box that provides three-phase power and neutral but lacks a dedicated ground line. Participants emphasize the importance of adhering to connection conditions and the necessity of protective earthing (PE) to ensure safety. The use of a PEN conductor, which serves dual functions as a protective earth and neutral, is debated, with recommendations for proper cross-section sizes (10mm² for copper, 16mm² for aluminum). The need for a residual current device (RCD) is highlighted, and the significance of measuring grounding resistance is discussed, with a target of below 10 ohms for effective protection. The conversation also touches on the implications of different network systems (TN-C, TN-S) and the importance of hiring qualified electricians for installation and measurement tasks.
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FAQ

TL;DR: In Polish LV grids, ≤0.4 s disconnection is required for circuits ≤32 A (IEC 60364); “PEN is a wire with two functions” [Elektroda, elvis13, post #15910873] Keep PEN intact, split to PE + N in the switchboard, add a 30 mA RCD.

Why it matters: Correct PEN handling prevents lethal touch voltages during construction.

Quick Facts

• Supply type in most rural plots: TN-C with combined PEN conductor [Elektroda, elvis13, post #15910873] • Minimum PEN size: 10 mm² Cu / 16 mm² Al (PN-HD 60364-5-54) [PN-HD 60364] • Mandatory disconnection time: ≤0.4 s for ≤32 A circuits (IEC 60364-4-41) [IEC, 2020] • Recommended RCD for site boards: 4-pole, IΔn = 30 mA, typ. cost €50–80 [Hager, 2023] • Earth electrode: used mainly as PEN backup; no fixed ≤10 Ω rule in TN-C [Elektroda, kkas12, post #15911825]

What network system is usually delivered when the utility gives 3 phases plus neutral?

The connection is almost always TN-C, meaning the neutral and protective functions share one PEN conductor up to your board [Elektroda, elvis13, post #15910873]

Should I bridge PE with N in a construction switchboard?

Yes. In TN-C you must split the incoming PEN into separate PE and N bars and link them with a removable PEN link or terminal; this equals “bridging” inside the board, not in downstream sockets [Elektroda, Karamba2014, post #15911018]

Where exactly do I separate PEN into PE and N?

Do it at the first switchboard you own (the site box). Use a dedicated PEN terminal, then feed PE and N to downstream RCDs and circuits [Elektroda, kkas12, post #15911027]

What cross-section must the PEN conductor have?

Regulations require minimum 10 mm² copper or 16 mm² aluminium for any PEN in TN-C systems [PN-HD 60364-5-54].

Do I need a separate earth electrode for the temporary board?

It is recommended but not obligatory for shock protection in TN-C; its main role is to stabilise voltage if the PEN fails [Elektroda, kkas12, post #15912132]

Is there a target resistance value for that electrode?

Standards give no single number; the loop impedance must still let protective devices trip within 0.4 s. Values below 30 Ω help, yet protection is verified by loop-impedance measurement, not earth-resistance alone [Elektroda, kkas12, post #15911274]

Is a 30 mA RCD compulsory on construction sites?

Yes. IEC 60364-7-704 and Polish rules demand 30 mA RCDs on all socket circuits at sites. A 30 mA unit trips in 30–40 ms under test conditions [Hager, 2021].

How do I confirm protection is effective?

An electrician measures fault-loop impedance Zs and RCD trip time. Zs × 230 V must stay below protective-device disconnection voltage (≤0.4 s for breakers ≤32 A) [IEC, 2020].

What happens if the PEN conductor breaks?

All exposed metal bonded to PE can rise to full phase voltage, creating a fatal touch hazard; any auxiliary earth only slightly reduces this risk [Elektroda, MARCIN.SLASK, post #15911857]

How do I link a TN-C supply to my future TN-S house system?

  1. Feed PEN from utility to site board.
  2. Split PEN to PE + N using a certified link.
  3. Run five-core cable (L1-L2-L3-N-PE) from the board into the building installation. [Elektroda, Karamba2014, post #15911018]

May I use a temporary rod earth before foundations are poured?

Yes; drive 2–3 steel rods ≥1.5 m into moist soil and bond to the PE bar. Treat it as provisional and retest after permanent foundation earth is installed [Elektroda, jdubowski, post #15910172]

Edge case: What if loop impedance is high even with PEN intact?

Install an RCD and shorten cable runs; if Zs still exceeds the limit, use a dedicated transformer or lower-rated breaker to satisfy the 0.4 s rule [IEC, 2020].

How much does a compliant site board upgrade cost?

Adding a 4-pole 40 A/30 mA RCD, PEN link, and test terminals typically costs €100–150 in parts plus labour [Hager, 2023].
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