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]
Can I run two separate wires (N and PE) back to the PEN bar in the meter cabinet?
No. Double-landing on one lug is forbidden and weakens the connection [Elektroda, mawerix123, post #15910720]
How do I link a TN-C supply to my future TN-S house system?
- Feed PEN from utility to site board.
- Split PEN to PE + N using a certified link.
- 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].
