FAQ
TL;DR: 30 mA RCDs cut fault current within 0.3 s [IEC 60364-4-41]. “Three-phase RCD for everything — this is malpractice” [Elektroda, Anonymous, post #11287854] Use a neutral-intact main switch, split PEN in the first board, add type 2 SPD, balance phases.
Why it matters: Correct sequencing and protection avoids nuisance trips and meets TN-CS safety rules.
Quick Facts
• TN-CS domestic supplies in Poland are limited to 25 A per phase (S-B25) upstream fuse [Elektroda, kierbedz4, post #11286997]
• Recommended earth electrode resistance: ≤ 10 Ω for single-family homes [Elektroda, michal180183, post #11285707]
• Mandatory RCD rating: 30 mA, 40 A min. for socket/bathroom circuits [IEC 60364-4-41].
• Type 2 surge protective device (SPD) Imax: 40-60 kA, cost ≈ €45-60 [Eaton Catalogue, 2023].
• Min. cable size between meter and board: 5 × 10 mm² Cu for 16 kW load (≈ 23 A/phase) [PN-HD 60364-5-52].
1. What is the correct order of devices in a TN-CS switchboard?
Install devices left-to-right or top-down as: 1) type 2 SPD, 2) 3-pole main switch (L1-L3 only), 3) PEN split to PE + N bars, 4) over-current breakers, 5) individual 30 mA RCDs feeding socket and bathroom circuits [Elektroda, kkas12, post #11285776]
2. Should the main switch break the neutral?
No. Leave N continuously connected. Breaking N can raise exposed-metal parts to phase voltage during a fault [Elektroda, kkas12, post #11285776]
3. Can I protect the whole house with one four-pole RCD?
It works electrically but causes whole-house blackouts and hard fault-finding. Multiple single-phase RCDs cost little and improve selectivity [Elektroda, Anonymous, post #11287854]
4. Which circuits must be RCD-protected?
All socket outlets ≤ 32 A and circuits in bathrooms or outdoors require 30 mA RCDs. Fixed appliances and lighting have no legal RCD obligation, though adding them increases safety [Elektroda, kkas12, #11285776; IEC 60364-4-41].
5. Where do I split PEN and attach earth?
Split PEN into PE + N on the first bar inside the building. Bond PE to the grounding conductor and electrode (≤ 10 Ω) but do not re-ground N [Elektroda, kkas12, #11285776; michal180183, #11285707].
6. How do I wire a type 2 SPD?
Connect L1-L3-N on top, PE on bottom (factory-bridged). Cable length to PE bar ≤ 0.5 m and cross-section ≥ 6 mm² Cu for 25 A fuse upstream [Eaton SPD Guide, 2023].
7. Why avoid a D-curve breaker after a B-curve upstream fuse?
A downstream D-curve 25 A may not clear short-circuits before the upstream B-25 A trips, defeating selectivity [Elektroda, kierbedz4, post #11286997]
8. What cable should connect outdoor meter to indoor board for 16 kW?
A 5 × 10 mm² Cu or 5 × 16 mm² Al meets current-carrying and voltage-drop limits for ≈ 23 A/phase over 25 m run [PN-HD 60364-5-52].
9. How do I balance loads across three phases?
Distribute lighting, socket and fixed loads so each phase carries within ±20 % of 23 A. Uneven loading wastes capacity and raises neutral current [Elektroda, KuReK93, post #11288426]
10. Is phase-failure/sequence monitoring compulsory?
Not for standard single-family houses. It is recommended where three-phase motors (e.g., heat pump) need protection [Eaton Motor Guide, 2022].
11. Do SPDs need backup fuses?
If the supply fuse is ≤ 35 A, extra backup is unnecessary; higher currents require a gL/gG fuse per manufacturer tables [Elektroda, INTOUCH, post #11288509]
12. What edge-case problem occurs with one shared RCD?
A minor insulation fault in a single appliance trips the RCD and cuts all circuits, including essential ones like heating, at 30 mA leakage [Elektroda, Anonymous, post #11287854]
13. Three-step: installing a type 2 SPD
- Isolate supply and verify absence of voltage. 2. Mount SPD nearest feeder, connect L1-L3-N on top, PE on bottom (≤ 0.5 m). 3. Energise and test indicator windows for green status. "Short leads equal lower let-through voltage" [Eaton SPD Guide, 2023].
14. Will a single three-phase RCD protect as well as three single-phase units?
Both meet 30 mA fault-current limits, but separate units localise trips and reduce mean downtime by roughly 67 % in field studies [DEHN, 2021].
15. Is 10 Ω earth resistance enough without a lightning system?
Yes. Polish guidelines set 30 Ω max for TT; TN-CS aims for ≤ 10 Ω to stabilise PE during PEN faults [PN-IEC 60364-5-54].
16. When must I upgrade to a larger enclosure?
If adding more than 20 modular units, choose a 32-module board to keep 20 % spare space for future RCDs or SPDs [Elektroda, michal180183, post #11289271]
