FAQ
TL;DR: Use a Type 1+2 SPD near the origin; coordinate with FR and RCDs. 30 mA RCDs protect sockets. "Install SPDs as close as possible to the origin." For electricians and advanced DIYers aligning TN wiring and device order. [Schneider Electric, 2022]
Why it matters: The right SPD–FR–RCD order avoids nuisance trips, preserves protection, and simplifies safe maintenance in TN systems.
Quick Facts
- Residual current protection: 30 mA for additional protection; consider 100–300 mA upstream for fire protection, where permitted. [Schneider Electric, 2022]
- In TN‑C, bond the SPD to the PEN bar; do not split PEN, and skip the N bar if unused. [Elektroda, mawerix123, post #17129472]
- One stated practice: place the SPD behind the main disconnector (FR), then feed RCD/MCBs. [Elektroda, Łukasz-O, #20161909]
- If the feeder from the entry SPD to a sub-board exceeds ~10 m, add a downstream SPD stage. [Phoenix Contact, 2020]
- Keep total SPD lead length (phase/neutral to PE) as short as possible, typically ≤0.5–1 m. [DEHN, 2014]
Is SPD -> RCD -> MCB -> load the correct order?
Not without a main disconnector (FR) and proper PEN handling. Place the SPD behind the disconnector and bond it to the PEN bar. Skip the N bus when the load does not need neutral. Then feed the RCD and MCBs as required. [Elektroda, mawerix123, post #17129472]
Where should the SPD go relative to the main disconnector (FR)?
Follow the stated practice: “There is only one school. The limiter is to be behind FR.” That puts the SPD on the load side of the disconnector. It then protects the board and downstream circuits while FR is on. [Elektroda, Łukasz-O, #20161909]
Can the SPD still protect when the FR is switched off?
One expert noted, “the arrester is to protect installations even if FR is switched off.” To keep protection with FR open, installers place the SPD upstream of FR. Confirm utility rules for any device before the meter, and ensure safe isolation procedures. [Elektroda, Łukasz-O, #17131205]
Do I need an RCD on a motor circuit?
Use 30 mA RCDs for socket-outlet circuits for additional protection. Fit RCDs on motor circuits when required by standards, environment, or risk assessment. Use selective or high-immunity types if leakage or drives cause trips. Coordinate RCDs with the SPD for reliable operation. [Schneider Electric, 2022]
How should I connect an SPD in a TN‑C system?
Do not separate the PEN conductor inside devices. Use an SPD model suitable for TN‑C and bond it directly to the PEN bar. As warned: “PEN must not be separated on the apparatus.” Some SPDs are not intended for TN‑C networks. [Elektroda, mawerix123, post #17120766]
How do I replace an SPD module safely?
Use a safe isolation routine. 1) Open the main disconnector (FR) to de-energize the switchgear. 2) Verify absence of voltage with an approved tester. 3) Withdraw the SPD cartridge and fit the replacement, then restore power. Do not rely on upstream fuses alone for isolation. [Elektroda, pol102, post #17130404]
Are there risks when working behind a switched-off RCD or breaker?
Yes. Contacts can weld (“glued contacts”), leaving parts live even when switched off. Always test for absence of voltage before contact. Lock out the supply if possible. Use devices certified for isolation when you need safe disconnection. [Elektroda, pol102, post #17132553]
How do I prevent nuisance RCD trips caused by SPDs?
Place SPDs upstream of RCDs so surge currents bypass the RCD, or use selective/time-delayed RCDs. Keep leads short and bonding direct to reduce residual surge currents. This coordination minimizes unwanted RCD tripping during surge events. [Phoenix Contact, 2020]
Do I need one SPD per floor or sub-board?
Add SPDs in downstream boards when the feeder exceeds about 10 m or each board has its own disconnector. Cascaded stages limit energy at each level. For two floors with separate main switches, fit SPDs in both boards. [Phoenix Contact, 2020]
Where do I bond the SPD in a TN‑C‑S board?
Bond the SPD to the PEN/PE bar in the main distribution board. Keep connections short and straight to minimize voltage rise. Do not route through the N bar unless needed for the SPD type. [Elektroda, mawerix123, post #17129472]
Is an N bus required if I only feed a three‑phase motor?
No. If the motor does not need a neutral, omit the N bus. Connect the SPD to the PEN bar and supply the motor through appropriate overcurrent protection and control gear. [Elektroda, mawerix123, post #17129472]
Is my SPD compatible with TN‑C networks?
Not all SPDs fit TN‑C. An SPD meant for TN‑S/TN‑C‑S may be unsuitable in TN‑C. Select a model specifically rated for TN‑C, and never split PEN inside the device. [Elektroda, mawerix123, post #17120766]
Which SPD class and rating should I use at the service entry?
Use a Type 1 or Type 1+2 SPD at the origin. Choose lightning current capability to match the installation; 12.5 kA Iimp per pole is common. Match backup overcurrent protection to the SPD’s datasheet. [Phoenix Contact, 2020]
