Summary

UPS systems inherently generate earth leakage due to internal EMI filters and connected IT loads. When combined with certain electrical network types (TN‑CS, TT) and sensitive RCDs, this may result in nuisance tripping. This article explains why tripping occurs, outlines regulatory requirements, and provides guidance on selecting and installing the correct RCD or RCBO to ensure safe and reliable UPS operation.

Issue

RCD / ELCB devices trip when a UPS is:

• Plugged in,
• Switched to battery mode, or
• Operating under heavy load.
  
  

Product Line

Smart‑UPS – All models, all serial numbers, Easy UPS, Symmetra RM / LX

Environment

Any installation where UPS systems are used in conjunction with RCDs, ELCBs, RCBOs or selective RCDs, including TT and TN‑CS networks.

Cause

1. Normal Earth Leakage from UPS Filters

UPS EMI filters contain Y‑capacitors between Line/Neutral and Earth, producing continuous, normal leakage currents. When combined with leakage from connected equipment, this can exceed the threshold of a 30 mA RCD.

EN50091 permits up to 3.5 mA earth leakage per pluggable device. Multiple loads plus the UPS can easily exceed 30 mA even when no faults are present.

2. Leakage Spikes During Transfer

During transfer to battery or inverter operation, UPS devices may create short neutral–earth voltage imbalances or switching pulses that sensitive RCDs detect as fault current.

3. High Leakage from Connected IT Equipment

Switch‑mode power supplies (SMPS) can produce brief but significant leakage spikes at startup. Standard RCDs may trip unless a selective (time‑delayed) RCD is used.

4. Installation Wiring Problems

Neutral conductors incorrectly mixed at the distribution board can cause RCDs to trip regardless of UPS performance.

5. TT Network Regulations

In TT earthing systems, fault current is limited by earth impedance and is insufficient to clear a fuse. Regulations therefore mandate:

• 300–500 mA selective RCD at the main supply, and
• 30 mA RCDs on final circuits.
  
  

UPS leakage plus connected loads may push total leakage above 30 mA, causing nuisance trips even though all equipment is functioning correctly.

Resolution

1. Do Not Install a 30 mA RCD on the UPS Input

Schneider Electric  recommends avoiding 30 mA RCDs upstream of UPS systems due to inherent leakage and switching behaviour. If an RCD must be used, choose an S‑type (selective, time‑delayed) model.

2. Recommended RCD Types (Based on UPS Size and Series)

1–3 kVA (SMC / SMT(L) / SMX / Easy UPS SMV – Line‑Interactive) / ( SRT(L) / SRV(L) ) Online

• Type A RCD, ideally S‑Type (selective)
• 30 mA (only if required by local regulations)
• Avoid Type AC
• Use a dedicated RCD

5–10 kVA (SRT(L) / SRTG / Easy UPS SRV – Online) / Symmetra RM

• Type B, or high‑quality Type A (S‑Type)
• 30–100 mA depending on regulatory requirements
• Increased switching noise means selective devices are strongly recommended

15–20 kVA (SURT / SRTG – Online) / Symmetra LX / SRTL

• Type B (S‑Type)
• 100–300 mA depending on installation and local codes
• Recommended due to potential DC leakage components

3. Use Selective (S‑Type) RCDs

S‑Type RCDs incorporate intentional time delay to tolerate brief leakage spikes from UPS transfers and SMPS startup currents.

4. Recommended Installation Method (Best Practice)

In TT systems - and in TN‑CS systems where leakage protection is required—the recommended configuration is:

1. Hard‑wire the UPS output using the appropriate Schneider wiring kit.
2. Feed the UPS output into a small distribution board.
3. Install individual RCBOs (RCD + MCB combination) for each downstream circuit.

This approach isolates leakage per circuit, prevents unnecessary trips on the main RCD, and satisfies regulatory requirements.

5. Troubleshooting Steps

1. Test the UPS alone on a dedicated RCD. If the RCD trips, replace the UPS. If it does not, check the site equipment or wiring.
2. Inspect for neutral mixing at the distribution board.
3. Measure cumulative leakage of connected loads.
4. Confirm RCD type and sensitivity match UPS rating and topology.

Additional Information

TN‑CS Network Characteristics

• Earth and Neutral are bonded at the supply point.
• Low earth impedance (~0.35 Ω allowable) means fault currents > 600 A can clear fuses quickly without mandatory RCD protection.

TT Network Characteristics

• Earth provided by a local electrode (15–20 Ω typical).
• Fault current (~15 A) is insufficient to trip a fuse or MCB.
• RCDs are mandatory for shock and fire protection.
  
  

Terminology for International Users

• RCD (Residual Current Device) – Standard term in Europe, UK, Australia.
• ELCB (Earth Leakage Circuit Breaker) – Older term still appearing in some markets.
• GFCI (Ground Fault Circuit Interrupter) – US/Canada term; typically 5–6 mA sensitivity (not suitable for UPS input protection).
• RCBO – RCD integrated with MCB; recommended for UPS‑fed distribution circuits.