RCD (Residual Current Device) on input side of the UPS

Cause: Regarding RCDs on the Input Side of a UPS

There are several reasons we do not recommend installing an RCD on the input side of the UPS:

• Cumulative leakage current
  The leakage current at the input of the UPS is the sum of the leakage from the UPS plus all connected load devices. This can easily exceed a 30 mA RCD trip threshold, especially because some IT equipment has relatively high leakage. Under IEC 62040-3, earth leakage must not exceed 3.5 mA per pluggable device, but this limit refers to the UPS itself only.
• Shared protection and existing installation leakage
  Standard 30 mA RCDs often protect multiple circuits. If there is already background leakage in the installation and from other connected appliances, the UPS or other devices may simply push the cumulative leakage over the limit, resulting in nuisance tripping.
• UPS operating modes and backfeed protection
  The effectiveness of an input-side RCD cannot be guaranteed across all UPS operating modes—particularly when the UPS switches to battery operation and the unit’s backfeed protection is active.
• Transient leakage from power electronics
  Power-electronic events such as capacitor charging and discharging can cause short-duration (millisecond) transient leakage. These transients can be sufficient to trip RCDs.
• Common-mode noise and surge suppression
  As part of its essential function, the UPS employs common-mode noise filtering and surge suppression, which can lead to nuisance tripping of an RCD installed on the UPS input.

Conclusion:
Considering the above, an RCD on the input side of the UPS may defeat the essential goal of equipment availability.

Resolution
If an RCD must be used due to a genuine earth-leakage requirement, a dedicated selective (time-delayed) type RCD for the UPS circuit is recommended. This type trips only after a slight, intentional delay, helping to avoid unnecessary tripping.

Note:
Australian Standards recognize the high-availability requirements of UPS and IT equipment and have developed an amendment to AS/NZS 3000:2018, released on 26th of June 2018. This can be verified directly with Australian Standards.

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