It is normal for a Smart-UPS (or any UPS system) to have leakage current on its input, and values up to 5 mA are within acceptable safety standards.
Why Leakage Current Occurs in a Smart-UPS
Leakage current is a small amount of electrical current that flows from the live parts of the UPS to ground, even when the device is operating normally. This happens due to:
- EMI filters: UPS systems include electromagnetic interference (EMI) filters to reduce electrical noise. These filters use capacitors connected between line and ground, which inherently allow a small current to flow.
- Surge protection components: Devices like MOVs (metal oxide varistors) also contribute to leakage current.
Internal circuitry: Some control and monitoring circuits may also cause minimal leakage.
Is 5 mA Safe and Normal?
Yes, up to 5 mA of leakage current is considered safe and compliant with international standards like:
- IEC/EN 60950-1 and IEC/EN 62368-1 (for IT and power equipment)
- UL 1778 (for UPS systems)
These standards typically allow:
- ≤ 3.5 mA for Class I equipment (with protective earth)
- ≤ 5 mA in some cases, especially for larger or more complex systems like Smart-UPS units
Practical Considerations
If multiple UPS units are connected to the same circuit, the combined leakage current can add up. This is why RCDs (residual current devices) or GFCIs may trip if the total leakage exceeds their threshold (usually 30 mA).
It is important to understand that, by itself, the UPS cannot reduce the leakage currents of the connected loads. Where the cumulative leakage currents of the loads cause facility GFCIs or RCDs to trip, adding the UPS will not solve the problem. In such cases, an appropriately sized and UL Listed isolation transformer is required in addition to the UPS.
For installations nuisance tripping of GFCIs or RCDs, consider using RCDs with higher trip thresholds or type B RCDs that are designed to handle such leakage.
