You're right, an RCB (Residual Current Breaker), also known as an RCD (Residual Current Device) or GFCI (Ground Fault Circuit Interrupter), protects against earth leakage faults downstream of where it's placed in the circuit.The recommendation to not place an RCB before a UPS, but to place it after, is due to a few potential issues and operational considerations

Why NOT Before the UPS:
Nuisance Tripping

UPSs, particularly online double-conversion UPSs, can generate a small amount of leakage current to earth as part of their normal operation. This is due to the internal components (capacitors, filters, etc.) and the way they condition the power. This normal leakage current, while harmless, can be enough to trigger a sensitive RCB placed before the UPS, resulting in a nuisance trip. This would shut down the UPS and any equipment connected to it, defeating the purpose of having a UPS.

Harmonics and Noise

UPSs (especially older or less sophisticated models) can inject harmonics and electrical noise onto the mains power line. These harmonics can be misinterpreted by the RCB, leading to false tripping.

UPS Fault Mode

In some fault conditions within the UPS, the internal circuitry might create leakage currents that cause the upstream RCB to trip. While this might seem like a good thing (protection!), it will instantly shut down the UPS and the critical load it's supposed to protect. It might be better to let the UPS continue to operate, even with a minor fault, than to immediately shut down the critical load. The downstream RCB will still protect against dangerous leakage currents.

Coordination Issues

Coordinating the RCB with the UPS's internal protection mechanisms can be complex. The RCB's tripping characteristics need to be carefully selected to avoid unwanted interaction with the UPS's fault detection and isolation.

Why After the UPS



Downstream Protection

Placing the RCB after the UPS provides essential protection to the equipment connected to the UPS and the people who might come into contact with it. It protects against earth leakage faults in the critical load equipment that is plugged into the UPS.

Isolation of Faults

If a fault occurs in the equipment connected to the UPS, the downstream RCB will trip, isolating the fault and preventing it from propagating back to the UPS or affecting other devices.

Reduced Nuisance Tripping

By placing the RCB downstream, you are less likely to experience nuisance tripping due to the UPS's own internal leakage currents and harmonics.

In summary, the core idea is

You want the UPS to reliably power your critical load as long as safely possible. Tripping an upstream RCB can defeat this purpose, even in non-critical fault scenarios. The primary purpose of the UPS is to provide backup power, not necessarily to act as the sole source of fault protection. The RCBs downstream of the UPS provide the necessary protection against earth leakage faults in the connected equipment.

Important Considerations



UPS Design

Some modern UPSs are designed to minimize leakage current and harmonics, making them more compatible with upstream RCBs. Consult the UPS manufacturer's documentation for specific recommendations.

RCB Sensitivity

Choose an RCB with an appropriate tripping current (e.g., 30mA) and tripping time for the application. Higher tripping currents might be more suitable for UPS applications to avoid nuisance tripping.

Selectivity/Discrimination

In installations with multiple RCBs, ensure that they are properly coordinated to provide selectivity. This means that the RCB closest to the fault should trip first, minimizing the impact on other circuits.

Local Regulations: Always comply with local electrical codes and regulations regarding the installation of RCBs.Think of it this way: You're protecting the connected equipment and personnel with the downstream RCB, while allowing the UPS to operate reliably, even if it means accepting a slightly higher (but still within safe limits) risk of a UPS-related fault not immediately shutting down the whole system. The downstream RCB will eventually trip if a dangerous situation develops in the connected equipment.Always refer to the UPS manufacturer's documentation and consult with a qualified electrician for specific recommendations regarding RCB placement and coordination in your particular installation. Flag for review