Tech Talk: Electrical Discrimination

Your UPS FAQs explained with our Technical Services Manager Jason Yates. This instalment focuses on electrical discrimination…

Does Installing A UPS Into A Circuit Affect Electrical Discrimination?

To put it simply, yes. Normally, discrimination within an electrical system is calculated to ensure that the correct protective devices are selected and subsequently adjusted from the incoming supply down to the final load connection point.

What Is Electrical Discrimination?

The discrimination path will typically be made via several distribution boards. The purpose of electrical discrimination is to ensure that the power is both interrupted and limited should a fault condition arise within the electrical system.

If we look at a typical electrical system, it comprises a sequence of protective devices from the supply source down to the final load. These devices will naturally become smaller in rating as we move from the supply source to the final load point.

Having discrimination will ensure that any affected circuit is disconnected within the required time. At the same time, it’ll make sure that all other circuits which are connected to the same source remain on.

Essentially, the lower rated protective device (protecting the final circuit) will trip or open before the larger supply protective device, this is basic discrimination.

UPS Considerations

When a UPS is installed, additional considerations must be made. When calculating the discrimination using the mains supply, for example, the designer has a fixed level of fault current available depending on the installation size and type. However, when a UPS is installed into an electrical circuit, the designer must then consider the operating modes of the UPS as these will affect the available fault current.

A UPS effectively has two operating modes on the mains supply when the mains supply is present and on battery when the mains supply has failed. The UPS is, therefore, supplying the power to the connected load from the connected batteries. In terms of fault current availability, this will be different during these two modes of operation.

UPS Modes Of Operation

The first mode of operation is when the mains supply is present. If a fault occurs on the outgoing side of the UPS then the device will instantaneously switch to bypass and the input supply will be directly connected to the output. In this condition, the fault current available is that of the mains supply and the mains supply will help to clear the fault.

During bypass operation, the designer must consider the rating of the static bypass components within the UPS to ensure that they are both capable of sustaining the fault current and ultimately, that they will not be damaged.

The specification for these components can be located with the technical documentation of the specific UPS being installed, typically this is measured as I²t (Amps² x time), which is effectively the energy let through limitations of the components used within the UPS’s static bypass line.

The second mode of operation is when the mains supply has failed or is not within tolerance and the batteries are supplying the inverter, which in turn is supporting the connected load. If a fault occurs on the outgoing side of the UPS then the unit will attempt to clear the fault using the fault current that is available from the inverter.

In this condition, the fault current available from the inverter will typically be significantly less than that of the incoming mains supply, which will significantly affect the rating of the downstream protective devices installed.

The reason the fault current is significantly less is due to the limitations of the inverter and hence the inverter must ensure that it is neither damaged nor any internal fuses are ruptured, which in turn would require the intervention of a Riello Certified UPS Engineer.

Design Specification

During battery operation, the designer must consider the rating of the inverter to ensure that they are both capable of sustaining the fault current and ultimately clear the downstream issue. The specification of the inverter can be located within the technical documentation of the specific UPS being installed.

Typically, this is defined as the ‘short circuit value’ and will generally be specified as a fault current level, which will be a percentage of the nominal output current for a set period of time. After this time has elapsed, the UPS output will be shut down in order to prevent any internal damage.

A key part of electrical discrimination design is to ensure that circuits are disconnected within a specified duration, for example, 400ms. In these cases, many UPS products can be specifically configured so that the output from the UPS shuts down within the required time and hence the circuit is made safe.

For example, if a UPS can sustain a specific fault current for 1 second, however, the fault current available is not sufficient to clear the downstream protective devices within the required time. Then the UPS can be specifically configured to shut down within 400ms (instead of 1s) to ensure the system remains safe, disconnects the power to the affected circuit and ultimately prevents unnecessary UPS oversizing.

In summary, the installation of a UPS does affect the electrical discrimination and hence the designer must give careful consideration to its installation and the potential issues that may subsequently arise.