Protection and Discrimination in Uninterruptible Power Supplies

In uninterruptible power supply (UPS) installations, ‘discrimination’ is a term used to describe the co-ordination of protective devices within a supply circuit to ensure any load developing a fault (through overload, short-circuit or earth fault) is isolated and removed in the shortest possible time without disruption to remaining loads. It also ensures protection of supply conductors.

The specification is known as a ‘discrimination path’ and it is vital to a power continuity plan that a suitable path is made available for the uninterruptible power supply to protect both it and the loads it is powering. The most commonly used means of discrimination are RCDs (Residual Current Devices) or RCBOs (Residual Current Breaker with Over-current Protection).

It is important the protective devices are correct, and suitably rated, for the application for which they are chosen. Fuses and circuit breakers have different operating characteristics (speed of response) and will react differently when required to disconnect the ac supply when a fault occurs. Whichever method is employed, it is also vital any failed protective device is replaced on a like-for-like basis to maintain discrimination.

The difference in overload capability means that care has to be taken when specifying which type to use, especially within a UPS application, to prevent nuisance tripping at power up.

A type-D circuit breaker (motor-rated) can sustain higher overloads for longer periods and is recommended for use with loads with high in-rush currents, such as some types of uninterruptible power supply, motor machinery and transformer installations. A type-B device, which is for general-purpose use, responds much faster and is therefore more appropriate for Switch Mode Power Supply-type loads (computers and telecoms hardware, for example). It is also ideal for general electric circuits where it can be used to prevent nuisance tripping and disconnection. For most uninterruptible power supply installations a type-C is the preferred choice.

Within most installations, the protective devices used will comprise of a number of fuses, circuit breakers or RCBOs, connected in series between the ac supply and load. They are specified to operate in a co-ordinated manner so that the correct device trips when intended. This ensures discrimination under fault conditions. Circuit impedance must be low enough so that the necessary currents can be generated when a fault conditions occurs. Measurement for this is performed using an Earth Fault Loop Impedance Tester.

For uninterruptible power supply installations there are two types of connection method, each of which dictates the installation and testing required when connecting to the mains power supply and loads:

Plug-in Uninterruptible Power Supplies (up to 16A input): which can be installed by any competent person and not just a certified electrical engineer, if the mains power supply sockets are already available.

Hardwired Uninterruptible Power Supplies (above 16A input): connected to both the mains power supply and load by hardwired terminal connection points. Here it is mandatory that a certified electrical engineer performs the installation and issues a test certificate.

Fault conditions are not only annoying and disruptive but for 24/7 businesses requiring consistent power protection they can be devastating, bringing the whole operation to its knees for want of relatively simple discrimination paths in the electrical circuitry.