By Louis McGarry, Sales and Marketing Director, CENTIEL UK 

The fundamental purpose of a UPS is to protect the critical load. However, protecting the critical load isn’t just about a mains failure. A UPS must also be designed to integrate with the protective devices that have been specifically selected to limit interruptions to the electrical infrastructure during fault conditions.  

The UPS should be sized so that it will produce enough energy during fault conditions to disconnect the appropriate protective downstream device.  I like to think of this ‘discrimination’ as having the ability to take out just a ‘finger’ to limit the damage, not the whole ‘arm’ or worse the ‘body’ of the system!  You will find an array of devices and cables downstream of any UPS which invariably should get smaller as you approach the load.  Think about the bones of the arm getting smaller as they reach the fingers which connect to the load.  During a fault, the appropriate device should automatically disconnect locally, preventing the whole system and UPS going off-line.

It seems obvious to assume a UPS should be designed based on power requirements alone (100kW load = 100kW UPS). However, when there is a fault within the electrical infrastructure this generates a high current, which the UPS needs to manage to ensure that all protective devices discriminate correctly. The inverter of the UPS can deal with a certain amount of current but if a higher demand is required for a longer duration the UPS transfers to static bypass. It’s the rating of the static bypass that is key to the design.

It is important that the UPS is sized to allow enough energy to flow through the static switch, to ensure that in the worse case scenario the correct protective device disconnects to clear the fault.  If a fault should occur downstream and the UPS system is undersized there wouldn’t be sufficient energy via the static bypass to allow the downstream disconnection. Unfortunately, this would cause the UPS to shut down to protect itself and the electrical infrastructure, resulting in the loss of the critical load. 

Ratings for protective devices are carefully calculated and selected to fit within the electrical infrastructure and so should the rating for the UPS. Assumptions should be avoided.  Just because a particular downstream device rating was sufficient for one project, it may not be suitable for another! Each project is individual and no one-size fits all.  

The correct approach is to design a UPS around the existing infrastructure and not the other way around. Sizing a UPS system should start with the rating of protective devices and end with the load demand, taking into account the full electrical infrastructure and not just how much power you anticipate you will need.  This will ensure the UPS is rated to work properly with devices such as MCCBs or ACBs or other automatic disconnecting devices.  Then, if there is an issue, the appropriate device will disconnect at the appropriate moment.  The whole infrastructure will remain protected as will the load.

The introduction of the BS7671 regulatory standard has acknowledged that a UPS forms part of the electrical infrastructure.  This has helped to some extent, but we are still seeing too many systems designed which don’t take discrimination into account.

The good news is that the latest UPS technology is designed to be fault tolerant with high short circuit capability.  Working with UPS manufacturers by bringing them into the discussion earlier to pool knowledge, ideas and come up with workable options, will ensure that your UPS is designed adequately for the electrical infrastructure.

To be certain a UPS will perform as expected when necessary, all the elements of design, selection, installation and maintenance of the solution need to be considered thoroughly.   Careful calculation is required by engineers who understand electrical discrimination and who can calculate the correct UPS rating based on the infrastructure. 

At CENTIEL our team are experts with many years of experience, trusted to advise organizations world-wide about the most appropriate UPS solutions to protect facilities.  Our highly trained engineers are always happy to advise clients on UPS rating requirements.  When it comes to protective devices, we work closely with clients and consultants to ensure UPS systems are all rated correctly and as a result, the critical load remains protected at all times.

Originally featured in UK Power News December 2020.

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