Circuit Breaker

Circuit Breaker

The nature of the beast, as far as uninterruptible power supply (UPS) is concerned, is power protection. Load connection to the source of uninterrupted power must be secure and provision needs to be made for power distribution to essential loads. Critical load connection will rely on either direct connection to the UPS output or a form or power distribution unit. Load connection may be via a socket or hardwired, depending upon rating and currents drawn. Downstream distribution depends upon the installation format that has been chosen whether centralised or decentralised.

Essential loads are those a power continuity customer cannot operate without and may include things like payment systems, transaction processing and other operational loads. Critical loads are those that require power protection, such as PCs perhaps and other back-office equipment but are not as crucial to operation as essential loads.

Uninterruptible Power Supply Connection

Decentralised UPS distribution usually incorporates plug-in type units, which rely on standard socket outputs to provide connections up to 16A in total. Larger current require a hardwired connection to the output terminals or perhaps an industrial-type connector. Where there are multiple hardwired loads, a distribution board will be used.

External Bypass Connection

Where an external bypass is installed, it provides the point of connection for the loads; either directly (plug-in or hardwired) or via a distribution board. The bypass will have a manual transfer switch or circuit-breakers, and may also include automatic transfer switching circuit to provide the installation with additional resilient if the UPS is accidentally removed or fails.

Distribution Boards

Distribution boards provide a common point of distribution for the loads and are used for centralised systems, primarily. Each feed within the cabinet may be individually supplied via a suitably-rated circuit-breaker to provide discrimination in case of a short-circuit. Within a large critical installation, two or more distribution boards may be provided, one for the uninterruptible power supply (critical loads) and one for the generator supporting essential loads.

Power Distribution Units (PDUs)

Power distribution units distribute power to multiple plug-in loads, such as file server clusters. They are supplied with a variety of socket lengths and combinations to accommodate any number of sockets. Each PDU is fused and may also incorporate another form of surge suppression, TVSS (transient voltage surge suppressors) for example. Within a rackmount cabinet, PDUs can be installed vertically or horizontally.

Intelligent power distribution units, which can be remotely controlled over a network, are becoming more prominent. PDUs should not be connected daisy-chain fashion where more than one is being used. They are normally individually fused and by powering one from another, a single-point-of-failure is being introduced into the power continuity system.

Automatic and Static Transfer Switches (ATS)

These can be used in place of power distribution units, especially for plug-in type loads. They are typically relay-based.

Static transfer switches cater for high power ratings and can be hardwired for either single-phase or three-phase inputs and outputs. A distinct advantage is that they are static switch-based and can be connected in parallel to provide added resilience.

A note or warning, however, with static transfer switches; care must be taken to ensure single-points-of-failure are not introduced on the output side of the uninterruptible power supply. Each device is fused or connected by a circuit-breaker, which should it rupture or open, will result in the load being dropped.

This article was compiled using information available in The Power Protection Guide - the design, installation and operation of uninterruptible power supplies (ISBN: 9 780955 442803).

By Robin Koffler and Jason Yates of Riello UPS.Robin Koffler is the co-author of The Power Protection Blog and the General Manager of Riello UPS Ltd, the UK subsidiary of Riello UPS (RPS S.p.A). For more information visit the Riello UPS - uninterruptible power supplies website.

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