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Application Engineering – Transfer Switch Application Manual

Transfer equipment is available in many configurations all serving a common purpose – to connect an electrical distribution system to alternative sources of ower. Proper selection, installation and use of this equipment are vital to the reliability of the system.

Load transfer equipment may be mandated by code or standard or desired for critical processes. Transfer equipment may be automatically or manually operated and may by supplemented by an uninterruptible power supply to achieve even greater reliability.

This manual includes a range of definitions, to assist in clarifying terminology that is common in the codes and throughout transfer equipment application.

Cummins Power Generation manufactures a range of transfer switches that provide safe, dependable power transfer between utility and generator supplies. The transfer switches allow either closed transition, where the generator is paralleled with the utility supply briefly, or open transition techniques.

Because the transfer switch connects with both the utility and the generator, which have differing electrical characteristics, correct application is essential. The documents below will advise you on the use of transfer switches for various loads, including delayed transfer for motor loads. They also describe maintenance techniques such as thermography.

Please read the Warranty Statement carefully before proceeding with any design or ordering materials or labor.

These documents are in PDF format. To view them, you will need Adobe Reader® software.

Chapter 1 – Introduction gives an overview of the content of this manual, warranty information and related codes and standards.

Chapter 2 – Electrical Design describes the initial considerations for the power transfer project. The position of the transfer equipment within the electrical system is one of the earliest considerations and the choice between single and multiple transfer switches is discussed. Sample system configurations, fault withstand, the coordination between protective devices and step loading are also reviewed. The chapter gives a number of examples of transfer switches and their application, together with examples of the use of bypass equipment as an aid to system reliability. The chapter ends with an analysis of ratings.

Chapter 3 – Transfer Equipment Switching Means explains the choices of transfer equipment available to the user, from a simple changeover contactor arrangement, through interlocked circuit breakers to single and double-throw and solid state transfer switches. The chapter reviews the open and closed transition methods, the construction of bypass arrangements and automatic mains failure equipment.

Chapter 4 – Transfer Equipment Controls describes the methods of controlling the transfer equipment which will vary according to the criticality and complexity of the installation. The choices range from a purely manual operation, through non-automatic electrically controlled devices, to fully automatic utility sensing switches that are capable of controlling both generator and utility supplies. The tolerance of various loads to voltage and frequency excursions is discussed together with the incorporation of various time delays to prevent premature switching.

Chapter 5 – Application Considerations discusses the effect of load on the transfer switch design. The effect of inductive loads, motor starting and methods of system grounding and switched neutrals are also detailed. The chapter also examines special cases, such as fire pump and healthcare loads, leading to closed transition switching where a break in supply is avoided. The chapter ends with details of short circuit performance and protection.

Chapter 6 – Installation Considerations explains the importance of location of the transfer equipment and the options for mounting the transfer equipment, which for smaller units may be wall-mounted and for larger units will be free-standing. Codes or standards may influence the choice of location of the unit as will the point of electrical interconnection in providing separation and avoiding excessive cable route length. Types of enclosure and the degree of protection offered are also mentioned.

Chapter 7 – Inspection, Testing and Maintenance describes the importance of maintenance and testing. Particularly where a reliable utility source is available, the transfer equipment may operate infrequently, leading to a false sense of security. The chapter details the importance of regular weekly inspections and monthly testing to ensure that, when the equipment is required in a true emergency, it is able to perform. More detailed thermographic testing on an annual basis is required to ensure that contact pressure does not deteriorate over time.

Chapter 8 – Appendices contains a bibliography and typical system layouts for basic and redundant standby, automatic priority selection, sequential prime power and the use a dual utility supplies in a duty / standby arrangement with generator backup.

For information regarding your application contact your nearest distributor.

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