Application Engineering

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

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Transfer of load between two sources requires the action to be completed without undue disturbance.

The chapter describes the how programmed-transition transfer switches are used to avoid current surges caused by out-of-phase connection. Switch controls may allow either a definite break in supply, or provide in-phase detection or closed transition.

Methods of neutral grounding are reviewed as this governs the type of transfer switch fitted. If the neutral pole requires to be switched, a fourth pole can be provided within the transfer switch.

The chapter ends with a detailed analysis of short circuit and surge withstand capability.

Requirements and Recommendations:

• The voltage at the terminals of motors and other inductive loads does not immediately return to zero when disconnected from a power source. Connection of a motor to a source before the voltage has decayed to a safe level can cause nuisance tripping of breakers and damage to the motor.
• Programmed transition transfer switches avoid this phenomenon by increasing the operating time of the transfer switch to allow the motor-generated voltage to decay before being connected to the oncoming source.
• Some transfer switches are provided with source synchronizing equipment which can be effective in addressing the problem in limited situations.
• Closed transition will prevent damage to motor loads and nuisance tripping of breakers.
• Closed transition transfer switches with fast operating times may still impose a heavy load step on the generator set, which can result in severe frequency and voltage dips and long recovery times.
• Solid state switches provide very rapid transfer of supply, but can only be used where supplies are in-phase.
• The method of system grounding must be verified and the appropriate design of transfer switch (1 or 2 pole, 3 or 4 pole) must be provided.
• In some regions, transfer switches used in conjunction with fire pump installations must be specifically listed for fire pump service.
• Bypass isolation provides a safe and effective means of allowing maintenance to the transfer switch while maintaining supply to the load.
• Closed transition transfer requires utility approval prior to installation.
• Where fast operating closed transition transfer cannot be employed due to step loading on the generator, extended parallel operation transfer switches will allow load to be ramped to the generator.
• With the exception of transfer switches constructed from correctly rated circuit breakers, all transfer switches will require upstream short circuit protection using either circuit breakers or fuses.

For information regarding your application contact your nearest distributor.

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