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Generator set(s) connections to LV system

Many different system designs are possible, but for highest reliability, systems are typically configured so that generator set(s) are connected at low voltage, with the minimum number of transformers and circuit breakers between the generator set and load to be served.

4 typical designs for connecting generator set(s) to the low voltage system
4 typical designs for connecting generator set(s) to the low voltage system (photo credit: Cummins)

Local laws often require that emergency loads are electrically separated from non–emergency loads, and given preference in service so that overloads will result in the non–emergency loads are shed, because this provides the greatest reliability of service to the most critical loads in the system.

In most cases a neutral conductor will be required. Since many loads and their controls at low voltage will be single–phase, requiring a return conductor. Careful consideration must be given to the need for system neutral grounding and neutral switching requirements. This design might also be used for a small prime power application.

Let’s see four most common designs for connecting generator set(s) to the low voltage systems:

  1. Generator set serving common loads
  2. Multiple generator sets serving common loads
  3. Single generator set standby applications
  4. Multiple generator sets, multiple ATS applications

1. Generator set serving common loads

Generator sets are commonly provided with a main circuit breaker that is mounted on the generator set and service to loads is provided through a separate distribution panel as shown in Figure 1.

Generator Set Serving Common Loads
Figure 1 – Generator Set Serving Common Loads

Generators are required to be provided with overcurrent protection, and that can be provided in many forms, which include a breaker mounted in the distribution panel, as shown in Figure 1.

Overcurrent protection is generally required for generator sets, but short circuit protection is not. For instance, there is not required to be protection for a short circuit between the genset and the main circuit breaker.

The significance of this is that the protection may be located at the generator set or in a remote panel. If the generator set circuit breaker is omitted, a disconnect switch may still be required by code at the generator set, to provide a point of isolation. Refer to local codes and standards for requirements for generator disconnects or isolation.

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2. Multiple generator sets serving common loads

Figure 2 shows a similar application with paralleling generators replacing the single generator set. In this situation the generator sets may be specifically selected to be of multiple sizes to allow for minimizing the fuel consumption at a site by closely matching the capacity of the operating equipment to the system loads.

Use of dissimilar–sized generator sets may require specific system grounding (earthing) arrangements.

Multiple Generator Sets Serving Common Loads
Figure 2 – Multiple Generator Sets Serving Common Loads

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3. Single generator set standby applications

Figure 3 represents a typical single set power transfer scheme for one utility (mains) supply at low voltage, as may be applied to many domestic, commercial and small industrial applications.

An automatic transfer switch (ATS), which may use contactors, circuit breakers or a dedicate transfer module, is used to transfer the electrical supply to the load from utility to generator.

Single Generator Set Standby Applications
Figure 3 – Single Generator Set Standby Applications

Three–pole generator and utility circuit breakers or fuse–switches are often used to limit the fault level present at the ATS. The ATS may be a 3–pole (solid, non–switched neutral) or 4–pole (switched neutral) device. Typically, 4–pole ATS equipment is used in applications where it is necessary to isolate the supply neutral from the generator neutral.

The selection of switched neutral equipment may be related to either safety considerations, or if the system is required to incorporate ground fault detection devices. The utility service provider should be consulted to confirm the type of grounding (earthing) system used in the utility distribution system feeding a site, and verify that the proposed grounding arrangements at a customer site are appropriate.

Power transfer switches and generator sets should not be connected to a utility service prior to this review (and utility approval, if required by local law).

Note that some local codes and standards require the use of multiple transfer switches due to requirements to isolate emergency loads from standby loads.

In these cases, the transfer switches may be located on the load side of the utility distribution panel, and the generator set may also need a distribution panel when the feeder breakers for the ATS equipment cannot be mounted on the generator set.

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4. Multiple generator sets, multiple ATS applications

Larger systems can utilize multiple ATS units and protection located close to the loads. These are often considered to be more reliable than those employing a single large ATS, because faults in the distribution system are more likely to occur toward the load end of a distribution system and the use of multiple switches would result in less of the system being disrupted when a fault occurs.

Figure 4 illustrates a design suited to larger installations, particularly where multiple buildings are served by the same generator installation.

Multiple Generator Sets, Multiple ATS Applications
Figure 4 – Multiple Generator Sets, Multiple ATS Applications

In this system, three ATS units are used, supplied by a common utility and generator system. This scheme can be further adapted to operate from separate utility systems.

Four–pole changeover devices are commonly used with three–pole generator and utility circuit breakers or fuse–switches. Each ATS has automatic utility failure sensing and will send a start signal to the generator system and will change over to the generator supply when this is within an acceptable tolerance.

This scheme enables a versatile generation system to be constructed and can readily be adapted to multiple sets.

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Cummins generator set 250kVA – Introduction and testing


Load testing a 500kVA generator

A follow-up to my August 2011 generator test video. Every six months (unless we’ve had a power failure in the mean time) we perform a load test of our main standby generator at the office. Rather than hiring a dummy load, we just use the electrical equipment in the office itself, which also gives us a chance to identify any “trigger-happy” breakers within the building.

We don’t perform a full-load test, but a combination of all the lighting, air conditioning and the IT server room gives us around 150kW to play with, almost a 40% load.


500KVA diesel generator set cold start

500KVA Cummins Diesel Generator set cold start/Backfire, been stood for a while and the temperature was -20°C. The controller shut it down after it revved up at the end.


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Reference // Electrical design of the on-site generation system by Cummins Power Generation

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About Author

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Edvard Csanyi

Edvard -

Electrical engineer, programmer and founder of EEP. Highly specialized for design of LV/MV switchgears and LV high power busbar trunking (<6300A) in power substations, commercial buildings and industry fascilities. Professional in AutoCAD programming. Present on

4 Comments


  1. Harjot Brar
    Mar 09, 2017

    In the case where 2 generators are connected to an ATS. How does NGR get connected in this case?

    Would you have 2 separate NGR boxes with their own sensing and tripping equipment?

    Thank you.


  2. AMIT KUMAR
    Feb 09, 2017

    Our mission is to be the leading provider of scientific information in the field of power and engineering in general. We publish, we share and we spread the knowledge.

    You’re welcome to read, write and contribute to EEP in any way!


  3. mustafa
    Feb 08, 2017

    download link for the reference book please

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