You're right, grounded B-phase delta systems are mind-bending, and it's crucial to get the grounding right. Let's break down your situation step-by-step

Understanding Grounded B-Phase Delta
Why it Exists

Grounded B-phase delta is less common than wye systems, but it's used in situations where there's a need for a high single-phase voltage (240V in your case) and a lower voltage three-phase (typically 240V, though sometimes 480V). The midpoint of one of the windings is grounded.

Key Difference

In a typical wye system, the neutral is derived from the center tap of the wye and is bonded to ground at the service. In a grounded B-phase delta, one phase conductor (B) is grounded.

Your Generator as a Separately Derived System (SDS) Correct Assumption

You're absolutely correct in treating the generator as a Separately Derived System (SDS) per the NEC (National Electrical Code), specifically Article 250.

Reasoning

The key is that the generator's source of power is electrically isolated from the utility source when operating. The transfer switch physically disconnects the utility.

Grounding Electrode System (GES) for the Generator Grounding Electrode is Required

Yes, Article 250 requires a grounding electrode system (GES) for the generator. This is to provide a low-impedance path to ground for fault currents.

Acceptable Grounding Electrodes

250.52 outlines acceptable grounding electrodes. A ground rod is commonly used, but other options like a concrete-encased electrode or a ground ring are also valid.

Grounding Electrode Conductor (GEC)

You need a separate grounding electrode conductor (GEC) to connect the grounding electrode to the generator.

Connecting the B-Phase and the GEC: This is Critical Key Concept

In a grounded B-phase delta SDS, you will connect B-phase to the generator equipment grounding system and earth ground.

Your Questions Answered



Can the conductor that grounds B-phase also serve as the GEC? NO. This is a violation. The NEC requires a dedicated GEC, sized according to Table 250.66. The purpose of the GEC is to connect the SDS to earth ground independently of the grounded B-phase conductor. How does the bonding work in the transfer switch? The transfer switch should not switch the grounded B-phase. It needs to maintain a continuous, solid connection between the B-phase conductor of the generator and the B-phase conductor of the utility system (on the load side of the transfer switch). It also should not switch the equipment grounding conductor. Separation is Key

You are correct. The GEC must be separate from the conductor grounding the B-phase. The GEC must originate at the grounding electrode and run directly to the generator.

Frame Bonding

Yes, absolutely bond the generator frame to the grounding electrode system. This can be done using a bonding jumper connected to the grounding electrode conductor (GEC).

Where to Connect the GEC at the Generator

The GEC should terminate at a point on the generator where the B-phase is grounded and the equipment grounding conductors are bonded. This is usually a grounding busbar within the generator.

At the Service Disconnect (Utility Side) No B-Phase Bonding

You are correct. In a grounded B-phase delta system, the B-phase is not bonded to ground at the service disconnect. The B-phase is grounded only at the transformer and at the generator (when operating as an SDS).

Electrode Separation Minimum Distance is Not Required But Recommended

There is no specific distance, but there is the 6ft rule for additional rods. Installing additional rods or other electrodes helps to lower overall resistance.

Why Closer is Often Better (Practically)

The primary reason to keep them relatively close is to minimize voltage differences during fault conditions. The further apart the electrodes are, the greater the potential difference between them, which can increase the risk of touch potential hazards.

Supplemental Ground Rod

You can drive a second ground rod 6ft apart from the first ground rod. No need to test them if you add a second rod.

Summary of the Installation1. GEC

Run a dedicated grounding electrode conductor (GEC) from the grounding electrode (ground rod) to the generator. Size it per NEC Table 250.66, based on the size of the generator's ungrounded conductors.
2.

Bonding at the Generator

At the generator, connect the GEC to the grounding busbar where the B-phase is grounded.3.

Bonding the Frame

Connect a bonding jumper from the generator frame to the grounding busbar (same point as the GEC).4.

Continuous B-Phase

Ensure the grounded B-phase conductor is not switched by the transfer switch. It must remain continuously connected between the utility and the generator.5.

Equipment Grounding Conductors

Ensure all equipment grounding conductors (EGCs) are properly connected throughout the system, back to the generator and/or the utility service. These EGCs provide a low-impedance fault current path.

Important Considerations Overcurrent Protection

Ensure that the overcurrent protection (circuit breakers or fuses) are properly sized for the generator and the connected load.

Transfer Switch Rating

The transfer switch must be rated for the voltage, current, and interrupting capacity of the system.

Local Codes

Always consult with your local electrical inspector to ensure compliance with local codes and regulations. They may have specific requirements for generator installations in grounded B-phase delta systems.*

Qualified Electrician

This type of installation is complex and potentially dangerous. It's highly recommended to use a qualified and licensed electrician with experience in grounded B-phase delta systems and generator installations.

In conclusion, you're right to be cautious and thorough. Grounded B-phase delta systems require careful attention to detail, especially when adding a generator as an SDS. By following the NEC guidelines and working with a qualified electrician, you can ensure a safe and reliable installation. Flag for review