This is a tricky situation with some significant potential pitfalls. Let's break it down

Understanding the Grounded B Phase System:
In a grounded B phase system, the B phase is directly connected to ground. This means B is your reference point for voltage measurements. You measure 240V to ground from phases A and C, and 0V to ground from phase B. This type of system is relatively uncommon in modern installations but was used historically in some industrial settings.

Challenges of Using GFCI Breakers with a Grounded B Phase System


GFCI Breakers Require a Neutral

GFCI breakers work by monitoring the current flowing to a load and comparing it to the current returning. Any difference exceeding the trip threshold (typically 5mA) indicates a ground fault, causing the breaker to trip. The "neutral" connection is essential for this monitoring process.2.

Grounded B Phase Doesn't Have a Neutral

Your grounded B phase system lacks a traditional neutral conductor. This is the core problem.3.

Ground Fault Detection in Grounded Systems

In a properly functioning grounded B phase system, a ground fault should, in theory, create a short circuit condition that a standard overcurrent protection device (circuit breaker or fuse) should clear. However, the sensitivity and speed of a GFCI breaker are typically much higher.

Answering Your Questions



Question 1: Do you think the GFCI would work correctly if the neutral on the breaker was tied to this secondary neutral [of the control transformer]?Highly Unlikely and Potentially Dangerous. Here's why: Current Imbalance

Connecting the GFCI's neutral to the secondary neutral of the control transformer introduces a foreign neutral connection into the 240V single-phase circuit. The GFCI will likely see an imbalance between the current flowing in the hot conductors (e.g., A and B) and what it expects on its "neutral" (which is now tied to the control transformer's secondary). This imbalance will almost certainly cause nuisance tripping.

Voltage Differences

The secondary neutral may not be at the same potential as the grounded B phase. This difference in potential can cause unpredictable current flow and further exacerbate the nuisance tripping.

Safety Concerns

Connecting neutrals from different electrical systems together (e.g., a 240V system and a 120V system) can create unexpected return paths for current and compromise the intended grounding and bonding of the equipment. This can create a shock hazard. This is a code violation as well.

Question 2: If this did work, if the breaker was tied across A-B phase would it function properly as well if the one phase is grounded?No. It is even less likely to work correctly, and even more likely to be unsafe. Grounded Phase Interference

The grounded B phase will almost certainly cause the GFCI breaker to trip immediately or very frequently. The GFCI is designed to detect any current returning through ground as a fault. With one phase directly connected to ground, any normal current flow would be interpreted as a ground fault.

Neutral Misinterpretation

The GFCI expects its "neutral" to carry the return current from the load. With a grounded phase and a neutral connected to a separate transformer's secondary, the GFCI will be completely confused.

Why It Won't Work and Potential Solutions

The fundamental problem is that GFCI breakers are designed for systems with a dedicated neutral conductor, not for grounded phase systems. They work by detecting small imbalances between the hot and neutral conductors. In a grounded phase system, the grounded phase effectively becomes the "neutral," but it is not a neutral in the way a GFCI breaker expects.

Recommendations (and Why They're Difficult)


Best (But Expensive and Complex): Convert to a Modern Three-Phase System

The most reliable and safest long-term solution is to modernize the electrical system to a standard three-phase system with a properly grounded neutral. This is a major undertaking, requiring new transformers, wiring, and potentially equipment modifications. It is often the most cost prohibitive.2.

Isolation Transformer for Each Receptacle (Potentially)

You could install a dedicated isolation transformer for each 240V receptacle you want to protect with a GFCI. The isolation transformer would take the 240V A-B, B-C, or A-C input and create a floating (ungrounded) 240V output. Then, on the secondary side of the isolation transformer, you would create a derived neutral. You would need to follow all applicable electrical codes regarding grounding and bonding of this derived neutral system. Finally, you could use a standard 2-pole GFCI breaker on the secondary side of the isolation transformer to protect the receptacle.

Important Considerations



Transformer Size

The isolation transformer must be sized appropriately for the load connected to the receptacle.

Grounding/Bonding

Proper grounding and bonding of the transformer enclosure and the derived neutral system are critical for safety. Consult a qualified electrician to ensure compliance with all applicable codes.

Cost

Isolation transformers can be expensive, and the cost would be multiplied by the number of receptacles you want to protect.3.

Don't Use GFCI Breakers in this way

In this situation, because you are dealing with machinery that would immediately trip if you had a small fault and you are already using a 20a breaker to protect the machines, simply making sure everything is grounded properly may be an easier approach as these machines do not need GFI protection per code.

Disclaimer: This information is for general guidance only. You must consult with a qualified and licensed electrician to evaluate your specific situation and determine the safest and most appropriate solution.* Working with electricity can be dangerous, and improper wiring can create a fire or shock hazard. Flag for review