You've hit on a common point of confusion and potential hazard when using portable generators with transfer switches and bonded neutrals. Let's break it down

Understanding the Problem: Double Bonding
Utility Power

Under normal utility power, the neutral and ground are bonded only at the service entrance (the main panel). This single point of bonding is crucial for proper operation of overcurrent devices (breakers) and GFCI protection. If the neutral and ground are bonded at multiple points, it can create parallel paths for neutral current, leading to nuisance tripping of GFCIs and potentially dangerous circulating currents on the grounding system.

Generator with Bonded Neutral

Many portable generators have the neutral bonded to the frame. This is fine when the generator is used as a standalone power source, powering loads directly from its receptacles. The bond provides a low-impedance path for fault current back to the generator, allowing overcurrent devices to trip and protecting people from shock.

Transfer Switch Complication

When you connect a generator with a bonded neutral to a transfer switch (with a bonded neutral at the main panel), you create a second neutral-ground bond. This is the "double bonding" that causes problems.

Why Double Bonding is Bad Nuisance GFCI Tripping

As you correctly surmised, the biggest issue is nuisance tripping of GFCIs. When the generator is running and connected to the transfer switch, some of the neutral current from the generator-fed circuits will flow back through the grounding conductor to the main panel neutral-ground bond, and back to the generator through its neutral-ground bond. This current imbalance is detected by GFCIs, causing them to trip even without a true ground fault.

Circulating Currents

The redundant bonds create parallel paths for neutral current. These circulating currents can flow through the grounding conductors, potentially energizing metal enclosures and creating a shock hazard.

Compromised Safety

While potentially not severe, any circulating current could, in theory, mask a developing ground fault. You also risk damaging equipment.

Your Scenario: GFI Outlets on the Generator

You're right to be concerned about the generator's GFCI outlets while the generator is connected to the ProTran2. Here's the breakdown:1. Unbonding the Generator

The correct procedure is to unbond the neutral at the generator when connecting it to a transfer switch with a bonded neutral. Follow the generator manufacturer's instructions carefully. Usually, this involves removing a bonding strap or wire. Make absolutely sure you know what you are doing; improper unbonding is dangerous.

Verify unbonding with a multimeter

Check continuity between the generator's neutral (the wider slot on the 120V receptacles) and the generator frame/grounding terminal. There should be no continuity after unbonding.2.

How it works with UNBONDED Neutral

If there is a ground fault while using the generator receptacles to power something, the GFCI will trip. The current goes: Hot (from the generator) to Fault, to Ground, back to the main panel's Grounding Electrode Conductor, and back to the generator's Neutral (but only through the bonding conductor at the service panel). Remember, the fault current will not flow from the generator's ground and directly back to its neutral because the neutral is unbonded. The fault current needs a path to the neutral, which will only occur through the main panel.

Addressing the Customer's Usage Case

With the generator neutral unbonded, the GFCIs on the generator will function properly when used simultaneously with the transfer switch-connected circuits. The GFCI will detect any current imbalance caused by a ground fault in the cord or equipment plugged into the generator's receptacles and trip, protecting the user.

Important Considerations & Recommendations Consult the Manuals

Read both the transfer switch and generator manuals thoroughly. Pay special attention to the sections on grounding and bonding.

Safety First

If you're not completely comfortable with electrical work, hire a qualified electrician. Mistakes can be dangerous or deadly.

Documentation

Clearly label the generator (near the neutral bonding location) stating: "NEUTRAL MUST BE UNBONDED FOR TRANSFER SWITCH USE"

Explain to the Customer

Explain the whole process to the customer. Explain the importance of unbonding the generator neutral when using the transfer switch and the purpose of the GFCI outlets on the generator. Explain that if there is a fault, they will be protected because the GFCI will trip.*

Confirm it's Legal: Local and national electric codes might have additional requirements. Consult with your local authority if necessary.By correctly unbonding the generator and understanding the principles of grounding and bonding, you can ensure a safe and functional installation for your customer. Flag for review