Okay, this is definitely a head-scratcher and potentially dangerous. Let's break down what we're seeing and what might be happening.

Observations & Breakdown



The Connector (Image 1)

This is a very unusual configuration for a 120V connector. It's designed to plug into a receptacle and provide power to two separate cords.

Important

The lack of a dedicated ground (EGC) for one of the cords is a

MAJOR SAFETY CONCERN. It's relying on a shared ground connection, which is a potential failure point. The fact that your megger shows continuity from the wires to ground, but not to each other is expected if nothing is plugged into the outlet(s) that this thing is meant to power. The Wiring (Image 2)

Hardwiring this unusual connector to the load side of a GFCI outlet is a recipe for problems. GFCIs are designed to monitor the current balance between the hot (black) and neutral (white) wires. Any imbalance (current leaking to ground) trips the GFCI. Because the breaker is tripping before the GFCI, that indicates a current short path is happening too quickly for the GFCI.

Possible Causes of the Breaker Trip



The Most Likely Culprit: Neutral-Ground Bond on the Load Side of the GFCI

This is the classic mistake. You should never have a neutral-ground bond after the GFCI. GFCI's are meant to see an imbalance between the hot and neutral, if the neutral is connected to ground on the load side, the electricity will flow in both the neutral and the ground paths back to the source. The GFCI detects this imbalance and trips.

Ground Fault on the Load Side

A ground fault is when the hot (black) wire accidentally comes into contact with a grounded object (e.g., a metal enclosure, a ground wire). This causes a high current flow directly back to the source via the ground path, tripping the breaker.

Overload

Although less likely in this scenario (since it trips immediately), it's possible the load being powered by this setup is simply drawing too much current, exceeding the breaker's capacity.

Faulty Wiring in the Load

It's possible there is something internally wrong within whatever is plugged into the outlet on this device.

Why the GFCI Isn't Tripping First (Most of the Time)



Trip Curve Differences

Circuit breakers and GFCIs have different "trip curves." A circuit breaker will trip almost instantaneously if there's a very high overcurrent. GFCIs, on the other hand, are designed to detect small current imbalances over time (e.g., 5mA). A direct short circuit (hot-to-ground) will often trip the breaker before the GFCI has a chance to react.

Fault Current Magnitude

If the short-circuit current is very high, the breaker's instantaneous trip function will activate, bypassing the GFCI's detection mechanism.

DANGER

This setup is highly dangerous. The lack of a proper ground for one of the cord plugs is a shock hazard. The hardwiring to the load side of the GFCI with the neutral-ground issue creates a fire hazard and renders the GFCI ineffective for protecting against shocks.

Recommendations


Disconnect Everything Immediately

Turn off the breaker and completely disconnect the unusual connector from the GFCI.2.

Do Not Use the Custom Outlet

The outlet is unsafe due to the lack of a dedicated ground wire. Dispose of it.3.

Inspect the Wiring

Carefully inspect the wiring from the breaker panel to the GFCI outlet.4.

Hire a Qualified Electrician

This situation clearly demonstrates the need for a qualified electrician. They can: Properly diagnose the cause of the breaker tripping. Ensure the GFCI outlet is wired correctly (especially verifying there is no neutral-ground bond on the load side).* Ensure the wiring is safe and meets all electrical codes.

In summary, this is a dangerous situation. Do not attempt to troubleshoot this yourself. Contact a licensed electrician to resolve the problem. Flag for review