Okay, this is a tricky situation, and you're right to be concerned. Let's break down the issues and potential causes, then look at solutions.

Understanding the Problem



208V to 230V Isolation Transformer

The core function of an isolation transformer is to isolate the secondary circuit from the primary circuit. This means no direct conductive path between them. You correctly chose a transformer with a 208V primary and a 230V secondary.

Unexpected Voltage and Current

Your readings of 230V Line-to-Neutral on the secondary are correct. The 120V Neutral-to-Ground before bonding on the secondary is a problem and indicates a leakage or imbalance somewhere. The 3.5A on the grounding electrode conductor after bonding is also a serious concern. Isolation transformers are supposed to isolate.

Why You're Seeing Voltage Neutral to Ground Before Bonding

This typically points to one or more of these possibilities:1.

Capacitive Coupling

Transformers, even isolation transformers, have capacitance between windings. This can cause a small voltage to appear on the secondary, referenced to ground. This is usually very low current and not dangerous on its own. It's usually in the range of single digit volts, not 120.2.

Leakage Current

A small amount of current might be leaking through the transformer's insulation (damaged or old insulation). Again, this would usually cause small voltages.3.

Ground Loop Issue

The issue here is more likely to be a ground loop issue, where the neutral has a return path that is the grounding system. This is especially dangerous.4.

Transformer Fault

There could be a fault inside the transformer itself. A winding could be shorting to the core, which is then connected to ground through the transformer's enclosure. This is less likely but needs to be ruled out.

Why 3.5A on the Grounding Electrode Conductor After Bonding?

This is the biggest red flag. This means neutral current is flowing through the ground path. This should never happen in a properly wired system.

Why You Shouldn't Just Bond Neutral and Ground on the Secondary (Without Investigation)

While bonding the neutral to ground seems to "fix" the voltage issue, it creates a dangerous situation where neutral current can flow through the grounding system. This defeats the purpose of having separate neutral and ground conductors. It can create shock hazards, and it can interfere with sensitive equipment. Also, 3.5 amps is not a "normal" amount of current to see on a grounding electrode conductor when the neutral and ground are bonded.

Troubleshooting Steps (in order of likelihood and safety)


Disconnect the Isolation Transformer Entirely

Completely disconnect the isolation transformer (both primary and secondary). Measure voltage from the neutral conductor of the 208V source to ground at the panel. This should be very low (ideally 0V, but a few volts is usually acceptable). If this is high, you have a problem in your building's wiring before the transformer. You'll need an electrician to investigate the 208V service and panel. Check the main bonding jumper at the main service panel to make sure it is present and tight.2.

Transformer Testing (Disconnect from Everything)



Insulation Resistance Test (Megger Test)

This is crucial. You need to test the insulation resistance between: Primary windings and core/ground. Secondary windings and core/ground. Primary windings and secondary windings.A low resistance reading indicates a breakdown in insulation. The values should be extremely high (hundreds of megohms or higher). A qualified electrician with a megohmmeter should perform this test.

Turns Ratio Test

Verify the transformer's turns ratio is correct and matches the nameplate. A specialized meter is required.3.

Inspect Wiring

Carefully inspect all wiring connected to the transformer on both the primary and secondary sides. Look for loose connections, damaged insulation, or incorrect wiring. Use a wiring diagram. Make sure the primary is properly grounded to the building's grounding electrode system.4.

Re-connect Transformer (Primary Only)

Connect the primary side of the transformer only, leaving the secondary disconnected. Measure voltage from each secondary terminal (L and N) to ground. These should be very low (a few volts due to capacitive coupling). If you see significantly higher voltage, or voltage that fluctuates, the transformer is likely faulty.5.

Re-connect Secondary (No Bonding Yet)

Now connect the secondary side. Measure voltage from secondary neutral to ground. Again, it should be very low. If it's high (like 120V), there's still a problem. Don't bond the neutral and ground at this point. If it's still high, disconnect everything connected to the secondary, including the RF equipment. Then re-measure the secondary neutral to ground voltage. If it's still high, the problem is in the transformer or the secondary wiring. If it's low now, the problem is in the RF equipment or the wiring to it.6.

RF Equipment Inspection

If the above tests point to the RF equipment as the source of the problem, carefully inspect its wiring and internal components. It's possible there's a fault inside the equipment causing the neutral to be elevated relative to ground.

Important Considerations and Safety



Qualified Electrician

This troubleshooting must be done by a licensed and experienced electrician who understands isolation transformers, grounding, and bonding. This is not a DIY project.

Lockout/Tagout

Always use proper lockout/tagout procedures when working on electrical equipment.

Personal Protective Equipment (PPE)

Wear appropriate PPE, including safety glasses, insulated gloves, and arc-rated clothing.

Transformer Disposal

If the transformer is found to be faulty, do not attempt to repair it yourself. Have it disposed of properly.

Addressing Your Concerns About the Filter

You are absolutely right to be concerned about the filter. Filters designed for line and neutral operation rely on a clean neutral reference. If the neutral has significant voltage on it, it can: Cause the filter to malfunction. Overheat components within the filter. Damage the filter. Potentially introduce noise into the RF system.

Bottom Line:

You have a significant problem that needs to be diagnosed and corrected by a qualified electrician. Do not operate the RF equipment with the neutral bonded to ground on the secondary of the transformer without resolving the underlying issue. The 3.5A on the grounding electrode conductor is a clear indication of a dangerous ground fault. Prioritize safety and thorough investigation. Flag for review