Fender Bassman Compact trips GFCI Breaker...sometimes

[Jon Snell]

I'll wager that a class Y in that position will still trip the GFI.

That is why they are no longer used with a three core power lead. They can conduct in miliamps and added to everything else connected on your mains supply, trip the RCD/GFI unit. Always remove them when servicing and fitting a three core power lead.

[tdlunsfo]

Can we get clarification as to whether the breaker was AFCI of GFCI? I saw both terms used in this thread and the terms often get comingled.

The breaker is a GE THQL 1120GFT HACR. Appears to be a "Ground Fault Breaker with Self-Test", so GFCI.

[g1]

That is why they are no longer used with a three core power lead.

Not sure what exactly you mean. There is lots of modern equipment with caps across the line and from line to ground. With 3prong or 2prong AC cords.

[Jon Snell]

If an isolation transformer were used there will be no path to ground and no earth leak so the trip will not trip.

[Jon Snell]

Not sure what exactly you mean. There is lots of modern equipment with caps across the line and from line to ground. With 3prong or 2prong AC cords.

If you look at RFI filters; 100n class X from Live to Neutral and both 22n class Y from Live to Earth and 22n class Y from Neutral to Earth. They are balanced and usually contain a common mode choke.
Most domestic appliances, washing machines etc, have these fitted to stop RFI from the electronics reaching the mains.
The old Fenders and Ampegs has a 100n paper capacitor to conduct the 'hum' from either live or neutral so unbalanced.
I have never seen a modern day item with just one capacitor between live and earth.

[Greg Robinson]

If you look at RFI filters; 100n class X from Live to Neutral and both 22n class Y from Live to Earth and 22n class Y from Neutral to Earth. They are balanced and usually contain a common mode choke.
Most domestic appliances, washing machines etc, have these fitted to stop RFI from the electronics reaching the mains.
The old Fenders and Ampegs has a 100n paper capacitor to conduct the 'hum' from either live or neutral so unbalanced.
I have never seen a modern day item with just one capacitor between live and earth.

There is leakage current through the class Y live-to-earth primary-side cap. The neutral to earth cap is also present in those filters in case of a non-polarised plug/outlet or neutral/earth bonding fault.
Note that there are maximum allowable leakage currents depending on the class of appliance, and this varies significantly in different jurisdictions. A common max for medical appliances for instance is 500uA, 3.5mA is another common standard for consumer appliances - don't take these as given, check local laws.
This places an upper limit on the value of these class Y caps depending on mains frequency and voltage. Too large and their reactance will be too low, allowing excessive earth leakage. It's also best practice to keep them as low a value as possible while still complying with EMC standards, or omit them entirely if possible to avoid nuisance tripping and to keep potential differences between neutral and earth to a minimum.