[morzh]

220nF is probably fine. At RF frequencies 10-20nF is plenty.
It is connected, usually, GND to chassis and central via a cap to your point of destination. Sometimes the original documentation of a DUT asks to leave the local circuit (like a resistor from the grid cap to GND) in place; sometimes they ask to remove it.

There are many culprits: leakage in your IsoTap and such, but if your radio is AC/DC with hot chassis (or chassis chopped via a 0.1cap from the outlet's pin it is connected to), you could have, especially with a large cap, like 220nF (this is in fact huge) a significant current imbalance in your GFI and it will get tripped.
There are various ways for the current to go.
Ideally it should not, but then I do not know your equipment and the integrity of it.

A smaller cap of 10nF (this is what I often use) might (I am not saying "will") help, but you should investigate nevertheless as the leakage is never welcome.

One of the leakage paths is a leaky tube you are trying to work with, or ... some RF coild connected from your grid to chassis and then via that 0.1uF to the outlet.....not sure.

And do check your IsoTap for isolation.