Hi RICK,
Yeah, I know that and my response must have come across as snotty and offhand, as it usually does when I'm preoccupied with getting my point across - that's something I need to work on so I apologise for that if it's caused offence. What you write sounds reasonable - my view is that under certain circumstances it is actually possible to obtain current paths or loops that are unexpected due to the arrangements of two neutral plates during the simultaneous operation of two electrosurgery units, even if they float independently. Effectively leakage between units, if you like.
Unexpected currents flowing between the neutral electrodes seems plausible to me since we accept that capacitive paths and inductive effects cause leakage currents around the patient all the time i.e. HF leakage current can flow through unexpected paths and is a potential source of burns (which is why I'm pushing this argument so much). Via an ECG electrode for example, if it's placed near the surgical field in the path of HF current between active and neutral (it shouldn't be as far as I'm aware, even if the monitor has electrosurgery filtering, protection and is isolated appropriately). This is because both electrosurgery unit and ECG monitor may share a common reference at HF due to capacitive leakage and inductive coupling effects.
So I guess what I'm saying is that you can have leakage currents flowing between two dual electrosurgery units, which may also be referenced to an alternative leakage path (this complicates the issue again - earth perhaps?) even though they're individually 'floating', and that differences in potential between the neutral plates could cause currents to flow between them. Call it a leakage path, whatever, it's still a possible source of current that could heat tissue under the wrong circumstances. These currents are due to two generators being referenced together in some way (via patient tissue) and possibly to some other leakage path(s) probably via capacitive coupling. Considering that two plates in contact with the patient and so are the actives when operated simultaneously and in contact with tissue. The frequency, phase and magnitude of the current is another issue and will be dependent upon the complex interaction between generators outputs if they're not in-phase with each other.
Because it's practically impossible to totally eradicate HF leakage paths (or prevent radiated EM fields associated with electrosurgery) - even with floating units - then users still must consider the ideal placement of the neutral plates and leads, relative to each other, the patient's bare skin (to help prevent damaged leads or capacitive effects causing burns), prevent them from draping over any earthed items or the patient for example, to limit capacitive leakage currents that could cause burns at a localised contact point where there's a possibility of a high current density path. With return current monitoring and split plate technologies the problems are not as great as they used to be; as far as I'm aware it's the older units out there that can cause problems since they may not have the necessary safety features built-in.
