Hi AndyDigital
You may be right.
That was my view also initially, at least for the first few dozen times
However it got worse, much worse and I changed the hose for a normal non-heated one and it stopped immediately despite using exactly the same everything else.
I was able to use the mask and plain hose for several weeks without being bitten.
Then after a few months the plain hose began biting too so I concluded it was actually being caused by the humidifier chamber or the machine itself and must have gotten worse in order to transmit through a hose without wiring.
At this point I became concerned not just about comfort and pain but about actual safety. I find the Cpap unpleasant enough as it is without being reminded repeatedly that it is trying to electrocute me or maybe just burn the house down.
So I changed machines and continued to use exactly the same hose and mask with a F&P Icon and have not been bitten since.
Admittedly it has only been a month or two and Murphy might have been busy elsewhere but I haven't had a tingle yet.
I will take the old Resmed 9 back to the supplier sometime in the next few weeks and I expect that they will replace it.
I have taken it back to them once already (some years ago) because the humidifier was playing up and shorting out the machine. On that occasion they just tinkered with it, replaced the hose and tank, and sent it back to me.