Insulation Diagram for Water Tub

[netseal]

Hello Forum,

I have got to do the insulation diagram for our medical device. Our device will be classified as 2a device.
The device is a water tub. On top of the water-surface is a latex mat, where the patient is placed on his back. Inside the tub are nozzles blowing water against the patients back. Outside of the tub is a pump for the pressure. The Water gets heated by a coil.
Pump an Heater are without separation connected to the mains (250V).

The heater coil is a rod, inside ceramic and with the resistive heating Element. The outer rod is earthed an placed in the Water. How do I explain this insulation in the Diagram.
It would be very friendly if sombody could help me a little.

Florian

 

[Peter Selvey]

It's a bit hard to answer this in a forum, it really needs a whiteboard and also would take a while to explain the basic principles. Your best bet might be to contact a local laboratory.

In principle if the motor and heater (and any controllers, EMC filters etc) are purchased as components from a reputable manufacturer that is familiar with IEC standards, and they are installed as instructed (e.g. connecting earth) then you should be OK.

Your equipment sounds like it will be Class I (earthed), and Type B (no patient isolation). In that case it's not too complicated, even though there is a patient involved.

Medical standards require 1500Vrms / 4mm creepage / 2.5mm clearance for the isolation between mains and earth, which is higher than some other IEC standards. You should confirm with your parts supplier that these requirements are met.

And of course if you are not familiar with electrical safety, get a third party lab to check.

 

[netseal]

Thanks for your answer.
I think the device is classified as Class I with BF (latex mat) (correct me if I'm wrong) and B for the chassis (steel) of a little computer (it is possible that the patient comes in contact with).

For the motor of the pump there is no problem I think, because the pump itself is made of plastic.
Now I have cutted the heatcoil. The clearance between resistive heating element and outer rod is varying widely. I think i do not have to specify it in the Insulation Diagram because it is filled with ceramic (dielectric)? Same applies to cables?
Florian

 

[Peter Selvey]

Around 25 years ago when I first started work in an Australian test lab, they had an X-ray system which they used to check heater elements for the spacing between the resistor wire and the metal case or sheath. But that was a long time ago.

These days I think most test labs will treat the internal part of the heater as solid insulation with no spacings. Usually heaters have either ceramic or powder insulation. If you are concerned, I would be looking at the manufacturer's drawings rather than a cut open sample, as cutting will destroy the internal spacings.

Also to note that the standard only allows basic insulation for heater elements (not double), so that a metal sheath must be earthed. This is reasonable because it's difficult to be confident about the internal construction, so at most only basic (1 means of protection) is allowed.

Finally, I was expecting Type B because of the water in the tub. The water will be earthed. It seems not really practical to claim BF in this case.

 

[netseal]

Many thanks,
I see now clearer. Your information is revealing.
Florian