IEC 60601-1 - Magnesium oxide used for the electrical insulation of heating elements

Roland chung

Trusted Information Resource
#1
Hi everyone,

IEC 60601-1 allows that insulating material in which heating conductors are embedded may be considered as one MOP. Normally, magnesium oxide is used for the electrical insulation of heating elements. Should the magnesium oxide be tightly sintered? or perhaps magnesium oxide powder compressed mechanically and sealed can be accepted? Either is subject to an 8.9.3.2 test.

Best regards
Roland
 
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Peter Selvey

Staff member
Moderator
#2
Not an expert about sintering, but I used to think one reason they use powder insulation is that the element expands while heating, so it has to be able to move. That would suggest not tightly sintered is better.

For this type heater, the humidity test is one of the most important. Normally the 48hr 93% RH test is a waste of time, but in this case it's an important to test the seals. I have seen one case of a heater that had poorly prepared seals and failed the test as the moisture got in to powder insulation. That again would suggest not tightly sintered? I would guess non-sintered powder form could draw more moisture if not sealed well.

Back when I started in testing (way back in 1987!) we used to X-ray these kind of heaters to see if the element was sitting neatly inside, and not close touching the case. No one seems to do that anymore, I guess the production method could be more reliable these days, or the risks from having inexperienced engineers running expensive X-ray machines is not outweighed by the benefits!
 

Roland chung

Trusted Information Resource
#3
Not an expert about sintering, but I used to think one reason they use powder insulation is that the element expands while heating, so it has to be able to move. That would suggest not tightly sintered is better.

For this type heater, the humidity test is one of the most important. Normally the 48hr 93% RH test is a waste of time, but in this case it's an important to test the seals. I have seen one case of a heater that had poorly prepared seals and failed the test as the moisture got in to powder insulation. That again would suggest not tightly sintered? I would guess non-sintered powder form could draw more moisture if not sealed well.

Back when I started in testing (way back in 1987!) we used to X-ray these kind of heaters to see if the element was sitting neatly inside, and not close touching the case. No one seems to do that anymore, I guess the production method could be more reliable these days, or the risks from having inexperienced engineers running expensive X-ray machines is not outweighed by the benefits!
Hi Peter,

Thank you very much for you reply. What do you think about the distances (creepage and clearance between heating elements and sheath) inside? For small heater, the cr/cl cannot meet 4.0/2.5mm. If treated MgO as insulating compound, after the thermal cycling and humidity preconditioning treatment as stated in 8.9.3.2, the difficulty lies in the inspection (cracks and voids) after the Hi-pot test. It looks like we need an X-ray here.

Best regards
Roland
 
Last edited:

Peter Selvey

Staff member
Moderator
#4
For a sealed system, creepage distances are not in theory required, since they are derived on assumption of pollution (dust etc.) falling onto the surface which is then subject to repeated cycles of heating/cooling, moisture and so on, eventually leading to a tracking and breakdown along the surface. The distances such as 4mm are intended to ensure a low probability of the distance breaking down over the lifetime. In a sealed unit such as heater, the pollution would not enter so a high creepage distance is not required. For MOOP, this allowance should be there in the tables, but unfortunately not for MOPP.

Clearance is different, it's based on 2.5kV impulse withstand (overvoltage category II). I'd probably start off with the manufacturer's specs/drawings and see how much margin there is. Note also that testing with 1.5kVrms is actually 2.1kV peak already, it might be possible to increase that 1.8kV or use 2.5kV impulse testing to show it's OK for 2500Vp as an alternative to X-ray testing. See how that goes!
 
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