Means of Protection For Heaters

[BuckeyeBurro]

Hello, my company makes a device that uses AC cartridge heaters to heat the water being delivered to an applied part. Due to the water's conductive nature, the heaters are considered to be in direct electrical contact with the applied part. The entire circuit order is as follows:

AC mains -> AC Filter -> Ceramic Heater Cartridge With Outer Metal Shell -> Water -> Electrically Conductive Applied Part (Type BF).

While we have tested our device to IEC 60601-1 ed 3.2, there is a clause that indicates we still do not have 2xMOPP from mains to the applied part. Specifically, clause 8.8.4.2, which includes "Insulating material in which heating conductors are embedded may be considered as one MEANS OF PROTECTION but shall not be used as two MEANS OF PROTECTION." The way I interpret this is that the ceramic insulation the heater core is embedded in can only serve as 1 MOPP, and therefore, we need another MOPP. Am I understanding this correctly?

If I am, could a simple way to address this be to replace our AC filter with one containing Y capacitors? My understanding is that this would only provide MOP with respect to mains and ground, but not mains to the Applied Part.

If Y capacitors are not our solution, I worry we need to look at a more complex solution, like a custom heater cartridge with an extra insulative material (e.g., encase the outside in silicone).

 

[Peter Selvey]

Normally in this case the metal shell would be connected to protective earth to form the second MOPP. This assumes you have an earth, it's not a double insulated (Class II design).

 

[BuckeyeBurro]

Normally in this case the metal shell would be connected to protective earth to form the second MOPP. This assumes you have an earth, it's not a double insulated (Class II design).

I meant to clarify that the circuit is treated as a Type BF applied part, rather than Type B. The actual accessory that interfaces our device to the patient is metal and blood-contacting, so we settled on BF instead of B. I believe grounding the metal shell would violate the floating requirement. Our device is Class I/has PE.

I have updated the original post to specify this.

 

[Peter Selvey]

Sometimes it's not practical to have Type BF. The intention of F type applied parts are floating from everything (not just earth) which is fine if it's a little sensor attached to the body that can be insulated/isolated, but water circuit, large metal parts (?) could be stretching it to be plausibly "isolated from everything else". Dialysis machines which have heaters, solenoid valves, sensors, dialysate etc face the same problem and are often Type B.

F-Type insulation addresses the theoretical risk that the patient is raised to mains voltage by another source (not your equipment), and the F-Type insulation ensures that at least your device at least has no path back to earth, including indirect paths, not just via the main earth. This was developed in 1970s when insulation was a bit more dodgy and some common long term monitoring devices like ECGs could actually solidly ground the patient. In the modern world, insulation quality is much better and F-Type insulation is more of a sales point than a real risk. It does help however for noise reduction, defibrillator protection, ESU isolation so there's no real backlash against the requirement.

The risk from having an unearthed metal enclosed ceramic heater in contact with an accessible liquid circuit is real and orders of magnitude higher than the theoretical risks on which F-type insulation is based on. If the metal case a mains heater was left floating, and someone got hurt, it could be reasonable grounds for negligence. Anything to do with mains voltage and water really needs a lot of care. Maybe consider a 24V secondary powered heater, not mains.

 

[BuckeyeBurro]

Sometimes it's not practical to have Type BF. The intention of F type applied parts are floating from everything (not just earth) which is fine if it's a little sensor attached to the body that can be insulated/isolated, but water circuit, large metal parts (?) could be stretching it to be plausibly "isolated from everything else". Dialysis machines which have heaters, solenoid valves, sensors, dialysate etc face the same problem and are often Type B.

F-Type insulation addresses the theoretical risk that the patient is raised to mains voltage by another source (not your equipment), and the F-Type insulation ensures that at least your device at least has no path back to earth, including indirect paths, not just via the main earth. This was developed in 1970s when insulation was a bit more dodgy and some common long term monitoring devices like ECGs could actually solidly ground the patient. In the modern world, insulation quality is much better and F-Type insulation is more of a sales point than a real risk. It does help however for noise reduction, defibrillator protection, ESU isolation so there's no real backlash against the requirement.

The risk from having an unearthed metal enclosed ceramic heater in contact with an accessible liquid circuit is real and orders of magnitude higher than the theoretical risks on which F-type insulation is based on. If the metal case a mains heater was left floating, and someone got hurt, it could be reasonable grounds for negligence. Anything to do with mains voltage and water really needs a lot of care. Maybe consider a 24V secondary powered heater, not mains.

This makes sense, and type B is more in line with the descriptions in the standard and what our competitors do. I am not sure why our NRTL pushed for type BF in the past, but I am sure this is something we can correct.

I appreciate the recommendations; it sounds like grounding the heater casing (and therefore the water circuit) is our best path forward. While a DC heater would reduce safety concerns, we are talking about a couple of thousand watts of heating, which would require excessively large PSUs.