Exceeding the DC Leakage Current Limit

Jaydub

Involved In Discussions
#1
We have a device that includes an enclosure at the end of a 10 foot cable. The earth impedance, with the 10 foot cable is less than .1 ohm and the enclosure is protectively earthed. An important customer is now requesting a 30 ft cable. The 30 foot cable increases the earth impedance above .1 ohms. Clause 8.6.4b allows impedance above .1 ohms, if allowable leakage currents are not exceeded when the relevant insulation is shorted. When I do this, I get a DC leakage current of 118 uA, exceeding the limit of 50 uA. Annex A indicates that the low limit is because prolonged current could cause tissue necrosis. In this case the patient would not be in contact with the device for more than 10 minutes if everything was working. In this SFC, the device will not function and patient would not likely remain in contact for even that long. I can't imagine that this is dangerous. Do you think I could justify through risk analysis that the DC leakage current is not hazardous?

 
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Peter Selvey

Staff member
Super Moderator
#5
There appears to be something wrong with the set up. Shorting the insulation in question should have blown the fuse. Also a dc value, even small, indicates a problem.

Anyway, if it is a non-detachable cord (hard wired at the equipment side), the limit is actually 0.2 ohms. The limit for 0.1 ohms was never intended to include the power supply cord, it is intended for internal earthing (e.g. from the equipment PE terminal to the frame).
 

Jaydub

Involved In Discussions
#6
Peter,

Thank you for the reply. Apparently I didn't explain the situation very well. What we have is a remote, DC powered module, connected by a cable to a main unit which is mains powered. The cable between the main unit and the remote module is what the customer wants to be 30 feet. The remote module is powered by 15 VDC, through the cable. The 15 Volt supply is fused and if it is shorted, by 0 ohms, to the enclosure the fuse will blow. I assumed that the worst case would be if a fault developed that would cause the maximum current that might not blow the fuse. For the test, I shorted the 15 Volt supply at the remote module and replaced the fuse with a resistor to draw 2.1 times the fuse rating. The DC leakage current, form the remote module to the protective earth terminal (at the appliance inlet) in the main unit is then >50uA.
 
Last edited:

Peter Selvey

Staff member
Super Moderator
#7
I guess the controller is used by the patient?

The 50uA limit is strictly for applied parts, which means the part the must contact the patient for the function (not parts that could touch, must touch).

If it is an applied part then a higher limit could be justified in risk management. The 50uA limit assumes a small contact area (1mm2), there is a lot of literature including modified limits in other standards which indicates that higher values are OK with a large contact area expected from a controller.
 

Jaydub

Involved In Discussions
#8
Thank you Peter. I will look into the literature and other standards you mentioned. In an internet search the only thing I found, on the risk from exposure to low value DC currents, was a study on what happens if a battery is swallowed and becomes lodged in the digestive tract.
 

Peter Selvey

Staff member
Super Moderator
#9
One reference is the appendix of the standard, also noting they refer to long term application (presumably, the controller is not held continuously).

Another reference is IEC 60601-2-10, which allows very high dc levels for use in stimulation partly due to the low current density (gel electrodes, typically around 500mm2 in area, so the current is spread), and partly due to a warning to move the treatment site in long application.
 
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