Leakage currents from 2 separate power supplies

HelviReg

Involved In Discussions
Hello,

We are currently prototyping a medical device that must include two separate power supplies (one supplying a specific component, one for the rest of the peripherals). We intend to connect both power supplies to mains via a single power cord and a terminal. The device does not have any isolation transformer.

This issue is that, based on leakage current results of each individual power supply, there is a risk that the final device fails this test during the 'open earth' single fault condition (the sum of the two power supplies leakage currents under this SFC is very close to 500 µА).

This prototype is intended to be used for a feasibility clinical investigation only.

I was then wondering if:
- Such potential fail test could be accepted by a competent authority if we ensure that the 'open earth' SFC cannot reasonnably occur (e.g. by testing the earth before each treatment)
- Defining the device as a ME System would be of any help. Would it allow us not to sum the 2 leakage currents ? It would require changing the connection to mains I guess.

Thank you in advance for your feedback on that !
 

Avidan B

System Eng, Medical devices safety&reg. consultant
Hi
My thoughts:
1. of course a SFC of open PE can acccure, you said both P/Ss connected to a single mains cable, what if that that cable has a broken PE wire....? so AFAIK you should test the leackage current of that foreseeable situation .
2. 500uA Leakage currect for two PSs sounds a bit high.. which PSs are you using? are those medical grade PSs? usually a medical grade PS shoulf not have a LC of more than 200uA.

Avidan
 

HelviReg

Involved In Discussions
Hi Avidan,

Thank you for your feedback.

The idea was to test PE both at mains level and at the product level before treatment.

The PS with the highest LC contribution is an Artesyn IVS3 which is almost at the 500uA limit during ground open SFC (460uA). That value was derived from its IEC60601-1 report.. so "medical grade" but with high LC :/
 

Avidan B

System Eng, Medical devices safety&reg. consultant
I'm not familiar with that PS series, anyway, 460uA seems too high. what is you nominal/max power consumption for each?

The idea was to test PE both at mains level and at the product level before treatment.
I dont understand this. you maybe mean to earth and patient leakage currents?
 

HelviReg

Involved In Discussions
Up to 3000W, which is why it's that high I guess..

Regarding testing, I meant testing ground from supply mains (infrastructure failure) and testing the product PE (system failure). In other words, ensuring that open ground failure is not present before starting a treatment.
 

CharlieUK

Quite Involved in Discussions
I suspect your issue is related to having quite a high power demand.

Why do you have to have two separate AC-DC supplies in a product with a single mains cord - can you not use a DC-DC convertor in place of the 2nd one?
 

CharlieUK

Quite Involved in Discussions
Up to 3000W, which is why it's that high I guess..

Regarding testing, I meant testing ground from supply mains (infrastructure failure) and testing the product PE (system failure). In other words, ensuring that open ground failure is not present before starting a treatment.
I'm not sure you can reasonably ensure that this is actually done each time before use.
 

Avidan B

System Eng, Medical devices safety&reg. consultant
Regarding testing, I meant testing ground from supply mains (infrastructure failure) and testing the product PE (system failure). In other words, ensuring that open ground failure is not present before starting a treatment.
Usually, Full leakage current tests are done once ("Type Tests") and regularly for each device (shorter procedure), at the end of production. Assuming properly designed, why a device needs a mechanism which tests the LC before usage?
 
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HelviReg

Involved In Discussions
Because one SFC is not met by design.. so we are trying to find a way to ensure it has not occurred before starting a treatment.

Doing so on marketed products is definitely not feasible but it seems possible on a single investigational device by imposing it on a protocol.
 

Peter Selvey

Leader
Super Moderator
From a safety point of view, it's probably not a good idea to be repeatedly testing earth leakage before use, as logistically this adds a risk simply by constantly playing around with the earth itself, which reduces the reliability of earthing, which is important for other things, not just sinking earth leakage currents.

If the combination is close to the limit, that in itself should not be a worry. It's not like 499uA is safe and 501uA is suddenly dangerous. A current of 0.5mA is already in the region in which you can feel it, but it's not going to cause anything other than pain at the contact point. You need about 30-40mA for serious stuff like fibrillation. The 0.5mA limit is somewhere around the region of "ow, what was that?" while say 1 or 2mA could cause a person to recoil from the pain and say a few choice words. But depending on the person and situation, the "ow" response could felt from as little as 200uA.

However, for legal reasons it is best to stay below the limit. If, say, the earth broke, someone got a light shock ("ow") but then bumps a switch and unintentionally injures the patient, and later it turns out the leakage was over the limit, the lawyers could have a field day.

In that regard, one problem may be that capacitors used in EMC filters are often high tolerance and temperature dependent. This means you can get a different result from different samples and also cold compared to warmed up. That's one reason designers usually steer very clear of the limit. Also if it is large equipment (3kW?) there might be other parts of the equipment such as mains heaters or mains motor that have significant stray leakage but these might not yet be accounted for, and need special consideration in testing to make sure it is measured properly. So, while it might be 460uA by math based on the reports, the actual system could be higher (or lower).

One option could be to add an extra earth, if it is being used in an environment where EP earthing is available. This is one of the options available for ME SYSTEMS if the cumulative earth leakage can exceed 0.5mA in the patient environment.
 
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