Internally Powered Medical Device and Dielectric Strength

A

AmirBaron

#1
Hi,
Should internally powered device (2xAA alkaline batteries) with plastic shells without any means of connection to supply mains (Or USB) be tested for dielectric strength? (I just can't see any exemption in standard)



Amir

 
Elsmar Forum Sponsor
#2
Re: Internally powered device and dielectric strength

Dielectric strength tests apply only to solid insulation with a safety function.

It sounds like that would only apply to your enclosure.

From the table of test voltages (Ed 3) - if your device provides for MOPP (means of patient protection) your test voltage is 1000V.

If you only provide MOOP (means of operator protection i.e. no patient contact) then there is no test.
 
A

AmirBaron

#3
I see.
What's the rational to test leakage current/dielectric strength any way ?
If the device is powered by alkaline batteries and connection to mains or PC is impossible, I don't understand how the patient/operator can be electrified, No current may flow through his body because there is no ground connection. Does it come to test optional current flow through some part of the body by loop closing with batteries? for example a palm holding the device?
 
Last edited by a moderator:

Peter Selvey

Staff member
Super Moderator
#4
Unfortunately the way that IEC 60601-1 is written is to consider voltages above 0.1V as unsafe, and then provide exceptions for voltages <60Vdc.

(I would prefer the other way around: consider up to 60Vdc safe in general, with special requirements when appropriate.)

Anyway, the main exclusion in IEC 60601-1:2005 is in Clause 8.4.2 c), which allows up to 60Vdc to be contacted without a tool. You will need prepare the RM document and instructions for use to meet the requirements detailed in the standard. There is a similar requirement in IEC 60601-1/A2:1995 under Clause 16 e), without the risk management.

If your device is used around fluids take care with the risk assessment, as this may be a case where some isolation is reasonable even for 3V (e.g. for a dental drill).

Once the exclusion in 8.4.2 is established, then there is no need for a means of protection to be evaluated for requirements such as earthing, leakage, dielectric strength, creepage and clearance (i.e. 8.6, 8.7, 8.8 and 8.9 are not applicable). This conclusion can be arrived at by the first sentence in Clause 8.5.1.1 (the exclusion for 8.7 can be found in Clause 8.4.2).

Test labs apply these exclusions all the time (both 2nd and 3rd editions), although possibly unknowingly. When a device has mains voltages (100-240V), these get all the focus and the secondary voltages (+5V, +/-15V, +24V etc) get largely ignored.

However, it seems that when there is no mains voltage around (i.e. battery operation only), some test labs feel uncomfortable having no tests for isolation, so suddenly 3V becomes dangerous and full isolation requirements get thrown at it. But this ignores the exclusions provided in the standard, is not reasonable nor consistent with testing of mains powered devices.

If your test lab is requiring 500V or 1000V isolation, gently discuss the above with them first (we all make mistakes), and if they don't agree look for another test lab.
 
M

medical1953

#6
Does this leave the only test to be done as Patient Auxiliary current between applied parts ?
 

Peter Selvey

Staff member
Super Moderator
#7
For patient auxiliary current: Depends on the equipment, but it is unlikely to be applicable or significant for a battery operated device.

If you want to provide more details (type of equipment, type of applied parts) then it may be able to determine if it is applicable.
 
M

medical1953

#8
Thank you, I was very interested in your post as I'm designing a 2 x AA powered medical recorder in plastic enclosure. It has type BF applied parts like ECG, leg movement, respiration etc. No PC or external PSU connection, just setup via wireless and data recorded internally on removable media. Besides the touchproof electrode connectors, there is one miniature multiway connector for a passive accessory optionally supplied with the unit. Is this a SIPP/SOPP?
I don't have a problem with understanding or meeting the Patient Auxiliary current tests, but concerned about 1500 VAC type dielectric strength tests (like orginal poster) and tests involving Mains on SIPPS or Applied Parts. Due to the size it's not really possible to have an isolation barrier between electrodes and SIPP/SOPP.
Do I approach this through the Risk Management File and ignore tests thats are really intended for Mains powered equipment.
Thank you for your help.
 

Peter Selvey

Staff member
Super Moderator
#9
It is likely to be "safe" based on a break down of the sequence of events leading to serious harm, and assigning reasonable probabilities. Requirements for Type BF are based on the following events:

- patient recieves a low impedance main voltage from somewhere (p = 0.000001 or less on a per treatment basis)
- your equipment then creates a low impedance path back to earth (p = 0.01 or less assuming plastic enclosure with no bare parts easily earthed)
- the resulting electric shock causes death or other serious harm (p = 0.1 or less)

This would give an overall probability of death somewhere in the order of 1 in 1 billion per treatment which is usually acceptable.

But ...

First problem is that you really should not use the Type BF symbol, as this symbol means you comply with the 1500Vrms / 4mm requirements. So keeping the Type BF symbol on there is like a false claim.

This will then potentially impact sales, particularily if you are selling to hospitals that may expect Type BF and do not understand the details.

Also, it is likely to affect regulatory applications, again because many people will not understand the underlying sequence of events, and simply expect an ECG to be Type BF.

The upshot is it is best to comply with Type BF requirements if possible.
 
M

medical1953

#10
Ok, I have been thinking and re-reading 601-1 Ed3, could you check my logic please:
As 8.3 b) effectively says if a Patient Connection is an electrophysiological signal from the Patient (ECG in this case) then its a TYPE BF APPLIED PART. So I have no choice there.
If my device is always worn by the Patient i.e. it is in direct contact, then I think it is an Applied Part in its entirety, which would therefore be F-Type Applied Part (8.3 b))
My device has several Patient Connections but they are all of the same Applied Part, and my understanding of 8.5.2.1 is that a single F-Type Applied Part with multiple functions, does Not require separation between them. So I think I do not need to apply tests of Patient Leakage curent, dielectric strength or creepage/clearance?
Any views on this?
 
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