Defining Expected Oxygen Leakage for Safety Testing

B

bassa

#1
Hello,

I am struggling with the test procedures to determine safety in oxygen rich environments.

Our product has socket outlets, and gas terminals (including oxygen)
Both are placed in separate compartments.

clause 11.2.2.1.b.2 states the following:
- [FONT=&quot]The oxygen concentration is measured for such a period that the highest
[/FONT]

[FONT=&quot]concentration of oxygen occurs. The least favourable control settings are selected.[/FONT][FONT=&quot]The leaking conditions of oxygen are selected such that they provide the minimum[/FONT][FONT=&quot] leak that could be detected by the [/FONT][FONT=&quot]OPERATOR [/FONT][FONT=&quot](e.g. because of a failure of the function[/FONT][FONT=&quot] of the device). If the concentration of oxygen exceeds 25 % in the presence of parts or[/FONT] [FONT=&quot]components that could be a source of ignition including at the moment energy is applied, it constitutes a failure.[/FONT]

The testing laboratory that is testing our product for 60601 3rd edition asks us what the expected oxygen leak rate could be. :confused:

When a leak is only to be detected only by a loss of function, the leak rate could be enormous; it's a supply hose at 8 bar pressure! I can imagine the hose system has a leak that is wildly roaring; while the connected device (needing only 200miliBar) is still working.

Anyone has experience on how to approach this?
 
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Peter Selvey

Staff member
Super Moderator
#2
Because air is already 21% O2, it does not take much of a leak to get up to 25%. The case (b) in the standard would really only work if the source of O2 is very small and the compartment is well ventilated.

Typically in normal condition, high pressure O2 will be confined to the pneumatic system (tubes, solenoid valves etc), so in normal condition OK. To handle the fault condition (e.g. hose disconnects), a typical approach would be to then enclose the pneumatic system inside a separate sealed box away from the main electrical parts. There is some electrical parts in the box (e.g. O2 controller, valves), but they are usually low power and not a source of ignition. Using a metal box would also limit the risk of fire in the worst case.

That approach follows (c) in the standard.

For the final risk management, you could consider (1) undetected failure of the pneumatic system (2) undetected failure of the pneumatic system enclosure, and (3) fault leading to an ignition source in the electronics. It's a fairly remote sequence of events. Also to note that exceeding 25% does not immediately mean "danger danger", it's just that we don't know about the effectiveness of flame retardants at increased O2 levels. Clearly at 90% O2, most don't work, but it could be that at 30 or 40% O2 the flame retardants are still reasonably effective.
 
C

cadevon

#3
We are having the same trouble identifying reasonable test conditions for our device (a neonatal CPAP).

We have venting slots in the upper chamber of our device which houses the electronics (approach B from standard), and the pneumatic components are below a lid with an o-ring seal (approach C from standard). There is grating on the bottom of the sealed portion to vent the gas out of the box.

Even with all precautions in place, I believe it's unreasonable to expect to pass if 100 PSI of oxygen is allowed to flow directly into the bottom (sealed) portion. This could occur in SFC where a filter was serviced improperly and the connection subsequently fails.

Is there a precedent for determining the actual pressure to test at? Other steps we could take in order to ensure safety even in this extreme situation (improve seal, fans to vent oxygen out, use of an orifice to limit flow)?

Thanks
 
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