Should the fault tests pass if temperature goes high in short time?

Roland chung

Trusted Information Resource
Dear all,

As you can see from the caption, my question relates to the judgment of fault test. Our product is powered by alkaline batteries. Because the spacings between traces of polarity on PCB can not meet basic insulation, short circuit test was conducted. Since no fuse employed, the peak temperature during short circuit test is up to 180 degree. But the duration of whole test is less than 5 min because of the batteries destroyed. There are no emission of flames and deformation of enclosures. Could I say this test is OK? But according to clause 52.4.1 in 60601-1 2nd edition, for thermoplastic materials, the ball pressure test specified in clause 59.2 b) is carried out at a temperature 25 degree higher than that measured during SFC tests. If so, the test shall be failure, because the ball pressure test is conducted at (180 25) degree.

Please kindly share your experience. Thanks in advance.
 

Stijloor

Leader
Super Moderator
Dear all,

As you can see from the caption, my question relates to the judgment of fault test. Our product is powered by alkaline batteries. Because the spacings between traces of polarity on PCB can not meet basic insulation, short circuit test was conducted. Since no fuse employed, the peak temperature during short circuit test is up to 180 degree. But the duration of whole test is less than 5 min because of the batteries destroyed. There are no emission of flames and deformation of enclosures. Could I say this test is OK? But according to clause 52.4.1 in 60601-1 2nd edition, for thermoplastic materials, the ball pressure test specified in clause 59.2 b) is carried out at a temperature 25 degree higher than that measured during SFC tests. If so, the test shall be failure, because the ball pressure test is conducted at (180 25) degree.

Please kindly share your experience. Thanks in advance.

Can someone help Roland?

Thank you!!

Stijloor.
 

Stijloor

Leader
Super Moderator
Dear all,

As you can see from the caption, my question relates to the judgment of fault test. Our product is powered by alkaline batteries. Because the spacings between traces of polarity on PCB can not meet basic insulation, short circuit test was conducted. Since no fuse employed, the peak temperature during short circuit test is up to 180 degree. But the duration of whole test is less than 5 min because of the batteries destroyed. There are no emission of flames and deformation of enclosures. Could I say this test is OK? But according to clause 52.4.1 in 60601-1 2nd edition, for thermoplastic materials, the ball pressure test specified in clause 59.2 b) is carried out at a temperature 25 degree higher than that measured during SFC tests. If so, the test shall be failure, because the ball pressure test is conducted at (180 25) degree.

Please kindly share your experience. Thanks in advance.

One more BUMP.

Can someone help?

Thank you!

Stijloor.
 

Peter Selvey

Leader
Super Moderator
Just out of interest, where was the 180C recorded? I remember many years ago performing a short test on 2x1.5V cells and finding that the contact spring material gets very hot (seems to be a material with a high positive temperature co-efficient, as it gets hotter resistance goes up and it gets hotter still).

Anyway: under abnormal / fault conditions, the standard has a limited list if things which can be considered a non-conformity (under the 2nd ed), which for this test would include:

1) damage to supplementary or reinforced insulation, associated with protection voltages above 60Vdc or 30Vac. So in your case this would not be applicable being only a few volts (e.g. 3Vdc) (see this thread for more discussion why)

2) fire: again in your case not applicable, fire needs at least 300C for ignition and usually much more

3) emission of hazardous gasses, somewhat subjective requirement as both the type, volume, duration need to be taken into account; at 180C you will usually get some, and may be sufficient cause a headache or similar transient low level severity issues; these would not normally be a non-conformity.

So I would consider melting of plastic enclosure in a battery powered device (e.g. 2 x 1.5Vdc cells) is not a non-conformity. However, with 180C, there may be many other issues (e.g. if the device is worn on the patient, burns etc) which should be carefully considered (e.g. under risk management) before accepting as a safe design.

It is also worth to note that the creepage/clearance distances for lower voltages are a huge overkill in the 2nd edition, so the whole test may be questionable.

IEC 60664-1 indicates the distance for reinforced insulation on a PCB is 0.08mm, somewhat smaller than the 3.4mm indicated in IEC 60601-1 2nd ed.

I have never found a good reason for the huge values in IEC 60601-1 for small voltages. They appear to be appropriate for pollution degree 4, i.e. continuously exposed to snow and rain. Go figure.
 

Roland chung

Trusted Information Resource
Thanks for all of the inputs. The device we developed is powered by 12V (8 x1.5V) and the temperature of batteries contact was measured during short circuit test at ambient temperature of 40 degree (placed the device into the oven). The batteries box is external to the main unit and does not come into contact with patient during normal use.

I know that the mentioned device does not shock the patient and user. The only concern is the temperature. I am wondering at what is the intent of the ball pressure test. The standard does not clearly say the ball pressure test is not applicable to the SELV devices.

Regarding the requirements of spacing, I completely agree with you. However, the standard is standard. It seems that we had better to follow the requirements. Otherwise, the NBs will ask us to provide evidence for the argument. Then, they may say IEC 60664 is not medical standard.
 

Peter Selvey

Leader
Super Moderator
In the above posting is a link to the thread explaining why 12Vdc does not require supplementary or reinforced insulation.

According to IEC 60601-1/A2:1995 (2nd ed), Clause 52.4.1, 8th paragraph, only supplementary or reinforced insulation of thermoplastic material requires the ball pressure test.

Therefore, the ball pressure test would not be applicable in your case.
 

Roland chung

Trusted Information Resource
The link is useful. Just interesting in the ball pressure test, did you mean that such test is only applicable to the MAINS powered equipment? If possible, could you tell the intent of ball pressure test? What characteristic of material being considered? If for fire purpose, many modern plastic have been tested according to UL94, it seems ball pressure test is not necessary.
 

Peter Selvey

Leader
Super Moderator
The rating of UL94 is related to the risk of fire, which can exist in secondary circuits as well as mains circuits. According to UL 60601-1:2003, any circuit which has a potential to produce more than 15W in normal or fault condition is a source of fire, and enclosures around such parts should be resistant to fire (i.e. comply with UL 94, see standard for specific rating such as V-0, V-2 etc).

The ball pressure test is related to electric shock, which in general applies only to parts enclosing or insulating voltages above 60Vdc/30Vac.
 
S

sankui

I had similar experience before. But the NB insisted on adding one fuse to prevent the short circuit of batteries even though the short test did not cause fire or melting, etc. The NB said, many fire accidents caused by batteries had been reported and the test result is not always be the case.
 

Roland chung

Trusted Information Resource
Hello peter,

A question pertaining to the UL rating (V-0, -1, -2, HB, etc.), if a mains operated device (>15W) passed all the fault tests specified in standard (no flame and deformation, etc.), whether plastic enclosure rated at UL94 HB is accepted? In other words, whether fire enclosure (V-1 or better according 60601-1: 2005) is needed?

Thanks in advance.
 
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