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Why two fuses are required - IEC 60601-1 cl. 8.11.5
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Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
Why two fuses are required - IEC 60601-1 cl. 8.11.5
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  Post Number #1  
Old 27th June 2018, 07:34 AM
Kunal Deshpande

 
 
Total Posts: 6
Question Why two fuses are required - IEC 60601-1 cl. 8.11.5

Hi all,

Why two fuses are required for Class I equipment, why not Class II.

Kunal

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  Post Number #2  
Old 27th June 2018, 08:11 AM
Pads38

 
 
Total Posts: 758
re: Why two fuses are required - IEC 60601-1 cl. 8.11.5

Hi Kunal,

In a Class I device a fault could be short circuits -

1) L-PE
2) N-PE
3) L-N

To protect against all three faults fuses are required in both mains supply lines - a fuse in the L line would not necessarily protect a N-PE fault.

For a Class II device only the L-N fault needs protection - so a single fuse may suffice.

Welcome to the forums.
Thanks to Pads38 for your informative Post and/or Attachment!
  Post Number #3  
Old 11th July 2018, 09:14 PM
Peter Selvey

 
 
Total Posts: 897
Re: Why two fuses are required - IEC 60601-1 cl. 8.11.5

The reason might be a little different:

Although effort is made to have the correct polarity (L&N) in the supply, it's not assumed.

So, if the L & N are swapped, a fuse in the N can still clear the fault to earth in the equipment. Obviously this does not apply for Class II equipment.

In normal appliances this is not required, as it is assumed the wall supply can still clear a fault (at least, when I was working with the old IEC 335 series 25 years ago ... it could have changed).

But for medical devices, it is considered important that the device can clear its own faults as far as possible, and not take out the supply of the theatre or ward etc.

Note that typically a N-PE fault should not cause the fuse to open. They are similar potential.
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