Why two fuses are required - IEC 60601-1 cl. 8.11.5


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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.

Peter Selvey

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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.