Spacing between the solder eyes of the fuse input/output leads

Roland chung

Trusted Information Resource
#1
Hello folks,

According to the standard, creepage and clearance between parts of opposite polarity are required before primary fuses. But test house told me that our product failed the standard because the spacing across the surface of the PWB between the solder eyes of the fuse input/output leads is less than 3 mm. The test house said if the spacing under the fuse is too tight, the fuse would be short-circuited and therefore lose its function.

I think spacing under fuse complying with functional insulation is sufficient. As there are no requirements for functional insulation in IEC 60601-1, it seems not reasonable to require the spacing under fuse to meet 3 mm.

Could I have your opinions?

Thanks,
Roland
 
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Pads38

Trusted Information Resource
#2
That seems a confusing issue. As you say only insulation that forms a Means of Protection or is between parts of opposite polarity in the mains part have a defined requirement.

I would ask your test house to identify the exact clause they think your device does not comply with. A process that is sometimes called "show me the shall" - that is ask for direct identification of the mandatory requirement.
 

Roland chung

Trusted Information Resource
#3
Thank you for your input.

I actually tried to argue the standard does not have specific requirement. Test house agreed with this but also said the standard does not state many other things clearly, for example, the fuse exploded is not allowed.
 
Last edited:

Peter Selvey

Staff member
Moderator
#4
Strictly speaking, creepage and clearance distances only apply to protection against electric shock, not protection against fire or other purposes. This comes from the definition of a "means of protection".

However, in a lot of switching power supplies, the fuse is an important means of protection. Generally, the circuit consists of low impedance parts up to the transformer and then the switching transistor. If the switching transistor shorts, the only reliable protection against burning out the transformer (and hence risking electric shock) is the fuse. If this is the case, the distance under the fuse can clearly be included as a means of protection and therefore creepage and clearance distances apply.
 

Roland chung

Trusted Information Resource
#5
Thank you for your insight.

Yes, the fuse is indeed an means of protection. But the working voltage between terminals of fuse shall be close to 0 V (fuse is no different from a wire). That means no distance required. Of course, the voltage will be raised to 240V if the fuse opened.

According to the definition, working voltage shall be determined when the equipment is operating under conditions of NORMAL USE.

How do you think about it?
 

Peter Selvey

Staff member
Moderator
#6
Actually, it goes the other way.

One could argue that shorting the transistor is normal condition (because it is a semiconductor and the spacing between FET D-S is unlikely to meet 1 MOP). In that case the spacing of the fuse should be 2MOP!

Of course in practice, FETs are highly reliable (if used well within specification) and it is reasonable to consider it 1MOP. The fuse is then also 1MOP, making overall 2MOP.

Similar to double insulation, the working voltage for each MOP is taken as the working voltage across both parts (i.e. mains voltage).

So be careful trying to argue out of this one. The test lab is only asking for 1MOP for L-N type insulation (3mm), not full basic (4mm) or reinforced (8mm).
 

Roland chung

Trusted Information Resource
#7
Good points. But I would think the probability of sequence events is extremely low:

1) the FET's Drain shorted to Source
2) the fuse opened
3) the spacing under fuse shorted
4) the transformer burn out and insulation system destroyed
5) people getting shocked
 

Peter Selvey

Staff member
Moderator
#8
The problem here is not the basic MOP theory, but more that the limit of 3mm is overkill for a printed circuit board. This is the fault in the standard.

If you follow the zero working voltage argument, no limit would apply, so even a distance of 0.1mm would be acceptable, clearly unsafe.

IEC 60664-1 says that on a PCB, a creepage distance of 1mm should be enough for 1MOP (pollution degree 2). That seems like an more appropriate limit for the situation, given as you say the full sequence of events.
 

Roland chung

Trusted Information Resource
#9
Yup, the distance under fuse now is 2 mm. We think it is sufficent and do not want to change the circuit board. But the test lab insists on 3 mm.
 
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