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Creepage distance from primary to secondary of offline SMPS

treez

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Hi
Can I just check that in an offline SMPS in UK,EU,USA, the primary to secondary conductor creepage distance should be >=8mm?

We have a student with us whose father is a Top Expert Chief Power Electronics Engineer, and he says the rules have just changed and its now 5mm.

Also, this student has layed out a 70W Offline Flyback for us. He has L,N & E connections to the primary mains input connector. The earth is then tracked onto the PCB and picks up the Y capacitors at the primary side. ...Also, this earth copper is then tracked to the primary FET heatsink PTH's to which the plastic TO220FP FET is affixed. However, he has then continued to track this earth on the bottom layer from primary to secondary side. First this earth connects to the secondary diode heatsink’s PTH holes. (TO220 diode). Then this earth continues throughout the secondary side where it connects to a PTH to which the earthed metal “cage” is soldered into. This cage is “holy” and is to reduce radiated emissions as it kind of “box covers” the top PCB side of the entire SMPS.

…So, the earthed copper is on primary and secondary sides. However, on both primary and secondary sides this earthed copper tracking/pouring is less than 1mm away from primary and secondary metal conductors.

He says his father told him that all this is right and the regs have just changed.

However, do you agree this is not the case, and his SMPS is very bad?
 
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treez

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I must admit it was a surprise to me to hear that earth can be tracked from the mains input connector...right round to the secondary side, and be 0.5mm away from tracks in the secondary side and 2mm away from any tracks on the primary side...giving a toal creepage distance of 2.5mm between primary and secondary side conductive tracks...and this is absolutely within regulations (because its earthed copper)
I have been assured that this is because being earth, it is the safety earth, and so any flash over to it would blow the fuse.
Of course , If a floating piece of copper were tracked between primary to secondary like that...then it would not be acceptable.
 

Easy peasy

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Creepage to earth is not "creepage" creepage and clearance apply to direct pri to sec areas

depending on output voltage - only 500V isolation to earth may be required - hence 0.5mm o/p to earth is OK - some power supplies have the o/p earthed ( 0mm )

For mains to earth I believe 2mm is allowed in some situations, pollution degree 1 for example, dry, no ingress of moisture or dust - i.e. a sealed case for the equipment.

Having an "earth barrier" from pri to sec, e.g. PRI/EARTH/SEC is an old recognised way of being able to use basic / non-reinforced insulation practises to ensure safety isolation - often the earth conductor is specified to carry fault currents to blow the fuse.
 
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treez

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Thanks, it dawns on me, that if the earth connection is the wire from the 3-conductor cable (L,E,N), and someone forgets to screw the earth in, then it is then illegal and presumably dangerous. So it surprises me that this case doesnt ask for "earth bonding"?

Also, with a non-sealed power supply, you would think pri to earth distance would have been the same as live to neutral clearance...ie 2.5mm?
 

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