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I'm designing a new series of products in metal chassis containing plastic fans. The fans' power supply is referenced to the negative DC rail of a variable-frequency drive, and the fans are mounted to the grounded chassis. That means the fan is required to withstand a ~3kV hipot between its mounting holes and its internal circuitry. The fan is not marked with such a rating. However, we have a long history of products using this configuration. Those products are UL-listed, and every unit is hipot-tested before it goes out the door. We've never seen a failure.

Is there an industry-standard best practice in this situation? Would it typically be considered acceptable to rely on an unstated voltage withstand, as long as it was tested on each unit? Or would best practice call for the fan to be isolated from the DC- rail (or alternately, insulation to be added around the fan)?

EDIT: To clarify, our user I/O is floating, not referenced to anything. (We do prefer to keep our customers alive and not blow up their equipment.) It's only the fan that's in question.

The fan is blowing air into the box, so the "guts" of the fan are opposite the fan from the fan grill. And the voltage on the fan is stepped down to 24V from the incoming DC bus. The only high-voltage differential involved is between the fan's internal power supply and the unit chassis.

On one hand, I want to say that a history of thousands of units and UL's testing to get the units listed should be sufficient; I think it's pretty clear we don't have a safety issue as long as the user grounds the box and fuses the units as instructed. On the other hand, I hate to rely on an unpublished spec for the fan. Both arguments seem valid. The question is, which one is the more "correct" way of thinking?

FURTHER EDIT: The fan I'm planning on using is here: http://datasheet.octopart.com/4715KL-05W-B50-E00-NMB-datasheet-14433727.pdf However, I don't see its relevance. I'm not trying to identify a solution to a design problem. I have two solutions I'm trying to decide between. Which solution is better from a standpoint of best practice?

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  • \$\begingroup\$ Is there a clearly delineated creepage distance over a known type of plastic that you can point to? Are the plastic fans UL recognized components? \$\endgroup\$ – Spehro Pefhany Feb 13 '14 at 22:23
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    \$\begingroup\$ Most DC fans run the wires for the motor down one of the legs of the frame, which, depending on how it's mounted, can put them very close to the chassis. This means that you're relying on the wires' own insulation, plus whatever the plastic frame adds, for the withstand voltage. Perhaps some additional strips of Kapton tape along the path of the wires would add some extra margin. But isn't this something you should be asking the fan manufacturer? \$\endgroup\$ – Dave Tweed Feb 13 '14 at 22:57
  • \$\begingroup\$ I'm with Dave. Get the manufacturer involved. \$\endgroup\$ – Matt Young Feb 14 '14 at 13:23
  • \$\begingroup\$ What is the UL listing number marked on the fan? \$\endgroup\$ – Adam Lawrence Feb 14 '14 at 13:32
  • \$\begingroup\$ Manufacturer and part number would also help. \$\endgroup\$ – Adam Lawrence Feb 14 '14 at 13:39
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From a unit-by-unit standpoint, if it passes hipot today, it should pass hipot tomorrow. As long as each unit passes hipot, you shouldn't have a safety or reliability concern.

From a product line standpoint, though, you can't be guaranteed that the fan will always be built the same way. It's possible different production runs will be built differently. Maybe a different plastic will be used, or a different geometry, or who knows what. If it's not a spec listed on the data sheet, you can't assume it will never change from batch to batch.

Since asking the question, I've found out about a case where exactly this happened to us. Many batches of fans passed hipot to our 3000VAC level, well past their spec, and we shipped units with no problems. Then a new batch came in, and every one failed 3000VAC hipot. We had products due out the door, and we had to scramble to insulate or isolate our fans.

For individual units, you can be fine using unstated specs that you test on a unit-by-unit basis. But as a matter of best practice for a product line, one should not rely on unstated part specifications.

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It sounds like the power to the fan is derived (without any form of barrier or galvanic isolation) from the rectified incoming AC supply. It's the same argument when your circuit needs a capacitor to earth from either live or neutral AC wire - what sort of component do you use in these circumstances? You use an approved (and UL listed) class Y capacitor.

I don't see that there is much of an option other than to galvanically isolate the DC supply for the fan. Your company/firm, as a buyer of components, are not in a position to get the fan approved because you don't manufacture the fan so this isn't a route either.

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You will have to galvanically isolate the low voltage part from the HV (High Voltage) part. If you value the life of your customers, I recommend insulating the interface electronics as well.

You must to cover the air hole for the fan with a mesh to protect against intrusion. If the fan blows out that hole, the electronics will most probably be on the outside, which is close to the mesh. I see no way to insulate the wiring of a normal DC fan with 3 kV against the protection mesh.

Since you probably get power from mains voltage, it is best practice to have case, interface, control etc on safe low voltage, completely separate from the HV part.

Putting HV anywhere near a weakly insulated part of your casing or LV electronics is a recipe for disaster. Dont.

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