EEVblog Electronics Community Forum
Electronics => Projects, Designs, and Technical Stuff => Topic started by: Jester on October 06, 2018, 04:38:12 pm
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I'm tweaking a board used in an industrial setting and I'm not sure how to interpret the creepage rules for relay contacts.
I diverge slightly......
Thinking about required creepage for a connector for example 120Vac MAINS supply to a board, only basic is required between the hot and neutral, no issue there.
Now thinking about creepage requirements between the contacts of a relay (not the contact to coil creepage). The relay is rated for 250/277Vac, the pin to pin contact spacing is 5mm, allowing creepage of about 2.75mm (> 100mils). This is obviously adequate from a functional perspective and even satisfies safety requirements for basic for pollution degree 2.
Now if working to a pollution degree 3 standard, basic or functional creepage is 151mils/3.8mm, however the relay contacts are obviously expected to operate so do creepage requirements even need to be considered between relay contacts?
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Relays are usually sealed, so we don't expect contamination inside (unless it's not, in which case you better buy a fat one?).
Can always rout between pins if you need more creepage. On the relay base itself, there probably aren't fins or slots to increase creepage distance, in which case either UL doesn't care(?!) or it would have to be potted under (probably with an approved inspection process). The good bet would then be finding a higher voltage/category part.
Tim
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For this kind of stuff you need to take a look to the rated insulating voltage of the relay contacts, and make a comparison with what your standard requires.
For example, you might need functional isolation for 230VAC, with an overvoltage category that requires testing at 2500VAC for 1 minute. If the relay is for example rated for 4000VAC for 1 minute, then you're fine.
You need to search and understand the safety standard to correlate between the creepage/clearance tables and the "equivalent" test voltage.
Usually safety standards let you choose between those options: either guarantee a measurable distance (such as creepage) or do an equivalent electrical test (for example 2500VAC for 1 minute).
Some relay datasheet also say something like "creepage equivalent to 5mm according to IEC xxx standard" or similar.