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These UL contact ratings are listed in the datasheet for a Tyco relay: T90 Series, 30A PCB Relay.

(Note: the link will ask you to download the PDF, as do most Tyco PDF links)

The contact life is different for different loads and this is as expected. However, if you look at the ratings for the 'ballast' load type notice that at 6A it's specified to 100,000 cycles yet at 3A it's only 6,000. Why so?

Don't lower currents usually increase contact life? Furthermore, for CFLs, should I be looking at "ballast" load-type?

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up vote 1 down vote accepted

To answer your question about whether a CCFL should be considered a 'ballast', I think not.

Very early CCFLs had inductive ballasts in them, however they were heavy and bulky and did not last long in the market once electronic ballasts became competitive. I think the rating applies to old-style reactance ballasts and not the modern-day electronic type.

It's not uncommon for the N.C. contact rating of a relay and/or the life to be considerably less than the N.O. contacts of the same relay.

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The 3A ballast rating is for NC, normally closed contacts, that is why it is rated less than 6A, normally open. The spring force on normally closed contacts is different to the force on normally open contacts when closed, explaining the lower life.

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OK, but then why does the 5th spec have the same as the 1st one? Is it because then the load tested is resistive? – Saad Aug 5 '14 at 16:28

UL508 is a spec written by/for the Underwriters Laboratory and the list in your question I reckon is the contents of the specification that can be assumed to be applicable to the relay in question.

The (NC) 3A spec is for 277V AC so it's higher than the relay's rated voltage of 240V AC and it's defined as connected to a load that is a ballast - I'm presuming here that this may cause an extended arc to be produced when the contact opens hence the 3A rating. However for the NO contact there are far more cycles of operation because there won't be any arcing present when it closes - just a small spark when the contacts get very close for the AC to breakdown the gap and, this will be very short-lived.

This is how I read it anyway. I hope someone has a more definite answer but basically contact closures are usually less problematic than when contacts open circuit because current is flowing and the contact is trying to interrupt that flow.

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Are these ratings the min. life or typical? It doesn't say min anywhere so it's probably typical. – Saad Aug 5 '14 at 17:19

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