Relay contact ratings

Question

I am looking at this relay part: FTR-LYCA012Y. I am tasked with finding a relay part with contact ratings of 5A for both 250Vac and 30Vdc, even better when UL approved.

The datasheet has a table at the end (Safety Standards) which seems to state the contact ratings as approved by UL. From this table, I can conclude that this relay satisfies our needed specs. However, upon looking at the contact ratings at the start of the datasheet (non-UL table), I see that it seems inconsistent with the UL approved ratings. In fact, the UL approved ratings are higher. Also, looking at the max switching power for DC (144W), I can compute that for 30Vdc load, I can only pass 4.8A. However, the UL rating specified it for 6A/30Vdc.

Can anyone clarify or correct my understanding. Thanks.

Answer 1

A relay has two working states.

1. When the contacts are closed; in this case, the area of contacts determines the max current it can endure. The more the contact area, the more the current can pass. If the current is more than it can handle, the contact usually burns out and will have more resistance.

2. Switching on/off the current. This is the part that most of the time makes a problem. The maximum power of switching depends on the material of the contact. Contact mostly is coated with some alloys like Gold, cobalt and etc. The more the melting point of the alloy, the more power the relay can switch. In the time of switching, when the power is reactive (inductor or capacitor load), it creates a spark. That can melt and fuse the contact. This even happens when the power is not very high; and after a while, it destroys the contact. In high power relays (like the ones in big distribution lines), the relays use oil to limit the spark and absorb the heat to prevent contact fusion.

So, the first one you are showing in the picture is for when the contact is close. The second one is for when the relay is opening/closing the contact.