Author Topic: Relay contact ratings  (Read 1869 times)

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Offline nukie

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Relay contact ratings
« on: May 27, 2013, 03:06:21 am »
Hi,

I just bought an Omron MY2 DPDT relay to fix an equipment. From the window, the contact rating as follow..

5A 240 VAC
5A 28 VDC

I don't understand why is there such a significant difference between AC and DC at the contacts?
When I google, the explanation is that DC current requires higher contact rating than AC but why so?
Is this because of the resistance difference between AC and DC?

Vincent.
 

Offline Rufus

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Re: Relay contact ratings
« Reply #1 on: May 27, 2013, 03:22:06 am »
When I google, the explanation is that DC current requires higher contact rating than AC but why so?
Is this because of the resistance difference between AC and DC?

Because an arc draw between the contacts as they open will extinguish on the next zero crossing of the ac current they are carrying. A dc arc can only be extinguished by the contacts moving far enough apart.
 

Offline nukie

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Re: Relay contact ratings
« Reply #2 on: May 28, 2013, 04:16:40 am »
Great explanation, thanks!
 


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