considerations when choosing a relay

Thread Starter

Kardo22

Joined Mar 12, 2014
41
Hi
Does anyone have a good place to read how to choose a relay? Safety facor and other considerations. Would be good if its official looking so that I can also refer to it.
Thanks
 

MrChips

Joined Oct 2, 2009
19,397
Coil voltage, AC/DC
Latching current
Holding current
Contact configuration
Contact voltage and current rating
Form factor
Size
Package
Mounting style
Price
Availability, stock, shipping

Edit: Eric has provided the better answer.
 

Thread Starter

Kardo22

Joined Mar 12, 2014
41
hi,
Look thru this pdf.

E
Thank
But it doesn't say much about how big the safety margin should be. I have a voltage withstanding tester and I'm using 1kV AC with max 6 mA current. The tester monitors the voltage and current and cuts off if it goes too high. Can the safety factor be lower in that case?
I've read that a relay should operate at 20-50% of its max. Should my safety factor be 2 in that case? Or should I choose a higher safety factor (I think with motors it can be 10x to account for startup surges) just in case (thou I don't think big surges are likely)?
 

ericgibbs

Joined Jan 29, 2010
8,873
When you say , safety margin, do you mean by what percentage you should over rate your relay for the V, I contact rating, insulation and/or its drop out and over voltage on the coil.?
 

ericgibbs

Joined Jan 29, 2010
8,873
yes, exactly that
The overrating of a relays parameters ie: Contact V/I, Coil under/over voltage, Insulation etc depends heavily upon the application.

eg:
contact switching AC or DC.
resistive or reactive contact loading.
contact switching cycles.
critical/ non critical applications.
environmental conditions.
etc....

Give us an example of an application.

E
 
Last edited:

Thread Starter

Kardo22

Joined Mar 12, 2014
41
The overrating of a relays parameters ie: Contact V/I, Coil under/over voltage, Insulation etc depends heavily upon the application.

eg:
contact switching AC or DC.
resistive or reactive contact loading.
contact switching cycles.
critical/ non critical applications.
environmental conditions.
etc....

Give us an example of an application.

E
I need to switch between 2 states (so form C needed)
- switched loads are AC 50 and 60 Hz
- voltage is max 2kV, current max 10 mA (this is output of High Voltage tester, the tester monitors its voltage and current and should interrupts current if it detects a fault. I think there should be some safety margin as the tester migth not be able to shut off the current if it spikes very fast).
- I think load can be considered resistive. There is some reactive component but its small.
-noncritical application, room conditions

Would greatly appreciate if someone could give me some tips what relay to choose.
How much difference does hot switching make? I'm tring to avoid it but it could still happen under some conditions (thou is rare).
 
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