considerations when choosing a relay

Discussion in 'Homework Help' started by Kardo22, Mar 14, 2014.

  1. Kardo22

    Thread Starter New Member

    Mar 12, 2014
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    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
     
  2. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    hi,
    Look thru this pdf.

    E
     
    Kardo22 likes this.
  3. MrChips

    Moderator

    Oct 2, 2009
    12,414
    3,353
    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.
     
  4. Kardo22

    Thread Starter New Member

    Mar 12, 2014
    26
    0
    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)?
     
  5. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    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.?
     
  6. Kardo22

    Thread Starter New Member

    Mar 12, 2014
    26
    0
    yes, exactly that
     
  7. ericgibbs

    Senior Member

    Jan 29, 2010
    2,499
    380
    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: Mar 17, 2014
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,498
    2,364
    If you after the official classifications, i.e. motor rated, load ratings etc, there are IEC and IOC classification tables.
     
  9. Kardo22

    Thread Starter New Member

    Mar 12, 2014
    26
    0
    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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