SEL751-A Arc Flash Relay

SEL-751A Arc Flash Relay

Does anyone have experience with this relay. More specifically, how are the optical sensors tested during routine maintenance? This looks like a very good relay, but I have concerns about how this unit will be thoroughly tested once it is in service. Thanks in advance for all feedback.


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About the Author

Chris is an Electrical Engineering Technologist, a Journeyman Power System Electrician, and a Professional Engineer. He is also the Author of The Relay Testing Handbook series and founder of Valence Electrical Training Services. You can find out more about Chris here.

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  1. I am not familiar with this relay but have heard that a flash is connected to the test set output to simulate the arc and the time is measured from the test set output closure to the trip contact output.


  2. I have heard of similar methods. This relay also has overcurrent supervision. So my test would consist of applying an overcurrent while simulating the arc flash. If I am thinking of this correctly, I would need to take the equipment being protected out of service so that I can test the sensors–since they are installed in the arc flash zone. Am I off base on this?

  3. A Friend sent me these procedures for testing a VAMP relay (Broken Link Removed by editor) which I assume is similar


    Activate one ARC sensor at time
    Clear the central- I/O-units after each activation
    Inject overcurrent to the central unit and I/O current unit.
    Make sure that the current setting is right by checking the led I> int for internal OC and I>ext for external OC
    For testing trip circuits, inject OC at the same time as the sensor is activated by a strong light source (spot light) or a camera flash

    Test Sequence
    Current output has to be activated simultaneously with digital output (triggering sensor input)
    Start timer when current output activated and Digital output closed (contact)
    Stop timer when Trip contact closes
    Injected Current should be minimum 50% higher than the Iset current (1A)

  4. I’ve heard that it is possible to time the relay as well but they had difficulty because of the inherent contact delay when triggering the flash. When using the MTS-5000, for example, the output contact has a 6 ms delay to transition between Prefault and fault. The fault current transitions instantaneously which creates a 6 ms difference between the current and the flash which screwed up the timing test. There are several methods to synch the flash and current output by using multiple fault states or actual output closure on a 2nd input to trigger the current.


  5. I just tested my first arc flash 751A relay. I had point sensors and I was able to use the SEL 4520 arc flash test module (this device really helped in the documentation of the test with results). The relay word bit for current supervision of arc flash is 50PAFP (arc flash maximum phase overcurrent pickup). Then you must raise the ambient light to a percentage of light level set in the relay. The relay word bit is TOL (time over light) and then corresponding numbers associated with the fiber port the sensor is on. For instance TOL1 is in the first port.
    You then run the test and trigger an arc. You can test the light level with a MET L command. Hope this helps

    1. The SEL-4520 is a huge help! It runs about $600 but it is worth every penny. It allows you to perform an End-To-End test by gating the current source on at the same time as flashing the light sensor.

      1. The procedure for testing the Arc Flash function can be found in the 751A technical manual, section 10.9. The SEL-4520 comes with it’s own instruction sheet.

  6. SEL has a test set for this relay arc sensing features It is a SEL-4520 Check it out at the web site. The 751 is a extremely reliable relay. The SEL-4520 delivery is about 2-3 weeks. Check out the arc flash videos on the SEL wen site. The tests were done in Kinectrics HV Lab in Toronto Ontario Very impressive.

  7. I have experience with the system arc protection VAMP 321

    The test must be with the equipment being protected out of service.
    This relay VAMP 321 also has overcurrent supervision. So my test consist of applying an overcurrent while simulating the arc flash.
    However all test set output has a delay around 7msec that must add to output time from relay in this case the VAMP 321 has 2 High Speed Outputs (Trip contact output) of 1~2msec; So the expected time is of 7~8msec.
    If you want to measure the total time of the test you should consider this; You can verify this if you have available a disturbance recorder with a debounce time of 1msec,

    The sensors are tested quite easily with a good quality electric flash with input auxilar for connect to output from test set, is not neccesary to buy elements additionals.

    Hope this helps

  8. Hello
    We have installed and commissioned a number of SEL751s using their arc-flash protection capability. We use Omicron CMC356 test sets together with their ARC 256x Arc Flash Initiator which simulates an arc by a xenon tube, very similar to a camera flash. This CMC accessory is positioned close to the arc flash sensor during testing and is connected to the external interface socket of the CMC test set, which provides the signal that will trigger the flash.
    The SEL751 relays have the option of selecting outputs that will be processed every 1/16th of a power cycle which can be used to get the fastest possible tripping time. We have tested relays with these contacts selected and get ~ 6 ms tripping time. The standard (1/4 of a cycle) contacts have given tripping times of around 11-12 ms.
    We have also used a Canon flash triggered by the CMC356 for certain arc flash sensors (GE) which needed far more light than the Omicron ARC 256x Arc Flash Initiator or smart phone camera flash could provide with successful results.

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