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AP7721 question about power surge | Racks, Rack Accessories, & Cooling

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AP7721 question about power surge

Discussion in Racks, Rack Accessories, & Cooling started by BliXem , 7/27/2018 8:29 PM
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AP7721 question about power surge

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  • BliXem


    I've got a AP7721, both A and backup B are running perfectly for 10+ months now. The screen is telling me that I use 7A. But I have a question. I need to replace a memory module in one of my 11 servers which are connected to my ATS. When I remove the power cable and replace it, I will insert the power cable again, you will hear 'crackle' sounds of the power when connecting to my PSU. Now the question: does this cause a power surge? If so, will my ATS go down and take all my 11 servers down? I'm a litte scared that this happens.... Or does B takes over?

    Hope you guys can tell me what to do in this case. My ATS is for security reasons not connected to the Internet.

  • Malvineous

    The crackle is because the server power supply doesn't have good inrush current limiting and the plug is either being inserted too slowly or isn't making good contact.  It is generating small sparks at the point of contact where the metal touches inside the plug and socket, and too much current is being drawn by the server power supply when there is not yet enough metal in contact to conduct that amount of electric current.  If you shove the plugs in quickly then you minimise this problem and usually only get one small "click".  In my own experience, crackles only happen when the plugs are not completely inserted, and sometimes they do need quite some force to get them to fully plug in.  Sometimes people assume they are plugged in properly when in fact they just weren't pushing them in hard enough!

    My understanding (I am not an electrical engineer) is that the inrush current is the same whether you hear a single crackle or not, but of course if you hear lots of crackles then you are probably causing repeated inrush events which increase the possibility that some current protection device will kick in.  So it's not anything to panic about, but it is better to securely insert the plug as swiftly as you can to minimise the number of crackles you hear.

    If you manage to cause too many crackles for too long a time, then it looks like an electrical overload (the opposite of a surge, where there is not enough power as opposed to too much power).  I believe the ATS doesn't have any sort of current limiting in it, so the overload would just be passed up the chain to the next power distribution device (UPS, circuit breaker, etc.)  If that decides there is too much power coming over the wire then it might trip, and the ATS will see there is no power coming from that source so it will switch to the secondary source.  If the overload is still there then you might manage to trip your second source too!  However this is extremely unlikely to happen from just inserting a power cable, unless there is an actual fault in the power supply.

    Realistically the crackles are so brief that you are unlikely to cause anything to break because of them.  Personally I prefer to minimise the number of crackles by inserting the plugs firmly and quickly, but for me this is more to ensure I don't provide bad quality power to the device I am plugging in.  I'm sure if you intentionally made lots of crackles, the power supply in the device itself would fail long before the power cut out.

    As a side note, I hate the relays APC use in the ATS devices, because the clicks they make when switching sound a lot like these crackles.  It always makes me think the plugs are crackling like crazy when I plug in a source and I think something is going wrong when it turns out it's only the relays all clicking undecided

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