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RCD trip during ATS switch to source B | Racks, Rack Accessories, & Cooling

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RCD trip during ATS switch to source B

Discussion in Racks, Rack Accessories, & Cooling started by Welly , 3/4/2021 10:30 AM
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RCD trip during ATS switch to source B

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  • My ATS SU044-1.
    source A connects to UPS A then A/C socket A;
    source B connects to UPS B then A/C socket B.

    UPS A suddenly went down due to hardware error (Smart UPS 3000 RM XL all LED flash); Both RCD of socket A and B break at the same time. The sensitivity of both RCD is around 30mA.

    I suspect the UPS A leak current, breaking RCD of socket A. Is it possible for my ATS to leak current to UPS B, as a result breaking the RCD of socket B? See if anyone has the same experience. Thank you.

    I suppose my ATS should be open transition so current leak through 2 input is impossible.

  • Hi Welly,

    That's quite an old ATS, so it may not be as good as it once was but it could also be related to your UPS. ATSs must be used with online UPSs. This is to do with transfer time and reaction speeds and phase alignment. 

    I'm going to assume the load connected to the ATS is an IT load or at least uses power supplies, if the ATS needs to switch from one source to another there will be a transition period (I don't know what it is on a unit that old) but during that transition all the capacitor in your load will start to drain and when the new connection is made there will be a high in-rush current to recharge them, this is probably what caused the RCDs to trip.

    The reason that you shouldn't use line-interactive UPSs with ATSs is that if the ATS move from once source to another it takes 8-12ms (on a modern unit) and the line interactive unit takes 8-10ms to transfer to battery. Now if the ATS switches source before the UPS does it will switch back wait and then switch back again. This means that a load can be without power for up to 32ms. I have seen cases where this has caused ATS failures, UPS failures, load damage, data loss, and tripping upstream breakers.


  • Hi Gaven,

    Both UPS A and B are line-interactive (Smart-UPS XL 3000VA RM 3U 230V).
    Is it possible to cancel the power source preference in ATS to prevent switching back, which causes the 32ms outage? Thank you.

  • No that's would be against how an ATS works hence the requirement to use them ATSs with online UPS not line interactive UPS.


  • Hi Gaven,

    Though the use of ATS in conjunction with line-interactive UPS is not recommended, it should be rare for RCD of A/C power source to trip due to the switching of ATS, right? Thank you.


  • It would depend on the load and the actual environment at that exact time but no I would not be surprised if caused an RCD to trip.

    - Gavan

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