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Ungrounded Neutral on 115 Vrms, 400 Hz Power Supply Secondary

RichHaze

On one of our facilities we have come across a 400 Hz, 115 Vrms power supply whose secondary side neutral does not appear to be connected to facility earth and is effectively floating.  Approx. 50 Vrms Neutral to GND is observed on the secondary side of the power supply unit. 

Some observations are:

  1. The power supply units are installed "fixed" within the electrical installation and wired in permanently (not portable).
  2. The PSU is class I equipment as is the downstream equipment connected to it
  3. The PSU chassis is connected to PE on the incoming supply
  4. The PSU output includes a thermal magnetic circuit breaker, implying the OEM intends for ADS to be used as the chosen method of protection against electric shock
  5. We can't see any OEM facility inside the power supply for the Neutral and PE to be connected on the secondary side
  6. There doesn't appear to be a route for a "Line" to "Exposed Conductive Part" fault currents to make their way back to the secondary side of the PSU neutral and so the thermal magnetic breaker and ADS is questionable
  7. There is no insulation monitoring installed on the secondary side
  8. We have gone back to the OEM to ask how their system design meets the intent of the wiring regulations for protection against electric shock but we are not getting an answer

Does anyone have any thoughts on why the OEM might have taken this approach or any specific regulation requirements we need to be cognisant of before proceeding further?  My initial thoughts are that we need to introduce another neutral to PE reference link on the output side of the PSU for the purpose of ADS and also to prevent issues with floating neutral voltages downstream.

Thanks,

Richard

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    it sounds like it uses the concept of an IT system so that it keeps going in the event of first fault. An MCB or fuse will still protect against overload,  but of course cable damage or other fault grounding either side of the supply will not trip anything, nor will it pose any danger. (well, until a second failure grounds the other pole anyway ! )

    Normally the two poles would be insulation tested with respect to the case of the kit at installation time- which may well be metal and earthed,  but with this sort of supply it is not really working as class 1

    As IT, .It would be normal to monitor the offset voltage to ground, and alarm if there was a serious insulation fault. You do not say what is being supplied with this supply or if the load and supply are part of a pair or if they are from different suppliers - this sort of thing needs both ends considering together.

    It is almost certainly safe unless there is something else wrong with it, - many small generators and inverters are also wired this way even with 230V 50Hz out but you are right to check and the suppliers/ installers should be able to explain.

    115v 400Hz sounds aircraft like - is it the sort of kit that may be used in one ? shore power to aircraft can be a bit odd, as for obvious reasons, currents through the frame that may ignite fuel etc are a big no-no, so earthing once, and avoiding loops results in things that look a bit  odd.

    Mike

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  • 0

    it sounds like it uses the concept of an IT system so that it keeps going in the event of first fault. An MCB or fuse will still protect against overload,  but of course cable damage or other fault grounding either side of the supply will not trip anything, nor will it pose any danger. (well, until a second failure grounds the other pole anyway ! )

    Normally the two poles would be insulation tested with respect to the case of the kit at installation time- which may well be metal and earthed,  but with this sort of supply it is not really working as class 1

    As IT, .It would be normal to monitor the offset voltage to ground, and alarm if there was a serious insulation fault. You do not say what is being supplied with this supply or if the load and supply are part of a pair or if they are from different suppliers - this sort of thing needs both ends considering together.

    It is almost certainly safe unless there is something else wrong with it, - many small generators and inverters are also wired this way even with 230V 50Hz out but you are right to check and the suppliers/ installers should be able to explain.

    115v 400Hz sounds aircraft like - is it the sort of kit that may be used in one ? shore power to aircraft can be a bit odd, as for obvious reasons, currents through the frame that may ignite fuel etc are a big no-no, so earthing once, and avoiding loops results in things that look a bit  odd.

    Mike

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