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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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  • 0

    ADS is assumed because the 400 Hz power supply feeds multiple items of class I equipment and the exposed conductive parts of these items are not isolated from the exposed conductive parts of other circuits and also have fortuitous connections back to facility PE via the local metalwork which is connected to the MET.

    Could still be IT? So while still classed as ADS, there's no hazard from a first fault to Earth (the system simply degrades to TN) and consequently no requirement to disconnect on 1st fault. If the PSU can only supply a limited current, then there might not be a need to disconnect promptly even on a 2nd fault - if the demands of reg 419 are satisfied (i.e. the output voltage collapses to something safe - e.g. <50V).

       - Andy,

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  • 0

    ADS is assumed because the 400 Hz power supply feeds multiple items of class I equipment and the exposed conductive parts of these items are not isolated from the exposed conductive parts of other circuits and also have fortuitous connections back to facility PE via the local metalwork which is connected to the MET.

    Could still be IT? So while still classed as ADS, there's no hazard from a first fault to Earth (the system simply degrades to TN) and consequently no requirement to disconnect on 1st fault. If the PSU can only supply a limited current, then there might not be a need to disconnect promptly even on a 2nd fault - if the demands of reg 419 are satisfied (i.e. the output voltage collapses to something safe - e.g. <50V).

       - Andy,

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  • 0 in reply to AJJewsbury
    AJJewsbury said:
    Could still be IT? So while still classed as ADS, there's no hazard from a first fault to Earth (the system simply degrades to TN) and consequently no requirement to disconnect on 1st fault. I

    BUT ... if this is two-phase or three-phase IT system, BS 7671 strongly recommends against distribution of the neutral. (See NOTE 1 to Regulation 411.6.1. NOTE 2 gives a clue to one of the reasons why.)

    So, if it is IT, with a distributed neutral, there would have to be some justification (and clear information) for its safe use and maintenance ... so again, all down to lack of information from the supplier.

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