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Power cable connection on a mobile installation to External equipment using a MIL-DTL-38999 connector.

Samantha Rendell

So I've inherited a a mobile installation project. The power cable connection on to External control equipment uses a dedicated port with a MIL-DTL-38999 connector. 

The inrush when the charging system is engaged trips the RCD when it is set to 30mA. My predecessor determined that the connection is not classified as a socket and increased the RCD trip current to 300mA.

Given that it uses the 38999 connectors and it is solely dedicated to the dismounted control systems that form part of the system is this a valid assessment? 

  • 0 in reply to gkenyon

    Though I could observe this is not wiring that is for supply of mobile equipment, rather this whole thing is mobile and has its own genset onboard - but of course ideally a similar safety standard should be observed. I don't think BS7671 is feels like quite the right approach.
    And there is a further thought - when was it brought into service ? It may well have been designed with the philosophy of some previous regs edition.
    Mike.

  • 0 in reply to gkenyon
    gkenyon said:
    Circuits for mobile equipment for use outdoors with rated current less than 32 A must be protected by 30 mA RCD in AC systems, and exceptions are not permitted ... Regulation 411.3.3

    A very good point! However, most things which are not planted into the ground are mobile if you have the resources.

    Fortunately, BS 7671 provides us with a definition. I doubt that Samantha's machinery falls into that category.

  • 0 in reply to gkenyon

    Worryingly that is not how that was interpreted. From the conversations I had when I started here, 411.3.3 was interpreted to mean that the mobile equipment section was also in reference to socket-outlets. I'm guessing it would have read 'Additional requirements for socket-outlets and for the supply of mobile equipment with sock-outlets for use outdoors ' or something similar if that had been the case.

    I've check it against the 17th edition which was in force at the time and the part of 411.3.3 referring to mobile equipment remains the same. Thank you Graham. While it isn't really what I wanted to hear it was very helpful.

  • 0 in reply to Philip Oakley

    Sorry about the delay been chasing DEFSTANs etc. The inrush is caused by a COTS filter circuit on the charger section of the Uninterruptible power supply distribution unit.

  • 0 in reply to Samantha Rendell
    Samantha Rendell said:
    The inrush is caused by a COTS filter circuit on the charger section

    Yep, the filter circuits are very common cause of deliberate leakage and initial surge for the residual/ground current.

    The purpose is to make RF currents on the Live-Neutral lines [conventionally viewed as being 'on the live' relative to an ideal Neutral, or supply relative to an ideal 0v] appear as if centre tapped and anti-phase on the cabling relative to chassis/ground/earth to minimise net radiation from the antennae-like cables.

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