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Guidance on Long-Run DC Cable Design for Lighting

Pabindu Gamage

Hi all,

I’m designing a greenhouse lighting system with a centralized DC PSU feeding multiple luminaires. The DC voltage varies between 200 V and 500 V.

The distance from the centralized panel to the first luminaire is around 50 m, and the total circuit length is approximately 100 m.

I have experience with AC cable calculations and design, but this is my first time working with long-run DC cabling. I would greatly appreciate expert guidance on:

  • Any special considerations when selecting cables or performing cable calculations for DC circuits — what is different from AC design?

  • Relevant standards or documents I should refer to.

  • Use of overload and RCD protection: do I need a specific type of RCD for DC circuits (Type B / RDC-PD), especially since I must provide minimum 300 mA RCD protection for greenhouse lighting?

I’m trying to expand my experience into DCcable design, so any advice or pointers would be greatly appreciated.

Thanks in advance for your support!

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  • 0

    Might be worth noting that for a similar - but different - scenario with DC circuits and elevated voltage and limited fault current, namely PV installations, the solution is to work on the basis of using the protection method of double or reinforced insulation, because ADS is so difficult to reliably achieve. This does of course bring its own problems (uninformed persons expecting the Supply to be Automatically Disconnected in the event of a fault being a significant one). There are cables and equipment marketed specifically to that application.

    In the UK, most PV installations are arranged IT with insulation monitoring. However, some systems, more so in other jurisdictions, are earthed on one pole with fault detection achieved by monitoring current in the earth connection (something like a DC MCB or fuse with auxiliary contact causing the main circuit to trip, or an earth fault relay, depending on the fault level and disconnection requirements). This is likely to be quicker than an IMD and easier to physically measure than a traditional residual (differential) earth fault detection, but may not meet your requirements for an agricultural facility. You also need galvanic isolation to ensure that the earth fault currents can't escape around your earth link via the AC system.

    Who specified the DC voltage? Assuming that they think it normal for a greenhouse, perhaps they can provide more guidance?

    Also, a significant gotcha that I don't think has been mentioned yet: The cable specification might be relatively straightforward but do make sure all your switchgear, connectors (etc) are rated for the appropriate DC application. Don't assume it'll be fine... it probably won't. Most of the time DC will derate the equipment, and a lot of switchgear isn't even tested/rated for DC.

  • 0 in reply to Jam

    Hi Jam, 
    Thank you for your support.
    After all the comments by everyone I am about to ask the PSU supplier to incoperate ADS with IMD for the system. 
    About the DC voltage, this is the operating voltage of lights. it wont be 500V all the time but 200V-500V (vary with dimming condition). 

  • 0 in reply to Pabindu Gamage

    Can I ask then why the unusual choice of a high level of DC, given the disadvantages in terms of switching protection and so on? Dimming lamps on AC is not especially difficult and I dont think I've ever been in a greenhouse lit by anything other than 50Hz mains or ELV.
    Mike.

  • 0 in reply to mapj1

    ITs to keep the power supply units centralized. given site level difficulties. 

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