Correct way to install negative ground equipment in -48VDC environments

Question

Hi,

I have a slight problem; I have a site where the battery backup system is a -48VDC system. Now, I have equipment that needs a 24VDC feed, so I need to use a power converter to step down the voltage.

The problem is that all power supplies from Phoneix Contact (our supplier) take positive voltage. As I've understood it, this doesn't really matter since I can just flip the input wires since the potential difference is the same regardless of the grounding scheme. My question is, how do I ground this, and what do I need to check to make sure that I don't short something out when attaching grounds together.

On another forum, I found a thread that stated as long as the power supply has isolated inputs and outputs (the 48/24VDC converter I've chosen has a 1,3kV isolation), and I leave the output side floating, I should be ok regardless of if the battery system is floating or tied to earth.

And as I understood it, even if the 24VDC side has it's negative terminal tied to ground.I should be ok due to the isolation between the inputs and outputs of the power supply?

jw0752 · Accepted Answer

Hi Sebastian, If your inputs and outputs of the converter are isolated, which means that none of the leads is considered as a ground, then you should be able to make it work. The piece of equipment that has the 48 volt positive ground system will supply the power to the input of the converter. The converters input positive will be tied to the machine's ground and the converters negative input will be tied to the 48 volts negative. The negative of the converter output will be tied to the ground of the 48 volt system and the positive of the output will supply power to your final unit. The only thing to keep in mind is that there will be a potential difference of 72 volts between the power to the 48 volt system and the power to the 24 volt system. The converter that doesn't carry the ground of the primary though to the secondary may be less common than the ones that do so make certain you get what you want.

John