The issue is if the power supply or any of the equipment connected to it has positive tied to ground and then you take another device with negative tied to ground and put that on the same power supply. Then you have + and - of the power supply dead shorted through the ground connections and that's when the sparks fly. Some people keep the -48V stuff in a separate rack. That's not necessary, but it's not a bad safety measure if you have happen to have enough space for two racks. It's nice to say
If the equipment has - tied to ground or tied to the chassis then it has to be isolated from your -48V power supply. You can still put it in the same rack as everything else, but it needs a separate power supply isolated from your -48V power system. So you get an isolated 48V to 48V power supply (Meanwell RSD-300C-48 is one example), or you plug that one device into an A/C outlet or inverter or what have you. I have test ONT's with -ground sitting in the rack with with a bunch of -48V (+ground) routers and it's all fine because they have their little DC transformers plugged into a wall outlet. Someone said this, but to check if the equipment has a ground reference set your multimeter for ohms and measure from each power input to the ground lug and/or chassis of the device. If you're reading an open circuit then you're fine. You're probably fine if you're reading a very large number of ohms --I'd hate to give a specific number and say that's always safe, but the Meanwell RSD I mentioned above is 100M Ohms from Input to Output per the datasheet and that device has not failed me. If you're seeing 0 or single digits from one of the power leads to ground then you have power bonded to ground on that device. An example fresh in my mind is Mikrotik CRS305 has - tied to ground, so although it will run on 48V I could not put it on our -48V rectifier. Lots of equipment intended for telecom will have + connected to ground. Also lots of equipment will have neither tied to ground and those often work with either +V or -V (but I'd go by what the manufacturer says first, and by the multimeter second). -Adam -----Original Message----- From: AF <af-boun...@af.afmug.com> On Behalf Of Mark - Myakka Technologies Sent: Monday, January 08, 2024 11:26 AM To: af@af.afmug.com Subject: [AFMUG] -48V kicking the dead horse I know we have been though this many times and I thought I understood it. -48VDC is the Negative side being HOT, correct? It is BAD to try to mix -48VDC and 48VDC There is no such thing as a -48V battery. A battery is a battery, correct? How about the ICT Platinum power supplies. They show as 48VDC, can they be used on -48VDC equipment? I remember Check saying something about a way to test to see if a piece of equipment that is Neg 48VDC is truly grounded as Neg 48VDC. -- Thanks, Mark mailto:m...@mailmt.com Myakka Communications www.Myakka.com -- AF mailing list AF@af.afmug.com http://af.afmug.com/mailman/listinfo/af_af.afmug.com -- AF mailing list AF@af.afmug.com http://af.afmug.com/mailman/listinfo/af_af.afmug.com
