“Negative” 48 Volt Power: What, Why and How

[sirhc]

Configuration Defined-48V DC Battery and Earth
Telecom and wireless networks typically operate on 48 volt DC power. But unlike traditional 12 and 24 volt systems which have the minus (-) side of the battery connected to ground (i.e. called negative ground systems), telecom batteries have the plus (+) side of the battery connected to ground, called a positive ground system, also designated as “negative 48 volt”. In this configuration the minus side of the battery becomes the “hot” conductor and the + is no longer hot, rather it’s at zero potential as it is connected to ground and referred to as the “common” or ”return” wire. Despite its complexity and propensity for confusion, described below, “neg” 48 volt is the common choice in DC power for wireless networks.

History
Why is the positive side of the DC circuit connected to ground in telecom applications versus negative ground used, as used, in automotive and other industrial dc systems?

Back in the day, when Telephony equipment was being developed, 48 was the chosen system voltage because it’s considered safe “low voltage”, and reduced amperage requirement of equipment powered at this voltage. This allowed use of smaller gauge wires, while still being capable of carrying power over long wire runs with minimum voltage drop as a percentage of the operational voltage. The early phone systems were configured as negative ground, this however resulted in corrosion on the conductors caused by electrolysis when the + wires were subjected to moisture (you can see evidence of this condition on your car battery where the corrosion builds up over time on the + terminal post). To remedy this problematic condition, the systems were changed to positive ground and the destructive galvanic corrosion was eliminated by the cathodic protection provided by grounding the + side of the circuit.

Precautions
This positive ground configuration does not cause polarity to change, plus (+) is still plus and carries a positive charge with respect to the negative (-) terminal or minus. Many a short circuits have occurred when installers assume that when changing to positive ground, polarity changes as a result, not true! Regardless of ground reference, connecting (+) plus to (-) minus will still result in either a short circuit or reverse polarity to equipment.
Another factor that can cause confusion (and potentially sparks) is use of red & black colored wires. In negative ground systems, red is universally understood as hot and +, in positive ground, this “red wire” is no longer hot, but is still +. So you can imagine an installer standing there with a black wire in one hand and red wire in the other looking at the input terminals on a “neg 48 volt” transmitter that are marked “HOT” and “RTN” asking himself “What goes where?” Answer is: black to plus and red to minus, which is a bit counter-intuitive. Thus, we recommend a common color be used for both conductors and the wires labeled with the polarity.

Another word of caution on system integrations that mix positive and negative ground equipment, Ground Isolation must be maintained between these operating system to prevent short circuits and equipment compatibility issues. In addition, there is the issue of continuity between chassis ground and system ground; they can be common or may be isolated (called a floating ground).

NOTE: The only switch we make that is rated for negative 48V is our WS-26-400-IDC. The power supply in it has polarity protection so if you hook it up backwards it simply will not power up and not harm it. Since the unit is for positive and negative DC sites the color code of the DC input terminals is coded for positive DC sites as they are most common.

WS-26-400-IDC is this model: https://www.netonix.com/wisp-switch/ws-26-400-idc.html

[Omniflux]

A common practice to reduce confusion on systems with both positive and negative grounded circuits is to use three colors.

The IEC standard for this is:

• Positive: brown
• Mid-wire (neutral): blue
• Negative: grey

However, as with passive PoE, there is a defacto standard using colors more commonly on hand:

• Positive: red
• Mid-wire (neutral): white
• Negative: black

When this setup is used and your terminals are color coded red and black, white is the only color that does not match

• Negative grounded systems use red and white - red connecting to red terminals and white connecting to black terminals
• Positive grounded systems use black and white - black connecting to black terminals and white connecting to red terminals

[sirhc]

Yes but the real problem is people will use one of our non ISOLATED DC switches like the WS-8-150-DC / WS-12-DC/ WS-12-250-DC / WS-26-500-DC and simply think if they flip the polarity it will work.

Well the switch will power up but as soon as you provide power to a radio on the tower that has POE DC negative bonded to rail / chassis / tower then POOF you fried the switch and maybe the radio.

Most radios WISPs use such as UBNT / MIMOSA / Cambium / IGNITE all bond POE DC negative to chassis ground and the shelter has positive bonded to ground and the cabinet ground and tower grounds SHOULD ALWAYS BE BONDED so when you power up the radio you create a DEAD SHORT - POOF

At negative DC sites you must use the WS-26-400-IDC.

You ALWAYS bond shelter / box / AC Service ground to tower ground system else you will also blow up equipment with GROUND CURRENT (Ground Loop) passing through the switch to the radio to the tower to the tower ground.

READ THE POSTS BELOW:
https://forum.netonix.com/viewtopic.php?f=17&t=5495&p=29602&hilit=Rules+of+thumb#p29602
https://forum.netonix.com/viewtopic.php?f=33&t=5496&p=29621&hilit=MOst+DC+chargers+also+bond#p29621

[dscott]

We are putting in a WS-26-400-IDC in a -48v site powered via an Eltek rectifier. The site has 24 & 48v hardware that will be powered via the Netonix, and other -48v hardware including backhaul radios and a switch.

I have questions about how this should be grounded. Currently the positive ground on the rectifier and -48v hardware (switch and backhaul radios) is grounded to the tower ground as it should be on a -48v site

Should the ground on the Netonix be connected to the tower ground as well? What about the positive 24 or 48 items that it powers on the tower? We are concerned about creating a bridge between the - and + polarities that might cause issues and I would just like some explicit guidance on the best practices for this configuration.

[sirhc]

We are putting in a WS-26-400-IDC in a -48v site powered via an Eltek rectifier. The site has 24 & 48v hardware that will be powered via the Netonix, and other -48v hardware including backhaul radios and a switch.

I have questions about how this should be grounded. Currently the positive ground on the rectifier and -48v hardware (switch and backhaul radios) is grounded to the tower ground as it should be on a -48v site

Should the ground on the Netonix be connected to the tower ground as well? What about the positive 24 or 48 items that it powers on the tower? We are concerned about creating a bridge between the - and + polarities that might cause issues and I would just like some explicit guidance on the best practices for this configuration.

If you are using a WS-26-400-IDC at a negative DC site you do not need a rectifier, it is designed to accept -48V as it has an isolated power supply and if you hook it up backwards it will not damage this unit like it would the WS-12-250-DC or WS-26-500-DC which will fry if hooked up backwards or will fry once you go to power radios as they do not have isolated power supplies. If you hook the WS-26-400-IDC up backwards it simply will not power up.

As far as what you can power with this unit you need to verify our POE pin-outs to insure your device will accept the power/polarity we are providing on the pin outs.

Our POE pin out polarity is in the spec sheet and in many posts like this post
viewtopic.php?f=6&t=1178#p8809

[clearnetworx]

We use ALGCOM -48V UPS. I tested the common and -48v outputs continuity to chassis ground and found no connection. Safe to use? We already have at least one tower site setup this way running fine.