ECN Forum Forums General Topics of the Trade General Discussion Area DC System - Grounding and fusing!

Which is grounded in a DC system, the (+) or the (-)? Which is refereed to as the common? And finally which one is fused? I always thought or was tought that the neutrons flow from (-) to (+) therefore I was thinking that the (-) is the hot...but I am seeing differently in prints.

These are all design issues. The TELCO grounds the + because they say it helps electrolysis problems. "Common" may be the ground but it is also used to identify a point where two or more supplies are connected together. Your PC supply is an example. There are several voltages, some connected + to common, the others - to common (AKA DC ground), which gets grounded to the case and EGC from the system board mounting tabs. The result is you have plus and minus voltages, referenced to ground.

I can help with this as I have designed Telco plants for about 30 years.

It is completely a design issue. You can ground either polarity or none of them. As a general rule here are the standards.

-48 Volt DC plants bond the positive polarity to ground, referred to as return or common. The hot or negative polarities have the OCPD installed.

+12 and +24 Volt DC plants bond the negative polarity to ground, referred to as return or common. The hot or positive polarities have the OCPD installed.

The third option is to float the plant where neither polarity is grounded. In which case ground fault detection is installed and both polarities have OCPD installed. Very rare and very expensive to implement.

My experience is with bulk DC (typicaly 120V, 20A branch circuits, 100 and 150A rectifier output) for lab use. The supplies that we use are rectified off of the AC mains, and of course filtered.

In this case distribution is thorugh a DC listed pannel with two busses. Two pole CBs are used one protecting positive and one negative.

Ground fault portection is provided by the neutral bond at the rectifier. One of our rectifers has the ability to float this bond, but it has an integral GFCI.

This setup is used maily for power DC motors, and sometimes for the input stage of DC/DC converters, any application that requres 'clean' DC I still use a small switchmode suply.

The - side is usually "grounded" because it helps reduce galvanic corrision. Water molecules are polarized- because of wave physics and molecular geometry, the electron spends relatively more times around the oxygen atom than the hydrogen atoms. As a result, the oxygen atom has a slight - charge and the hydrogen molecules a slight + charge. So, if we give the steel chassis of a car (or a building, plane, ship, flashlight, etc), it repels the oxygen atoms in water and is less likely to oxidize and corrode. The + terminal, on the other hand, agressively oxidizes, but it's a relatively small area and can be maintained with dielectric grease or cleaned off.

Vehicles and other items that aren't solidly grounded (cars, etc) will always ground - and fuse +.

On our DC circuits (UPS batteries, 384 or 480VDC), we ground the - terminals and run the + through the breakers, but I think it's more habit than anything else- it's a benign environment and considering it's grounded, grounding the + would actually make more sense from a corrosion-prevention standpoint.

Most of our DC sources are left floating. Of course our biggest source, traction power, has the negative return at a near ground potential. Since the return path is also part of the supporting structure of trucks wheels and running rails, it would be very hard and dangerous to elevate it well above ground. Since touching the third rail is as dangerous as messing with Social Security, it is kept elevated and isolated on stand-offs called frogs. If one of these frogs gets fried, it will not taste like chicken.
Joe

This is rather interesting with all you US people saying that the Positive is always the Pole Grounded, here it would be the Negative,
people have been arguing about this since electricity was discovered, as in which way does the current flow?.
There is Conventional Current Flow(+ve to -ve) and there is Electron Current Flow (-ve to +ve), the US being (obviously) different to anything the English did, took on Conventional current flow ideas, that is why most cars in the US have a positive ground in them, while the rest of the world is negative ground.

Mike there has not been a 'positive ground' car made in the US since I would bet the 1940s or earlier.

I was under the impression that was an English practice. I believe Triumph Motorcycles where positive ground.

Indeed. Many British vehicles were positive-chassis right up until the 1960s.



Don't N.Z. telephone exchanges use positive ground on the batteries?



As does the traction power for the 3rd-rail system employed on the southern region in Britain as well.

The London Underground (subway), however, actually uses a 4-rail system with neither pole solidly grounded. Balancing resistances are used to set the potentials in a 1/3 to 2/3 proportion, resulting in nominal voltages of about +420V and -210V relative to ground.

The outside conductor rail is the positive, central rail is the negative:

As for US built vehicles, as far as I know GM were always negative ground while Ford and Chrysler were positive ground until around 1955 when the transistor became popular and most cars went to 12 volt.