Why Grounding

Re: Why Grounding

Welcome to the forum.

Grounding the negative side of DC gives it zero Voltage potential, making it safe to touch. Then if a short between positive and any other point (such as a metal casing of an inverter) should develop the current flow should instantly go high and trip the circuit protection. Shuts everything down safely without risking fires from arcs or high current and without possibly shocking anyone touching anything.

This is the same principal as with AC grounding, but most of the time the DC Voltage is low enough that the hazard is minimal anyway. For instance your car has a 12 VDC system but obviously is not Earth grounded: it's almost impossible to get a shock from that low a Voltage and the system runs below the sustainable arc threshold. The higher the DC Voltage, however, the greater the risk. A PV array may be well above system Voltage (70+ for a 48 Volt system for example) and quite capable of sustaining an arc (hence the new inclusion of arc fault protection on MidNite equipment) or delivering a nasty shock. Grounding minimizes but does not eliminate these risks.

Some of us have argument with the NEC regulations for DC ground fault because the negative is now required to not be grounded. This opens up a new set of potential problems in efforts to eliminate one.

And since this is a thread about grounding, I will now sit back and let the disagreements begin.

Re: Why Grounding

If you touch either positive or negative of a battery you won't feel anything because you are not completing a circuit. The problems arise when something goes wrong and something that is not supposed to be connected to power is; then you can touch both sides accidentally and get zapped.

Normally conductive casing is tied to ground so that if any positive wiring comes in contact with it the circuit protection will trip. If the connection is made by you personally you will feel it. Low Voltage is a tingle, but the higher it goes the more dangerous it becomes.

This is where the NEC DC GFCI comes in to play; by judging that equal current is flowing through both negative and positive lines it presumes there is no "incorrect parallel path" as in through your body. This works fine on the GT arrays it seems to have been intended for. Through batteries in and things are different (two power sources, one current based the other Voltage based, need to interrupt both lines of both sources simultaneously to shut down all power - complicated).

Touch positive to negative at any Voltage and you get a spark. You can start a fire with a couple of 'D' cells and some steel wool. How much spark/arc, current heating, or shock potential depends on how much power is available from the source. A 12 Volt car battery won't shock you, but you can weld a wrench across its terminals. A 400 Volt PV array can stop your heart even at very low current.

Human skin resistance is variable.

Add grounding and what you get is a lot of debate about how best to do it. Most DC grounding practices have been centered on sinking current to trip circuit protection in the event of a short. Now there are efforts being made to protect against electrocution (ground fault interrupt) and fire hazard from arcing.

Two other aspects of grounding: RFI control and lightning protection. Both of these topics are really complex. There is a very long and involved thread about RFI here: http://forum.solar-electric.com/showthread.php?5887-Radio-Noise-from-PV-System Note that it is not as simple as "add ground = noise goes away". Lightning has been and no doubt will be discussed numerous times. The short version is you can't do anything about a direct strike, you can only minimize the damage potential from stray Voltage caused by nearby strikes.

Oh and someone is bound to mention EMP's and grounding's relation to them too. Another of those complicated "do-I-really-have-to-know-this?" subjects.

Electricity is dangerous. You can not make it 100% safe. You can only minimize the risks.