Samlex PST-1500-12 grounding?

Re: Samlex PST-1500-12 grounding?

Sorry for not replying to your post/question earlier... This is not a simple question and takes a bit of time to type up a clear answer.

First... Why we ground.

1. We ground so that no piece of metal can become energized with respect to other metal. I.e., you don't want to get a shock when holding an electric mixer while touching the kitchen faucet (I remember back around 1960--When using my Mom's mixer, getting shocked doing exactly that).

2. We ground so that if any "hot" electrical wire touches a large piece of metal (kitchen sink, electrical wiring box, gas fueled heater/stove, short circuit inside a metal electric drill, etc.), that instead of energizing the metal, the current will be carried back to the breaker panel and trip the breaker and prevent shock/fire from current finding "other paths".

3. Grounding to that the maximum voltage on the wiring does not exceed the 120 VAC rating (US/North America). If there was a lightning strike or a cross between the high voltage (12,000 volts or higher) and the 120/240 voltage wiring on the pole, the grounding of the 120/240 wiring will prevent the voltage from exceeding 120 VAC inside your home (of course, it is not perfect, you can still get "surge current" for fraction of a second, and lightning does not follow "normal" rules for current flow--But is it better than not doing that).

4. Grounding is required by a few devices to operate correctly. Some tube type florescent fixtures need grounding of the metal fixture so they start reliably (florescent lights are a form of "arc light" and need to start an arc--the grounding helps that happen). Also some types of ignition systems (spark ignition systems for some gas stoves/water heaters) use the grounded neutral to properly sense if the flame is burning or not.

5. Some power systems use ground as one of the current conductors... Specifically, autos, trucks, RV's etc. use the metal chassis of the vehicle as the "return line" for their 12/24 volt systems. This makes for even more confusion as many/most RV's are metal skin but wood construction inside--So electrical fixture/devices still need a ground wire too.

6. Single point grounding of the AC neutral, typically at the main power panel, also allows us to use GFI outlets (ground fault interrupter). Basically the way a GFI works is there is a small "current transformer" wrapped around the AC "Hot" + "Neutral" (Black and White wires). A CT measures the "difference" in current flow between the Hot and the Return wire. In normal operation all current that flows "out" the black wire" is balanced exactly by the return current in the "white or return" wire. All is good. If there is current flow from the black wire to "ground" (green wire, water in pool or sink, etc.), this causes current to flow in the CT coil and "trips" or turns of the AC voltage to the load. Hopefully preventing you from getting electrocuted. If the neutral is connected both at the AC panel and the load (aka ground loop), you may prevent the GFI from detecting the small AC current (0.005 amps will "trip" the GFI), or it will cause the GFI to trip on its own. Note I don't like to use a single GFI outlet on the output of the AC inverter for an RV application. A valid GFI trip (drop a mixer in the sink) will turn off all power from the inverter--And plunge the RV into darkness (if using AC lighting). It is better to have a GFI outlet at each point of use--Will not turn off all power at once, and make it easier to isolate the fault (inside at sink, outside when using power tools, etc.).

So, that is why we ground. In most cases, the appliance will operate just fine without grounding, and many smaller/portable electrical systems (AC inverters, small generators, etc.) do not even ground the AC neutral. Larger systems will usually ground the AC neutral to the metal frame/case of the AC generating device. Which can cause problems.

The problem is called a "ground loop". Generally, you do not want the electrical power to have two parallel paths for current flow. Various reasons for this include--Metal door + device mounted to door--Electrical flow through the hinge can ruin hinge (or bearings such as on a boat propeller shaft) by spot welding the sliding portions or causing corrosion (electrolysis). It is also not a very good electrical connection.

Ground wires are intended to be a second current path to only be used in an "emergency". If you have two parallel paths, you will not know if one path "breaks" until the second path breaks--Then things are "not safe".

Another issue especially common with AC + DC systems... Remember that Power=Voltage*Current. At 120 VAC, 5 amps is not "Big deal". Power=120V*5A=600Watts. However, if that is power drawn from a 12 VDC powered inverter, the DC current path needs to support more than 50 amps (P=12V*50A=600Watts). So, if you have multiple ground connections between AC and DC power systems (aka a ground loop), it is possible for your AC grounding system (using 14-12 AWG wiring--designed for 15-20 Amps maximum) can be overheated by the 50+ amps flowing through the DC side of the power system (parallel ground/return path). That is one of the major "problem" with ground loops.

In a home--We do AC Neutral Grounding by connecting the AC Neutral to Electrical Box Ground and to the Ground Rod/cold water pipes in one place--Inside the main electrical panel.

In a portable power system, we do that by grounding the AC Generator Neutral to the Metal Frame/Vehicle Frame (if RV/vehicle with metal frame). We generally do not tie metal frame to a ground rod unless this is a fixed system (say trailer home). Earth grounding is only a "good idea" if there is lightning in the area and this is a fixed installation (piece of property with no utility power) or other specialized needs (if you are making a stage with lighting/sound system, you probably want to install a temporary ground rod/ground plate for safety).

OK, why grounding, and what we are trying to have happen (safety), and not happen (ground loops).

Please ask questions--This is not simple--And if done wrong, can be worse than doing no grounding. Also, I do make mistakes typing this up and could have forgotten the world "don't" in the wrong place.

By the way, for general electrical information on AC power (power factor, apparent power, MSW vs TSW wave forms, non-linear current flow, lead acid batteries, etc.)--The Samulex manual for your inverter is very good.

http://www.samlexamerica.com/documents/manuals/11001-PST-1500-2000-12-24-0513.pdf

It covers many posts and threads worth of discussion on our forum. It is not electricity 101--But is a very good overview with lots of drawings and useful information.

Interestedly, the only thing it virtually does not cover hardly at all, is grounding.

Next post, how to make that all happen.

-Bill