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Surge Protection, Grounding, Lightning, extension cords
#11
(06-11-2019, 10:19 PM)Roadtripp Wrote: it sounds like the surge came from the new building and extension cord backfeeding the panel and inverter AC side.

In a regular house there is a ground rod at the service entrance and neutral is tied to the ground rod there.  In the main panel all the safety grounds are tied to the neutral in that one place.  

With an inverter or portable generator set up there may be no main panel, there might be no ground rod anywhere.  Safety grounds might be all tied together, maybe not.  There might be no neutral connection to ground.  If it is an RV it might be plugged into an RV 30 amp pedestal that might have safety ground connected correctly to neutral in the main panel.  That safety ground might be connected to the vehicle, or not.  

What probably killed this inverter is a common mode high voltage from the extension cord antenna.  Common mode means both the hot and the neutral went to 10,000 volts together but still only 120 from neutral to hot.  Without a neutral to ground connection the voltage rises until there is a spark.  With the extension cord antenna many volts built up with nowhere to go until the voltage got high enough to cause a spark.  A wire connecting the inverter output neutral to inverter dc input minus would prevent a large voltage build up across the inverter by dissipating it slowly over time.  

The spark always happens inside the most expensive piece of equipment.  Electricity takes the path of least resistance.  Silver is an excellent conductor.  Thirty pieces of silver have very low resistance.  Logic.

Surge protectors respond to differential high voltage not common mode.  If the 120 from neutral to hot gets too high like 240 the surge protector MOV (metal oxide varistor) will conduct.  The inverter itself connected the neutral to hot driving it to 120.  There was nothing to drive it too high.  Lightning did not hit just the hot wire in the extension cord.  Differential high voltages are usually caused by bad wiring that connects 240 to a 120 RV input.  That has nothing to do with an extension cord antenna in a storm.  Typically MOVs don't have enough current capacity to trip a 20 or 30 amp breaker so they just go open circuit.
Say good night, Dick.
[-] The following 2 users say Thank You to Trebor English for this post:
  • rvpopeye (06-13-2019), Roadtripp (06-26-2019)
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#12
OOOOO as Kaylee would say Tron Porn !
Juicy "under the hood" details too !
It's rare finding the good stuff anymore................

If anybody had trouble making it through that , it all checks out !
Give Trebor the password to your breaker panel.
Stay Tuned
rvpopeye



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(What a "Stinkin' " honor !)
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  • Roadtripp (06-26-2019)
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#13
Thanks Trebor, Popeye, and Co. I was gone for a week or two. That’s is a interesting elucidation by Trebor. I’ll need to try and grok it a couple times. There’s a few terms I’m not familiar with. Like ”torn porn.”  
 I’m trying to think like an electron. I’m trying.....aaahhhhh
 So, often the AC electrical ground is bonded with the DC negative common in a battery backup energy system. If this thread is still alive what happens if the AC neutral is bonded with the DC negative common???  You dont want both AC electrical ground and AC neutral bonded to the DC negative?? Is that right? 
 I was not totally clear in the original post that the system inverter that fried after lightning was connected to the grid on input side and extension cord on the output side. A mobile system may or may not be connected to the grid depending how it is designed and what equipment is used. And if grid is available or not. A extension cord is often connecting the inverter to a generator however.  Essentially a mobile system may or may not have extension cord on input and output. Double lightning antenna lol. Lit up and at both ends. Yeehaw!!
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#14
(06-26-2019, 08:24 PM)Roadtripp Wrote: You dont want both AC electrical ground and AC neutral bonded to the DC negative?? Is that right?

You do but it must be done right.  

You can think of an RV like an electric clothes washer or any other appliance.  It has a plug in cord and a conductive external surface.  It can have internal faults that will cause the conductive external surface to have enough voltage and current capacity to be dangerous or lethal. 

In general, faults, short circuits, will draw too much current and blow a fuse or trip a breaker.  There is no such thing as a special fuse that will protect you from an open wire.  An open wire can cause great havoc.

A key concept for safety ground wiring is that the safety ground wire must never be used to carry "neutral" or "return" current.  For example, you have an electric drill with a three prong cord.  The green wire, connected to the round safety ground prong, is attached to the structure of the tool and the conductive outer surface.  There is no connection to the motor windings or speed control circuit.  The hot wire, the black one, brings in the electric power at a high voltage and the neutral wire, the white one, returns the current at about zero volts.  Way back in the system the white wire is connected to the power company neutral and a ground rod.  If the socket you plugged the drill into was a three prong socket you might presume that it was wired correctly.  If the house was built in 1947 it might not be.  Back then they only used the white and the black, neutral and hot.  The person who replaced the two prong socket with the three prong socket could have done one of four things: A) run a green or bare wire through the wall to the panel for safety ground, B) leave the safety ground contact of the socket open, C) if there was a metal tube conduit for the wires connect the safety ground to that, or D) connect the white wire, neutral, to the neutral and the safety ground contacts of the socket.  Option D violates the rule.  When an open wire failure happens there is a problem.  If the neutral wire comes open, loose screw, and is no longer tied to the system supply neutral nothing happens until someone picks up the drill and squeezes the trigger to turn it on.  Then the hot voltage goes through the switch, through the speed control, through the motor to the white wire in the drill cord to the miswired socket.  There the open neutral is not at zero volts but is powered through the drill.  There the neutral connected to safety ground lights up the ungrounded green wire.  That connects to the body of the drill zapping the person holding the drill.  The high voltage safety ground is also connected to the other appliance plugged into that outlet.  My circular saw case is tied to the safety ground so it becomes shocking too.  

A bad safety ground can make the skin of the appliance hot.  That's worse than no safety ground.  Allowing neutral current in the safety ground wiring is bad.  The standard practice is to tie the safety ground to the neutral as far away as possible.  An RV park has a main panel where commercial power comes in.  Same for a house, the main panel where the power comes in is where they get tied.  That makes it so the neutral current returns using the neutral wire rather than the safety ground all the way back to the supply.  If there is a panel in a shed it is a "subpanel" and neutral and safety are not tied there.   

Back to the clothes washer / RV comparison.  The RV body should be tied to the green wire.  The green wire should never carry neutral current.  If a hot wire comes loose and causes a short circuit to the body of the RV that should blow a fuse or trip a breaker disconnecting the supply.  The breaker panel inside the RV is a subpanel and should not allow neutral current on a safety ground wire.  They must be kept separated in the subpanel.  They must ultimately connect or there is no path to allow current in order to blow the fuse or trip the breaker.

Next, unplug the RV from the campground pedestal and start the built in Onan generator.  The transfer switch should connect the Onan hot, Onan neutral and Onan safety ground to the RV hot, neutral, and safety.  The Onan must have safety connected to neutral at the Onan, on the Onan side of the transfer switch.  Instead of a built in Onan you might have a portable Harbor Freight 3500 watt generator.  When you plug the RV cord into the 30 amp socket of the portable generator that plug is the transfer switch.  It completely disconnects the RV pedestal (including the neutral - safety ground connection) and connects to the new source.  The portable generator must have the neutral to safety ground connection. 

If you have a 3 conductor extension cord between the portable generator and the RV AND the safety ground to neutral bonding is done in the RV rather than at the generator AND the neutral wire in the extension cord fails AND the generator does not have a safety ground to neutral connection AND the generator is sitting on moist dirt or concrete AND there is an AC load in the RV turned on like a light bulb, fan, or coffee maker AND you stand on the ground and touch the doorknob of the RV then you complete the circuit.  You want nothing between the safety ground to neutral bonding point and the source of electricity.  At the RV camp ground you are at the mercy of whoever did their wiring.  If you have the generator then it is up to you to put the bonding point at the generator end of the extension cord.  Any circuitry between the neutral to safety ground bonding point and the source of the electricity is an invitation to have neutral current on the safety ground wire which will be bad when the neutral wire fails open.  

Many portable generators do not have the neutral to safety connection.  You need it to trip a breaker when there is a fault.  The facts are you need it and they didn't give it.  Do it yourself.  The generator has multiple outlets.  Get a plug, put one wire in it, safety to neutral.  Plug that into a generator socket.  Now you have it.  But first, use your multimeter and measure, generator off, the resistance from neutral to safety.  It may already be connected.  Open frame noisy contractor generator is more likely than polite quiet inverter generator.  

Now that you have the AC side neutral and safety ground set right turn to the low voltage DC side.  With only 12 volts you really don't need a separate safety ground wiring system.  Your 12 volt side could be completely isolated from everything.  That's the way golf carts are wired.  There are no connections to the chassis.  If any part of the electrical system contacts the chassis there is no current, no fire.  If the plus 36 volt end of the battery makes contact with the chassis because a wire chafes against a sharp edge, no problem.  The second contact will cause a problem.  If the minus 36 volt end of the battery also makes contact there will be great current.  Isolation has advantages.  The main problem is maintaining isolation.  You never know when it is no longer isolated.  Also, you never know how much voltage there is between the two systems.  Static can build up a charge.  While driving wind can deposit a charge on your solar panel.  Tires rolling also transfer charges.  Nearby charged clouds can build up serious voltages.  You don't need a lightning strike on your solar panel to have a charge.  If your 12 volt system is connected with wire that has 600 volt insulation there will likely be an arc somewhere if you shuffle across some carpet and touch part of the system.  

A deliberate high capacity connection between battery minus and chassis is an effective way to eliminate a high voltage difference between the AC and DC systems.  Anyone using  a vehicle alternator as a charge source must have this.  Any equipment that connects to both AC and DC systems (battery chargers, inverters) must be able to withstand thousands of volts or something must be done to eliminate the high voltage difference.  Connect your battery minus to the chassis.  If you really want it open, go ahead.  It will cost some zapped equipment.  That's not a warranty issue, don't return it because you broke it.  Just keep on buying more stuff.  

Where you bond things matters.  You do not want the 12 volts minus side from the battery charger feeding through the green AC safety ground wire to get to the battery.  From the battery charger to the battery use a red and black pair.  The bus bar where all the 12 volt minus things come together is a handy place to connect the DC minus to the chassis and the battery charger minus.  If the inverter has a 200 amp battery connection that is possibly direct to the battery avoiding the bus bar.  There may be a shunt resistor between the bus bar and the battery to measure the battery current.  All battery current, charging or draining, must pass there.
Say good night, Dick.
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  • rvpopeye (06-27-2019), Roadtripp (07-27-2020)
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#15
This link is for a interesting site explaining how to minimize emi and rfi interference in solar electric systems:
https://www.solar-electric.com/learning-...tems.html/
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#16
Good article. I always wrapped my power wires thru ferrite chokes. 4 gauge was the max I could use back then. Probably better wire nowadays.


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