Fast question for he electricians

[Gpax7]

If there are two each 120 volt lines and a neutral coming in from the pole, where and how in the connection box coming into the house is the input power grounded? In wiring a connection box, the two hot leads go to their own busses. Where does the neutral go? If the third wire ground from the load receptacles is connected to its own bus and earth ground in the box, what is the interface connection between the input power ground and the load ground? Or
is there one?

 

[Nords]

If you're sticking your fingers behind the main panel then I sure hope you have a reference like page 91 of "Wiring A House".

The neutral wire connects to one of the neutral buses on either side of the panel. There should be a tie bar connecting the neutral buses at their bottoms so it shouldn't matter which neutral bus the neutral wire connects to. If you don't have a tie bar between the neutral bus then you need to convert your subpanel to a main panel by adding that tie bar.

The neutral is grounded at the utility transformer and at the main panel. The panel itself has a grounding wire from the neutral bus to a no-foolin' main house grounded earth connection (which also connects to the house's cold-water pipe). The panel also has a "panel bond" screw from the neutral bus to the panel designed to make sure that the panel case is grounded to the neutral and doesn't somehow develop a voltage.

Page 8 shows how it works. A 120V load takes current from the hot leg's wire to the load and back through the neutral wire-- in other words the current is flowing directly from the transformer to the panel and then out through the load and back to a grounded neutral. (The electrons complete the circuit by running from the house's earth ground back out to the transformer ground.) A 240V load uses each of the 120V lines (whose net difference is 240V due to their opposite polarity from a center-tap transformer) and doesn't include the grounded neutral at all.

So the grounded neutral has current flowing through it (which can be checked with a clamp-on ammeter), but it has zero potential with respect to ground (and hopefully also with respect to you, which can be checked with a voltmeter). You're OK as long as you don't become a more attractive grounding path than the 120V neutral wire or get into parallel with a 240V load.

It's pretty hard to be an attractive 120V ground unless you're soaking wet... GFCI breakers (& GFCI receptacles) are designed to detect the small leak-off current that has a couple microseconds to try to kill you before the GFCI trips. They're pretty cool little pieces of equipment.

If you happen to ground one of the live feeds to a 240V load then you are well and truly screwed for a few more milliseconds-- after that it's EMS's and probably your survivor's problem.

Submarine electrical buses are ungrounded on the theory that it's a more reliable way to get power to the equipment without having to worry about maintaining a grounded neutral return. It saves a lot of wiring on 450V AC and 120V AC feeds, too. (Some local compartment receptacles are grounded separately.) The ungrounded buses are tested hourly by watchstanders to make sure that a ground fault isn't developing, but sometimes the ground faults come looking for you and find you before it's time for another set of logs. However NAVSEA decided that equipment survivability was more important than the occasional fireball.

I hope this satisfies your curiosity without giving you the feeling that you're capable of wiring a panel. If you're planning to tackle some home improvment then do yourself a favor-- seek professional help or go read books from your local library or from Home Depot before trying this at home. I've seen too many guys get killed (or worse, injured) to try to figure this out on my own.

 

[Gpax7]

Nords, thank you.

 

[mathjak107]

alot of time grounds cause more problems than they are worth...most electronics should be on whats called isolated ground receptacles with a dedicated ground wire thats not grounded to the building in anyway but directly to earth ...usually rfi and emi like to accumulate in the ground ckt as a building makes an excellent antenna and do to differences in ground potential can actually have their own little circuit of noise flowing in the ground side....we sell quite a few circuit breaker panels here in nyc with oversized neutrals because of all the extra noice current flowing and burning out the panel neutrals ....isolated ground receptacles have a seperate ground thats grounded directly to earth as opposed to using the common grounding of the building...

 

[mathjak107]

another coomon problem alot of us have is ground loop hum...thats when you have your stereo connected to your tv and every time you have both on together you hear that annoying hum.......if anyone has that problem i found a way to get rid of it after a major annoyance over the last year..no point going into it unless someones having the problem too than ill explain....

 

[riskadverse]

another coomon problem alot of us have is ground loop hum...thats when you have your stereo connected to your tv and every time you have both on together you hear that annoying hum.......if anyone has that problem i found a way to get rid of it after a major annoyance over the last year..no point going into it unless someones having the problem too than ill explain....

I'm game.  I don't have the problem currently, but have had similar problems years ago.  It seems like running a grounding wire between the two devices was a solution.  What was / is your solution?

 

[mathjak107]

get 2 radio shack coax to 300 ohm adapters,make sure they have no common ground,i can get you the part number of the correct ones if you need...connect the twin leads of both transformers to each other making a little isolation transformer....in with your coax on one end and out with the coax on the other...no more hum...works like a charm....