Neutral - ground bond...clarification?

[jim mccloy]

I do not consider myself a bonified electrician. I have an associates degree in maintenance electricity from an acreddited trade school, but do not have the benefit of years of experience in the trade.

I was browsing thru some archived posts and stumbled onto a thread that provoked some thought.

The thread concerns emergency generators (residential application). I recently had to use mine and while I was talking with one of the power company employees he mentioned that he could tell that I had been running a generator.I asked him if he read voltage or current with a meter or some other instrument and he just said that he could tell that there was something back feeding. It had been raining out very strongly and the ground was saturated.

I figure he was reading some current returning on the grounded conductor as I do not have the type of transfer switch that switches the neutral (I believe) It's a gentran model.

Ok...my questions: While reading through some archived posts I came across a thread that talked about whether or not the grounded conductor should be swithched in order to prevent backfeed onto that conductor. It was stated that if the neutral was bonded to the generator than the grounded conductor from the premises wiring must be switched in order to prevent the backfeed.

If I have an unsafe installation then I want to correct it. I didn't perform the original installation.

Also, on my generator,(it is a 5000 watt devilbiss) the two receptacles on the genny are three prong, but where the twist lock connector plugs in at the gentran xfer switch it's four prongs...obviously one blade of the receptacle on the xfer switch is not being used. Why is the xfer switch receptacle provided with four blades?

 

[Ed MacLaren]

Re: Neutral - ground bond...clarification?

Jim, the switching of the neutral is not done to prevent "backfeeds".

The neutral must be switched if the generator neutral is bonded to the frame in order to prevent the equipment grounding conductor (EGC -green) from being in parallel with the neutral when the load is on utility power. (Diagram 2) This configuration is called a separately derived system.

When the neutral is not switched, it (the neutral) must not be bonded to the generator frame. (Diagram 1)

Why is the xfer switch receptacle provided with four blades?

There should be a four-wire cord between the generator and transfer switch. The fourth wire is the EGC. (green in diagrams)

Ed

[ June 27, 2004, 05:25 PM: Message edited by: Ed MacLaren ]

 

[jim mccloy]

Re: Neutral - ground bond...clarification?

Ed, Thank you for the diagrams, I have further questions though. How is a four wire cord connection possible when the 120/240 volt receptacle on the genny is only a 3-slot configuration?

And am I correct in thinking that with my current installation, there will still be return current returning through the grounded conductor in my service panel out to the utility lines?

jim.

 

[Ed MacLaren]

Re: Neutral - ground bond...clarification?

How is a four wire cord connection possible when the 120/240 volt receptacle on the genny is only a 3-slot configuration?

You or your electrician could replace the existing receptacle with the correct one.

And am I correct in thinking that with my current installation, there will still be return current returning through the grounded conductor in my service panel out to the utility lines?

No, when the load is on generator power, any unbalanced load current on the neutral flows back only to the source it came from, the generator.

Ed

 

[jim mccloy]

Re: Neutral - ground bond...clarification?

I understand now. but what I'm wondering is...how in the world did the lineman detect that I had been operating a generator. I know for a fact that he could have heard the unit operating being that the transformer is only about 100 yards from my house...but he said that he detected some "Back feeding". I even had my main shut off. That's why I thought he found return current coming back through the grounded conductor.

He could have been just telling stories...I dunno.

Jim.

 

[dereckbc]

Re: Neutral - ground bond...clarification?

It is possible, especially if you have a metalic water pipe sharing a transformer with another neighbor.

 

[sandsnow]

Re: Neutral - ground bond...clarification?

I have a question in regards to Ed's post. I understand that w/a solid neutral XFR sw the neutral cannot be bonded at the gesnet. When the XFRsw is in genset position, a ground fault at the load would travel to the service on the EGC, then back on the neutral to get back to the source(genset), then to the OCD in order to trip. From what I have learned that could be a high impedance path if it is long enough. I'm thinking commercial application here, where large distances could be involved.
Could you please clarify if I'm thinking this wrong?

 

[Ed MacLaren]

Re: Neutral - ground bond...clarification?

a ground fault at the load would travel to the service on the EGC, then back on the neutral to get back to the source(genset)

Yes, that is correct. These diagrams are copied from diagrams published by Square D, in their transfer switch wiring instructions.

It would be wise to install the generator and transfer switch as close to the service as possible.

then to the OCD in order to trip.

It is assumed that there is an OCPD at the source, the generator.

Ed

 

[hurk27]

Re: Neutral - ground bond...clarification?

Jim
As far as what the line man said about back feeding, if the generac transfer panel (if it is the type I'm thinking of) has the multiple wires that you must hook up for circuits in and out (from the circuit breaker/ to the circuit) then it is very possable to back feed the utility. if for any reason in was crossed with the out cables then you stand a chance of electrocuting a line man as your generator can back feed the power transformer on the pole which will heat up the primary line when the line man thinks it's dead. these panel have a set of wires for each circuit one from the original breaker and one to connect to the circuit you want to feed from the generator. if these get swapped then what I said will happen.

And it is easy for a line man to find out which one was back feeding as the meter will run backward. all he has to do is call in to find out the last meter reading and he's got you.

So I would have the transfer panel double checked if it is this type and make sure it is wired correctly.

[ June 29, 2004, 12:24 AM: Message edited by: hurk27 ]

 

[jim mccloy]

Re: Neutral - ground bond...clarification?

Thanks Wayne, I'm going to double check the wiring in the GENTRAN switch per the installation instructions and see if something is crossed up.

Also, I guess if I wanted to truly investigate whether or not there is someting backfeeding out to the utility lines, I could run my genny and shut the main off and do an amp test.

I'd then know for sure.

Jim.

[ June 29, 2004, 01:20 AM: Message edited by: jim mccloy ]

 

[jes25]

Re: Neutral - ground bond...clarification?

quote:
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How is a four wire cord connection possible when the 120/240 volt receptacle on the genny is only a 3-slot configuration?
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You or your electrician could replace the existing receptacle with the correct one.


Ed I thought the generators that had a bonded neutral and ground had a 3 wire plug and the ones with a floating neutral had a 4 wire plug. If this is true you could not just change the receptacle. Correct?

 

[Ed MacLaren]

Re: Neutral - ground bond...clarification?

I thought the generators that had a bonded neutral and ground had a 3 wire plug and the ones with a floating neutral had a 4 wire plug.

That is quite probably correct.

Jim's generator is likely one that was designed for cord connected loads only, and never intended for connection to an interior wiring system by way of a transfer switch.

However, I maintain that if it is going to be connected to his wiring system, it should be done with a 4-wire cord and receptacle.

Ed

 

[jes25]

Re: Neutral - ground bond...clarification?

I agree it needs a 4-wire, my point was I dont think it is as simple as changing the plug. You would have to change the plug and remove the internal bonding jumper which I dont think its a good idea to tamper with equipment like that.