Grounding/Neutral in building structure supplied by feeder

[electricalist]

 

[kwired]

So I finished the job put in a ground rod and ran a #6 ground with the 1/0. When I hot checked everything I did a continuity test on the grounding and grounded conductor at the sub panel and I had continuity between the two even though they are floating. I got to thinking about it, there is continuity because the grounding conductor feeding the sub panel is tied in at the main panel with the grounded and grounding conductors on the same bar. So even though they're separate at the sub panel they are still bonded together, correct? So why even float them at the sub?

They are not "floating" they are bonded together back at the service as well as grounded back at the service. Though "floating" isn't exactly a trade term and could have different meaning to different people or in different situations, but to "float a neutral" typically would mean it is not grounded to most people.

The reason we separate "grounded" (typically the neutral) conductors from "grounding" conductors is to reduce rise in voltage on equipment that is bonded to the equipment grounding conductor. The "grounded" conductor is intended to carry current - if it is carrying current it will have some voltage drop on it. If you then bond equipment housings to the same conductor with that voltage drop imposed on it you will have voltage between that equipment and true ground. It may only be a volt or two in most cases, but in the right situations that is too much - just ask dairy farmers what "stray voltages" do for their cows.

 

[electricalist]

They are not "floating" they are bonded together back at the service as well as grounded back at the service. Though "floating" isn't exactly a trade term and could have different meaning to different people or in different situations, but to "float a neutral" typically would mean it is not grounded to most people.

The reason we separate "grounded" (typically the neutral) conductors from "grounding" conductors is to reduce rise in voltage on equipment that is bonded to the equipment grounding conductor. The "grounded" conductor is intended to carry current - if it is carrying current it will have some voltage drop on it. If you then bond equipment housings to the same conductor with that voltage drop imposed on it you will have voltage between that equipment and true ground. It may only be a volt or two in most cases, but in the right situations that is too much - just ask dairy farmers what "stray voltages" do for their cows.

Is that what happened to blue bell?

 

[kwired]

Is that what happened to blue bell?

Sure why not? People like to blame electricians for everything else.

 

[electricalist]

There will be no blue bell at the ranger game.

 

[zcanyonboltz]

They are not "floating" they are bonded together back at the service as well as grounded back at the service. Though "floating" isn't exactly a trade term and could have different meaning to different people or in different situations, but to "float a neutral" typically would mean it is not grounded to most people.

The reason we separate "grounded" (typically the neutral) conductors from "grounding" conductors is to reduce rise in voltage on equipment that is bonded to the equipment grounding conductor. The "grounded" conductor is intended to carry current - if it is carrying current it will have some voltage drop on it. If you then bond equipment housings to the same conductor with that voltage drop imposed on it you will have voltage between that equipment and true ground. It may only be a volt or two in most cases, but in the right situations that is too much - just ask dairy farmers what "stray voltages" do for their cows.

Thanks making more sense, but then the grounded and grounding conductors are not really separated at all shouldn't they be separate at the main panel as well to be truly separated? I mean the current can still get back to the panel on ground and the ground can also add to the voltage drop right? I was thinking how does a plug tester know when ground and neutral are reversed? They both are connected to same bar? It can't know one is bare or green and one is white.

 

[zcanyonboltz]

Adding this to my to get tool list.

 

[electricalist]

A co worker gave me the tester for Xmas.
I won't say it's great. All you do is turn it on and that's it, It will give ac, dc, and continuity, so you have to be sure of what you are testing. If you have a dead circuit but there are incandescent lights on the circuit, it beeps continuity. If it's a dc ballast it will say dc and give volts,. Same with ac.
It confuses most that have used it.
The leads in the picture go to my ideal circuit tracer but they are handy for my tester.
I find when trouble shooting things where mwbc are used the 20' leads really help When looking for open neutrals or open circuits by getting one lead to one device and have the tester at the other.

 

[Little Bill]

Thanks making more sense, but then the grounded and grounding conductors are not really separated at all shouldn't they be separate at the main panel as well to be truly separated? I mean the current can still get back to the panel on ground and the ground can also add to the voltage drop right? I was thinking how does a plug tester know when ground and neutral are reversed? They both are connected to same bar? It can't know one is bare or green and one is white.

They have to be bonded at the main panel, but no where after that. If they were not bonded at the main panel you may as well not even run an EGC. The reason it is bonded to the grounded (neutral) is to provide a low resistance path back to the source in case of a fault. For instance if the hot conductor were to make contact with the metal box for a receptacle and the EGC is bonded to the outlet box as it should be, the EGC will carry the fault back to the main panel. With the EGC bonded to the neutral it will cause the breaker to trip and shut off the power to the receptacle.
If the EGC and neutral were not bonded at the main the fault would just go back to the ground (EGC) bus and on any other metallic path in the circuit. It would present a danger should anyone touch this. With no path to the breaker for it to sense a fault, the metallic items would remain with a voltage potential between it and a person should someone come in contact with it.

With the EGC and neutral separated at a sub panel, a fault will still have a path back to the source via the EGC. It just goes to the main first, then back through the neutral (because they're bonded) to the sub. The breaker in the sub, or the breaker in main panel, or both will trip. But won't present a shock hazard (in normal operation) on metallic objects because it is isolated from the current carrying neutral.

 

[ActionDave]

Thanks making more sense, but then the grounded and grounding conductors are not really separated at all

You are correct. An Equipment Grounding Conductor is an "extra neutral" just for tripping the breaker. Touch a hot conductor to the neutral and the breaker trips. Touch hot to green and the breaker trips.

Difference is the white is the one that gets hooked up to the light to complete the circuit and make the bulb glow or the heater so it gets warm or to the motor so it will spin, The green/bare doesn't get to do any of that. No load current on the green, only fault current.

....I was thinking how does a plug tester know when ground and neutral are reversed? They both are connected to same bar?

It doesn't.

It can't know one is bare or green and one is white.

You are correct. It can't.

 

[iwire]

You are correct. An Equipment Grounding Conductor is an "extra neutral" just for tripping the breaker. Touch a hot conductor to the neutral and the breaker trips. Touch hot to green and the breaker trips.

Difference is the white is the one that gets hooked up to the light to complete the circuit and make the bulb glow or the heater so it gets warm or to the motor so it will spin, The green/bare doesn't get to do any of that. No load current on the green, only fault current.

I like that.

To add to it the importance of using a separate conductor for ground faults has to do with the fact if you used only one conductor for both (as the power companies do) an open neutral would create a shock hazard.

 

[electricalist]

Plug testers can be mis leading.
If they worked as well as they appear to then surely volt meters would have 3 leads..
The only one I have is to trip a gfci.

 

[kwired]

Thanks making more sense, but then the grounded and grounding conductors are not really separated at all shouldn't they be separate at the main panel as well to be truly separated? I mean the current can still get back to the panel on ground and the ground can also add to the voltage drop right? I was thinking how does a plug tester know when ground and neutral are reversed? They both are connected to same bar? It can't know one is bare or green and one is white.

The grounded conductor has to split into current carrying and equipment grounding conductors somewhere, NEC permits that to be as far downstream as the service equipment or first disconnect of a separately derived system. Then there are exceptions for circumstances that once let you bond them again at separate buildings or for elecric ranges or clothes dryers - but current exceptions are for existing installations that allowed that practice when they were new. If you want even better design/safety potential - move that point all the way back to the transformer terminal (usually the X0 terminal) and keep "grounded" and "grounding" conductors separate from that point on.

 

[electricalist]

I like how you explained that. Way back when,,.I use to say I think it's a bad idea where imo people over ground like emt x 6 into a 12x 12 with a ground wire in all 6 then grounding bushings and blue weren't all that crap together

 

[zcanyonboltz]

Thanks for the replies and great explanations, that is a LOT clearer now!

 

[electricalist]

You are welcome and for me it's a fair trade to answer the question that I can. Learning and 're confirming