Why is the ground circuit low impedence?

Status
Not open for further replies.
S

syakoban

Member
I've followed a number of the bonding and grounding posts and believe I understand the basic issue of bonding and the need for a low impedence ground circuit.

What I don't understand is what makes the ground circuit lower in impedence than the neutral circuit. If they are the same gage wire, is it the fact that there are other metallic devices connected to the ground that lowers impedence? How would that work with plastic boxes?
 
Re: Why is the ground circuit low impedence?

You have two issues here.

The first is the basic concept of bonding. Ungrounded and grounded circuit is isolated and insulated from metallic parts and the EGC. This is required in order to keep normal operating current from flowing on the frames of equipment and other metallic parts of the systems. If in the case of a ground-fault, a low impedance path must be established to clear this fault as quickly as possible. This is the function of bonding.

The second is the concept of grounding. At the service, grounded and grounding circuits are bonded. At this point, the currents will divide in ratio to the impedance of those circuits. In this case, it is required to keep metal parts free of objectionable currents. This can be accomplished by assuring the grounded neutral path is the lower path of impedance thus reducing the amount of current flowing on the grounded parts.
 
Re: Why is the ground circuit low impedence?

Originally posted by syakoban:
If they are the same gage wire, is it the fact that there are other metallic devices connected to the ground that lowers impedance? How would that work with plastic boxes?
Click to expand...
When plastic boxes are used, and say, NM-B cable, and there is only one receptacle on the whole circuit, then you are absolutely right. . .the impedance of the ground is the same as the impedance of the other two wires, when the cables are #14, #12 & #10. With #8 NM-B, the ground will be #10 and the ground impedance will actually be greater than the insulated conductors.

Now, think of a kitchen counter. Imagine, at the right side of the sink on the back splash of the counter, there is a 3 gang box holding two switches and a small appliance counter receptacle. One switch controls the light over the sink and is on a lighting circuit. The other switch controls the disposal which is on its own 15 Amp 120 Volt circuit. The receptacle is supplied by a 20 Amp 120 Volt small appliance branch circuit.

The ground wires in the 3 gang box are all tied together per 2005 NEC 250.148. If one of the hot wires in the 3 gang box faults to a ground in that box, the fault current will travel back on two #14s and the #12 ground wires in parallel.

The interconnection of the grounds in the 3 gang box lowers the impedance of the ground below the individual ground wire impedances.

The more interconnections of parallel grounds, the lower the overall ground impedance.
 
Re: Why is the ground circuit low impedence?

The more interconnections of parallel grounds, the lower the overall ground impedance.
Click to expand...
So in very basic terms... is that the practical difference between bonding and grounding? The bonded circuit offers more "pathways" than a grounded circuit, effectively lowering impedence and being a more enticing route for electricity to return.(??)
 
Re: Why is the ground circuit low impedence?

What I don't understand is what makes the ground circuit lower in impedence than the neutral circuit.
Click to expand...
In many cases its not.
Don
 
Re: Why is the ground circuit low impedence?

Originally posted by syakoban:
What I don't understand is what makes the ground circuit lower in impedence than the neutral circuit.
Click to expand...
Agree with Don here most of the times it will not. On the other hand like an industrial or commercial installations where a raceway is used like conduit and EGC are ran then the ground impedeance will be much lower because you have multiple paths other than the EGC.
 
Re: Why is the ground circuit low impedence?

BY Bryan: The second is the concept of grounding. At the service, grounded and grounding circuits are bonded. At this point, the currents will divide in ratio to the impedance of those circuits. In this case, it is required to keep metal parts free of objectionable currents. This can be accomplished by assuring the grounded neutral path is the lower path of impedance thus reducing the amount of current flowing on the grounded parts.
Click to expand...
In the highlighted section of your post, I'm a bit confused at this. It is not the lower impedance of the grounded conductor that will reduce the amount of objectionable currents flowing on the grounding. It is the requirement that the grounding and grounded conductors have to be kept separate after the service disconnect that prevents objectionable currents from flowing on grounding within a structure? :confused:

[ November 11, 2005, 09:42 PM: Message edited by: hurk27 ]
 
Re: Why is the ground circuit low impedence?

Within the structure yes, for the most part. But what about at the service itself?

Sections such as 215.2(A)(1), 220.61(B), and other various sections permit the reduction of the grounded neutral. Yet, most of Part III in Article 250 works to reduce service grounding to the lowest possible impedance.

These requirements and allowances together will create objectionable current. Why, becasue it can be avoided. I was just pointing out that the code contradicts itself when it comes to the service verses the wiring after it.
 
Re: Why is the ground circuit low impedence?

I was just pointing out that the code contradicts itself when it comes to the service verses the wiring after it.
Click to expand...
That is because the electrons change their behavior after the customer buys them. :D
Don

[ November 12, 2005, 05:12 PM: Message edited by: don_resqcapt19 ]
 
Re: Why is the ground circuit low impedence?

Bryan Your right, At least it sure seems that way, There are a few grounding requirements in the NEC that I have issue with and this is one of them, One way to stop stray current is to leave the neutral current off the grounding at the transformer, In one of mikes news letters it was talking about some POCO's supplying dairy farms using 5 wire service drops which on a 120/208 Y would be separating the neutral and grounding after it leaves the transformer, I think it's a great idea and I have stated that many times in the past, But it seems that our states are more concerned about cows not producing milk then kids getting shocked around pools. :D
 
Re: Why is the ground circuit low impedence?

"That is because the electrons change their behavior after the customer buys them. [Big Grin]
Don
Don you have got to tell that to Mike Holt next week in Chicago. That explains it all.
 
Re: Why is the ground circuit low impedence?

Originally posted by syakoban: What I don't understand is what makes the ground circuit lower in impedance than the neutral circuit.
Click to expand...
Who cares? Comparing the impedance of the neutral and the ground means nothing.

The ground wire exists for no other reason than that an ungrounded (i.e., "hot") conductor might come into contact with the case. If it does, and if you are touching the case at the time, you will get a shock that you will not enjoy. Presuming the ground wire is there, and is correctly wired, there will be three parallel paths for current. One is through your body. Another path, the original circuit path, is still there. Current will still flow through the equipment, and will return to the source through the neutral. But the current flowing in the neutral is also flowing through the impedance associated with the equipment. That is the normal amount of current flow, and is certainly not enough to trip the breaker. The third path for current, the one that includes the ground wire, does not first flow through the impedance of the equipment. If flows through a low impedance path that only includes the phase conductor, the case of the equipment, and the ground wire. That current will be high enough to trip the breaker, and thereby save the life of the human operator.
 
Status
Not open for further replies.
Top