Question re: voltage on groung circuit of 120 volt system

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Floyd hock

New member
I am struggling understanding an item that I see happening every so often and few instances here lately, it involves the voltage potential on the ground of a 120 volt circuit. My question is how is there voltage on the ground circuit of a 120 volt system? The ground is used for incident energy (i.e. arc flash, lightening etc) not normal voltage or current path. The circuit path should be hot leg through the device returning on the neutral leg. Could someone please explain the logic that it is okay to have the voltage on the ground during normal operation.
 

gar

Senior Member
Location
Ann Arbor, Michigan
Occupation
EE
120705-0737 EDT

How are you measuring the apparent voltage on the ground (EGC), and with respect to what reference point? What is the magnitude? Does it vary with time? Does it pulse once per second, or some other periodic rate?

For your background education go in your backyard. Put a screwdriver in the soil in one location. Put a second one in maybe 100 ft away. Use a high impedance meter, like a Fluke 27, to measure the voltage between the two points. In my yard this is usually less than 1/4 V. Be careful. Under unusual conditions this voltage could be large enough to shock you. It could even be high enough to kill you.

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Strathead

Senior Member
Location
Ocala, Florida, USA
Occupation
Electrician/Estimator/Project Manager/Superintendent
I am struggling understanding an item that I see happening every so often and few instances here lately, it involves the voltage potential on the ground of a 120 volt circuit. My question is how is there voltage on the ground circuit of a 120 volt system? The ground is used for incident energy (i.e. arc flash, lightening etc) not normal voltage or current path. The circuit path should be hot leg through the device returning on the neutral leg. Could someone please explain the logic that it is okay to have the voltage on the ground during normal operation.

Two basic ways.

1. Capacitive and inductive property of electricity. Voltage is induced in the ground wire just by the natural properties of physics.

2. And becoming less acceptable all the time, is the use of the ground for small trickles of power. For example, the pilot light on Decora switches that "glow" in the dark, or the operation of an occupancy sensor that requires a very minor amount of electricty to operate. This is the exact reason that the 2011 version of the NEC is requiring a neutral to switch locations. Up until now, it was considered acceptable.
 

don_resqcapt19

Moderator
Staff member
Location
Illinois
Occupation
retired electrician
Exactly how are you measuring this votlage. There should always be voltage between the ungrounded conductor and the grounding conductor. There will also be votlage between the grounded and grounding conductors. This voltage will be equal to the voltage drop on the grounded conductor between the main or system bonding jumper and the point where you are measuring the voltage.
 
T

T.M.Haja Sahib

Guest
Wasteful neutral to ground voltage

Wasteful neutral to ground voltage

Efforts should be taken to reduce the voltage between the grounded and grounding conductors, because this voltage reduces the phase voltage and hence the useful energy.
 
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