We are often left in the air about exact meanings of grounding problems. This article simplifies the differences between the grounded and grounding conductor.
The two aspects of grounding are system grounding and equipment grounding. This article concerns the former. We may not always have a system ground but we will always have equipment grounding.
In a grounded system, a conductor carries current. Write the words “grounded” and “grounding” on a piece of paper. Next, underline the last four letters of “grounded.”
The “n” stands for “not,” and “ded” stands for “dead.” So it carries current. In a common three-wire circuit, such as an ordinary grounded, 120-volt outlet, we have a hot conductor (ungrounded conductor), a grounded conductor, and a grounding conductor.
It is not proper to call the white (or gray or three white stripes) grounded conductor in a two-wire 120-volt circuit, a neutral. You must have two or more hot wires to have a neutral. The white wire in a 120-volt receptacle carries exactly the same current as the hot wire. If 6.33 amps leave the source, 6.33 amps must return. So, it is properly called the ‘grounded’ conductor.
It is obviously not dead. If you lift this wire back at the panel, in either system, your equipment will stop running because it is the return path to the source. It is one of the two current-carrying conductors.
In a three-wire, 480-volt corner-grounded delta system, three wires make the motor run, two are ungrounded, and one is grounded. If the motor has a full-load current of 5.40 amps, all three conductors will read 5.40 amps. Two conductors will read 480 volts to ground, and one reads zero.
Picture a two-wire, 120-volt circuit from a transformer left temporarily ungrounded. If we check the voltage from either conductor, to earth ground, we will not get a reading. There is no path from ground back to the source.
Between the two conductors, it will read 120 volts. If the load were a 120-volt, 1,000-watt heater, we would have approximately 8.33 amps traveling on the black wire and returning on the white wire. If you put an ammeter on either conductor it will read 8.33 amps.
Now, if we ground this circuit, we could ground either conductor without changing the voltage at the load. It will still read 120 volts between the two conductors. It will not change the amperage; it will still read 8.33 amps on both conductors. The difference now is that the ungrounded conductor reads 120 volts to ground and the grounded conductor reads zero.
Ground or earth is now the same potential as the conductor that was connected to earth because they are connected together. That is why we properly call this conductor the “grounded” conductor. The other is the “hot” or “ungrounded” conductor.
Underline the “ing” in “grounding.” The “i” stands for “in,” the “n” for near, and the “g” for ground. Thus: in or near ground. This conductor is at or near ground potential, which is considered as zero. It is not current carrying, except in cases of a fault. If it normally carries current, the National Electrical Code (NEC) calls it “objectionable current.”
In a two-wire circuit, one black and one white conductor of a grounded system, we usually refer to the black wire as the hot, and the white one as the return path, or the common conductor. We should stop using the word “neutral” here. If the white, grounded and green (or bare) grounding conductor to touch at the load, then unwanted parallel paths for the current to return on result.
If we want voltage and current to only be on the insulated conductors, then why connect equipment-grounding conductors and the grounded conductor at the source, but not at the load?
So that fault current induced anywhere in the system on any grounded metal or metals will return on this path to the grounded conductor, at the source only. The higher the current, the quicker the fuse will blow, or the breaker will trip, to remove this source. If we have a high-resistance path back to the source, then the current may not be high enough to open the circuit.
Also, all of our connections must be tight, so that the current is high enough to open the circuit.
The purpose of grounding one of our conductors in the system is stated in Section 250-2(a). It reads in part:
…that will limit the voltage imposed by lightning, line surges, or unintentional contact with higher voltage lines and that will stabilize the voltage to earth during normal operation.
It’s also to dissipate static charges. This grounded conductor plays little, if any part, in removing any dangerous voltages on equipment.
CORCORAN is a Code consultant for EMC Code Consultants in Burlington, Wash. He can be reached at (360) 757-3605.