Update #18 Main Disconnect Grounding

Bob Desnoyers Elevator & Escalator Inspections, Inc.

CODE UPDATE

Update Warning

Update #18 Main Disconnect Grounding

Let's define a few terms first:

(Most of the definitions and all of the citations in this update are found in the NEC - 2005 (NFPA 70))

Bonding (Bonded): The permanent joining of metallic parts to form an electrically conductive path that ensures electrical continuity and the capacity to conduct safely any current likely to be imposed. (NEC)

Effective Ground-Fault Current Path: An intentionally constructed, permanent, low-impedance electrically conductive path designed and intended to carry current under ground-fault conditions from the point of a ground fault on a wiring system to the electrical supply source and that facilitates the operation of the overcurrent protective device or ground fault detectors on high-impedance grounded systems. (NEC)

Effectively Grounded: Intentionally connected to earth through a ground connection or connections of sufficiently low impedance and having sufficient current-carrying capacity to prevent the buildup of voltages that may result in undue hazards to connected equipment or to persons. (NEC)

Ground: A conducting connection, whether intentional or accidental, between an electrical circuit or equipment and the earth or to some conducting body that serves in place of the earth. (NEC)

Ground: A large conducting body, such as the earth or an electric circuit connected to the earth, used as an arbitrary zero of potential. (American Heritage Dictionary)

Grounded: Connected to earth or to some conducting body that serves in place of the earth. (NEC)

Ground Fault: An unintentional, electrically conducting connection between an ungrounded conductor of an electrical circuit and the normally non-current-carrying conductors, metallic enclosures, metallic raceways, metallic equipment, or earth. (NEC)

Ground-Fault Current Path: An electrically conductive path from the point of a ground fault on a wiring system through normally non-current-carrying conductors, equipment, or the earth to the electrical supply source. (NEC)

Conductors

Grounded Conductor: A system or circuit conductor that is intentionally grounded. (Identified by a continuous white or gray outer finish or by three continuous white stripes on other than green insulation along its entire length. "The Neutral Wire" "White Wire" (See 310.12))

Equipment Grounding Conductor: The conductor used to connect the non-current-carrying metal parts of equipment, raceways, and other enclosures to the system grounded conductor, the grounding electrode conductor, or both, at the service equipment or at the source of a separately derived system. (Shall be permitted to be bare, covered, or insulated. Individually covered or insulated equipment grounding conductors shall have a continuous outer finish that is either green or green with one or more yellow stripes. "The Ground Wire" "Green Wire" (See 310.12))

Ungrounded Conductor: Conductors that are intended for use as ungrounded conductors, whether used as a single conductor or in multiconductor cables, shall be finished to be clearly distinguishable from grounded and grounding conductors. ("The Hot Wire" "Black Wire" (See 310.12) - I could not find a formal definition for ungrounded conductor in the NEC...but we do know it normally carries current and in most applications is not intentionally grounded.)

Did you fall asleep yet?!

Grounding is not really down to earth in it's simplicity is it? (A little humour for our British/Australian friends.)

Now I'm not going to pretend to be an "expert" on grounding, however, I would like to address a few problems that seem to come up on a regular basis.

Grounding is a really big deal in the electrical trade. It should be in our trade as well. Some of the mistakes or omissions that are made when attempting to provide an effective ground-fault current path can cause injury, loss of life, or property damage. Please note that almost all conductor nomenclature in the electrical code is in reference to ground (ungrounded conductor, grounded conductor, equipment grounding conductor, etc.).

I'd like to zero in on just few of the "grounding/bonding" issues I have encountered in the past that I feel are easy to correct.

Several times when inspecting private residence elevators and platform (wheelchair) lifts that are supplied with 240 or 120 VAC single phase power, I noticed that the grounded conductor was connected to the equipment grounding conductor at the main line disconnect. This is not allowed. (See 250.24(A)(5) - Of course there are exceptions to almost all of the NEC requirements but the few narrow and specific exceptions allowed by this requirement do not concern us in the vertical transportation trade.) The particular disconnect I am recalling was wired on the line side using a three-conductor with ground nonmetallic-sheathed cable (Type NMC - See Article 334). This particular controller required 240 VAC single phase power so the grounded conductor was not required to be carried over to the controller. Someone decided to connect the grounded conductor to the grounding bus along with the equipment grounding conductors. The "Heads Up" below will explain why you should not do this - with the aid of a slightly different but easier to understand circuit.

Heads Up ...

Connecting the equipment grounding conductor to the grounded conductor on the load side of the service disconnecting means...what's the big deal? They both eventually end up at the service were they are both connected to ground...don't they? Well let's see what the big deal is. We will assume that in all 4 circuits represented below a utilization device drawing approximately 10 amperes is plugged into receptacle #2.

Circuit #1 represents a section of a properly wired receptacle branch circuit. The equipment grounding conductor is provided by the metallic raceway, metallic fittings, metallic boxes, and bonding connections. Section 250.118 permits the use of certain metallic raceways enclosing conductors as the equipment grounding conductor. The ungrounded conductor (black conductor - "hot wire") is connected to the shorter rectangular contact in the receptacle. The grounded conductor (white conductor - "neutral wire") is connected to the longer rectangular contact in the receptacle. The equipment grounding conductor (metallic raceway, fittings, and boxes - "green wire" in many installations) is connected to the grounding contact in the receptacle by the green bonding connection.

Proper Circuit

Circuit #1
Properly Wired Receptacle Branch Circuit

Circuit #2 represents the same arrangement as above but there is a problem with the equipment grounding conductor at receptacle #1. The locknut is loose on the fitting leading into the outlet box. The circuit will operate normally as long as a ground fault is not encountered. Depending on the resistence of the ground fault and the resistence of the loose fitting, the branch circuit overcurrent protective device may or may not sense sufficient fault current to clear (open) the circuit. This is why many electricians run an actual wire conductor to serve as the equipment grounding conductor. Most of the major vertical transportation equipment contractors are specifying in their power requirements that a separate equipment grounding conductor be installed to each main line disconnecting means. This separate conductor will be carried through and connected to all the other major components in the system.

Loose Locknut

Circuit #2
Improperly Wired Receptacle Branch Circuit
(Faulty Equipment Grounding Conductor At Receptacle #1)

Circuit #3 represents the same arrangement as above but not only is the equipment grounding conductor faulty but the grounded conductor has gone to ground, either accidentally or intentionally, in receptacle #2. Most of the current in the branch circuit will be returning to the source buy way of the grounded conductor since it is still intact. However, a small portion of the total current will flow through the ground at the grounded conductor. We have all heard the saying, "Current takes the path of least resistance." But remember - current takes all paths back to the source. The "path of least resistance" may get the lion's share of the current but other paths will be conducting current as well depending on their total impedance (resistance to the flow of alternating current). This particular circuit has a faulty equipment grounding conductor but the boxes might be attached to a metal framed building structure and the current could be returning to the source through the building frame as well. In this particular case we have only the equipment grounding conductor (the raceway) as the second return path. The current in the raceway is described as "net current."

Net Current: "When referring to wiring, net current is the unbalanced resultant current carried by a circuit when some of its neutral return current has been diverted to another circuit or conducting path"...from an article by Karl Riley - which contains a more in-depth description of this phenomenon. Net currents can cause undesirable magnetic fields which could disrupt devices such as computer monitors. Net currents can also cause the heating of raceway by way of induced currents caused by magnetic fields and the destruction of insulating materials due to this heating. Section 250.6 describes the prohibitions concerning "Objectionable Current Over Grounding Conductors." All of the conductors in a branch circuit should be grouped together in the same raceway. For additional information on net currents perform this Google Search.

Improperly Grounded Neutral

Circuit #3
Improperly Wired Receptacle Branch Circuit
(Faulty Equipment Grounding Conductor At Receptacle #1)
(Neutral Grounded To Metallic Box At Receptacle #2)

Circuit #4 represents the same arrangement as above but, in addition to the faulty equipment grounding conductor and the grounded conductor going to ground, the grounded conductor on the downstream side of receptacle #1 is also open. With this combination of circuit faults, the equipment grounding conductor, which is the raceway, is now conducting all of the current being utilized by our device (10 amps) which is plugged into receptacle #2. The net current is quite high but it is no higher than normal so the branch circuit overcurrent protective device will not open. Since the locknut at receptacle #1 is loose we have an "arcing fault" which is developing an extremely "hot spot" in our system. This "hot spot" is probably out of sight (behind gypsum board or some other type of wall covering) and is a potential point of ignition for a fire. Also, if someone is unlucky enough to open the raceway at some point in the run back to the panelboard they would be subject to a potential of 110 VAC between the two sections of the open raceway.

Improperly Grounded Neutral & Faulty EGC

Circuit #4
Improperly Wired Receptacle Branch Circuit
(Faulty Equipment Grounding Conductor At Receptacle #1)
(Neutral Grounded To Metallic Box At Receptacle #2)
(Neutral Open At Receptacle #1)

If the grounded conductor had not been connected to ground at receptacle #2, receptacle #1 would have continued to operate as intended and receptacle #2 would have just stopped working. With the situation that we have in Circuit #4 both receptacles continue to work but we have a potential disaster on our hands.

Figures #1 and #2 represent two typical disconnects used in the vertical transportation industry. (I "borrowed" these images from this Square D Enclosed Safety Switches PDF document.) The correct location for the grounding (bonding) screw is indicated. I have found many disconnects that are missing this bonding screw. If the electrician is running a separate equipment grounding conductor and it is connected to this bus it should be used as intended. The disconnect is not properly grounded when the bonding screw is missing. Without the bonding screw the neutral bus, which is now the grounding bus, is above ground. See Figure #3 and Figure #4 for a close-up of these connections.

Square D General Duty Safety Switch - Class 3130

Figure #1
Square D General Duty Safety Switch - Class 3130

Square D General Duty Safety Switch - Class 3130

Figure #2
Square D General Duty Safety Switch - Class 3130

Figure #3 is a close-up of a typical grounding bus arrangement with respect to the relationship between the various parts. The grounding (bonding) screw is designed to pass through the bakelite and securely bond the grounding bus to the enclosure wall. If the correct length screw is provided, and installed, the bus will be securely bonded. If a screw that is longer than required is installed and the enclosure is mounted on a concrete wall the screw may seem tight but the bus will not be securely bonded because the screw has "bottomed-out" against the concrete.

Square D General Duty Safety Switch - Grounding Bus

Figure #3
Square D General Duty Safety Switch - Grounding Bus

Things To Do

1 - Insure that the main disconnecting means is properly grounded and bonded. (See 250.4)

2 - Use the listed grounding screw or device provided in the disconnect, enclosure, or box for grounding. (See 250.8 and 250.148(C))

As you can see from the image below (Figure 4), also "borrowed" from this Square D Enclosed Safety Switches PDF document, the neutral can be grounded with the addition of a bonding screw. Also, if a neutral bus is required then a grounding kit can be purchased and installed in the field. Generally speaking, this would only be required when the disconnect is used as service equipment and marked, "Suitable for Use as Service Equipment". In this case the grounded conductor and the equipment grounding conductor are connected together. We are not installing service equipment. (See "switches, enclosed" in the UL Whitebook - General Information for Electrical Equipment Directory 2005)

Square D Insulated Groundable Neutral & Grounding Kit

Figure #4
Square D Insulated Groundable Neutral & Grounding Kit

3 - It is not mandatory for the grounding (bonding) screw to be green in color, however, use the listed screw that was provided by the manufacturer for their listed device. The code only requires the screw (bonding jumper) to be green if it is the main bonding jumper or the system bonding jumper. We are not working with either one of these bonding jumpers. (See 250.28 - Be sure to read the definitions for "bonding jumper, main" and "bonding jumper, system" in the NEC definition section.)

I found the following card (Figure #5) with a green bonding screw pushed through the crosshairs in a main line disconnect switch enclosure. I have also found the green bonding screw secured in place in one of the terminals of the neutral bus as if it were a conductor. This is the screw to use. If it is missing ask the electrician for one.

Bonding Screw Notice

Figure #5
Bonding Screw Notice

Things Not To Do

1 - Never connect the grounded conductor to ground or to the equipment grounding conductor. (See 250.24(A)(5))

2 - Never use a sheet metal screw to "land" an equipment grounding conductor. This includes "Tek" screws, drywall screws, or any screw with a coarse thread. (See 250.8)

3 - Never drill a hole in an enclosure to "land" an equipment grounding conductor with a bolt or screw. The enclosure will be provided with a grounding screw hole or other listed grounding device. The listing approval of the enclosure may be invalidated by this action. (See 250.8 and 250.148(C))

4 - Never use a 1/4-20 bolt and nut with a flat washer as a grounding terminal - it is not a listed device. "Grounding conductors and bonding jumpers shall be connected by exothermic welding, listed pressure connectors, listed clamps, or other listed means." (See 250.8)

5 - "The structural metal frame of a building shall not be used as the required equipment grounding conductor for ac equipment." (See 250.136(A))

6 - Rule 620.53 requires "a single means for disconnecting all ungrounded car light, receptacle(s), and ventilation power-supply conductors for that elevator car." This device "shall be a fused motor circuit switch or circuit breaker" and "shall be located in the machine room." Rule 240.22 states, "No overcurrent device shall be connected in series with any conductor that is intentionally grounded ("white conductor" or "neutral conductor") unless..." One of the exceptions is if the "overcurrent device opens all conductors of the circuit..." If you are using a fused disconnect as this "single means" then the "white wire" should not be fused. If for some reason this fuse opens the "black wire" will still be hot because its fuse may not open. I would recommend running the "white wire" through the disconnect without connecting it to a contact.

So you see there are many things to consider when grounding a circuit, system, enclosure, or device.

Envelope E-mail Bob Desnoyers with your main disconnect grounding comments

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