With the problems an improper neutral-to-case connection can create, it's important to closely follow the grounding requirements of Art. 250 in the 2002 NEC.
How do you prevent fire, electric shock, or improper operation of circuit protection devices and other equipment? By stopping objectionable current (neutral return current) from flowing on electrical equipment, grounding paths, and bonding paths as required by the NEC in 250.6. To do this, you must keep the grounded (neutral) conductor separated from the metal parts of equipment, except as required for service equipment in 250.24(B) and on separately derived systems in 250.30(A)(1) and 250.142. Making the proper neutral-to-case connections is the key.
Consequences of improper neutral-to-case connections. There are several consequences of improper neutral-to-case connections that range in severity from problems with equipment to the death of an employee.
Fire hazard. Improper wiring that results in the flow of neutral current on grounding and bonding paths can cause enough excess heat to cause a fire. Fire occurs when the temperature rises high enough to ignite adjacent combustible material in an area that contains sufficient oxygen.
Electrocution. Death from an electric shock can occur when the touch voltage is above 30V rms, and as little as 30 mA flows though the body. These conditions can easily exist when improper neutral-to-case connections are made and the neutral is opened. Improper operation of protection devices. Nuisance tripping of a protection device equipped with ground-fault protection can occur if neutral current returns on the equipment-grounding conductor instead of the neutral conductor. Current can return this way because of multiple and illegal neutral-to-case bonds.
A circuit breaker with ground-fault protection (480Y/277V, 3-phase system over 1,000A) uses the residual current method to detect a ground fault. On a 3-phase, 4-wire system, the trip unit will sum the currents in the 3-phase conductors and in the neutral. When no ground fault is present, the summation of currents flowing on A+B+C+N will equal zero. Any current flow not equal to zero is considered a ground fault.
Where multiple neutral-to-case bonds have been made, neutral current will flow on the equipment-grounding path. Depending on the impedance of this path versus the neutral conductor path, the ground fault protective relay may see current flow above its pickup point and cause the protective device to open the circuit.
If there are multiple neutral-to-case bonds and a ground fault occurs, the protection relay might not operate, because some of the ground-fault current won’t flow on the equipment grounding conductor. Some fault current returns on the neutral conductor, partially bypassing the ground fault protective device.
Power quality problems. When objectionable neutral current travels on the metal parts of equipment because of improper neutral-to-case connections, the electromagnetic field generated from circuit conductors will not cancel out. This uncanceled net current flowing on metal parts of equipment and structural parts causes elevated electromagnetic fields. These low-frequency electromagnetic fields can negatively affect electronic devices.
Common improper neutral-to-case connections. The most common improper neutral-to-case bonds occur in panelboards, separate building disconnects, transformers, and generators. Neutral current will flow on metal underground water piping systems where the water service to the building is metallic. However, this occurs only if the underground water pipe system is metallic and connected to other buildings. Although this isn’t an NEC violation, you need to be aware of such situations.
Panelboards. Bonding of the neutral terminal to the case of a panelboard, which isn’t part of service equipment or separately derived systems, creates a parallel path for return neutral current. The result is neutral current (net current) flowing on the metal parts of electrical equipment and on the grounding and bonding conductors (see Fig., right).
Connection at separate buildings. Where an equipment-grounding conductor is run with the feeder conductors to a separate building [250.32(B)(1)], some people make the common and dangerous mistake of making a neutral-to-case bond in the separate building disconnect. This ties the neutral and equipment grounding conductors together, allowing objectionable neutral current to flow on the feeder equipment-grounding conductor.
Separately derived systems. Making a neutral-to-case bond for a separately derived system at more than one location creates a parallel path for neutral return current.
Transformers. If a neutral-to-case bond is made at both the transformer and at the secondary panelboard, neutral current will flow through metal raceways (and on the grounding and bonding path) on its return to the power supply.
Generators. If the grounded (neutral) conductor in a transfer switch is not opened, the grounded (neutral) from the generator will be solidly connected to the utility’s service grounded (neutral) conductor. Under this condition, the generator isn’t a separately derived system, and a neutral-to-case bond must not be made at the generator or at the generator disconnect [250.20(D) FPN 1].
If a neutral-to-case bond is made at both the generator and generator disconnect, objectionable neutral current will flow through metal raceways—and on the grounding and bonding path—to the power supply.
Required neutral-to-case connections. The NEC offers guidelines on how to make neutral-to-case connections in the following applications:
Service disconnecting means [250.24(B)]. Services supplied by a grounded utility transformer must run a grounded (neutral) conductor from the electric utility transformer to each service disconnecting means. You must bond the grounded (neutral) conductor to each disconnecting means enclosure (neutral-to-case connection) by a screw or strap supplied by the equipment manufacturer [250.28].
The grounded (neutral) service conductor must be sized to safely carry the maximum ground-fault current likely to be imposed on it from where a ground-fault may occur (110.10). Thus, you must size the grounded (neutral) conductor per Table 250.66, based on the total area of the largest ungrounded (hot) conductor. Also, the grounded (neutral) conductors must be able to carry the maximum unbalanced neutral current in accordance with 220.22.
If a grounded (neutral) service conductor, that serves as the effective ground-fault current path is open or not provided, then you can’t clear a ground fault, and the metal parts of electrical equipment and metal piping and structural steel will become—and remain—energized.
Transformers or other separately derived systems [250.30(A)]. To provide the low-impedance path necessary to clear a ground fault from the separately derived system, you must bond the metal parts of electrical equipment to the grounded (neutral) terminal (XO) of the derived system. You can make the neutral-to-case bond at the source of a separately derived system or at the first system disconnecting means. The bonding jumper used for this purpose shall be sized per Table 250.66, based on the area of the largest ungrounded conductor.
If you don’t install a bonding jumper from the equipment-grounding conductor to the grounded (neutral) terminal of the separately derived system, then you can’t clear a ground fault, and the metal parts of equipment, as well as metal piping and structural steel, will become and remain energized (see Fig., right).
To protect against electric shock, fires and the improper operation of equipment from objectionable current on metal parts, make your neutral-to-case connections only at service equipment and separately derived systems in accordance with 250.142.