Separate Buildings and Structures (4-26-2K)
By Mike Holt, for Power Quality Magazine
Metal parts of the electrical system in separate buildings or structures supplied by a feeder from another building must be grounded to the earth to limit imposed voltage. Also, the metal parts of the electrical system at separate building and structures must be properly bonded to a low impedance path that will remove dangerous voltage on the metal parts [250-2 and 250-32(a)].
Grounding Limits Imposed Voltage
Note: Graphics not provided on the internet.
DANGER: Failure to properly ground the metal parts of the electrical system to the earth can result in electric shock, fires and the destruction of expensive electronic equipment from lightning or high voltage line surges.
Author's Comment:A grounding electrode at separate buildings or structures is not required where only one branch circuit (with an equipment bonding conductor) supplies the building or structure [250-32(a) Exception] Figure 2.
Bonding Removes Dangerous Voltage
To protect against electric shock from a phase-to-ground fault, dangerous voltage on metal parts of the electrical system must be removed in less than 1/60th of a second by opening the circuit overcurrent protection device. To accomplish this, the impedance of the fault current path must allow the phase-to-ground fault current to rise to a value of at least 5 times and preferably 10 times the rating of the overcurrent protection device, Figure 3
In order for the circuit overcurrent device to open and prevent dangerous voltage from remaining on metal parts of electrical equipment, the equipment at the separate building or structure must be properly bonded. National Electrical Code allows two methods of accomplishing this [250-32(b)]:
250-32(b)(1) Feeder Equipment Grounding Conductor. An equipment grounding conductor (bonding wire) [250-118] installed with the feeder conductors to the separate building or structure can be use to provide the low impedance path to clear a phase-to-ground fault. To prevent dangerous neutral current from flowing on the metal parts of the electrical system, the grounded (neutral) conductor at the separate building or structure must not be bonded to either the equipment grounding conductor or to the grounding electrode system, Figure 4
Author's Comment: The feeder equipment grounding conductor (bond wire) to the remote building or structure must ultimately terminate to the grounded (neutral) conductor at the service equipment or source of separately derived system, Figure 4
250-32(b)(2) Feeder Grounded (neutral) Conductor. Where a feeder equipment grounding conductor is not run to the separate building or structure, the feeder grounded (neutral) conductor can be use to provide the low impedance path to clear phase-to-ground faults.
DANGER: The use of the grounded (neutral) conductor for equipment bonding is a dangerous practice and should not be done.
Author's Comment:Interior metal piping system must also be bonded to a low impedance path in accordance with Section 250-2(c) and 250-104(a)(3), Figure 5.
- A grounding electrode must be available at all separate buildings or structures supplied with a feeder. A grounding electrode is not required at separate buildings or structures supplied with one branch circuit that has an equipment grounding conductor.
- The grounding electrode system at the remote building or structure must be bonded to the separate building or structure disconnect.
- An equipment grounding conductor (bond wire) run with the feeder supply conductors to the separate building or structure must terminate to the separate building or structure disconnect. No neutral-to-ground connection is permitted.