safety webinar #3 - gb 2008 (2).ppt [read-only]

Grounding Services

Presenter: Michael J. JohnstonNECA Executive Director,

Standards and Safety

Objectives:

• Review general requirements for grounding electrical services and service equipment

• Review requirements specific to services supplied by grounded utility systems (sources)

• Review requirements specific to services supplied by ungrounded utility systems (sources)

• Review requirements for grounded conductors, main bonding jumpers, and equipment bonding jumpers

• Review essential conductors and connections for grounding electrical services.

General Grounding Provisions

• Electrical systems are grounded (connected to ground) in a fashion that limits voltages imposed by lightning, line surges, and contact with higher voltage lines.

• Grounding electrical systems stabilizes to voltage to ground during normal operation. [250.4(A)(1)]

• Grounding equipment limits the voltage potential to ground by keeping the grounded conductor and conductive parts of equipment at or as close to earth potential as possible. [250.4(A)(2)]

Definition of Service

• The conductors and equipment for delivering electric energy from the serving utility to the wiring system of the premises served.

• The definition of service includes the statement that electric energy to a service can be supplied only by the serving utility.

• If electric energy is supplied by other than the serving utility, the supplied conductors and equipment are considered feeders, not a service.

Services Supplied by Grounded Source

• Grounded conductor is required to be brought to the service equipment.

• The grounded conductor must be routed with the ungrounded phase conductors.

• The grounded conductor must be connected to the service equipment enclosure.

• The grounded conductor must be connected to the earth by a grounding electrode conductor.

• The grounded conductor must be sized not smaller that required by 250.24(C).

Services Supplied by Ungrounded Source

• The utility system or source is ungrounded.

• No grounded conductor is required to be brought to the service equipment.

• The service equipment is required to be grounded.

• A grounding electrode conductor connects the equipment enclosures to a grounding electrode system meeting the requirements of Article 250,Part III.

Grounding Electrode System Required

• All grounding electrodes specified in 250.52(A) that are present at each building or structure service shall be bonded together and used as the grounding electrode system for the service.

– Metal water pipe– Structural metal frame electrodes– Concrete-encased electrodes– Ground rings– Other electrodes such as rods, pipes, or plates

Conductors at Grounded Services

• Four conductors must be present for proper grounding at electrical services supplied by grounded utility sources.

– Grounded conductor– Main bonding jumper– Grounding electrode conductor– Equipment grounding conductor

• Each of these conductors will be covered separately.

Grounded Conductor Requirements

• Required to be run to the service equipment enclosure.

• Required to be bonded to the service equipment enclosure.

• Required to meet the minimum sizing requirements in 250.24(C).

• Overcurrent devices generally not permitted in the grounded conductor.

• Must be identified in accordance with 200.6.

Grounded (Neutral) Conductor Functions

• Carries the maximum unbalanced current on the neutral of the systems served.

• In ground-fault conditions, the grounded (neutral) conductor at each service functions as an effective ground-fault current path.

• It must be run to each service disconnecting means for this reason [250.24(C)]

Main Bonding Jumper

• The main bonding jumper connects the grounded service conductor to the equipment grounding conductor (enclosure) at the service equipment.

• Main bonding jumper can be a wire, bus, screw, or other suitable conductor. [250.28(A)]

• Main bonding jumpers in listed equipment can be used without calculating size.

• Wire-type main bonding jumpers shall be sized according to the provisions in 250.28(D) which indicates use of Table 250.66 or the 12.5% rule for larger services.

Suitable for Use as Service Equipment

• Equipment is required to be suitable for use as service equipment. [230.66]

• Service equipment includes grounding and bonding provisions for use as service.

• Main bonding jumper is part of listed equipment and can be used without calculation of size.

Grounding Electrode Conductor

• The grounding electrode conductor connections at the service shall meet the provisions in 250.24(A)(1) through (A)(5).

• Connect at an accessible location at any point from the load end of the service drop or lateral to an including the grounded conductor terminal bus at the service disconnect. [250.24(A)(1)]

• Where the service transformer is outside the building, an additional grounding connection shall be made at the transformer or elsewhere outside the building. [250.24(A)(2)]

Load-Side Grounding Connections

• The grounded conductor generally shall not be connected to normally non-current-carrying metal parts of equipment, to the equipment grounding conductor, or be connected to ground (earth) on the load side of the service disconnecting means. [250.24(A)(5)]

• Note that the grounded conductor is permitted to be used for grounded on the supply side of the service disconnect as indicated in 250.142(A).

• Separation generally required beyond that point to control normal current present in the grounded conductor (often a neutral).

Equipment grounding Conductor

• The last conductor that will be present in the service equipment is the equipment grounding conductor, which could be the service equipment enclosure.

• All four conductors have to be installed and connected properly within the service equipment enclosure.

Service Bonding Requirements

• Service bonding rules are provided in 250.92.

• The following conductive parts of equipment are required to be bonded together:

– Service raceways– Cable trays– Cable bus framework– Auxiliary gutters– Service cable armor or sheath

• Note: The bonding on the supply side of the service must be robust (strengthened method) due no overcurrent protection ahead of the service.

Service Bonding Requirements

• Service bonding rules are provided in 250.92 (cont).

• The following conductive parts of equipment are required to be bonded together:

– Service equipment enclosures– Meter sockets or metering equipment enclosures– Raceways that join metering equipment to the service

equipment enclosures

• Note: Some service equipment includes the meter and service disconnect in the same enclosure.

Bonding Methods

• Electrical continuity of conductive parts at service equipment shall be ensured by one of the following methods:

– Using the grounded conductor

– Threaded couplings or threaded bosses on enclosures

– Threadless couplings and connectors for metal raceways and metal-clad cables

– Other listed devices, such as bonding-type locknuts, bushings, or bushings with bonding jumpers

Bonding Methods (cont.)

• Bonding jumpers shall be installed around concentric or eccentric knockouts that are punched or otherwise impairing effective bonding connections between conductive parts.

• Note that standard locknuts are not permitted as to sole bonding means required by Section 250.92(B).

Grounded Conductor for Bonding

• Permitted on the supply side of the service disconnect in accordance with 250.142(A).

• Section 250.92(B)(1) indicates a method of bonding at the service equipment using the grounded conductor (usually a neutral conductor).

• The method of bonding is common between a separate meter socket enclosure and the service disconnecting means enclosure.

Supply-Side Equipment Bonding

• Metallic parts shall be bonded together using one of the methods in 250.92(B).

• Equipment bonding jumpers (wire-types) on the supply side of the service disconnecting means shall be sized not smaller than required by 250.102(C).

• Use Table 250.66 or 12.5% rule for larger services.

• Connections shall be in accordance with Section 250.8.

Other Service Bonding Connections

• Metal water piping systems must be bonded in accordance with 250.104(A).

• Other metal piping, including metal gas piping systems are required to be bonded in accordance with 250.104(B).

• Structural metal building frames must be bonded in accordance with 250.104(C).

Summary for Grounded Services

• Four conductors have to be installed and connected at the service equipment for proper grounding and bonding:

– Grounded conductor (usually a neutral)– Main bonding jumper– Grounding electrode conductor– Equipment grounding conductor

Ungrounded Services

• Rules for services supplied by ungrounded systems (utility sources) are provided in 250.24(E).

• Grounding electrode conductor is required to connect the equipment to a grounding electrode system meeting the requirements of Part III of Article 250.

• This connection shall be made at any accessible point from the load end of the service drop or lateral to the service disconnecting means.

Ungrounded Services (cont.)

• A minimum of three conductors must be present in the service disconnecting means enclosure as follows:

– Grounding electrode conductor– Supply-Side Equipment bonding jumper(s)– Equipment grounding conductor(s)

• Note: More bonding conductors may be required for water pipe bonding, other pipe bonding and bonding structural metal.

• Sizing requirements are the same as those for grounded services.

Summary for Grounded Services (cont.)

• Grounded services include a grounded conductor that must be:

– Routed to the service disconnect enclosure– Bonded to the service disconnect enclosure– Sized in accordance with 250.24(C)– Identified in accordance with 200.6

• Bonding at service equipment shall meet the requirements in 250.92(A) and (B).

• Other metallic parts to be bonded include metal water piping, other metal piping, and structural steel.

Summary for Ungrounded Services

• Ungrounded services do not include a grounded conductor.

• Must have a grounding electrode conductor connected to a grounding electrode system meeting the requirements in Part III of Article 250.

• Bonding at service equipment shall meet the requirements in 250.92(A) and (B).

• Other metallic parts to be bonded include metal water piping, other metal piping, and structural steel.

Grounding Services

Presenter: Michael J. JohnstonNECA Executive Director,

Standards and Safety