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Instrument Transformer Installation

When instrument transformers are required, they shall normally be mounted in an instrument transformer cabinet or in a metal-clad switchgear compartment. The Energy Technology Services Department will help choose a location if space limitations do not provide for a conventional installation.

Current and voltage transformers will be furnished and delivered to the job location by Company Representatives. The customer shall mount the metering transformers, then furnish and install 1.5 inch rigid or intermediate galvanized metallic conduit from the instrument transformer cabinet or compartment to the meter location designated by the Energy Technology Services Department. Thin wall, flexible PVC or non ferrous conduit is not acceptable for metering. Long runs may require use of 2 inch conduit, pull boxes, and a pull line.

Instrument Transformer Cabinets

Instrument transformer cabinets shall not be used as junction boxes. Connections to other meters or current transformers should not be made in the instrument transformer cabinet. However, for more than one conductor per line installations, it is permissible for individual conductors to feed different circuits on the load side of the current transformers.

Instrument transformer cabinets need to be of sufficient size to contain all the instrument transformers and conductors, giving consideration to possible future removal and replacement of the transformers. The factors which determine the cabinet size are the size and number of conductors, the point of entrance and exit of conductors, and the size, type and number of instrument transformers. Recommended minimum sizes for instrument transformer cabinets are shown in Table III. For all installations higher than 480 volts, and for any questions about a specific installation regarding the minimum allowable size of the instrument transformer cabinet, consult the Energy Technology Services Department. See Figure 15.

Instrument transformer cabinets shall have hinged double doors. Cabinets that are 10 x 24 x 32 inches or smaller may have a hinged single door. All instrument transformer cabinet doors shall be sealable with pad lock type meter-sealing devices. Adequate clearance shall be provided and maintained in front of the cabinet to allow the cabinet doors to be fully opened, and to allow the removal and installation of the instrument transformers.

Instrument Transformers In Cabinets

Instrument transformers shall not be mounted directly on the back surface of instrument transformer cabinets, but shall be mounted on a 3/4 inch plywood panel or on mounting plates or brackets. The polarity marks (red dot) shall face the line side. Instrument transformers must be mounted so that they may be readily removed or replaced. Where bar-type current transformers are used, the connections shall be on the back side of the primary bar of the current transformers, with the nuts of the bolts on the front side. Where through-type current transformers are used, the conductors shall be broken on the line side of the current transformer and reconnected with adequate bolted connectors. See Figure 18.

Instrument Transformers In Switchgear

When instrument transformers are installed in a customer's metal-clad switchgear, the customer shall arrange to have the supplier furnish detailed drawings of the switchgear and instrument transformer arrangement. These drawings shall be forwarded to the Energy Technology Services Department for acceptance and approval before constructing the switchgear. The Energy Technology Services Department on request will supply information on the size, type and number of instrument transformers to be furnished.

The instrument transformer compartment shall be completely separated by a rigid barrier from the rest of the switchgear. It shall have hinged doors which are sealable with pad lock type sealing devices. It shall be large enough to contain the required number of through-type current transformers and voltage transformers for services of 277/480V and higher. The design shall be such that the transformers can be readily installed or changed after the switchgear is in place. Removable sections of bus bar shall be provided as the primary conductor of the current transformers, and shall be the same ampacity as the bus bar entering and exiting the compartment. It also may be necessary to parallel narrower bars to accommodate the CT's supplied by the Company.

The instrument transformers shall be installed by the customer. They shall be located on the line side of the main switch or circuit breaker. The service-entrance conductors shall be completely enclosed in conduit or raceway from the service point to where they enter the switchgear instrument transformer compartment. All service and metering conductors exiting the instrument transformer compartment shall not be returned to the compartment. Voltage connections for metering shall be provided in the compartment. These connections shall be on each phase bus on the line side of the current transformers, and for three-phase, four-wire services shall also be on the neutral. In installations where the service is four-wire but the load is three-wire, a neutral conductor shall be extended to the CT compartment.

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