sce grounding procedures tech brief

Technical Brief

SCE Grounding Procedures

Aclara Power-Line Systems Inc. Confidential and Proprietary

Copyright 2009. All Rights Reserved.(Y20220-TEB Rev A, March 2009)

The information in this document is subject to change without notice and should not be construed as a commitment by Aclara. Aclara assumesno responsibility for any errors that may appear in this document.

No responsibility is assumed for the use or reliability of software on equipment that is not supplied by Aclara.

Information contained in this document is private to Aclara Power-Line Systems Inc., St. Louis, Missouri. This information may not be published,reproduced, or otherwise disseminated without the express written authorization of Aclara.

Any software or firmware described in this document is furnished under a license and may be used or copied only in accordance with the termsof such license.

PROPRIETARY NOTICE

DISCLAIMER

Table of Contents

Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Support . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1Required Equipment. . . . . . . . . . . . . . . . . . . . . . . . . . . 1

Control and Receiving Unit Grounds . . . . . . . . . . . . . . . . . . . . 2Main CRU Ground Lug . . . . . . . . . . . . . . . . . . . . . . . . . . 2

Torque Specifications . . . . . . . . . . . . . . . . . . . . . . . . . 3CRU Door. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4

Torque Specification . . . . . . . . . . . . . . . . . . . . . . . . . 4Termination Panel. . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Torque Specification . . . . . . . . . . . . . . . . . . . . . . . . . 5Distribution Panel Assembly . . . . . . . . . . . . . . . . . . . . . . . 6

Torque Specification . . . . . . . . . . . . . . . . . . . . . . . . . 6CRU Card Cage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Universal Modem Assembly. . . . . . . . . . . . . . . . . . . . . . . . 8

Torque Specification . . . . . . . . . . . . . . . . . . . . . . . . . 8RS-232 Surge Suppressor . . . . . . . . . . . . . . . . . . . . . . . . . 8

Torque Specification . . . . . . . . . . . . . . . . . . . . . . . . . 8OMU Ground . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Torque Specifications . . . . . . . . . . . . . . . . . . . . . . . . . . 10

MTU Grounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Single-Phase Transformers . . . . . . . . . . . . . . . . . . . . . . . . 13

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SCE Grounding Procedures Technical Brief

OverviewThe unique nature of TWACS signaling requires a specific method of grounding the Substation Communication Equipment (SCE). This Technical Brief illustrates the correct method of grounding the SCE, and procedures to help the utility verify their equipment is grounded correctly.

Support

If you have any questions about this procedure or any Aclara product or service, please contact Aclara Customer Care:

Required Equipment

The procedures in this document utilize the following tools:

• Digital Multi-Meter

• Torque wrench (20 - 400 inch-lbs)

Email: [email protected]

Phone: 1-800-892-9008

Address: Aclara Power-Line Systems Inc.945 Hornet DriveHazelwood, MO 63042USA

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[email protected]

[email protected]

[email protected]

mail to:[email protected]

mailto:[email protected]



mailto: [email protected]

Control and Receiving Unit Grounds

Control and Receiving Unit GroundsThe Control and Receiving Unit (CRU) must have a direct and dedicated connection to the substation ground grid.

Main CRU Ground Lug

The main CRU ground lug is located inside the CRU on the lower left wall as shown in Figure 2.1.

Figure 2.1 Main CRU ground lug location

• Verify the main CRU ground lug is attached directly to the substation ground grid and is not tied to either the Modulation Transformer Unit (MTU) grounds or the Outbound Modulation Unit (OMU) ground. The CRU ground lug must be connected to the ground grid using #4 - #8 AWG copper wire.

• All of the CRU component grounds should be connected to the main CRU ground lug.

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Figure 2.2 Main CRU ground lug

Torque Specifications

• The hex nut on the main CRU ground stud should be tightened to 75 in-lbs.

• The substation ground connection must be tightened to 50 in-lbs. for a pan head screw or 75 in-lbs. for a set screw type lug.

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CRU Door

The CRU door ground is located behind the lower door hinge as shown in Figure 2.3. The CRU door ground should be connected directly to the main CRU ground lug.

• Make sure that the door of the CRU is connected to the main CRU ground lug.

• Using an ohmmeter, verify that the resistance between the CRU door ground and the main CRU ground lug is close to zero.

Figure 2.3 CRU door ground

Torque Specification

• The CRU door ground must be tightened to 75 in-lbs.

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Termination Panel

The Termination Panel ground is located where the termination panels mount to the back of the swing-frame door. See Figure 2.4.

Figure 2.4 HDTP ground

• Ensure that the termination panel grounds are connected to the main CRU ground lug.

• Using an ohmmeter, verify that the resistance between the main CRU ground lug and each termination panel ground is close to zero.


The screws connecting the termination panel(s) to the swing-frame door must be tightened to 25 in-lbs.

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Distribution Panel Assembly

The Distribution Panel Assembly (DPA) Ground is located at the back of the upper portion of the CRU.

Figure 2.5 DPA terminal block

• Aclara recommends that the CRU be powered by a 3 wire, 120 V service.

• Verify that the main CRU ground lug is connected to the DPA terminal block.

• Verify that the AC surge suppressor ground is connected to the ground on the DPA terminal block.

• Using an ohmmeter, check the resistance between the main CRU ground lug and the AC surge suppressor ground (The green wire shown in Figure 2.5). The resistance should be close to zero.


• The connections on the DPA must be tightened to 20 in-lbs.

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CRU Card CageFigure 2.6 CRU Card Cage

• Using a voltmeter, check for the presence of AC voltage at the main CRU ground lug and any metal part on the front of the CRU card cage. No more than 0.10 VAC should be present at these points.

• Using a voltmeter, check for the presence of DC voltage at the main CRU ground lug and any metal part on the front of the CRU card cage. No more than 0.10 VDC should be present at these points.

• Verify that the screws securing the cards to the CRU card cage are firmly tightened.

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Universal Modem AssemblyThe Universal Modem Assembly (UMA) rack is located in the top of the CRU.

Figure 2.7 Universal Modem Assembly grounds

• Verify that the UMA rack is connected to the main CRU ground.

• Using an ohmmeter, verify that the resistance between the universal modem rack ground and the main CRU ground lug is close to zero.

Torque SpecificationThe ground connections on the UMA rack must be tightened to 20 in-lbs.

RS-232 Surge Suppressor

The RS-232 surge suppressor provides surge and fast transient protection for RS-232 communication between the modem, radio, or ethernet communication device and the CRU.

• Verify that an RS-232 surge suppressor is present. If an RS-232 surge suppressor is not present, please contact Aclara to obtain one.

• Verify that the RS-232 surge suppressor is connected to a chassis ground (either the universal modem rack or one of the mounting rails in the CRU).


If grounding the RS-232 surge suppressor to one of the painted mounting rails in the CRU, sand off the paint and use a #10-32 screw and clip nut. These should be tightened to 25 in-lbs.

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OMU GroundThe OMU must have a direct and dedicated connection to the substation ground grid. The OMU ground lugs are located on the back side of the OMU as shown in Figure 2.8.

Figure 2.8 OMU ground lugs location

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OMU Ground

Figure 2.9 OMU ground lugs

• Verify the main OMU ground lugs are attached directly to the substation ground grid and are not tied to either the MTU ground or the CRU ground. The OMU ground lugs must be connected to the ground grid using the same gauge cable as the ground grid.

Torque Specifications

• The OMU ground lugs must be tightened to 400 in-lbs. (33 ft.-lbs).

• For all other OMU torque specifications, see the label affixed to the inside of the OMU door.

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MTU Grounds

Figure 2.10 MTU grounds

Verify that:

• H0 (primary neutral) is connected directly to the substation ground grid with the same gauge cable used for H1-H3.

• H0 does not connect to any other ground.

• X0 is connected directly to the Neutral Outbound Signaling Switch Assembly (OSSA) inside the OMU and is not tied to any other ground.

• Concentric neutrals (if present) are connected to a tank ground and are not connected to H0.

• Any factory installed ground straps between X0 and the tank ground and/or between H0 and the tank ground have been removed.

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MTU Grounds

Figure 2.11 MTU grounding overview

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Single-Phase TransformersThe same basic grounding rules apply when using three single-phase transformers in place of one three-phase transformer. See Figure 2.12.

• The tank ground of each transformer must be connected to the substation ground grid.

• The CRU and OMU must have dedicated connections to the substation ground grid.

• All other tank grounds should be grounded to the nearest point on the substation ground grid.

• The H0 terminals of the three transformers may be tied together, and should be connected directly to the system neutral.

• The X0 terminal of each transformer must connect directly to the neutral OSSA inside the OMU, and must not connect to any other ground.

• If ground straps are used to connect X0 or H0 to a tank ground, they must be removed.

Figure 2.12 Three single-phase transformers

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Single-Phase Transformers

Figure 2.13 Single-phase transformer grounding overview

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sce grounding procedures tech brief

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