E-24—Policy on approval and use of 2½ element metering

The copy of this document located on Measurement Canada's website is considered to be the controlled copy.

Category: Electricity
Bulletin: E-24 revision 1
Date: 2002-11-29

Table of contents

1.0 Scope

This bulletin applies to 2½ element electricity meters used for revenue metering in Canada.

2.0 Background

The 2½ element meters were designed to reduce costs associated with metering, by using one less instrument transformer and associated components internal to the meter. Although 2½ element metering is a legal and approved form of metering used in Canada, there is a potential for error as a result of its noncompliance to Blondel's theorem when certain conditions of application are not met. This potential for error was considered negligible while presenting significant cost benefits to the electricity utilities. However, the electricity industry and Measurement Canada (MC) have recently questioned this potential error and the appropriateness of using 2½ element metering in light of today's market and technology.

A national survey was completed by industry representatives of the Canadian Electricity Association and supported by MC to determine the actual impact of 2½ element metering on the Canadian market. The results of that surveyFootnote 1 were discussed by MC and industry during the Canadian Forum on Trade Measurement 2001 and a number of recommendations were agreed on to reduce the impact of this potential error.

The purpose of this bulletin is to provide policies to reduce the potential for error in the application of 2½ element metering.

3.0 Terminology

Blondel's Theorem:

In a system of N conductors, N-1 meter elements, properly connected, will correctly measure the power or energy taken. The connection must be such that all potential coils have a common tie to the conductor in which there is no current coil.

Self-contained Meter:

A meter designed to be connected directly to a power circuit, without the use of external devices such as instrument transformers or shunts.

Transformer Type Meter:

A meter designed to be used with instrument transformers.

4.0 Approval policy

4.1 Applications for approval of self contained 2½ element meters will no longer be accepted by Measurement Canada after December 31, 2002.

4.2 Applications for approval of transformer type 2½ element meters will continue to be accepted by Measurement Canada.

5.0 Installation policy

5.1 Effective April 1, 2003;

  1. all new, and
  2. all reconstruction of existing, 3-phase 4-wire wye configured metering installations shall use metering that is compliant to Blondel's theorem such as 3-element metering.

Note: An example of a reconstructed metering installation may include service upgrades and, or instrument transformer replacement. Reconstruction of existing metering installation, sites that do not have sufficient physical space to accommodate the additional voltage transformer will not be required to meet 5.1 above.

5.2 All 2½ element meters installed on 3-phase 4-wire wye configured services prior to April 1, 2003 may remain in service under the following conditions:

  1. 2 ½ element meters continue to be verified and reverified as per Measurement Canada's legal requirements.
  2. The risk of error associated with 2½ element metering remaining in service has been minimized, by ensuring that feeder loads are reasonably balanced.

6.0 Additional considerations

6.1 For the application of 2½ element meters, contractors are advised of the potential for error due to zero sequence voltage. If the contractor assesses the service or loading to be of such nature that the potential for sustained error could be higher than the dispute tolerances prescribed under the Electricity and Gas Inspection Regulations, then the service should be converted to a 3-element metering installation.

6.2 The long-term expectation is that:

  1. Self-contained 2½ element meters will be replaced with self-contained 3-element meters through obsolescence of the meters, and
  2. Transformer type, 2 ½ element meters will be replaced with transformer type 3-element meters, on an opportunity basis, through obsolescence or reconstruction of the metering installation.
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