S-S-03—Prerequisites to the use of sampling inspection

S-02—Application of specifications for statistical sampling and measurement uncertainty in the verification and reverification of electricity and gas meters

Category: Statistical methods
Specification: S-S-03 (rev. 2)
Document(s): S-S-01, S-S-02, S-S-04, S-02
Issue date: 2009-10-28
Effective date: 2009-10-28
Supersedes: S-S-03 (rev. 1)

Table of contents

• 1.0 Scope
• 2.0 Authority
• 3.0 Normative references
• 4.0 Prerequisites
• 5.0 Revision
• Annex A (normative) Sampling plans
  • A.1 Scope
  • A.2 Conditions
  • A.3 Inspection and conformity evaluation

1.0 Scope

1.1 This specification establishes requirements that shall be satisfied prior to the use of sampling methods for the inspection of homogeneous lots of measurement devices.

1.2 These requirements shall apply to the qualification of production processes and to the re-qualification of production processes following a discontinuation or suspension of sampling inspection.

2.0 Authority

This specification is issued under the authority of section 19 of the Electricity and Gas Inspection Regulations.

3.0 Normative references

3.1 ISO 7870:1993, Control charts—General guide and introduction.

3.2 ISO 8258:1991, Shewhart control charts.

3.3 ISO 7966:1993, Acceptance control charts.

3.4 ISO 2859-4:2002, Sampling procedures for inspection by attributes - Part 4: Procedures for assessment of stated quality levels.

3.5 S-S-02, Measurement Uncertainty and Meter Conformity Evaluation Specifications.

3.6 S-S-04, Sampling Plans for the Inspection of Isolated Lots and Short Series of Lots.

3.7 S-S-01,Specifications for Random Sampling and Randomization.

3.8 S-A-01, Criteria for the Accreditation of Organizations to Perform Inspections Pursuant to the Electricity and Gas Inspection Act and the Weights and Measures Act.

4.0 Prerequisites

4.1 The device manufacturer or servicing organization shall be an accredited meter verifier meeting the requirements of accreditation listed in S-A-01 (reference 3.8). The quality management system shall have a clearly stated focus on the devices of interest that are anticipated to be inspected by sampling under S-S-04 (reference 3.6).

Note: Accredited meter verifiers should validate non-accredited meter suppliers' conformance to requirements.

4.2 A detailed description of the device production process, associated quality controls and uncertainty reviews shall be fully documented and provided to Measurement Canada to support the use of sampling inspection, including:

1. full identification of all measurement device quality characteristics inspected and controlled by the manufacturer or service organization, including when and where in the production or re-servicing process that they are inspected, and the following details for the quality characteristics:

   1. associated specification limits;
   2. inspection procedure details, including whether inspection is done by 100% inspection, sampling inspection, or statistical process control (SPC) methods, with consideration of S-S-02 (reference 3.5);
   3. where SPC methods are used for monitoring and controlling a characteristic's acceptability in accordance with references 3.1 to 3.3, details regarding:
      1. determination of rational subgroups;
      2. sampling frequency;
      3. determination of control chart parameters (e.g., central line, upper and lower control limits);
      4. relevant process capability measures;
      5. empirically-collected data regarding distributional shape; and,
      6. the most recent quality records to support that adequate process control has been achieved and maintained.

   Note 1: Capability measures for performance should be 1.4 or more with 90% confidence.

   Note 2: References other than those listed in references 3.1 to 3.3 may be employed providing the intent of those references are met.

2. where the manufacturer's or service organization's inspected and controlled quality characteristics differ from those used in Canadian meter verification and reverification specifications, the manufacturer or service organization shall supply objective evidence as to the relationship of the performance at its selected quality characteristics to those of the national requirements, along with any reductions in specification limits for testing that are determined necessary to assure conformity to national specification limits.

3. in accordance to S-S-02 (reference 3.5) the associated measurement system capability details in terms of repeatability, reproducibility, and overall system uncertainty with respect to the fully identified measurement equipment in use and its traceability to internationally-recognized standards.

4.3 The sampling plan approach may be used to provide objective evidence and information with respect to 4.2(a)(iii) above. In such case, the sampling plan specified in Annex A may be applied to a run of production to qualify the manufacturer's or service organization's output for sampling inspection. All other requirements of 4.2 and this specification shall still be met.

4.4 If the requirements of 4.1 through 4.3 are met then the process shall be qualified for the sampling plan having a limiting quality (LQ) of 3.15% for the inspection of non-performance characteristics and a LQ of 3.15% or 8.0% as the case may be, for the inspection of performance characteristics under of S-S-04 (reference 3.6).

4.5 Quality records necessary to support conformance to this specification shall reflect all details described in this document.

5.0 Revision

The purpose of revision 2 is to:

• amend section 3.0 to update the title of S-S-01.
• add in section 3.8 normative reference S-A-01, "Criteria for the Accreditation of Organizations to Perform Inspections Pursuant to the Electricity and Gas Inspection Act and the Weights and Measures Act".
• amend section 4.1 to align wording with requirements of S-A-01. In addition, under this section, the note addresses recommended practices concerning non-accredited meter suppliers.
• renumber section 5.2 as 4.5.
• delete sections 5.0 and 5.1, as requirements for accredited meter verifiers to develop and implement policies and procedures are stipulated in S-A-01. Wording in section 5.1 concerning non-accredited meter suppliers, moved to the note under section 4.1.

The purpose of revision 1 was:

• section 4.1, to add, "The audit results shall be fully documented and found acceptable to Measurement Canada".
• in section A.3.4.1 to change sample size from 200 to 125.

Annex A (normative) Sampling plans

A.1 Scope

A.1.1 This annex specifies sampling by attributes plans for qualifying or requalifying a manufacturer's or service organization's device production process for sampling inspection with respect to relevant specification requirements.

A.1.2 The qualification process is designed to permit inspection of measurement devices by sampling according to a limiting quality (LQ) of 3.15% or 8.0% for type 1 marginal conformities and a LQ of 20% for type 2 marginal conformities.

A.2 Conditions

The following conditions shall exist before sampling by attributes procedures may be used for sampling inspection qualification:

1. the production process is producing or is capable of producing devices with stable quality.
2. the production process output is or will be in the form of homogeneous lots submitted in the order of their production.
3. production conditions are stable.
4. the production run is long enough to qualify under this Annex and is completely inspected under the terms of this Annex.

A.3 Inspection and conformity evaluation

A.3.1 Annex A of S-S-04 (reference 3.6) shall be used for the inspection of lots for the purpose of:

1. process qualification or re-qualification with the exception that table A.1 below shall be used.
2. for the purpose of moving to LQ 8.0% from processes operating at LQ 3.15%, use the applicable Option 1: LQ 3.15% plans of S-S-04 table A.1 or B.1, reference 3.6.

A.3.2 The quantity of devices to be presented shall be the greater of:

1. if available, the historical average or typical lot size.
2. if available, the expected lot size.
3. a lot size of at least the quantity indicated in A.3.3 or A.3.4 below.

A.3.2.1 For processes which have been operating at LQ 3.15%, the devices most recently presented may be used for the purpose of meeting A.3.1(b) above.

A.3.3 For the purpose of process qualification, re-qualification, or for the purpose of using the LQ 8.0% option indicated in A.3(2) of S-S-04 (reference 3.6) the process is considered qualified if at least two consecutive lots have been accepted and the cumulative count in the current lot is:

1. zero marginally conforming units of type 1 and the number of samples devices inspected is at least 125.
2. at most three marginally conforming units of type 2 and the number of samples devices inspected is at least 50.
3. zero nonconforming units found in the sample devices inspected.

A.3.3.1 Alternative plan for (a) above: one marginally conforming units of type 1 and the number of samples devices inspected is at least 200.

A.3.4 For the purposes of using the LQ 8.0% option indicated in B.5.1 of (reference 3.6) the process is considered qualified if at least four consecutive lots have been accepted and the cumulative count in the current lot is:

1. zero marginally conforming units of type 1 and the number of samples devices inspected is at least 80.
2. at most three marginally conforming units of type 2 and the number of samples devices inspected is at least 50.
3. zero nonconforming units found in the sample devices inspected.

A.3.4.1 Alternative plan for (a) above: one marginally conforming units of type 1 and the number of samples devices inspected is at least 125.

A.3.5 In the current lot if the number of samples devices inspected is less than the identified sample size required in A.3.3 or A.3.4 above then the accumulation shall extend to the previous lots.

A.3.6 Device test limit values for performance type 1 characteristics shall be the appropriate limits specified in the device specification for LQ 3.15 or 8.0%.

A.3.7 For the purpose of process qualification the accredited meter verifier may provide historical evidence indicating a high quality process exists provided documented evidenced confirms that the intent of A.3.1 through A.3.6 above have been met.

A.3.8 Whenever a nonconforming unit is found or the number of marginally conforming units exceed the value specified in A.3.3 or A.3.4 above, then a new sequence of lots and a new count of inspected devices shall be initiated. If a second attempt to qualify or to requalify the process is unsuccessful then corrective action shall be implemented prior to production of the next lot.

Notes:

n

Sample size

Ac₁

Acceptance number (maximum permissible type 1 marginally conforming units)

Re₁

Rejection number (minimum impermissible type 1 marginally conforming units)

Ac₂

Acceptance number (maximum permissible type 2 marginally conforming units)

Re₂

Rejection number (minimum impermissible type 2 marginally conforming units)

Where the sample size equals or exceeds the lot size, perform 100% inspection of the lot.

Ac = 0 for all nonconforming units.