AMETEK MXCTSH Administrator Manual - Manual (Page 24)

MX Compliance Test System System Administrator Manual
uncertainty. Such was the case with the 1998 IEC 61000-3-2 class D harmonic current limits. In
many tests there will be uncertainty components that need to be evaluated on the basis of
previous data and experience, in addition to those evaluated from calibration certificates and
manufacturers, specifications.
Accreditation bodies are responsible for ensuring that accredited laboratories meet the
requirements of ISO/IEC 17025. The standard requires appropriate methods of analysis to be
used for estimating uncertainty of measurement.
4.3 Statements of Compliance - Effect of Uncertainty
This is a difficult area and what is to be reported must be considered in the context of the client's
needs. In particular, consideration must be given to the possible consequences and risks
associated with a result that is close to the specification limit. The uncertainty may be such as to
raise real doubt about the reliability of pass/fail statements. When uncertainty is not taken into
account, then the larger the uncertainty, the greater are the chances of passing failures and
failing passes. Using better equipment, better control of environment, and ensuring consistent
performance of the test usually attains a lower uncertainty.
For some products it may be appropriate for the user to make a judgement of compliance, based
on whether the result is within the specified limits with no allowance made for uncertainty. This is
often referred to as shared risk, since the end user takes some of the risk of the product not
meeting specification. The implications of that risk may vary considerably. Shared risk may be
acceptable in non-safety critical performance, for example the EMC characteristics of a domestic
radio or TV. However, when testing a heart pacemaker or components for aerospace purposes,
the user may require that the risk of the product not complying has to be negligible and would
need uncertainty to be taken into account. An important aspect of shared risk is that the parties
concerned agree on the uncertainty that is acceptable; otherwise disputes could arise later.
4.4 Uncertainty of the MXCTS System
Uncertainty is a consequence of the unknown sign of random effects and limits to corrections for
systematic effects and is therefore expressed as a quantity, ie an interval about the result. It is
evaluated by combining a number of uncertainty components. The components used in the
MXCTS system were quantified by a combination of evaluation of the results of several repeated
measurements and by estimation based on technical data provided by the original manufacturers
of the components used in the MXCTS system.
The result of this analysis yields the accuracy specification of the MXCTS measurement system,
in particular that of the current harmonic measurement system. Software calibration coefficients
are used extensively to allow calibration of each individual measurement channel. The resulting
accuracy specification is a worst-case specification, meaning that typical MXCTS systems will
perform better. To ensure compliance of each MXCTS system to these measurement
specifications, each MXCTS system is individually tested before delivery using commonly
accepted measurement methods and NIST traceable calibration equipment.
The MXCTS accuracy specifications as published in the MXCTSH and MXCTSL User Manual
form the basis of the uncertainty assessment. The method of combining the uncertainty
components is aimed at producing a realistic rather than pessimistic combined uncertainty. This
usually means working out the square root of the sum of the squares of the separate components
(the root sum square method). The combined standard uncertainty may be reported as it stands
(the one standard deviation level), or, usually, an expanded uncertainty is reported. This is the
combined standard uncertainty multiplied by what is known as a coverage factor. The greater this
factor the larger the uncertainty interval and, correspondingly, the higher the level of confidence
that the value lies within that interval.
The uncertainty specified for the MXCTS system in the tables below uses a coverage factor of 2,
which corresponds to a confidence level of approximately 95%.
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