hillbille, I started writing this last week, it's still work in progress, but thought it may be of value in answer to your question.
Calibration.
Accuracy of Test Instruments.
Domestic Installers have a responsibility to ensure the accuracy and consistency of test instruments used to carry out the range of measurements required by BS 7671 for certification purposes.
BS 7671 does not require regular calibration of test instruments. However, in order to maintain confidence in the accuracy of all test instruments used for initial verification and condition reporting purposes. Those responsible for testing should put in place an effective system to confirm and record their continuing accuracy and consistency. Remedial action can be taken immediately if there is any indication that an instrument is no longer sufficiently accurate.
They should therefore already have in place an effective system, which enables them to confirm the continuing accuracy and consistency of all test instruments used for certification and reporting purposes.
Part P scheme assessors will check the effectiveness of the system each installer has in place to ensure the accuracy and consistency of their test instruments. These checks will be in addition to the checks already made of the suitability of test instruments to carry out the range of tests required by BS 7671.
There are a number of alternatives for such control systems, including:
- Maintaining records of the formal calibration/re-calibration of test instruments as recommended by the instrument manufacturers, supported by calibration certificates issued by recognised organisations with measurements traceable to national standards. Certificates issued by UKAS accredited laboratories are preferable.
- Maintaining records over time of comparative crosschecks with other test instruments used by the business.
- Maintaining records over time of measurements of the characteristics of designated reference circuits or devices. For example, the consistency of continuity, insulation resistance and earth electrode test instruments could be checked against a proprietary resistance box or a set of suitable resistors. Earth fault loop impedance test instruments could be checked by carrying out tests on a designated socket-outlet (on a non-RCD protected circuit) in the contractor's office. RCD test instruments could be checked by carrying out tests on an RCD unit plugged into the designated socket-outlet.
ESC, BPG7, states, “Each test instrument should be clearly and uniquely identified for record and traceability purposes.”
If an installer is unable to demonstrate to the satisfaction of the assessor that an effective control system is in place, which ensures the continuing accuracy and consistency of the test instruments used to carry out the tests required by BS 7671, the assessor will record a non-compliance in the assessment report.
Tony Cable, NICEIC technical officer talking at Elex last year:
“What a lot of contractors misunderstand is that the NICEIC inspector does not always look at the annual calibration certificate but focuses on the checks made on the instrument using a check box or another method, that the records are kept and consistent. This does not mean that the NICEIC don't expect the tester to be calibrated that should be taken as read. I always tell contractors that calibration is a requirement, which apart from the obvious could also make any manufacturer guarantee invalid, and should always be carried out by the due date.
What is Calibration?
Calibration is the process of comparing the accuracy of instruments readings to known Standards whose traceability is maintained by National or International standards.
Definitions from United Kingdom Accreditation Service website:
Calibration: - defined as specific types of measurement performed on measurement standards, material measures and measuring instruments to establish the relationship between the indicated values and known values of a measured quantity. The term covers calibrations carried out using appropriate reference equipment at any location.
Adjustment: - defined as the operation of bringing a measuring instrument into a state of performance suitable for its continued use.
Sometimes, however, the word calibration is misused to describe the process of altering the performance of an instrument to ensure that the values it indicates are correct within specified limits (e.g. adjusting an instrument until its reading agrees with that of an instrument or standard of greater accuracy than the instrument calibrated). Although the nature and magnitude of the adjustment, is determined by a pre-adjustment calibration, sometimes known as an as found calibration.
The procedure thus runs:
- First calibration (to determine the approximate magnitude of the adjustment needed).
- Adjustment.
- Second calibration.
Results from the calibration will be needed for the instruments reproducibility to be estimated (essentially its change in characteristics over a long time interval).
In summary, calibration is checking the tester on every function and every range to see that it is within specification, certifying it as fit for continued use. Adjustment is not calibration. This is the biggest misunderstanding of people outside the metrology environment.
Statistics from UKAS; suggest that 65% of instrument do not require adjustment, that 10% needs some adjustment, 5% need component changes to allow them to be brought back within limits, 5% require semiconductor device, display or memory replacement, 5% require firmware restore or upgrade, 5% require replacement leads, probes or fuses and 5% require repair/replacement of protective cases.
Without Calibration, you do not have a recognised method of ensuring that the equipment used during these tests is accurate. You may fail installations or appliances that are perfectly fine or more importantly you may issue a pass Certificate on an installation that should have failed and could possibly be dangerous. Put simply, for such a small cost, let's all ensure we do this right.
I have a Checkbox; Can I not use this to calibrate my instruments?
Checkboxes were never intended to replace an annual calibration check. They are intended to be used to confirm on-going accuracy between your annual calibrations. Most checkboxes only have a very limited number of values that can be used to check against, compare this to the whole list of readings that you receive from a calibration laboratory that will be checking your equipment on calibrators, which costs tens of thousands of pounds each.
The checkbox was a great idea to minimize the risk of any catastrophic failure resulting in site revisits being required and as such was a great addition to the quality system. Nothing more nothing less.
Use of Calibration Laboratory.
By using a calibration lab, you can be sure that every range on your equipment is comprehensively checked, giving you and your customer peace of mind in the results obtained.
More and more site managers are requesting calibration certificates of equipment used on their sites. Showing just a checkbox or a record showing half a dozen readings obtained from a checkbox is not considered good enough to comply with company H&S policy.
The leading test equipment manufacturers including Megger, Fluke, Ethos, Di-log, Kewtech & Seaward all recommend annual calibration in a calibration laboratory to ensure ongoing accuracy verification of their equipment. Their only exception is for the tri-annual calibration of check boxes. Generally, approved voltage testers that indicate voltage in broad bands do not require calibrating. However, if the tester incorporates a meter it should be subject to annual calibration to maintain accuracy of the meter.
What is UKAS?
The United Kingdom Accreditation Service (UKAS) is the sole UK National Accreditation body recognized by the government to assess, against internationally agreed standards, organisations that provide certification, testing, inspection & Calibration Services. Acute Sales Limited obtained UKAS recognition in 2008
What is the difference between UKAS & standard calibration?
UKAS Certificates are generally more expensive, as there are specific standards to be complied with to issue a UKAS Certificate. Any Certificates issued will bear the UKAS Logo and will be fully traceable back to National Standards; UKAS carefully monitor our laboratory to ensure end-to-end compliance.
Standard certificates are also of an extremely high calibre, and use standards traceable back to national standards.
All procedures employed and results reported, comply with the international standards ISO/IEC17025
Why should I use a supplier registered to UKAS?
Choosing a UKAS registered firm as your chosen calibration service means that you are confident of a professional quality service of the highest quality. UKAS Accreditation is an assurance to businesses that suppliers meet internationally recognised requirements for integrity and competence through regular UKAS assessments. All UKAS certificates issued to the high standards required and all measurements traced back to national and international standards.
http://www.esc.org.uk/fileadmin/user_upload/documents/industry/best_practice/BPG7_10.pdf
