CHRIS HALLIDAY: Is calibration of test instruments important?
Should test instruments be checked for calibration and, if they aren’t, what are the risks? Chris Halliday sorts out the madness.
Electricians use various test instruments, such as voltmeters, tong meters and resistance meters, including insulation resistance testers.
They also use loop impedance testers, RCD testers, phase rotation meters, proximity testers – and perhaps even energy and/or power quality loggers.
These instruments have to perform in all sorts of harsh conditions in Australia: from dusty, dirty environments to temperatures below freezing or well beyond 40°C. They get roughed about in the field and bounced about in vehicles and toolboxes.
We then expect them to function accurately.
Test instruments have improved dramatically throughout my career, from ‘average reading’ analogue instruments, which were easily knocked out of calibration, to true rms digital meters that can withstand a one-metre drop onto concrete.
How great it was in the 1970s when we got taut band analogue instruments (Google that) and relished having a more robust instrument.
However, calibration is now more important than ever before due to the increasingly litigious nature of Australian society.
Statutory or Standards requirements
The National Measurements Act 1960 and associated regulations and guidelines are focused on measurement accuracy of quantities that can be traded, eg: a litre of petrol, or a kilo of meat from the butcher.
This act does not include the normal types of measurement made by sparkies, such as volts, amps or ohms. Yet it does cover some electrical energy measurements taken by power companies, including kilowatt/hour or Var/hour.
The Standards AS/NZS3017 Electrical installations – Verification guidelines and AS/NZS3019 Electrical installations – Periodic verification suggest that test instruments should be calibrated at regular intervals.
Of course broad requirements are imposed by workplace health and safety legislation. But how can you verify compliance if your test equipment is not calibrated?
Tests in accordance with the Wiring Rules, AS/NZS3000, and AS/NZS3017 are required to ensure safety in an installation after electrical work has been carried out.
Let’s look at a couple of hypothetical situations:
In Scenario 1 you are running a new circuit to a socket outlet using ‘Dodgy Brothers’ cable.
You use an uncalibrated insulation tester and the results show that the cable is fine – yet it has a very low insulation resistance.
The insulation breaks downs, the cable starts sparking between active and earth, and the installation burns down two days after installation.
An electrical consultant working for WorkCover or the coroner checks the quality of your processes and the calibration date of your insulation tester. It is found to be out of date or possibly never calibrated.
The consultant gets a calibration report on the tester by a National Association of Testing Authorities (NATA) certified laboratory. The results show that the tester is faulty and would always show a good result.
Where does this leave you? In deep trouble.
In Scenario 2 you have recently tested a residual current device (RCD) in a building. The tests were to confirm minimum trip times for known applied currents.
Soon after the tests were conducted, there was a fatality due to an electric shock.
If uncalibrated equipment was used as the reference, can it be confirmed that the tests were performed correctly and the RCD was compliant?
I have heard sparkies say “I know my test gear is good. I don’t need to calibrate it.”
However, you can’t be sure – or prove that measurements meet the required specifications – unless you manage the risk appropriately.
Calibration intervals are generally specified in three ways:
- The sparkie specifies how often the calibration is to be carried out;
- The sparkie and the calibration company agree on a calibration interval;
- The manufacturer’s recommended calibration frequency is followed.
Other than this, NATA has the document General Equipment – Calibration and Checks, which details a calibration interval of 12 months for digital multimeters and other types of meters that measure electrical parameters.
The intervals should really be specified around the calibration performance of individual makes and models of test instruments. Certain types that don’t hold the calibration should be verified more regularly than those that do.
I remember once asking my calibration company to determine whether I could extend the calibration interval on large number of power quality loggers that I was responsible for. I was trying to increase the availability of the instruments in the field and reduce my calibration costs.
Unfortunately the company responded that I needed to shorten the interval due to poor calibration performance of the loggers. You can’t win them all.
Who can calibrate?
It’s best to use a NATA calibration laboratory, as it will have the specialised knowledge, equipment, procedures and traceability to ensure success.
Naturally these services come with a price, but quality work always does.
In some cases it may be cheaper to buy a new instrument, with a calibration certificate, than to have your old faithful checked.
It pays to contact the calibration company before sending all your instruments off at once, or you might be in for a hefty bill. The whole-of-life costing for test instruments should include calibration costs.
A list of accredited calibration laboratories can be found on the NATA website at http://www.nata.com.au
Multifunction testers are cheaper to buy than separate instruments for each function.
However, you will be without all test functions when the one instrument is sent away for calibration. Larger organisation can often afford to have a spare instrument for these periods. Small contractors could send their testers for calibration when they go on holiday (if the calibration people aren’t also on holiday).
It is good practice to send the full instrument kit with the test instrument for calibration. Certainly CTs will need their calibration checked, and you can ask the company to also do a safety check of all accessories.
Another matter is when a sparkie is asked to install an energy meter (kWh measurement) to on-sell electricity. This opens up a can of worms that doesn’t need to be opened.
There are several issues here, including:
- The need for a licensed meter provider;
- Meters needing to meet specific criteria for utility meters in order to be compliant under the National Measurement Act; and
- The need for a retail licence.
You can advise your client that all this is able to be done, but it could be expensive and not easy.
Outside Queensland, electricians have no strict requirement, other than broad provisions of workplace health and safety legislation, to have their electrical test instruments calibrated.
Australian Standards AS/NZS 3017 and AS/NZS3019 suggest that test instruments should be calibrated regularly.
If instruments aren’t calibrated regularly, then electrical test results cannot be guaranteed. On the off-chance that things go pear-shaped, this leaves electrical contracting businesses open to prosecution, fines and loss of reputation. It also leaves them open to civil action in which a victim or relative may seek compensation for injury or loss of life.
I recommend that all sparkies get their test instruments checked at least annually.
Thank you to Tony Archer of Essential Energy for his assistance in the development of this article.
Disclaimer: This article is written on behalf of the Institute of Electrical Inspectors, although the views expressed are not necessarily those of the institute.
Any information in this article is given in good faith and is not all-encompassing. Electricians should make their own decisions based on legislative requirements, Standards, codes of practice, risk assessment, knowledge, etc. Otherwise seek further assistance.