Test Instrument Calibration

Electricians use various test instruments such as voltmeters, tong meters, resistance meters including insulation resistance testers, loop impedance testers, RCD testers, phase rotation meters, proximity testers. Some electricians may even use energy and/or power quality loggers. Do these test instruments need to be checked for calibration? If instruments are not calibrated, what are the risks? Is there an alternative? We investigate these issues in this article.

Test instruments are asked to perform in all sorts of harsh conditions in Australia from dusty, dirty environments to those in temperatures below freezing or beyond 40°C. They get roughed about in the field and bounced about in vehicles and toolboxes. We then expect them to work accurately.

Test instrument 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 up to a one metre drop onto concrete.

I remember in the old days how great it was when we got taut band analogue instruments (Google that) in the 70’s and relished having a more robust instrument.

Calibration, however, is more important today than ever before due to the litigious nature of today’s society in Australia.

Statutory or Standards Requirements to Calibrate

The National Measurements Act 1960 and associated regulations and guidelines are focused on measurement accuracy of quantities which can be traded e.g. a litre of petrol, or a kilo of meat from the butcher.

This act does not include the normal types of measurements made by sparkies such as volts, amps or ohms. Yet it does cover some electric energy measurements taken by power companies, such as kilowatt hours or VAr hours, where the user is billed for the energy usage.

Both AS/NZS3017 Electrical installations – Verification guidelines and AS/NZS3019 Electrical installations – Periodic verification suggest that test instruments should be calibrated at regular intervals.

There are of course broad requirements imposed by Workplace Health and Safety legislation about ensuring safety – how can you verify compliance to such requirements if your test equipment is not calibrated?

Daniel reading a scope meter that has been calibrated

Risks of NOT Calibrating

Tests in accordance with the Wiring Rules, AS/NZS 3000, and AS/NZS 3017 are required to ensure safety within an installation after electrical work has been carried out. AS/NZS 3760 specifies requirements for testing and tagging of portable appliances and leads and also residual current devices (RCDs), to ensure safety. The electrical risk cannot be properly managed with certainty if test instruments cannot be confirmed to be within calibration.

Let’s look at a few hypothetical situations:

In Scenario 1, you are running a new circuit to a socket outlet using “Dodgy Brothers” cable.

You use an un-calibrated insulation tester and the results show the cable is fine, but in fact the cable has a very low insulation resistance. The insulation breaks downs further over the coming days, the cable starts sparking between active and earth, and subsequently the installation burns down.

An inspector or electrical consultant working for SafeWork or the coroner checks the quality of your processes and also checks the calibration date of your insulation tester. Finding it is out of date or possibly never calibrated, the consultant gets a calibration report done on the tester by a certified National Association of Testing Authorities’ (NATA) 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 an RCD at a premise. The tests performed were to confirm minimum trip times for known applied currents.

Soon after these tests were conducted there was a fatality due to an electrical shock.

If uncalibrated equipment had been used to do the testing, 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.”

 But how can you be sure or prove measurements taken meet nominated specifications unless you manage the risk appropriately?

In Scenario 3, a network provider investigates a shock off a pontoon to the water – a shock voltage of 0.5 volts is measured. A person was cleaning the pontoon whilst in the water cooling off. Strong muscular contraction/locking on, can occur at a mere 2 volts for large area contact in salt water. The meter accuracy was not good and the meter should have read 5.4 volts. Imaging if someone receives strong muscular contractions and can’t move and subsequently drowns – a very common cause of death in water due to electricity.

How Often?

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 recommends the calibration frequency.

Industry practice is to have a calibration interval of 12 months for digital multimeters and other types of meters which measure electrical parameters.

The interval should really be specified around the calibration performance of individual makes and models of test instruments. A particular make and model of test instrument that doesn’t hold its calibration should be calibrated more regularly than one that holds its calibration well.

I remember at one stage asking my calibration company to determine if I could extend the calibration interval on a 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 to reduce the calibration costs. Unfortunately, the calibration company came back and said that I needed to shorten the interval due to poorer calibration performance of the loggers. You can’t win them all.

Who Can Calibrate?

It is best to use a NATA approved laboratories as they have the specialised knowledge, equipment, procedures and traceability that will ensure success. Of course, their services come with a price, but quality work always does.

In some cases, it may be cheaper to purchase a new instrument, with a factory calibration certificate, than to have your old faithful rechecked. A factory calibration may be as accurate as a NATA laboratory calibration but doesn’t hold quite the same ‘standing’.

It will pay to check with your 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 really consider calibration costs in the analysis.

A list of accredited calibration laboratories can be found on the NATA website at http://www.nata.com.au

Other Issues

Multi-function testers are cheaper to buy than buying a separate test instrument for each function. However, you lose 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. Smaller contractors may send their instruments for calibration when she/he goes on holidays (if the calibration people aren’t on holidays).

It is good practice to send the full instrument kit with the test instrument for calibration. Certainly, current transformers will need their calibration checking and you can ask your calibration company to also do a safety check of all accessories and the test instrument itself.

Another matter is where a sparkie might be asked to install an energy meter (kWh measurement) to on sell electricity in an embed network scenario. This could be as simple as a granny flat, a small shopping complex or caravan park. In such situations, retail and network licence exemptions are likely to be needed. These exemptions may be deemed/automatic, registerable or individual. Various rules apply, including using a National Meter Identifier (NMI) approved meter for the measurement. Check the AER (Retail) Exempt Selling Guideline and the Electricity Network Service Provider – Registration Exemption Guideline.

Alternatives

Many small electrical businesses have limited resources and time. For these businesses, calibration costs and the need to have additional test instruments for periods where instruments are away for calibration, may be crippling.

Some contracts with large clients will require the small contractor to have tests instruments regularly calibrated. In such situations, the small contractor can build in the additional calibration costs into the contract renumeration.

However, for those smaller contractors with limited resources, calibration by a NATA lab with a calibration interval of two years might be appropriate. This may be supplemented with some occasional checks using a ‘check box’.

It should be well noted that verification using a check box does not replace a full calibration nor does it really assess accuracy, it simply checks whether the instrument is working or not and the reading is close to what should be expected. Check boxes also have a limited range of comparisons that can be performed (refer to Table 1).

Calibration instruments, normally, need to be more accurate that the instrument under test by a factor of four times. Most check boxes won’t meet this requirement.

Conclusions

Except for Queensland, electricians have no strict requirement, other than broad requirements under 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 the electrical business open to prosecution, fines, loss of reputation and open to a civil case where a victim or relative may seek compensation for injury or loss of life.

Check boxes provide a means to keep a check that test instruments, with functions that can be checked, are providing readings close to what should be expected in between calibrations. Check boxes only have a limited range of tests. They DO NOT replace NATA calibration. Check boxes should also be sent for regular calibration checks.

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