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Three into one goes easily

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In the last twenty years, electrical installation testers used by contractors and electricians around the nation have changed completely. Despite the reservations of times gone by, Mark Johnson of Metrel UK Ltd explains why today, it’s beneficial to put all your eggs in one basket.

Back in the day, the average contractor would have three testers: a loop tester, an RCD tester and a combined insulation and continuity tester, typically lunch box shaped and yellow in colour. Suddenly loop testers had to contend with operating through RCDs, giving birth to the much-loved D-lock version of the non-trip test.

Everything was simpler then including the electronics in the testers, but even so they were less reliable. There were all-in-one testers around then, but customers resisted them saying, “with three testers, one can go for repair or calibration and I can carry on working”. Yes, they were right about repair, but they forgot they usually had all three testers calibrated at the same time.

There were some manufacturers who combined the RCD and loop tests into one machine as they used much the same circuitry. This had a limited success around the time Part P was added to the building regulations, but now that has waned, and the kits have been withdrawn.

Despite the early objection to “having all your eggs in one basket”, end-users have been swayed by the financial logic of a single multi-function installation tester. And their electronic design and reliability has improved by leaps and bounds in recent years. So why pay for three knob and case sets and three LCD screens when you can buy one, and calibration costs are only slightly more than a single instrument calibration?

Convenience is a key factor, the multi-function tester means that when testing you have all the test kit you need with you, so you do not need to go back to your van, or keep an eye on the testers you are not using.

Originally multi-function testers were box-shaped with a portrait layout, some had handle extensions on the side and they used custom monochrome LCD displays. But now they have generally evolved to a bean-shaped landscape aspect, making them more comfortable to hang around the neck in use. The best testers now use full-colour dot-matrix screens that can display large amounts of information, such as results and test parameters at the same time. It goes without saying that rechargeability is a must.

All multi-function testers offer the same basic tests; some perform them more competently than others and some extend the range of tests in that test group. For example, a basic tester may measure voltage between two probes, while a more sophisticated tester will, when the three connections are made, display voltages between each probe, and the phase rotation. 

Basic machines offer just two insulation tests, 250 V and 500 V, whereas a more useful tester will extend the range from 50 V to 1000 V so that insulation testing can be started at a lower voltage to prevent damaging any vulnerable components that may have been left in circuit. A really good tester will also offer a voltage ramp test to confirm that surge protection devices operate at their nominal voltage.

Turning to low ohms, or continuity, all testers have to test at 200 mA for certification purposes, but premium devices offer a lower current option between 7 mA and 15 mA for generally checking the connections during installation, prolonging the battery life. The 7 mA test current also guarantees a continuity test can be performed through an RCD. Additionally, some can do automatic forward and reverse continuity.

Earth loop impedance testing is another key function of the multi-function tester. A high current loop test for unprotected circuits and Ze measurement is normal, but only the better multi-function testers will display the value of the components that make up the impedance result, resistance and reactance at the same time. It is quite normal these days to have the prospective short-circuit current value calculated (though one should be aware of whether a nominal or measured voltage is used for the calculation) and displayed at the same time. Some testers still have a different switch position and require an extra loop test to be performed for a PSCC value.

Non-trip loop tests are available on all machines these days; they all use a three-wire connection with the exception of the two-wire test from Megger. Non-trip tests all use different algorithms to produce the result. The algorithms are all patented, consequently they are different, and they produce different measured results, giving more or less repeatability. Repeatability of the test is generally how users judge their acceptability. 

The algorithm of some manufacturers suffers inordinately from RCD up-lift, where the reactance of the coil in the RCD affects the final measurements and they have built into their premium testers what they call a confidence meter to overcome the problem. 

With the advent of EV charging equipment, there is a new limitation on the non-trip test, the ability to loop test without tripping not only the 30 mA a.c. RCD but also the 6 mA d.c. protection. Only an elite few can achieve this.

That brings us to RCD testing in its own right. These days all but the very cheapest multi-tester offers all the required tests – ½ In, In and 5 In as individual tests and an auto-RCD test – but ramp tests are less common, though they are really useful for diagnosing nuisance tripping. Having the ramp test included in the auto-RCD test is rare despite its usefulness. Many users prefer the idea of having all the results and test parameters displayed on a single screen rather than having to scroll through the results. It makes it quicker to identify the passes and fails.

It seems the longer the list RCD types that the machine tests the better. With the advent of increasing amounts of EVSE installation and testing work, machines that will automatically take you through the procedure testing both 30 mA a.c. RCD and the 6 mA d.c. protection and successfully perform a loop test too are few and far between.

The point of a multi-function tester is to save users time on the job and back in the office. Over the years, many testers have been designed to store test results on board, but until recently none have been practical or able to completely remove pen and paper from the process on-site. This is because memory management did not reflect the circuits and boards users were working on, and the labelling of circuits and the adding of comments was impossible. The new dot-matrix, touch-screen testers have changed everything; it is easier to build a representation of the drawing one is working to, and to post results to each location. Downloading and uploading to a PC running the certification software is then done most effectively through a wired connection.

Over the years, the multi-function tester has essentially become a computer, capable of performing electrical tests.

 

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