“Loop testing creates more paranoia among electricians and produces more calls to our helpline than any other form of testing,” says Simon Wood of Megger. However, as he’s pleased to report, things are definitely changing!
If you carry out three loop impedance test’s on the same circuit and get a different result every time, it’s hard to know which one to record or, possibly, be confident about any of the results. Yet inconsistent loop test results are common, as are results that simply look wrong – often, they’re considerably higher than you would have expected.
So why do these problems occur?
In fact, the standard two-wire high-current loop test available on most test meters today, gives accurate results that are rarely, if ever, inconsistent. As such, this is the go-to test that any electrician should use wherever possible. Unfortunately, however, these tests can only be carried out on circuits that are not protected by an RCD or an RCBO.
To get around this, test equipment manufacturers have developed different methods of non-trip loop testing, but these normally have to use very low test currents – low enough so as to avoid the RCD or RCBO tripping during the test. The voltage changes produced by these low currents, which is what the loop tester actually measures, are very small and hard to measure accurately, particularly if there are any harmonics - or there is noise present - on the circuit.
There’s also another common problem, which is often called RCD uplift. This means that because of the measurement technique used, the result will include the internal impedance of certain types of RCD and RCBO in the displayed value, giving a result that can be far higher than the actual loop measurement alone. It’s easy to see that there are good reasons why loop impedance testing causes more issues for electricians than any of the other tests combined, but what can be done about it?
There has been only two key changes in measurement technology for loop impedance testing over the last 30 years. In the early 1990’s we saw the first Robin ‘D-Loc’ meters hit the market, closely followed by numerous 3-wire no-trip solutions, still common place today. Later that decade the first 2-wire no-trip tester that overcame the newer ‘electronic’ RCD’s was introduced by Megger. This technique has undergone three updates over the years and gives the test engineer the ability to undertake a test where no-neutral is present, something obviously not possible with the three wire test!
Today, there is a third step-change that will be added to the history books. Megger has been aware of the limitations of the current test methods that are still apparent in the real world for the test and inspection engineer on a daily basis. Whether that be the RCD impedance issues or the inconsistency due to noise etc. – and just as importantly but not yet mentioned, the time any of these no-trip methods take to complete a test. To overcome all three of these Megger has now delivered the answer in the form of its new MFT1741 multifunction installation tester.
For non-trip loop tests, the MFT1741 uses a new and proprietary technique, which all but removes the impact of noise or harmonics. Another benefit of the new advanced method is that it completely ignores the internal impedance of the RCD or RCBO protecting the circuit, so high readings due to RCD and RCBO uplift are a thing of the past.
Finally, the meter introduces a brand new feature in a “confidence” display – an arc that starts off wide but then reduces as the instrument becomes more and more confident in the accuracy of the result. When the arc reduces to a single point, a high-confidence result is displayed. With the new test method, this can take as little as 8 seconds on circuits with little noise present. Should the meter encounter a high noise circuit, this is indicated by a noise symbol being shown and this may be accompanied by the confidence arc expanding mid-test, before finally reducing to a single point. This ensures the user is always aware of what is occurring on the circuit during the test.
One final additional feature is the meters ability to now carry out a loop impedance test on circuits protected with 10mA RCD’s – often seen in Schools, medical environments and other special locations. Again, this is a first for the industry and will allow test and inspection certificates to be fully completed.
The latest loop test technology, used in the Megger MFT1741 multifunction installation tester, now provides the most comprehensive solution to loop impedance testing available today.
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