The whole works

Megger has launched the MTB7671 test box to ensure instruments are maintaining their accuracy and performance between calibrations.

Rather than just focusing on the passive measurement of various resistances, the MTB7671 also measures the voltage and current outputs of the tester. What’s more, the MTB7671’s resistance checks are not cramped down at one end of the measuring range, but are spread along it.

With loop testing, the majority of test boxes arrange for the meter that’s being checked to measure the loop impedance of a local circuit. Then a 1 ohm resistor is added, and the measured result should show a 1 ohm increase.

However, many loop testers read up to 3000 ohms. Testing accuracy at two points right at the bottom of this range says almost nothing about accuracy at higher readings. The MTB7671 checks at the local loop value Zs, Zs + 1 ohm and at Zs + 180 ohms.

Another example is RCD testing, where the problem is not with the results provided by the test box, but with the inconvenience of getting those results.

When used to check an RCD tester, most test boxes trip any RCD protecting the circuit from which they are powered. One solution is to power the test box from a circuit that's not RCD protected, but this can often be inconvenient. With the MTB7671, however, tripping simply does not occur.