MCB tests (application examples)


		WARNING
		Death or severe injury can occur if the appropriate safety instructions are not observed. The COMPANO 100 test set can output life-hazardous voltages and currents. Before operating any such electrical equipment, carefully read the Safety Instructions chapter in this manual (see Safety instructions).

MCBs (miniature circuit breaker) are factory-tested, and in general that is sufficient for the rest of their service life. Special circumstances, however, may require MCB tests. COMPANO 100 is capable of testing approx. 90 % of the ≤13 A circuit breakers, and 80 % of the 16 A circuit breakers.

The higher the fault current detected by the MCB, the faster the circuit breaker trips. Most MCB have both a magnetic and a thermal overcurrent detection, so that above a certain level of current, the trip occurs particularly quick (see graph of a typical overcurrent-time characteristic).

With such a characteristic, the MCBs avoid nuisance trips, for example on inrush currents beyond the nominal current. The tolerances of these two curves are significant. In the example given, the trip times at three times the nominal current can vary from 6 seconds up to 30 seconds, still being within the tolerance.

Manual MCB testing with QUICK

For this test, disconnect the MCB from any other circuits.
Then connect the MCB to the I OUT output of the COMPANO 100 test set:
To do a manual single test, use the QUICK application module. All you need to do is configuring the trigger to I OUT Overload, set an output current, and wait for the breaker to trip.

The overload indicator is delayed by 200 ms in either direction, coming and going. This is necessary to suppress short spikes of the detection, on one hand, and, on the other hand, to reliably evaluate that the overload has finally cleared instead of just disappearing temporarily like, for example, during zero crossing. This means, if you do a time measurement using "Overload" triggers, add to or deduct from the result 200 ms, accordingly.

Automated MCB testing with FLEX

This example shows how to automate the measurement of three interesting points. Use the FLEX application module to program a sequence that detects the overload of the output once the breaker trips. Such a semi-automatic test makes sense if the number of breakers to test is higher.

Three test points are tested:

first, the current for the magnetic trip (in our example five times nominal current)
the final point where the thermal overcurrent detection will trip and the magnetic one should not (in our example three times nominal current)
and a longer time period of nominal current for a stability test (when the breaker is still warm from the thermal trip before).

The first step of such a FLEX sequence is a step issuing five times the nominal current, with a trigger I OUT Overload, and a timeout of 2 seconds to protect the circuit and the cables in case something goes wrong.

Example screenshot

When the breaker trips, the operating time is recorded.
In the next step, the MCB needs to be manually closed again. To detect the closing, add a sequence step 2, and set an output current of I OUT = 1 A, a timeout of 10 minutes, and a trigger I OUT Overload Go. That way you have sufficient time to close the breaker.

Example screenshot
To avoid having to hold the MCB in your hands while it trips, set an additional sequence step 3 with 0 A for 5 seconds.

Example screenshot
Add a sequence step 4 with an output current of three times the nominal current. In that sequence step the MCB's thermal mechanism should react at some point between 6 seconds and slightly below 3 minutes.

Example screenshot
Add two more sequence steps with values as before: sequence step 5 with an output current of 1 A, a timeout of 10 minutes, and a trigger Overload go; and a sequence step 6 with 0 A for 5 seconds.
As a last sequence step, sequence step 7, add a stability test with an output current of 13 A for 10 minutes. The MCB should not trip in that time, nevertheless set I OUT Overload, this time expecting that the overload will not occur.

Example screenshot

For a 13 A MCB, for example, with the above shown overcurrent-time characteristics, in a tabular form the sequence looks as follows:

Step	I OUT	f	Timeout	Trigger	Transition
1	65 A	50 Hz	2 s	I OUT Overload	Step
2	1 A	50 Hz	10 min	I OUT Ovl. Go	Step
3	0 A	50 Hz	5 s	Off	Step
4	39 A	50 Hz	3 min	I OUT Overload	Step
5	1 A	50 Hz	10 min	I OUT Ovl. Go	Step
6	0 A	50 Hz	5 s	Off	Step
7	13 A	50 Hz	10 min	I OUT Overload	Step

Prior to starting the test, make sure the MCB is closed.
Press the Start/Stop key. The MCB should trip immediately.
Close the circuit breaker within the next 10 minutes. About 5 seconds later, you will hear the MCB humming loudly as the magnetic trip is working but not strong enough for the trip.
Finally after a while — within 3 minutes — the circuit breaker will trip again.
Close the circuit breaker within the next 10 minutes. The MCB should not trip again at its nominal current of 13 A.

To see the results of the individual sequence steps, press the Previous or Next soft keys. The results can be seen in the following sequence steps:

sequence step 1: magnetic trip time at 5 times overcurrent
sequence step 4: thermal trip at 3 times overcurrent
sequence step 7: (hopefully) timed-out.

Most breakers with a nominal current of 13 A and 16 A trip long before the internal limit of the test set is reached, that is, before a thermal overload of the COMPANO 100 occurs. However, if you had a breaker with a very long trip time but an overcurrent-time characteristic still within the tolerance, COMPANO 100 may not trip that breaker.
Therefore note: COMPANO 100 can well be used to prove that an MCB works properly by initiating the MCB's trip. However, it cannot reliably verify in all cases that the breaker is not working properly, in particular when a thermal overload stops the test set's current output before the MCB trips.