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How to Perform the Minimum Trip Voltage (MTV) Test

Introduction

To ensure that circuit breaker operation is within specification under the most rigorous conditions placed on its tripping ability, circuit breaker trip coils are specified to work with a minimum tripping voltage much below the nominal battery voltage. According to the standard IEC 60694 closing coils shall operate correctly between 85% and 110% of the rated supply voltage (AC or DC). Also, opening coils shall operate between 70% DC (85% AC) and 110% (AC or DC) of the rated supply voltage.

The minimum trip voltage test is described in a number of international and national standards such as IEC 56, ANSI C37.09, etc. Traditionally, this test was performed by tapping into the main tripping battery. However, this method should not be utilized as it presents a considerable risk to testing personnel, battery, and the breaker being tested.

Performing the Test

A better way to carry out the MTV test is by using SAT or POB devices. These devices are developed to supply voltage to breaker coils and spring-charging motor during commissioning testing and field service. SAT and POB generate filtered DC or AC voltage (50 Hz or 60 Hz; true RMS). It eliminates the need for station batteries or converters during a test. The output voltage is selectable from 10 V to 300 V DC or 10 V to 250 V AC (50/60 Hz; true RMS).

Minimum trip voltage test of the circuit breaker’s coils can be performed in two ways:

  • MANUAL: A pre-selected DC voltage in the DC MAN (SAT) or Volt DC (POB) menu is brought to the selected coil by pressing START (on the SAT) or TRIG (on the POB) button. The voltage level is increased manually until the coil is actuated.
  • AUTOMATIC: Select minimum voltage Ustart, maximum voltage Ustop and step voltage ΔU on the instrument in the DC AUT (SAT) or MTV DC (POB) menu (i.e. as shown in Figure 1). Press the START (TRIG) button. A pre-selected minimum DC voltage is applied across the desired coil, lasting 100 ms. If the coil is not actuated, after 900 ms of waiting time, the voltage is automatically increased for a pre-selected step (value 1 V to 19 V). The instrument repeats these steps until the coil is activated or maximum voltage Ustop reached (Figure 2).

NOTE: Please make sure SAT (POB) is connected to the control circuit to include auxiliary contacts “a” and “b” within connection, exactly as shown in Figure 3. If SAT cables are directly connected to the coil terminals a result will not be obtained. SAT (POB) registers the minimum trip voltage at the moment when auxiliary contact opens and the coil current drops to zero. The auxiliary contact in the tested coil circuit has to be in a closed position. Therefore, the MTV test is performed on the opening coil when the circuit breaker is in a closed position, while for performing the MTV test on the closing coil the circuit breaker has to be in an open position. Also, performing the MTV test on the closing coil requires an operating mechanism to be charged.

Connecting SAT to CB Control Circuit
Figure 3: Connection of SAT (POB) to circuit breaker’s control circuit

Test Results

The test can have three possible outcomes:

  • The coil is actuated and the minimum trip voltage and maximum coil current are registered;
  • The maximum pre-selected test voltage is reached without coil actuation, which is indicated by “No activation” message on the display, and announced by a sound and light alarms. The reason can be the disconnected coil circuit (e.g. auxiliary contact is open), insufficient high maximum pre-selected test voltage or a damaged coil;
  • The coil is actuated with no registration of the minimum trip voltage and maximum coil current. After the circuit breaker operation is executed, SAT (POB) will continue to “pump” the coil. This outcome indicates SAT (POB) is not connected in a proper way as shown in Figure 3, but directly to coils. If voltage supply is not stopped manually by the user (by pressing the STOP button), the message “No activation” will appear after reaching the maximum pre-selected test voltage. In this case, to prevent possible coil damage, it is needed to stop voltage supply from SAT (POB) after coil actuation.

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May 27, 2020

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