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How to Measure Static Resistance of Main Contacts on High Voltage Circuit Breakers

Safety Precautions

DV Power micro-ohmmeters must be used in observance of all existing safety requirements/regulations from national/local standards for accident prevention and environmental protection.

Make absolutely sure that the application equipment is de-energized and properly grounded before proceeding with any installation or maintenance.

While measuring the static resistance of high voltage circuit breakers, we recommend grounding both sides of the circuit breaker. It is the most secure way to avoid the appearance of voltages (inducted by nearby high voltage electric fields) on the voltage sense inputs of the micro-ohmmeter.

There are two possible methods of performing the grounding that are going to be explained.

High Voltage Circuit Breaker Static Resistance Measurement with One Side Grounded

In cases when only one side of a circuit breaker is grounded, it is necessary to close the main circuit breaker contacts before the current and voltage sense cables of the micro-ohmmeter are connected. This way, the other side of the circuit breaker will be grounded as well.

Figure 1.1. shows a case when one side of the HV circuit breaker is grounded. The input of the device is connected to the grounded side.
Before the measurement is initiated, it is necessary to open the main contacts of the HV circuit breaker, and then to open the disconnector switches on both sides of the circuit breaker. Visual check is required to make sure that the circuit breaker has been completely separated from the system.

Measurement Procedure

  1. Ground one side of the circuit breaker
  2. Close the main contacts of the circuit breaker
  3. Ground the micro-ohmmeter
  4. Connect the mains voltage supply cable first to the micro-ohmmeter, then to the supply source
  5. Connect the “-” current cable between the instrument and the grounded side of the circuit breaker
  6. Connect the “+” current cable between the instrument and the other (non-grounded) side of the circuit breaker
  7. Connect the voltage sense cables to the circuit breaker inside of the already connected current cables
  8. Turn on the micro-ohmmeter

Disconnecting Procedure

  1. Turn off the micro-ohmmeter
  2. Disconnect the “+” voltage sense cables from the circuit breaker
  3. Disconnect the “-” current cable from the non-grounded side of the circuit breaker
  4. Disconnect the current cable from the other (grounded) side of the circuit breaker
  5. Disconnect all current and voltage sense cables from the instrument
  6. Disconnect the mains voltage supply cable first from the supply source, and then from the instrument
  7. Disconnect the ground (PE) cable from the instrument
  8. Open the main contacts of the circuit breaker

If this procedure is not followed, the operators and the instrument will be endangered.

One Side Grounded Circuit Breaker
Figure 1.1 High voltage circuit breaker contact resistance measurement with one side grounded

NOTE: The circuit breaker must be closed during the preparations and during the measurement itself. If the circuit breaker is in the OPEN state, the operators and the instrument will be endangered.

In this case, the grounded side of the circuit breaker will be on zero potential, while the other side will have the potential amounting to the value of induced voltage. This will cause a difference of potentials between the voltage sense inputs of the instrument. This difference can be very large (depending on the induced voltage value) and it can endanger the operators and the instrument.

High Voltage Circuit Breaker Static Resistance Measurement
with Both Sides Grounded

During the HV circuit breaker contact resistance measurement, an efficient way of ensuring protection from the inducted voltages is to ground the tested circuit breaker on both sides. This will ensure the safety of the operators and protect the instrument as well.

The diagram for connecting the instrument to the high voltage circuit breaker with both sides grounded has been shown in Figure 2.1.

Before the measurement is initiated, it is necessary to open the main contacts of the HV circuit breaker, and then to open the disconnectors on both sides of the circuit breaker. Visual check is required to make sure that the circuit breaker has been completely separated from the system.

It is recommended to ground both grounding cables at the same point. This will improve the static resistance measurement accuracy since it removes the influence of grounding resistance variations.

Circuit Breaker Resistance Measurement
Figure 2.1 High voltage circuit breaker contact resistance measurement with both sides grounded

Measurement Procedure

  1. Ground both sides of the circuit breaker
  2. Close the main contacts of the circuit breaker
  3. Ground the micro-ohmmeter
  4. Connect the mains voltage supply cable first to the micro-ohmmeter, then to the supply source
  5. Connect the current cables to the circuit breaker
  6. Connect the voltage sense cables to the circuit breaker inside of the already connected current cables
  7. Connect the current clamps to one of the circuit breaker grounding cables (it does not matter which one)
  8. Turn on the micro-ohmmeter

Disconnecting Procedure

  1. Turn off the micro-ohmmeter
  2. Disconnect the current clamp from the circuit breaker grounding cable
  3. Disconnect the voltage sense cables from the circuit breaker
  4. Disconnect the current cables from the circuit breaker
  5. Disconnect all current and voltage sense cables from the instrument
  6. Disconnect the mains voltage supply cable first from the supply source, and then from the instrument
  7. Disconnect the ground (PE) cable from the instrument
  8. Open the main contacts of the circuit breaker

If this procedure is not followed, the operators and the instrument will be endangered.

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December 1, 2020

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