Section 3—Performance Tests for Thermal-Magnetic Circuit Breaker

Square D UL® Listed 500 Vdc circuit breakers are designed, manufactured and calibrated for use on ungrounded uninterruptable power supplies (UPS). The maximum nominal (loaded) voltage is 500 Vdc and the maximum floating (unloaded) voltage is 600 Vdc.

These circuit breakers are UL Listed when applied with all three poles connected in series as shown on the label of the circuit breaker. The series connection is customer provided and external to the circuit breaker.

Square D UL Listed 500 Vdc circuit breakers are special circuit breakers for dc applications only and must be tested using dc current.

• Select the correct time-current trip curve. The trip curves show both the thermal and magnetic trip ranges of the circuit breakers.

• Use a dc power supply to test the circuit breakers as follows:

  Circuit Breaker	Time ConstantTime constant is defied by UL 489 as “The time constant of the circuit is the time measured on the oscillogram where the current is 63.2 percent of the prospective current.”*
  ≤ 400 A dc	≤ 20 ms
  450–2500 A dc	≤ 25 ms
  dc ripple constant ≤ 1% rms.

• Make sure ambient temperature and circuit breaker temperature is 25°C ± 3 degrees.

• Remove the circuit breaker from the enclosure. If removing the circuit breaker is not practical, test the circuit breaker in the end-use equipment. If the test results fall outside of the trip curve tolerance, remove the circuit breaker from the enclosure and retest.

• Use correctly sized cable (per National Electrical Code® [NEC®] tables) with a minimum of four feet (1.22 m) of cable per connection.

• Connect dc power supply to circuit breaker with all poles connected in series as shown on the circuit breaker label.

• Make sure connections to circuit breaker are properly torqued.

• Apply dc test current to trip the circuit breaker. The tripping mechanism in the circuit breaker reacts to the magnetic fields created by the current flowing through the circuit breaker. Apply a dc test current to the circuit breaker of approximately 70% of the expected value. If the circuit breaker does not trip, increase the test current on successive trials until it does trip. When the circuit breaker trips

  1. Reset and close the circuit breaker.

  2. Reapply the dc test current to trip the circuit breaker again.
  3. Record the current and compare to the trip curve.

Recommended steps to accurately field test circuit breakers (see test procedures in this manual for specific information):

• Use correct test equipment.

• Make sure ambient temperature and circuit breaker temperature is 25°C ± 3 degrees.

• Remove the circuit breaker from the enclosure. If removing the circuit breaker is not practical, test the circuit breaker in the end-use equipment. If the test results fall outside of the trip curve tolerance, remove the circuit breaker from the enclosure and retest.

• Use correctly sized cable (per NEC tables) with a minimum of four feet (1.22 m) of cable per connection.

• Test each pole individually.

• Make sure connections to circuit breaker are properly torqued.

• Test circuit breaker and record findings. The tripping mechanisms in the circuit breaker react to the magnetic fields created by the current flowing through the circuit breaker. When current flow is near the trip point of the circuit breaker, the magnetic fields can cause false tripping due to vibration. Do the following steps to minimize the effects of false tripping:

  1. After each current pulse or when circuit breaker trips, reset and turn the circuit breaker on.

  2. Reapply the test current to trip the circuit breaker again.

  3. Record the current and compare to the trip curve.

Severe environmental conditions can reduce the dielectric strength of molded case circuit breakers. Check insulation resistance during electrical system testing.

To check the insulation resistance, perform the following steps:

1. De-energize and isolate the circuit breaker.

2. Clean the circuit breaker as described earlier.

3. Using a megohmmeter with a capacity of 500–1000 Vdc, apply voltage from:

   1. Each phase-to-ground with the circuit breaker on (circuit breaker contacts closed).

   2. Phase-to-phase with the circuit breaker on (circuit breaker contacts closed).

   3. Between each line and load terminal with the circuit breaker off (circuit breaker contacts open).

4. Record resistance values. Resistance values of less than one megohm (1,000,000 ohm) should be investigated.

DANGER

Hazard of Electric Shock, explosion, or Arc Flash Apply appropriate personal protective equipment (PPE) and follow safe electrical work practices. See NFPA 70E or CSA Z462. This equipment must be installed and serviced by qualified electrical personnel. Turn off all power supplying this equipment before working on or inside equipment. Always use a properly rated voltage sensing device to confirm power is off. Replace all devices, doors and covers before turning on power to this equipment. Failure to follow these instructions will result in death or serious injury.

Inverse-Time Overcurrent Trip Test

1. Completely de-energize and remove the circuit breaker from service. Conduct the following tests at 300% of the circuit breaker ampere rating to verify the performance of the thermal tripping element on thermal-magnetic circuit breakers using a high-current, low-voltage ac power supply of less than 24 V.

2. Test in open air at 25°C (77°F) ambient temperature.

3. Trip times are measured from a “cold start.” A cold start, as defined by Underwriters Laboratories Inc. Standard 489 occurs at 25°C ± 3° (77°F ± 5°). Therefore, before beginning overcurrent testing, the circuit breaker must be in 25°C (77°F) ambient temperature long enough for all parts to reach that temperature. Circuit breakers that have been in higher ambient temperatures may take two to four hours to reach the steady state temperatures mentioned above.

4. Connect the circuit breaker to a power supply by using a minimum of four feet (1.2 m) of cable on each connection. Size the cable according to the ampere rating of the tested circuit breaker. Refer to the National Electrical Code Table 310-16; use the 75°C column for proper conductor sizing. Improperly sized cable will affect test results.

5. Test each pole of the circuit breaker individually at 300% of rated current using a high-current, low-voltage ac power supply.

6. Record and compare the trip test values to those in Inverse-Time Overcurrent Trip Test. As long as the recorded trip times are below the maximum trip times, the circuit breaker is providing acceptable thermal protection.

If verification of the manufacturer’s data is required, compare the trip times to the 300% trip range shown on the trip curve for the specific circuit breaker. If field test results fall outside the characteristic trip curve tolerance band, the test conditions and methods should be carefully evaluated for accuracy. A small error in test current results in a large error in trip time.

Instantaneous (Magnetic) Trip Test

This test simulates short-circuit conditions using a low-voltage test supply. To keep stray magnetic fields from affecting test results, test cables exiting the circuit breaker must be parallel with the current path of the circuit breaker for a minimum of 10 in (254 mm). Test results can also be influenced by the wave shape of the supply current. Use a power source with true sinusoidal output and a true RMS or analog ammeter to ensure accurate results. To verify the performance of the instantaneous (magnetic) trip element, proceed as follows:

Test NA and NC circuit breakers in the end-use equipment or lying flat on a piece of 1/8 in. (3 mm) thick steel.

1. Set the circuit breaker instantaneous (magnetic) trip adjustment, if provided, to the high setting. Tests conducted at the high setting ensure instantaneous trip protection exists at all lower settings.

2. Connect the circuit breaker to the low-voltage test source with any convenient length of conductor.

3. Test each pole individually by the pulse method as follows:

   The pulse method requires that the test equipment have a controlled closing and a pointer-stop ammeter, a calibrated image-retaining oscilloscope, or a high-speed, sampling-rate digital ammeter. The pulse method involves the following steps:

   1. Connect one pole of the test circuit breaker to the test equipment.

   2. Set the current control of the test equipment to a value approximately 70% of the instantaneous trip current setting.

      Example: If the instantaneous (magnetic) trip setting is 2000 A, set the test equipment to 1400 A.

   3. After the circuit breaker is properly connected and adjusted, apply current in approximately 10-cycle pulses.

   4. Starting at 70% of the instantaneous trip setting, increase the current of each pulse until the circuit breaker trips. After each pulse, move the circuit breaker handle to the full reset position and then to the on position.

   5. Repeat step D to recheck and verify this value. Start with the current level below the value measured in step D to ensure a “no trip” on the initial pulse.

4. Record current level and trip time. To ensure protection of the rated conductor, the current necessary to trip the circuit breaker instantaneously must not exceed 140% of the high setting for circuit breakers 250 A frame size and below, and 125% of the high setting for circuit breakers 400 A frame size and above. These settings are printed on the faceplate label of the circuit breaker. If currents higher than these maximum levels are necessary to trip the circuit breaker, consult your local field sales office.

If field test results fall outside the characteristic trip curve tolerance band, the test conditions and methods must be carefully evaluated for accuracy.

Rated Current Hold-In Test

This test should be performed only on circuit breakers that have been nuisance tripping under normal conditions.

Conduct the test in a 25°C (77°F) ambient temperature using a high-current, low-voltage ac power supply. Follow the same procedure used in the Inverse-time Overcurrent Trip Test, through . Connect all poles of the circuit breaker in series using cables with the appropriate ampacity for the application. These cables should be 4 ft. (1.22 m) long per terminal (8 ft. [2.43 m] total between poles). All connectors must be properly torqued according to the circuit breaker label specifications.

The circuit breaker should not trip when 100% of the device’s rated current is applied for one hour for circuit breakers rated less than 100 A, or two hours for circuit breakers rated more than 100 A. If the circuit breaker trips, reset and move the handle from the off to on position several times while under load, then repeat the test. If the tripping condition continues, contact your local field sales office.