Electrically held contactors – Siemens

Electrically maintained contactors – Siemens contactor type CLM

Most lighting and heating applications require a contactor to control the loads. One type of contactor is an electrically held contactor, similar to a magnetic starter. Unlike magnetic starters, however, light and heater contactors are designed for lighting and resistive heating loads rather than motor loads.

Siemens LE class lighting and heating Contactors are available with 2 to 12 poles from 20 to 400 amps.

They can be used with 480 VAC and 600 VAC tungsten ballast type lighting loads as well as 600 VAC resistive loads. Enclosures are also available.

Electric light contactors are similar to the magnetic contactors and starters discussed previously. When current is applied to the coil, all normally open contacts close. In the event of a power failure, these contacts open, thus cutting off the power supply to the connected loads.

Because the constant current required to keep the contactor activated causes a humming noise, these contactors should not be used for applications such as libraries, hospitals and some commercial buildings where this noise will be an issue.

Circuit breaker or vacuum contactor? 

Application considerations

To compare the application of medium voltage circuit breakers and fused contactors, it is necessary to understand the fundamental characteristics of each switching technology.

Left vacuum contactor 6kV 400A; Right – 12 kV, 40 kA indoor vacuum circuit breaker

Electrically held contactors - Siemens

The comparison presented here shows the main characteristics of medium voltage circuit breakers and medium voltage fused contactors that influence the application. Of course, the entries in the table are generalized and the information varies according to the nominal voltage and current characteristics of the equipment.

However, the comparison is valid for a comprehensive understanding.

Medium voltage circuit breakers are preferred when:

  1. Typical loads include transformers, capacitors, larger motors, generators, or distribution lines.
    (The required ratings exceed those of 400A or 720Aat vacuum contactors up to 7.2 kV)
  2. The continuous load current is high (eg, bigger transformers, bigger motors)
  3. Switching is not very frequent (weekly or monthly, for example), high endurance (1000 operations) is satisfactory
  4. Continuity of the process is critical (eg, no time for fuse replacement).
  5. Reduced voltage (RV) starting is not required (starting a passenger vehicle complicates the configuration of the switch bus)

Medium voltage NEMA class E2 controllers (fused contactors) are preferred when:

  1. Typical loads include motors or smaller transformers.
  2. The continuous load current is low or moderate (eg, smaller motors or transformers.
  3. Switching is very common (eg, daily or several times a day); very high endurance (100,000 operations) is required.
  4. Continuity of the process is consistent with the time it takes to replace the fuses.
  5. Reduced voltage starting is necessary to reduce starting service (and voltage fluctuations) on the system.

Historically, circuit breakers have been used for medium voltage motors in some industries, especially in power plants .

As these stations have aged, and the station operation has changed from basic service to peak service, many of these motor start circuit breakers have experienced total operations well in excess of the endurance required by ANSI / IEEE standards. As a result, these applications have had higher maintenance costs than if medium voltage fused contractors had been used originally.

On the other hand, users in the process industries have long favored the use of fused contractors for such applications and long service with reduced maintenance costs.

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