A unique way to use 3-pole circuit breakers
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It’s not a totally new idea: In order to save space and reduce cost, a three-pole circuit breaker can protect three single phase loads. What if the loads – e.g. electric motors – have different technical data and need different types of protection? E-T-A’s three-pole circuit breaker/switch combinations offer a convenient solution. The pole chambers of these circuit breakers can be built with different bimetals working as trip elements so that the loads are appropriately protected.
If a machine only has a single drive motor, it is fairly easy to select the appropriate overcurrent protection. First, a designer needs to decide what kind of overcurrent protection should be used. Normally, a designer can choose between circuit breakers for equipment protection (CBEs), temperature limiters and blade fuses. If a circuit breaker is selected, the correct current rating, trip curve and actuation method must be defined.
Selection becomes more complex if a machine has more than one drive motor or if several loads must be protected within a system or plant. In this case, a few more questions must be answered: Is group protection sufficient or are individual protection devices required? Should only faulty loads be disconnected in the event of a failure or should an entire group of loads be disconnected? The answer to these questions strongly depends on the application. There is no general solution available. What is appropriate for one machine, may be just the opposite for another.
In equipment with multiple loads, it generally makes sense to protect each load separately and individually. Unlike group protection, individual protection ensures that each electrical load is appropriately protected against possible damages caused by overheating. It also widely eliminates nuisance tripping. If single pole protection is sufficient, single pole circuit breakers for equipment (CBEs) are a reliable solution. Example: If three motors must be protected within one machine, the Design Engineer must specify three single pole circuit breakers for this application.
However, there is an unique alternative: Instead of three single pole circuit breakers, an engineer can use one three-pole circuit breaker. This significantly reduces mounting time and space requirements. In addition, a three-pole circuit breaker is normally less expensive than three single pole breakers. Also, in the event of a load failure, all three loads will always be disconnected from the supply voltage, including the non-faulty loads – ensuring complete machine shutdown.
However, a three-pole circuit breaker can only be used if the current ratings of the circuit breakers correspond with the performance requirements of the loads. If the three loads are identical electric motors, an engineer can use any standard three-pole circuit breaker. But what happens if the motors require different circuit breaker ratings? Traditionally, standard three-pole circuit breakers could not be used because the three pole chambers have identical ratings. But not anymore.
The scenario above is possible with E-T-A’s thermal circuit breaker/switch combinations, e.g. the three-pole rocker-actuated 3130 or the 3140-multipole model with push button operation. The pole chambers can be fitted with different bimetal trip elements. This allows customers to select the appropriate current ratings from the entire circuit breaker current rating range for each pole of a three-pole circuit breaker. The rocker-actuated 3130 three-pole circuit breaker can have a 5A bimetal in the first pole chamber, a 10A bimetal in the second chamber and a 16A bimetal in the third chamber. This allows ideal and precise specification of the circuit breaker ratings to the loads requiring protection. E-T-A’s 3130 and 3140 circuit breaker types carry international approvals, including VDE, UL, CSA and CCC, even when fitted with different bimetal trip elements and are suitable for global use.
Many customers take advantage of the different bimetals/ratings in multipole devices to implement a so-called “Dual Voltage Breaker”. For example, this can benefit a North American treadmill manufacturer selling its products into the domestic market in the US and also in Germany.
The drive motors of these treadmills always have the same power. However, the standard supply voltage is nearly twice as high in Germany as in the US (Germany: AC 230 V, USA: AC 120 V), the European version requires “half the current” of the North American version for the same driving power. This has a direct effect on the current ratings of the circuit breakers required.
In our example, the manufacturer of the treadmills requires a 12A circuit breaker for the US version and a 6A breaker for the German version. In order to reduce the number of components required and improve the equipment availability, the manufacturer decided to use a “Dual Voltage Breaker” for his treadmills which makes one single circuit breaker out of multiple. He actually used the following configuration of the 3130 thermal three-pole circuit breaker:
- rating of first pole: 12 A
- rating of second pole: 12 A
- rating of third pole: 6 A
North American treadmills are double pole protected in this application with a 12 A rating (using pole chambers 1 and 2, while pole chamber 3 remains unused) and treadmills for the German market require single pole protection rated 6 A (by using pole chamber 3, pole chambers 1 and 2 remain unused).
This example illustrates how versatile using multipole circuit breakers can be. If they are specified to the application, they help reduce space requirements and costs compared to other solutions with single pole circuit breakers.