Looping a power supply through a circuit breaker twice. 1

[hollandhvac]

I have got a control design and notice that the single phase power supply went through a three phase circuit breaker twice. I have asked our electrical engineer here, but he told me there was a reason for it but he cannot remember what the reason is. Can anybody here tell me what the reason is?
I have attached a copy of the design drawing and marked the circuit breaker.

 

[DanEE]

Haven't seen it for AC circuits, but that is relatively common in DC circuits to increase the instantaneous interrupt rating.

Circuit breaker arc suppression is more of a challenge with DC since there are no zero crossings to assist in killing the arc.

You'll find the practice documented in the specs of some breakers with interrupt ratings listed for single pass and multiple pass through the breaker. We use MCB breakers spec'd and UL listed for this as the shunt trip for 48 volt DC plants as the EPO.

One would presume that the instantaneous interrupt requirements for your circuit similarly requires..

Pull the spec for the C/B and see if the 2 pass configuration is listed and supported by the manufacturer.

 

[rbulsara]

Probably to make the all three overload elements to see the current, so that it does not think that one of the poles (phase) is missing and prevent trip on single phasing.

(Similar wiring is used in DC circuits for entirely a different reason, to provide multiple breaks.)

Rafiq Bulsara

http://www.srengineersct.com

 

[burnt2x]

rbulsara nailed it!

 

[waross]

Using all three O/Ls gives better action on marginal overloads even on breakers without differential trip. The overload heaters generate heat. Some of this heat is lost through the ambient air in the heater section of the breaker until an equilibrium temperature is reached. Using all three heating elements ensures that the breaker tripping follows the design Time/Current curves.
When differential trip is a feature see Rafiq's comment.
Many O/Ls intended for motor protection have a differential feature and all three poles MUST be used. The loop back is common when single phase motors are fed from a three phase motor protection device.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[dpc]

I believe this may be a requirement when using a 3-pole breaker with a single-phase circuit for the reasons already mentioned. Consult the instruction manual for the breaker.



David Castor

http://www.cvoes.com

 

[hollandhvac]

All above thanks for your answers, it is clear to me why this is done.

 

[stevenal]

I see one pole is for the neutral. The neutral is generally not disconnected, since an open neutral by itself is a hazard. This design lessens the chances of that happening by giving L2 twice the opportunity to successfully open.

 

[jraef]

Stevenal, it depends on where you are. Some systems in other countries require opening the neutral.

As is the case with "circuit breakers". Here in North America, what we call a low voltage "circuit breaker" is a UL489 listed device and in that design, looping through is unnecessary because they do NOT have differential trips that would bias the trip threshold if current were not passing through all 3 legs. As Bill said it is OK and maybe even preferrable if you have a 3 pole device, but not absolutely necessary.

But in IEC countries, the term "circuit breaker" also includes devices that we can only call "manual motor protectors" because they have adjustable thermal trips (something precluded on UL489 non-solid state "circuit breakers"). Most of those are only available as 3 pole devices and because they are more often used for motors, also include the differential trip feature. Given the very low current trip rating shown in your drawing and the range, indicating adjustability, that appears to be the type of "circuit breaker" you are using here. So it is absolutely necessary to have current pass through all 3 poles on that type of device.


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