Cascade breaking method

The primary function of the breaker is to safely interrupt an accident current. The technical standards for electrical equipment state that a breaker with a sufficient breaking capacity for the wiring must be installed. However, there are cases when MCCB breaking capacity can be insufficient when the power packs for the power system increase. In addition, the cost efficiency of the entire system is also an important point. Incorporation of cascade breaking technology between two breakers installed serially to difference positions in the electric circuit is considered.

Basically, when using the cascade breaking method, the selective tripping system is sacrificed and both are not established simultaneously. Each maker has announced combinations of this cascade tripping method by combining two breakers and data to backup the operation. MCCB wiring path is applied according to this combination. Cascade breaking is also prescribed in Interpretation 37 of the Electrical Installations Technical Standards.

4. 3. 1 Combination of cascade breaking type breakers

(1)   Combination of MCCB units

Focusing on the fact that MCCB opening time is extremely fast, the arc generated at MCCB₂ contact and the arc generated at MCCB₁ contact are superimposed on the short-circuit current generated at the X point short-circuit accident in Fig. 4. 13. These two cooperate to trip the circuit. This reduction of arc energy applied on the branch MCCB₂ is the definition of the cascade breaking method.

The operation that takes place between the two MCCBs for ideal cascade tripping is explained below.

If a short-circuit current larger than MCCB₂ breaking capacity occurs at the X point, MCCB₁ opens after ta seconds, and the arc voltage Va occurs. The short-circuit current is limited by this Va, and suppressed to the peak value I₀. Subsequently, MCCB₂ opens after tb-ta seconds, and arc voltage Vb is generated. Total breaking is completed after tc-tb seconds, but an arc is generated at both MCCB₁ and MCCB₂ during that time. When the current for MCCB₁ is limited, the arc energy is shared simultaneously to assist MCCB₂.

Coordination between the two MCCB units in cascade breaking method refers to this action. MCCB₁ must have a current limiting function, and the opening time must be as quick as MCCB₂.

MCCB combination for cascade protection is limited to the combinations recommended by the makers. The following conditions must be satisfied for the cascade operation coordination between MCCB units is established.

q  The peak current value limited by MCCB₁ and MCCB₂ must be less than MCCB₂’s mechanical strength.

w  The maximum passage I2·t during the short-circuit current tripping by MCCB₁ and MCCB₂ must be less than MCCB₂’s thermal strength.

e  The i intersection with MCCB ₂ total breaking characteristics curve and MCCB₁ opening time must

be within MCCB₂ breaking capacity.

by MCCB₁. r The arc energy (Ú   tc tb Vbidt) generated in MCCB₂ must be less than MCCB₂ resistance backup and protected by MCCB₁

t  MCCB₁ must have sufficient breaking capacity by itself in respect to a short-circuit in the bus.

If a short-circuit current exceeding 10,000A is estimated in the branch circuit, it is often economical to use the cascade breaking method. In this case, a breaker with the capability to interrupt a 10,000A or larger short-circuit current is required as the backed up breaker. However, when using two breakers in combination at the same place as one overcurrent breaker, coordination is established between the backup breaker and backed up breaker, and the 10,000A or higher breaking capacity limit does not apply.

The following locations are viewed as the same place:

q Within the same panel board, the same power distribution panel or the line board.

w Within the same cubicle, control center or the line board

e Within the same electricity room (incoming power room, transformer room)

(1)   Combination of fuse and MCCB

There are cases when a fuse is used as MCCB upstream overcurrent breaker for the following purposes:

              The fuse overload range is operated by MCCB so that the fuse does not blow or deteriorate.

•    Within the short-circuit range, to provide cascade protection of MCCB in areas where the short-circuit current is extremely large.

The required conditions are as follow within Fig. 4. 15.

q The fuse’s tolerable short-time characteristics (a) must not intersect with MCCB characteristics within the overload range.

w The cross point current Ic with the fuse blowing characteristics (b) and MCCB characteristics (d) must be 80% or less of MCCB rated breaking current Is.

e The fuse’s total breaking I2·t and passing current peak value ip must be within MCCB tolerable limit.

r The arc energy generated by the current limited by the fuse and the arc voltage of MCCB that interrupts it must be within the MCCB tolerable limit.

Conditions q and w above can be reviewed based on information available in catalogs, etc. However, conditions e and r cannot be quantitatively reviewed on paper. Thus, in the same manner as cascades between MCCB units, when applying a cascade between the fuse and MCCB, the combinations are limited to those that have actually been tested and verified.