MCCB Protection

CONCEPT OF PROTECTION COORDINATION

1.1

An electric circuit has various irregular phenomena including overload, short- circuit, ground fault, overvoltage and undervoltage. These irregular phenomena do not occur frequently, but if sufficient measures are not provided,the damage can be extensive. In addition, spreading of the accident to the upstream 

system cannot be avoided. Various protective devices are installed on an electric 

circuit  in preparation for the rare accident. However, if these are not selected or used properly, they will not serve as a protective function.

2.1

According to the “High-voltage Incoming Facility Policy”, Protection Coordination is defined as “the accident circuit must be accurately tripped,
and the supply of power continued to healthy circuits past the accident circuit. Adjust the operating characteristics curve of the protective devices to prevent load devices, circuit devices, and breakers from been damaged.”

3.1

MCCB is used to protect the wiring from burning under an overload or by the short circuit current.   The overload and short- circuit current passing through 
is interrupted at the installation place so that the spread of the accident can be limited to as small a range as possible. However, it is necessary to use a 
suitable protection method for the power feed conditions required by the load while taking into consideration matters such as layout  of the protective devices
and cost efficiency.

4. Selective tripping method

4.1  Basics of Selective Tripping Method

The selective tripping method is a protection method with which only the

protective device directly related to the accident circuit functions. The other healthy circuits continue power feed. For example in Figure below, only MCCB₂ 

functions in reaction to the accident at the S₂ point, and the upstream MCCB₁ and the MCCB₃ for the other branch circuits do not function.

Selective tripping should be used for all overcurrents including the 

overload and short-circuit. However, in consideration of cost efficiency, measures should be taken to expand the range in which the relation can be retained.

Considering the selective tripping method

The above figure takes a look at using the normal MCCB.  The operating
characteristics curve of both MCCB₁ and MCCB₂ are compared. If this
relation is as shown in Figure below both do not intersect, so it appears
that the selective tripping relation can be retained in all areas.However, 
since it is confirmed that MCCB₁ does not function, a non-operating  
characteristics curve must be drawn instead of an operating characteristics curve. In other words, the so called unlatching time (returnable time)
 of the MCCB₁ must be understood.

The unlatching time refers to the maximum overcurrent passage time that does not result in an operation when a set overcurrent flows to MCCB for a set time With MCCB, the 
operating time within the long-time tripping range is long, so the difference between 
the operating time and the unlatching time can be ignored. However, in the instantaneous tripping range, the tripping time itself is usually 20ms or less and is very short, so the unlatching time cannot be ignored. As shown in Fig. 4. 3, in the instantaneous tripping range of the operating characteristics curve, the unlatching time 
must be drawn accurately, and must be compared with the branch circuit’s 
MCCB tripping characteristics curve. As stated below, normally T₁ is 20ms or less and depending on the frame size the difference is not great. T₂ is several ms, 
so MCCB₁ and MCCB₂ relation is normally as shown in Fig. 4. 4. Both breakers 
can retain the selective tripping relation only to the cross point of MCCB₁ 
unlatching time and MCCB₂ all tripping time. In other words, it is retained only to MCCB₁ instantaneous tripping current value Ii.

           As explained above, with a main circuit MCCB₁ and branch MCCB₂ as shown in Fig. 4. 1,

the selective tripping range extends to the instantaneous tripping current value for the main circuit MCCB₁ However, the S₁ point accident current is considered to the S₂ 

point’s short-circuit current, so the selective tripping function must be retained for the 

entire range or for all overload currents. As shown in Fig. 4. 5, to retain the selective tripping relation for the full range, the MCCB₁ unlatching time can be extended so that it does no cross with MCCB₁ operating characteristics curve. For example, T’₂ could be 

extended by approx. 30ms. This is the only method available for the MCCB with a short-time delay tripping characteristics

4.2 Breaker for Protection Coordination

(1)   Electronic circuit breaker

The electronic circuit breaker has a short-time delay tripping characteristics that can adjust the pickup current as shown in Fig. 4. 6, and is suited for the selective tripping. Since instantaneous tripping is used for large short-circuit currents, the breaking capacity does not drop when the sacrificed high-speed tripping as occurs with the conventional short- time MCCB. 

The electronic type is equipped with outstanding features as the long-time operating time, short-time operating current, short- time operating time and instantaneous tripping current can be adjusted, so selective tripping can be used in various applications. Fig. 4. 7 shows a photo of the electronic circuit breaker’s characteristics setting section. Fig. 4. 8 shows an example of the settings. Fig. 4. 9 shows the coordination relation. 

With the circuit configuration shown in Fig. 4. 8, the coordination for operating characteristics is completely attained between the 1st step (NF1600-SEW, 1600A setting), 2nd step (NF630-SEW, 500A setting), and 3rd step 
(NF250-SV, 150A) as shown in Fig. 4.9. Selective tripping up to 20kA is 
possible between the 1st and 2nd steps, and up to 10kA between the 2nd 
and 3rd steps.